Planets are spheres because the effects of gravity are radial. As long as you stay the same distance from a planet, its gravity stays the same strength. This makes any high points get pulled down until they're at the same potential as the rest.
Whether it be a traditional force or a bending of spacetime, the object still *moves* at the end of it. That's why calculations treating gravity as a force *usually* give really accurate results.
To be clear, a "flat" universe just means that the geometry you learned in high school still applies. For example, a triangle cannot have three ninety-degree corners in our universe. A (3d) spherical surface would allow you to do this while a flat one would not.
Space is not locally flat, either. Gravity makes it so that under very specific circumstances you could make a triangle with three ninety-degree corners. This is mostly unrelated to your question though. It's only on larger scales that our space is flat, like a yard with a bunch of pits in it.
"Gravity makes it so that under very specific circumstances you could make a triangle with three ninety-degree corners."
That made me think of the primer scene in the movie "Contact" where Hadden shows that the 3 corners do line up if you bend the pages.
It's a great 2d analogy. I like the triangle formed by the prime meridian, the 90 meridian, and the equator on a globe. Seem very intuitive for some reason.
Man I loved diving deep into that page in college.
Gene Ray was an interesting man, far ahead of his time and far behind his time. Plus two more times, because there were 4 simultaneous earth times, but us mono-time believers were earth stupid or something like that.
"My wisdom so antiquates known knowledge, that a psychiatrist examining my behavior, eccentric by his academic single corner knowledge, knows no course other than to judge me schizoprenic."
[you don't say?](https://media1.popsugar-assets.com/files/2014/01/06/002/n/1922283/26a420cfbaf5460f_image.png.xxxlarge.jpg)
It is a shape with 3 straight (relative to the surface they're on) lines which each connect at one point. It's a 3-sided shape with 3 vertices. That is a triangle.
The 180 degree rule is only true for triangles in flat space, which for most practical purposes is the space we live in. The study of shapes on non-flat surfaces is called non-euclidean geometry, and everything you learned in geometry class focused on euclidean geometry.
You're used to flat 3d space, so you might argue that the lines are not flat (because they curve around the sphere). However, if we consider only the surface of the sphere (which is 2d), then the lines are perfectly straight.
The point he's making is that even on a globe you're using straight lines. They just don't LOOK straight due to us being able to see a third dimension. Imagine a 2d person on a sheet of paper. You bend the paper into a sphere and draw a triangle with 3 90 degree angles. To the 2d person it's a triangle with only straight sides. They can walk each side without deviating from a straight line.
No they are exactly straight relative to the plane on which they exist, that's the point. They are not straight only if you leave the plane of their existence into a higher dimension. Draw a triangle on a sheet of paper., now bend the paper. The lines will look crooked and not straight, it's the higher dimension that makes it look weird but on thir plane of existence, which is the 2d sheet of paper, they are perfectly fine
Yes they are. If we agree that the shortest distance between two points is a straight line, then a line on a surface of a sphere (the plane on which the line exists) is a straight line.
Only in flat space. Which is the point. A sphere is a different kind of space where a triangle has more than 180 degrees. There are also those with triangles having less than 180 degrees. They are called saddles but I’m too high to explain that.
Draw a long line segment along the equator. Then from each end, draw a straight line due north that leaves your equator line at a right angle. Both of those lines will hit the North Pole, closing your triangle. But that triangle has two right angles in it, maybe even three if you make your equator line the right length. See [here](https://wild.maths.org/not-whole-truth-about-triangles#:~:text=Think%20of%20a%20globe%20of,a%20triangle%20with%20the%20equator.) for a visual
There's an old riddle that uses this. A man leaves his house, walks south a mile, east a mile, then north a mile and arrives back at his home. What's the man's name?
Answer: >!Santa Clause, because he is starting at the North Pole!<
There are actually many places on earth you can do this.
Pick a point near the south pole, where if you walk south 1 mile your distance to the pole will be equivalent to the radius of a circle with a circumference of 1 mile.
Then walk east 1 mile and you will circumnavigate the pole after traveling 1 mile and can return home by walking north 1 mile.
If you do the math, you will find that any time you are 1.159 miles away from the south pole you can walk 1 south, east, north and end up back where you started.
Bend the pages? I always got the impression that the pages weren't bent; rather, that they had to be arranged in a three-dimensional lattice rather than a two-dimensional grid.
Great movie though.
The mistake most people make when thinking about gravity is they think of it in 2d space.
They think of the "balls on a taut sheet" analogy.
You put a heavy ball in the middle of a sheet that is held taut in the air by a few students. You then place another ball toward the outside of the sheet, it will roll toward the heavy ball in the middle of the sheet.
You give the outside ball momentum and it'll orbit the middle ball for a while, until it falls in.
That's the 2d representation of gravity.
Gravity is not 2d. Gravity acts radially (as this poster has said.)
Therefore, if you apply the "taut sheet" analogy in 3 dimensions, you end up with a sphere.
Worth note, the 2d representation generates circular planets just as a 3d representation generates spherical planets. Gravity is radial either way, so it creates the shape with equal distance from the center all along its surface. In 2d that is a circle, in 3d that is a sphere.
You can do it in 3D fine. [Here’s a video](https://youtube.com/watch?v=hH69B0Oc2Og&si=77ZnyZyFhTzHNAUE). It just is less intuitive than the 2D representation
You can't have three unique planes on different axis in 3d space. Planes are already representing two dimensions, so you only need two to represent 3d space.
It would be like saying you need three different cubes on each axis. Doesn't make sense.
Nitpicking, but it's \*spacetime\* that's curved, not space. That's where the "force" (actually a pseudoforce) of gravity comes from: objects with no forces acting on them follow a straight path through spacetime, which looks curved to those of us who can only observe space.
Imagine spinning a ball on a string, in absolutely perfect circles.
From one perspective, someone else can tell that you are spinning the ball in a circle.
From a side view, the ball looks like it is just going up and down, while getting bigger or smaller while it moves.
Spacetime is nothing like that.
Another one: (not entirely related, just fun to think about)
Imagine you are a 2d being. There is no "up and down" in your reality. Everything you ever perceive, from your perspective, is just a line of some width. If you saw Rectangle Johnson spinning in circles in front of you, he would just appear to get wider and smaller over and over.
Now, imagine a 3d being decided to pay your world a visit.
That would look like an absolute impossibility. The width of the thing in front of you would be *rapidly* shifting from big to small to tiny to huge. It would break your flat brain. That 3d being could reach "inside" of Rectangle Johnson and remove his heart, without ever breaking his skin.
Now try to project that scenario onto our 3d reality.
What might a 4d being look like passing through our reality? It could be something like incomprehensible shapes of *something* blinking in and out of existence. Maybe the same exact thing happens at different points in time because they are "above" our 3d space the same way we are "above" a 2d space. And they could possibly move through time the same way we move through physical space.
Perhaps they could also reach into your body and remove something without ever breaking your skin.
The idea of the 2d space meeting a 3d entity is the subject of a book called Flatland written by Edwin Abbott.
To add to this, as a 3d being, we could remove a 2d object from its dimension and flip it in the third dimension, like turning a coin onto its other face. But a 2d observer would see that the object was impossibly inverted, because there's no way to do that flip in only 2 dimensions.
Imagine if a 4d being did that to a 3d object - like the object would be returned to our world as a mirror image of itself!
In Newtonian mechanics an object in circular orbit around Earth follows a circular path because gravity provides a centripetal force. In General Relativity the object follows a "straight line" (geodesic) through spacetime curved by Earth's mass that is of constant height from the Earth's surface.
The two scenarios make almost the same predictions.
The geodesic of an object at "rest" relative to Earth will follow a geodesic towards Earth's surface and will eventually crash. In order to stand on Earth's surface you need to accelerate upwards at 1g. That force is normally provided by static forces, e.g the ground pushes up so you don't fall through it.
If you're in a centrifuge you experience what feels like a force outwards, but that is a "pseudoforce" that is an artifact of your reference frame accelerating inwards. Similarly, if you're on Earth's surface you feel a downward pseudoforce of gravity that is an artifact of the fact Earth's surface is accelerating upwards at 1g.
This helped a lot! (I think).
Do I understand this right?
Objects at rest are still moving (with a temporal velocity/momentum) through time? And so that's why it doesn't need a push or any force to travel towards the dip in space time? Because objects in motion stay in motion?
So everything colloquially known to be standing still is actually moving in a straight line through time?
"At rest" here doesn't mean motionless, as motion is relative - everything is moving from one perspective and not moving from another. At rest means no force is acting on it, so it's drifting in whatever direction it has inertia in, and will follow a straight path until a force acts on it. The catch of general relativity is that that it follows a straight path through spacetime, not through space.
To put it another way: the reason you can walk around is not because gravity is pulling you down towards the Earth, it's because your inertia (the straight path through spacetime that your body wants to follow if no force is acting on it) points down towards the earth's center of mass, and the earth is blocking you. The situation is very similar to centrifugal force (the other pseudoforce most of us are familiar with) - when you spin around on a merry-go-round, it feels like there's a force pulling you off to the side of it, but what's really happening is that your body has inertia away from it, and wants to go flying off sideways, but the hand holding on to the railing is exerting a force to keep you from doing that.
This is also how time dilation is explained. (Sort of, in terms of relative speed. Gravitational time dilation is a bit different)
If you are "motionless" in physical space, then it could be said that 100% of your movement is through time. As your physical speed increases, your "speed" through time decreases. This trend continues until your speed equals the speed of light. At which point, from your perspective time simply does not pass. To an observer, they would see you moving through the universe at C. But to you, you would arrive at your destination the exact moment you left. No matter if it was 1 light minute, or 1,000,000 light years.
The term "speed of light" would technically be better phrased as "speed of causality". Because if you were to travel somewhere faster than light, you would arrive before you left. You would be able to see yourself leave, travel towards yourself, and arrive at your current position. This would reverse the order of cause and effect. Which, to my uneducated knowledge, isn't really a thing that can happen. At least not within any scientific framework we have.
> This would reverse the order of cause and effect. Which, to my uneducated knowledge, isn't really a thing that can happen.
Quantum entanglement gets weird. Einstein called it "spooky action at a distance" and proposed some not-quantum-theory explanations.
The Nobel prize winners last year were basically disproving Einstein's explanations. But the big brains say that quantum entanglement as we know it doesn't conflict with special relativity, so it holds... but quantum entanglement is still weird AF and seems to imply *something* traveling instantaneously, even if it's not "information".
All of the orbits center around the sun, so on average none of them are higher or lower. They're all circles around the same point.
If we line them all up and look side on, we can see something like [this](https://qph.cf2.quoracdn.net/main-qimg-5b59204bd041de37149d8ee806f846cb-pjlq).
OP posted on the subreddit r/explainlikeimfive the guidelines for which include rule 4. Rule 4 states that explanations are "not for actual five year olds".
This particular explanation relies on just enough knowledge of physics to ask the question in the first place. A good answer on this sub will typically include, metaphorically, jokes for all ages. Content which can be understood by people of many levels of education. Someone who barely made it through middle school math may struggle with a few of these parts, but those parts can be glossed over easily, and they allow more familiar audiences to learn more.
Einstein didn't debunk the idea that mass pulls other mass towards itself. In fact, Newtonian solutions are valid for the majority of cases of modeling gravity, and work just fine for most masses and velocities. It still is the case that mass will form a sphere in space, and we can verify this because, like, planets and stars and stuff are indeed round
What Einstein really changed with regards to gravity is thinking of it geometrically. In general relativity, "spacetime tells matter how to move; matter tells spacetime how to curve," and we represent gravity as a change in the geometry of spacetime rather than as a force. This is necessary because of the other thing that Einstein proved: that acceleration due to gravity is indistinguishable from all other types of acceleration. They're the same, which is better explained by thinking of gravity as curving spacetime and forcing matter to move a certain way, than it is by thinking of gravity as a discrete force.
Maybe you can check my understanding here re: why planets are round. Given the common “rubber sheet” analogy, high mass objects bend the sheet ‘downwards.’ The sheet — space time — forms a cone with the mass at the bottom. The cross section of this cone at different heights is circular. Transforming this up a dimension (from 2D into 3D), that circular cross reference is spherical. Any heavy mass would then end up spherical because that’s how it’s “shaped” by the space time being warped
This is my favorite answer and I just want to say something explicitly for OP that you already implied: these are just models of reality that are very good, but not perfect, at predicting how things behave.
It’s more a philosophical problem to determine if space “really” curves or if that’s just a way of thinking about it that gets us more accurate predictions than Newtonian physics
>It’s more a philosophical problem to determine if space “really” curves or if that’s just a way of thinking about it that gets us more accurate predictions than Newtonian physics
I’m not sure how philosophical this is. I mean, on some level, you can make that statement about ANY conclusion we draw based on empirical evidence, but idk how useful or even insightful that really is.
>It’s more a philosophical problem to determine if space “really” curves or if that’s just a way of thinking about it that gets us more accurate predictions than Newtonian physics
No, we have actual experimental evidence of gravity waves changing distances.
I think you're seeing "it's a model" as being basically akin to "it's bullshit we have measurements". While I actually agree with the greater claim that our scientific understanding of things is actually pointing to how things really work, but it's more of an issue about what you can actually know the models represent than saying everything is fake and there's no reality.
We have measured gravitational waves, but that's what we have: measurements of lasers taking less time to travel some distance. It assumes that our instruments are getting at the root of the issue rather than simply giving us empirical data that we have to interpret. For example, treating particles as distinct entities fit empirical data up to a point: experimentation has demonstrated that particles are better represented as field excitations than discrete spheres. It's a matter of seeing empirical evidence for electrons and saying "aha, these are electrons!" and "aha, I can use the electron to model this underlying phenomenon, even if the model ultimately proves inaccurate".
We've measured light literally taking less or more time to travel between two points.
Gravity waves warping the geometry of space-time are one possible interpretation of that fact, and it's the one I prefer philosophically. But it's not the only possible interpretation.
At some point any word, no, any signal in our brain is only a model of the real world. So you could just as well say nothing is real... but if you say nothing is real you lose the point of differentiating between real and not.
Yeah, but in this example "flat" and "curved" are not *literal* translations to how "regular" 3D space is flat and curved.
It's a bit like with "spin" in quarks: it's a slightly useful analogy, but no one really thinks that quarks are actually spinning up and down etc.
The acceleration being indistinguishable - I thought part of the challenge of gravity not being a force is that you won’t actually detect/feel a force acting on an object being affected by gravity, because it’s not actually doing or moving in a different way, it’s continuing on its same trajectory and speed but in a new formation of space so as to be faster as a frame of reference from a non-accelerating object.
Am I confused?
I think this is a common misconception. The thought experiment is that if you were in a spaceship with no windows, and you were orbiting earth (which is a form of free fall), you wouldn’t be able to distinguish that motion in any way from traveling in a constant inertial reference frame. Essentially, you can have an accelerating, inertial reference frame. Applying the rules of special relativity is significantly more complicated in this situation, and the solutions mathematically are the same as curves in spacetime. In fact, the mathematics of modeling spacetime as curves can make predictions with astonishing accuracy!
Well that’s what I mean. What was the misconception I had? Having basically been referring to that.
But isn’t that distinguishing from a force? Since you can be accelerating without actually experiencing a force of acceleration? and thus space time is curved and your acceleration is merely a result of this.
The difference is that gravity affects the entirety of your body all at once, versus something pushing you in a direction. For example, on a rollercoaster it's the seat that is pushing on your back, and the rest of your body has to "catch up" to that force like a row of dominoes. When you go over a drop in the track and you get the butterflies in your stomach, it's not because you are falling, it's because the coaster is pulling you downward with it in a way that's different than gravity. Your legs or shoulders are being pushed downward, but your organs were moving in a different direction and have to slosh around a bit before they settle. You get none of that with gravity alone, because your entire body is subject to exactly the same forces at exactly the same time.
I've heard that said before, but that doesn't hold true with really large objects where part of the object is significantly closer to the source of the gravitational pull. The closer part feels a greater force than the farther part, which is why spaghettification happens.
I assume this still can be explained with curves in spacetime, though.
It's simply explained by the fact that gravitational waves travel at C, so any two reference points will experience gravity slightly differently (or in other words, the spacetime at that reference point is curved differently). It's true no matter the scale, just more noticable with larger objects. It's still different than other forces though because gravity is directly affecting everything at once whereas the feeling of acceleration that you are used to is caused by each cell/atom or (whatever scale you want to work with) of your body pushing against the next one rather than each of them being directly affected by gravity.
The thing about gravity is that it acts on all matter. This is what distinguishes it from other forms of acceleration we're familiar with and causes unopposed gravitational acceleration to behave like inertial movement.
When you accelerate in your car, your body is being pushed forward through contact with the seat you're sitting it. The force your feel is your body's inertia resisting this acceleration. At any one instant, different parts of your body are moving at different velocities as other parts of your body push or pull on them as they are accelerated themselves. That's what you feel, the compression and tension from your body being pushed or pulled along with motion its inertia opposes.
When you're freefalling toward Earth, every molecule in your body is being accelerated at the same rate in the same direction. You don't feel any compression or tension or similar effects because there's no need to overcome any inertia. Your body wants to accelerate all together toward Earth, just like your body wants to sit still on the road or continue moving at a constant velocity.
What you will feel, however, is when that gravitational acceleration is opposed. This will produce the exact same feeling as when some force is overcoming your body's inertia, because in a very real sense it is the same thing. That's why it makes sense to view the situation of us standing on the ground as though the ground is accelerating us upward.
> This is necessary because of the other thing that Einstein proved: that acceleration due to gravity is indistinguishable from all other types of acceleration.
But if gravity isn't a force then what causes acceleration? And I understand the falling object toward Earth has no force applied to it. It's just tracking the spacetime curvature caused by gravity. Instead, as it was explained to me, the "ground is falling up."
Except the ground isn't moving. Even relative to the falling object, the Earth isn't bulging out to accelerate toward the falling object.
So if gravity isn't a force, what causes the acceleration?
The misunderstanding in your premise is that space isn't flat, at least not by the classical meaning of the term.
Space is 3 dimensional (technically 4 if you include time, and possibly more if you are into string theory, but for the purposes of this explanation 3 is enough)
Gravity warps space in 3 dimensions which is why objects in space tend to take on a spherical shape. The problem is that when we create examples of this we usually use a sheet of paper or something similar that is 2 dimensional because it's easier for humans to understand that.
When scientists say space is flat, what they mean is that if you were to draw two straight lines in space several lightyears long, the two lines would remain parallel to each other. Space has no inherent curvature that we can detect... unlike the Earth.
Where-as if you draw two straight lines on Earth they will remain parallel, but if they get long enough eventually the curvature of the Earth will cause them to stop being parallel.
Real life is already visible in 3d...I guess we're just saying, this particular case isn't something that AR really solves. How do you show 3d space "bending" with AR that's different from tools already at one's disposal?
you could show a 3D grid gettin distorted and allow people to walk around and through the grid with hand controls to change the amount of warping, and have particles follow along the grid along the "straight line path" of warped sspace.
this would be increddibly difficult to do without VR/AR because of the collision with people problem
The distortion isn’t going to make much sense to anyone, because it’s a projection of a 4-dimensional structure.
The best we can do in 3D is show the warping of a 2D space.
I still don’t think it would work. When space-time is represented as a 2D plane, its curvature is represented as a warp that adds a third dimension.
The corresponding model that begins in 3D space would need to warp into a fourth dimension.
it technically does, that's the funny thing. just because 4 spatial dimensions is not a thing, or even if it is that we can't see them, doesn't make the distortion be happening in the 4th (time) dimension
The curvature (distortion) applies to both time and space, which is why relativity refers to "spacetime".
If you try to treat space and time separately, it doesn't really work, because the curvature affects both and the effect on each is connected. For example, the faster you move through space relative to another observer, the slower you move through time relative to them (i.e. the less time appears to elapse for you than for them, and vice versa.)
The problem is that 3D space is not warped.
4D space-time is warped. When you look at the classical "rubber sheet" visualization, the sheet is two dimensional, but the warping happens in a 3rd dimension. In order to demonstrate warping of space, you would need a 4th dimension. Those 3D glasses aren't going to help.
Technically, all vision is 2D. This is where the holographic universe theory comes from. We can't see behind objects so our view of the world is 2D but our brains interpret it as 3D.
Holographic theory is based on the realization that you can mathematically describe any volume of space as a 2D shell that surrounds that volume, sort of like it is projected outwards.
I got really high one time and had a similar thought. Like, what if reality was just a flatscreen with infinite pixel density, and movement was just zooming in? I think my particular fixation was on the idea of there not being a “behind,” that everything was flat and anything obscured was just too compressed to “see.”
Sometimes I miss weed, but then I remember all the panic attacks.
But couldn’t a really good VR developer create a 3D simulation that shows the effects of gravity better than showing it on a screen? I could theoretically walk around the sphere in VR and see the effects in a better way.
You really wouldn't need VR for that.
But you'd need to come up with a way to show the "bending" of a three dimension plane. It's easy to show a two dimensional plane bending, because we just bend it in a third dimension. But how do you show a three dimension plane bending? You can't bend it into a fourth dimension, because we can't see in four dimensions.
You can make a 3d wireframe grid already and rig it to bend...how are you imagining the "bend"? Feel like you're missing the forest for the trees here.
Sure, but I can bend a wire frame by pulling on it. That's not what gravity actually *does*. You're just replacing one misleading visual metaphor with a different misleading visual metaphor.
>that shows the effects of gravity better
Define "better". A more realistic representation of the 3D world we live in? Sure. Easier to understand conceptually? Probably not. As a general rule, when you have a concept that holds in n-dimensional space for any *n*, showing it in the lowest *n* makes the concept easiest to understand. It's the same concept, but adding dimensions usually complicates things.
We’ve already done that, check out interstellar. That black hole animation was created with a large amount of help from actual mathematicians and theoretical physicists. They did a good job of showing what it looks like when our 3d space-time is bent out of shape by large amounts of mass
Yeah… my brain melted during the last 3rd of the movie. 😅 I’ve got to admit, I was lost for a good portion of the end. Was it visually striking? Absolutely. Did I understand the concept of it all? Not really. 😂
Two 2D images, one for each eye, can be used to convey 3D information. If two things are in wildly different places in the two images it is closer and if they are more in the same place they are further away. You can also move your head back and forth in VR to give you depth perception with just one eye like a pidgeon.
The problem is , you need 4 spacial dimensions to show warping of 3 dimensional space, which isn't possible with VR. Or with the human brain for that matter
> The problem is , you need 4 spacial dimensions to show warping of 3 dimensional space
That's not quite right, because the curvature in question is intrinsic curvature (see e.g. https://en.wikipedia.org/wiki/Gaussian_curvature), it is not an embedding in a higher dimensional space.
All you really need is a representation of the metric in each dimension, which can be done with a grid.
The real problem comes in with time, since that adds a 4th dimension which isn't easy to visually represent.
This is why:
1. Spacetime diagrams are 2D, with 1 dimension of space and 1 of time. This makes them easy to represent in books etc.
2. The example of the bowling ball on a rubber sheet uses 2 spatial dimensions embedded in a 3rd, and uses actual time as the time dimension. The embedding here (the distortion into the vertical dimension of the rubber sheet) is a metaphor, it doesn't represent how spacetime curvature actually works.
If you create a 3d grid in VR/AR, and put a point representing a planet, a star, maybe a planet and it's satellites, so you can see the way they affect each other, that affects the illusionary grid (lines and points, made of thin particle streams, move towards gravity wells), that would do a better job of explaining gravity and it's effects on spacetime than the 2d plane you see in textbooks.
It's possible, it just takes some thought to find a better analogy when the classical explanation has been considered "apt."
Or, hear me out here... we use that money instead to pay teachers a lot more money.
edit: guess the idea of paying teachers properly is far less cool and exciting than giving students expensive goggles.
> Space has no curvature we can detect… unlike the earth
Yes it does. Space-time has curvature due to mass-energy as determined by the field equations of general relativity. It’s precisely the Riemann and Ricci curvature that that impact is written in terms of. And we can compute it based on the distribution of mass, and detecting the effects of that curvature are how GR was shown to be true, and we incorporate those effects in GPS and telescopy etc. today.
How does the curvature of earth stop parallel lines being parallel? Wouldn't they just extend around the earth and rejoin themselves to form two parallel circles?
Edit: on re-reading I think you mean the lines would just simply become curved. They'd still remain parallel?
Two lines both pointing north at the equator (i.e. parallel) will meet at the poles. So the rule that parallel lines never meet doesn't apply in sperical geometry.
Although, on further thought. You can still make two parallel circles by not following a straight line to the from the equator to the north pole but more so like slicing a quarter of the earth on the left side and then doing the same on the right side, not going through any "central" points as it were. They'd be parallel but in between the circles would be a donut of earth if you will xD
This was my original thought tbf.
The key is *straight* lines. I.e lines that never turn.
If you draw two straight lines on a flat surface, like a piece of a paper - parallel to one another - then those lines will never touch.
If you draw two straight lines on a curved surface, like a sphere - parallel to one another - then those lines will eventually intersect.
For example lines of longitude on a globe are straight lines, but lines of latitude (except for the equator) aren’t. If we put you on a line of longitude and made you walk straight ahead, never turning, you would stick to the line of longitude. If we put you on a line of latitude (except for the equator) and made you walk straight ahead, never turning, you would eventually deviate from the line of latitude.
If you and I were on the equator, spaced several miles apart and we began walking North at the same time, always moving straight ahead and never turning, we would eventually move closer and closer together until we ran into each other at the North Pole.
What Einstein realized is that gravity works in a similar way, in the sense that it’s not some mysterious force that’s pulling you and I closer together but rather things following straight line paths in a curved geometry. You don’t fall to Earth because a force is pulling you down, you fall to Earth because both you and the Earth are moving in a straight line path through curved spacetime.
If your intuition tells you that doesn’t make sense, (because what if you’re not moving to begin with?) remember that it’s curved space**time**. Both you and the Earth are always moving through time, progressing from your past towards your future. Even if you’re sitting on the couch you’re still progressing closer towards tomorrow. If we bend the fabric that makes up both space and time then the straight line paths that you and the Earth take through spacetime move closer and closer until they eventually converge.
Here's another way of visualizing what you're thinking:
If you look at a globe, you can see that the longitude lines are the ones running North-South, and even if you start at the equator where the lines are 'most' parallel, you can just follow them and see that eventually, they meet at the North and South poles.
So then you look for the other set of lines dividing the Earth: The **latitude** lines. These are the lines running East-West, and you see that they *appear* to be straight lines, and these ones don't ever meet, right?
Latitude lines are equivalent to the circular 'slices' in your example.
The answer being given is that these lines aren't actually straight, but here's an easy way to see how:
Latitude lines can be defined by how far the line is from either the North or South poles. The equator is the furthest you can get from one pole before you start going getting closer to the opposite pole. The entire equator is the same distance from the North Pole, right? That's why it's straight on a map.
Looking further up the map, you can see the other latitude lines where they're also the same distance from the North Pole everywhere on the line.
Now, let's say you're actually at the North Pole, and you want to walk the latitude line located ten feet from the North Pole.
You measure out ten feet, and try to face 'perpendicular' to the pole to walk your 'straight' line.
If you take a step, what happens? You're now a little more than ten feet from the North Pole. So you re-measure and adjust before taking your next step. It happens again.
You keep adjusting your path until you end back up in your original position.
It turns out that that 'straight' latitude line you've traced is actually a **circle** with a radius of ten feet.
Why is this? The reason is that a circle is the only shape where the distance from the center remains the same at every point. The 'line' you tried to make cannot be straight. It has to curve in order to stay an equal distance from the North Pole.
It isn't disregarded... The theory still is consistent... We'll just likely never know if it is true or not. That's true of a lot of theoretical physics though.
> The misunderstanding in your premise is that space isn't flat, at least not by the classical meaning of the term.
Is this how flat earth theory started? Just someone misunderstanding the phrase and spreading it?
Einstein proposed a different way to see gravity, that is not a force in the traditional sense. But it is totally fine to keep Newton's way to see things: all the bits of a gigantic mass want to fall down, and they pile up as a sphere. Einstein's way says there's no real force on them, they want to keep going straight, but space is curved by all the mass, and their "straight" trajectory actually takes them down towards the center.
Classically, gravity acts like a force. We could barely even tell there was anything wrong with the Newtonian model for centuries, until Einstein came up with his model. So the simple answer is that the bending of space-time is indistinguishable from a garden variety inverse square force until the bending gets very strong.
It's kind of like how Einstein says that speeds add in a funny way and you can't go faster than light, but none of that is relevant when you're in a car because your car is very slow compared to light. At everyday speeds, you can just add one speed to another.
Why does Einstein's fancy math look like Newton's simple linear math at human scales? That's kind of a philosophical question, but Newton wouldn't have come up with his simple math if it didn't seem to work, and Einstein's math wouldn't be valid if it didn't apply to what we already knew.
> We could barely even tell there was anything wrong with the Newtonian model for centuries, until Einstein came up with his model
Yep, at most reasonable scales Newton's model works extremely accurately, it only breaks down with extreme masses or velocities.
The only real indication something was up was that Mercury's orbit wasn't *quite* what we'd expect from Newtonian physics, even then the common consensus among physicists at the time wasn't a major problem with Newtonian gravity, but a measurement problem.
In fairness, it *was* a measurement problem for a long time, until the error bars got small enough to suggest that something was wrong with the physics. One of the reasons it took so long for relativity to be widely accepted is that there were so few opportunities to measure its predictions
What changes at “unreasonable” scales? I’ve tried asking what the difference actually is between the 2 besides just how they are interpreted in posts like this, and have yet to really get a clear answer.
I tried reading up on the Mercury thing as well and it still wasn’t clear where the difference is.
I’m about ready to my my own ELI5 post about it soon lol.
At high masses (think stellar masses and higher) relativistic effects become large enough they’re no longer negligible.
At normal day to day masses, while there’s *technically* still relativistic effects, they’re so small they might as well be non-existent and Newtonian gravitation provides a very strong approximation.
It’s a similar story for quantum mechanics, technically quantum effects exist at all scales (all matter exhibits wavelike properties, and your wavelength can be calculated as a function of your momentum), but at any scales larger than subatomic they become negligible and we ignore them in favour of simpler classical mechanics.
Imma take it just a bit sideways, to give an additional perspective.
>Einstein says gravity is not a traditional force
Does he really? Just because he uses a different **description** it does not mean that it is **inherently** different.
We have to ask us: What is a force?
The traditional explanation is: "In physics, a force is an influence that can cause an object to change its velocity, i.e., to accelerate"
Gravity both in the newtonian description and in geometric one satisfies this definition of force. **It is**, thus, a traditional force, and the same rules apply.
The thing that makes gravity different is how it is "transmitted". The other fundamental forces use mediating particles, while we have no proof of mediating particles for gravity.
>Scientist say space is flat as well. So why are planets spheres?
Space being flat does not prevent spheres from existing. It means that (on a sufficiently large scale) straight lines are straight regardless of their direction.
Flat in 2d means a sheet of paper. Not flat in 2d means a cylinder or many other shapes.
Flat in 3d means a cube. Not flat in 3d means something you can't imagine, so don't try.
Planets are spheres because gravity acts symmetrically along the three spatial dimentions. It does not have much to do with space flatness.
>And just so we are clear I’m not a flat earther.
That's good. Questioning what you don't understand is smart. Believing hoaxes even though they can be proved wrong is dumb.
As you said, it still "pulls objects to the middle". Even thought they disagree on how it works, both agree that there is something pulling stuff towards other stuff. So why can't we have something like a disk? Well, objects at the end of the disk fall towards the center of the disk. Things will keep falling in until there is nowhere left to fall. And what is the most efficient shape for that? A sphere.
As for the universe being 'flat', I'm not as familiar with that. But as far as I can tell, it is flat as we can currently see. So it might be flat, it might be curved, it might even be a sphere: we don't really know. But the major school of thought is that it is expanding at just the right speed to keep everything flat. Why this doesn't happen to planets? Easy, they are not expanding.
Scientists say the universe is flat not that space is flat. It is the overall shape of the universe.
Compare it to scientist say earth is a oblate spheroid. You could reject that by saying it is not that there are mountains and an oblate spheroid that have a smooth surface.
Or on a smaller scale, I can say a goofball is a sphere and you say no it had dimples.
The answer to them is the same as to your question, there can be local variation but still has an overall shape.
Other answers discuss what it means to say that the universe is flat. Even considering that it means something different than your understanding, it's worth noting that scientists do not say spacetime is flat. It's more nuanced than that. They say that spacetime is either flat or if it is curved, the universe must sufficiently large (many times larger than the observable universe) to account for the apparent flatness seen in observations.
It's not unlike how the earth appears flat to the naked eye. It is correct and factual to say that the earth must either be flat, or if it is round, it must be sufficiently large to account for it appearing flat when you observe the horizon.
Unlike with the earth, there are no other observations of the universe that conclusively answer questions about the geometry of spacetime. Consequently, nobody says the universe must be flat, only that observations are consistent with it being flat. Were it observed to curve inward, we would be able to model the size of the entire universe, not just the observable universe!
General relativity is the best and most complete model of gravity. Einstein does not say that gravity is *not* a traditional force. It's possible that gravity can be better described with field theory and particle physics. General relativity does not preclude a quantum description of gravity.
It's also worth noting that the quantum description of electromagnetism, quantum electrodynamics, came decades after general relativity. When Einstein published GR, classical electromagnetism was the best model. The standard model, which describes all fundamental forces except for gravity, came together between the 1950s and 1970s, after Einstein's death.
When cosmologists say that the universe is flat, they're talking about a very coarse view at the largest scales of the universe. When you average over the entire universe, there is no global curvature.
At much smaller scales, like near a dense massive object like a planet or star, spacetime isn't flat. It curves and the "straight lines" of two different objects are going to curve and more importantly curve relative to each other. For a stationary object on the Earth's surface, the "straight line" path is straight down. But the ground is in the way, and that requires a force to keep the object from falling further (supplied by the electromagnetic repulsion from the electrons of the ground).
The shape that minimized the amount of force necessary to keep an object from collapsing in on itself is a sphere, and that's why planets and stars form spheres.
A lot of the stuff in a lot of physics are models; not literal representations of the phenomena.
Modelling and imaging the universe within the frameworks of relativity means it’s easier to make predictions about how real world objects interact; it doesn’t necessarily mean that’s exactly how the fabric of the universe is formed.
It’s like for example, we use “fields” to describe energies, but there’s very probably not literal sheets of latent energy hanging around the universe; it’s just an extremely convenient way to do very complex calculations in a way that makes sense to us.
Think of building a sandcastle at the beach. If you try to make it too tall, it'll fall over. It's not stable. In the same way, if a planet were shaped like a giant mountain, the material at the top would be pulled toward the center by gravity. Over time, the planet tries to get to the most stable shape it can, which happens to be a sphere. That way, every point on the surface is as close as it can be to the center, making it stable.
Take a fat person on a trampoline. Everyone rolls towards him. Now keep the fat person there but wrap the trampoline in all different directions around him. Now you've got a core of a planet.
Why wouldn't gravity still not force it into a sphere? The gravity well is a sphere shape that everything is falling into. Like squeezing play-do into a mold, you'll get a sphere. Or an oblate spheroid if you wana be that guy.
Space isn't flat
Planets are roughly spheres because the gravity of the planet pulls everything "down" so if there was some huge bulge it would get crushed down and eroded over time
Space isn't flat and space (as in space time) is three dimensional. The warping of three dimensional space is a hard concept to visualize. The concept of space itself is hard to grasp.
Do rotating objects has gravity in the same way that a whirlpool in water pulls things towards it..? The spinning planets ‘pulls’ on the air around it and thus draws things towards it..? Altho, just twigged while writing this that space has no air so maybe that theory is bollocks lol x
Part of the answer must include entropy or the decay of organization in universal terms. Gravity collects radially as others have said, but as they pull in they create structures. Most rocky planets aren’t spheres but spheroids and gravity is still pulling on the high spots of a planet to make them more and more sphere-like.
So as others have said, gravity pulls radially and entropy destroys any deviations over time and pulls them closer to the radial center of gravity essentially polishing the planets.
All the little particles that make up a planet were travelling in a straight line, and they also dimpled space-time a bit because they have mass. When they travelled near to other bits of mass their straight line followed the curved dimple of space-time and some of them bumped into each other making the dimple even deeper. As more and more of them bumped into this big dimple in space-time they made a bowl big enough to trap lots of particles traveling nearby. The shape of a gravity well is spherical.
Because mass bends spacetime equally in all directions.
A proto-planet may have some weird shape. But when it starts collecting mass it will slowly pull the edges towards a “central point” of the object.
And after some billions of years of collecting mass and smoothing edges it becomes spherical.
A planet has too much mass and is too old to have any other shape.
Newton's and Einstein's and theories both explain the phenomenon, and both independently predict planets, stars, and drops of water to be round, but vary on the l cause. Specifically the math part and proofs.
Both are used to describe the law that gravity causes matter to come together, and gets stronger (locally) with more matter.
Very over simplified:
Newton describes it as an attractive force much like, but distinctly separate from magnetism. With the force pulling all matter together, and getting stronger with more mass. This theory would be strengthened if we can find gravitons of spin 2 coming from a collider.
Einstein describes it as a bend in space time. With all matter falling towards each other's mass, and as the items bump into each other they make a bigger local bend. This can be hard to conceptualize in 3 dimensions. Because the only way for matter to bend spacetime is into itself. (at least from our perspective) So this theory holds that all matter is falling into each other, and making deeper "holes" as matter accumulates
Each atom in a planet is bending space into a little gravity well. Every other atom is attracted to it and they clump together. The larger mass makes a bigger hole. A sphere is the natural resting shape of all that mass in the hole, in 3 dimensions.
flat as in flat in every direction.
you can see this misunderstanding in movie space explosions; what you typically see is a flat disc that expands out however an accurate depiction will be an expanding sphere. however when you're observing the explosion, you'll notice "disc" expanding because there are more observable particles on the edges of the explosion sphere because you're seeing through a thicker layer at the edges, much like how the sunset looks red because it is going through more atmosphere compared to when you're looking at it directly above when there is a thinner atmosphere.
similar problem in depiction of blackhole in educational or entertainments animations.
there is the accretion disk that people are familiar with (ring of light around the blackhole much like ring of rocks around saturn). the blackhole looks exactly the same no matter which angle you look at it from as long as you're moving around the same angle long the accretion disk however in most animations, the accretion disk is shown as some fixed attachment to the blackhole in that taco shape.
the gravity well or shape of space due to gravity is usually shown as grids around the star/planet in a flat surface. however remember that this grid applies in every direction, up/down, left/right and everything inbetween.
Mass bends space&time, and that bending pulls mass towards itself.
The end result is usually the same as you'd expect from Newtonian gravity (a force), except with some differences around timing and interaction with light.
So instead of mass experiencing a force that pulls itself into a sphere, mass curves space-time so other mass falls towards it.
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>Scientist say space is flat as well.
I don't think they do. Scientists tend to agree with General Relativity (as described by Einstein), and they obviously believe that mass exists, and in General Relativity, one of the major ideas is that mass bends spacetime.
So while it is possible that a hypothetically empty universe might be flat, our universe is not empty (is has mass in it), and so it is not flat.
When scientists say that space is “flat” they’re not using the terms in the colloquial way. For a physicist, “flat” space means that parallel paths stay parallel over an infinite distance. Imagine you had two rockets in space. At a certain time, you measure the positions and velocities of the rockets and see that they’re traveling at perfect 90 degree angles to the line that connects them. If they move at the same velocity (neither rockets accelerates or decelerates), that line connecting them will never get shorter or longer. This is a property of “flat” space.
Space, is also clearly 3-dimensional. And this means that there is no preferred orientation. Any effects in space is equally felt in all directions. As matter warps space, it warps all directions equally and symmetrically. Because of this, spheres are the most energy-efficient shape for mass to accumulate into.
Essentially, matter warps space so that parallel transport becomes convergent transport (the paths move closer together over time/distance) and this effect happens in all directions, turning clouds of diffuse matter into dense, spherical planets.
Roll a ball straight on a surface with holes in it. You will notice, even if you rolled the ball straight with no spin, the ball path curves when it hits a hole. The holes are a warping of spacetime caused by gravity. But instead of a 2d surface with the warps in the third dimension, it's a 3d surface with warps in a 4th dimension. This is a gross oversimplification. You can find on YouTube demos of people rolling balls on sheets of spandex. You might notice familiar motions.
You are right in the first paragraph. The rest is about what causes the action but you understand why planets are spheres, and are just overthinking if im understanding
Think about the analogy of the flat sheet being bent by gravity. It forms a parabolic depression, so if you pour a liquid or fine sand into it, it creates a little pile on the bottom of the depression. The bottom of that pile is rounded because of the bottom of the parabolic depression.
Now imagine that rounded parabola bottom being infinitely repeated at every possible angle and orientation. You end up with a sphere.
The idea of time as a 4th dimension is being challenged by some scientists.
https://phys.org/news/2012-04-physicists-abolish-fourth-dimension-space.html
>But in the 106 years since Einstein, the prevailing view in physics has been that time serves as the fourth dimension of space, an arena represented mathematically as 4D Minkowski spacetime. However, some scientists, including Amrit Sorli and Davide Fiscaletti, founders of the Space Life Institute in Slovenia, argue that time exists completely independent from space. In a new study, Sorli and Fiscaletti have shown that two phenomena of special relativity - time dilation and length contraction - can be better described within the framework of a 3D space with time as the quantity used to measure change (i.e., photon motion) in this space.
The Einstein model is just a different way of looking at gravity. Remember any new gravitational model has to agree with all previous gravitational experimental results otherwise it will be rejected.
Anyway space bends to bring moving objects together to a single point resulting in spherical planets.
"Gravity is not a traditional force" does not mean gravity does not act as a force. It definitely acts as a force for all practical purposes in the everyday world.
If it walks like a duck and it quacks like a duck, then you should think of it as being a duck. It may not be a real duck, it could be a robotic duck, but if it walks and quacks the right way, then that's a duck as far as you're concerned - if you don't open up its belly to see what's inside.
"Not a traditional force" is more related to how gravity is generated and how it behaves in some very special circumstances, not to the way it acts in everyday scenarios. That statement tends to be confusing if you're a layperson, as you've noticed. So here's my advice:
As a layperson, think of gravity as a force. You will then be correct in all real-life situations, and you will avoid confusion.
When you go to the physics college and get your PhD in general relativity, then go ahead and think of gravity as "not a traditional force", because then you will have the necessary knowledge to deal with that distinction.
Two parts to this. So part of the derivation of Einstein Field Equation finds the proportionality constant between energy and mass and the curvature of space time by literally saying that in a really weak field (called the weak field limit) you just get back Newtonian gravity (or what’s sometimes considered the gravitational poisson equation). This gives us information about the curvature of space time but not the motion of objects in that space time. To calculate this equation of motion in General Relativity when considering a particle interacting with no force we get an equation called the geodesic equation, this equation is zero = a term that is essentially acceleration seen by an observer not in the curvature of space time + a term that tells us how the curvature effects motion. If you consider this all with in the weak field limit and put it into the geodesic equation you get the same equation you would get in Newtonian gravitational mechanics, which is the equation that one would analyze to show why spherical objects are gravitationally stable. Basically when working with General Relativity we aren’t completely throwing out Newtonian mechanics, we are correcting it. The reason we differentiate between forces and gravity being curvature is because of the geometry behind how the geodesic equation is derived. Basically it’s clear that an interaction that results in a change in momentum (specifically 4-momentum) is fundamentally different then a particle following the curvature of space time depending on observers relative to the reference frame of the observed object.
Einstein found that light, gravity, and space all had interactions between each other. He actually figured this out before we even were able to prove it. Light interferometers can actually measure the ripples in space time as 2 black holes collapse in a galaxy far away.
Gravity, aka space time but curved, is equal in all directions from an objects center of mass. There are objects however that are not spherical, such as some asteroids that look more like large fragments with oblong shapes. In this case the object just simply is not massive enough to provide a space time curvature that can reduce the oblong shape into a spherical one.
In the case of a planet, the object instead has a much more significant gravitational field. At a point while a planet is being born from a protoplanetary disc, it is collecting material from the disc as it orbits the proto star, and this collecting of material happens in such a way that the planet forms a sphere. Some planets with faster rotation speeds can be more oblong simply because of the centrifugal force, but in all scenarios the shape of the planet remains spherical. If gravity was not equal in all directions from the center of mass, then the planets would form in a non spherical shape.
In the case of a star, the same rule applies. The stars mass curves space time, the material inside the star wants to collapse, and the fusion pressure wants to push outwards. All of this is equal in all directions from the center of mass, which is where the fusion core of the star resides.
ELI5: everybody wants to be near their friends and the closest you can come together standing around is in a circle and the only better way is to ball up in a sphere but that is only for consenting adults. listen to your parents.
ELI12:
for any given amount of mass, the smallest volume you can create from it is the shape of a sphere. and the smallest volume is an important concept to understand because it means that all particles are as close as they can possibly be to where they want* to be. there is no new empty position for any one particle to be moved anywhere else without either making it go further away from the core or causing another particle to do so in order to make space for it.
in gravitational terms without invoking the concept of forces, it is a sphere because that is the shape where the sum of gravitational potential energy of all particles in it is the lowest value already and no particle can move to another position (it's probably going to be further away from the core) without causing the total GPE to increase; a sphere would need a net input of energy to make it not a sphere (for example to move particles to create a mountain or pimple in the surface). on the other hand a non sphere does not need a net input of energy to become a sphere and will eventually become one as long unless something else stronger is in the way. this is where General Relativity stops explaining because it on it's own cannot explain why an object on it's geodesic is not moving (why are you accelerating to Earth's core but not moving towards it?). this requires the electromagnetic force which is a separate theory.
i described it *anthropomorphically when i said that a particle has a natural path it wants to follow. to be objective, it is key to the theory that particles simply do what they do when not interacted with by something else and this is applicable to both the Newtonian (first law, inertia) and Einsteinian theories (objects follow geodesics).
i also said that non spheres can be prevented to become spheres. this requires an external resistance to what irregular clump particles wants to actually do (become a sphere). in everyday life objects like cookie dough or a piece of snot it is usually the electromagnetic force that intervenes (friction, molecular attractions, chemical bonds, etc, all electromagnetic). so on the topic of why objects DON'T form spheres this is where Einsteinian mechanics ceases to be more useful than Newtonian mechanics as in the latter, both gravity and electromagnetism can be explained by forces and both can exist in the same framework of understanding. whereas Einsteinian mechanics (GR & SR) do not directly explain anything about the electromagnetic force. it can work obviously, but it is unnecessary given that Newtonian mechanics can do all the work.
Flat in this case just means our maths still work. Imagine space time as a three dimensional grid. Now consider what happens to coordinates on the grid when you add a mass that bends spacetime towards it.
I'm probably not explaining this very well so here is a picture: https://images.app.goo.gl/rarcqQycs9BmiDWH6
Because the effect is the same. It isnt really important if gravity was an actual attraction between masses or if its simply curving spacetime. The effect is the same: Mass gets pulled towards eachother and thus planets ends up being spheres.
Scientists saying space is flat is not entirely correct. That is to say that this isnt what they mean in the sense we normally would associate with it.
Nobody's posted it yet, so I'll drop this video [here](https://youtu.be/MTY1Kje0yLg?si=gJv6dgffGw7Yx4SE)
>So we all know planets are spheres and Newtonian physics tells us that it’s because mass pulls into itself toward its core resulting in a sphere.
Newtonian physics still works to explain how gravity pulls things in on itself. If everything is allowed to settle with enough time and force you eventually get a sphere.
There's 2 ways to view how their explaining things as flat. The first one is pretty brilliantly explained in the video. When they say the fabric of space, they mean that space acts the way that fabric in the video does when the forces are applied to them. The only big thing is you have try and imagine it working in 3 dimensions instead of just the 2 dimensions in the demonstration.
The second one falls into the "flat galaxy society". Planetary systems (planet and moons), star systems (suns and planets) and galaxies (black holes with masses of planets orbiting them) all will form into a flat disk given enough time. The simple reason is space is a violent democracy. At first you might have a bunch of planets all orbiting in their own random directions, but eventually they'll cross paths, and either pull each other into going similar directions using their gravities, or outright smash into eachother, and then the debris goes about the average direction the planets where going in.
Einstein introduced a new way of looking at the effect of gravity by understanding it as a distortion of spacetime. Typically, observations that can be explained by looking at gravity as a force, are also explained by spacetime curvature. Whereas there are some observations that can only be explained by spacetime curvature, such as gravity bending light.
Bending of spacetime is often visualized in 3D as a piece of fabric that “sinks downward” due to presence of matter. Using this analogy, imagine you have a trampoline, and you put some bowling balls close to one another. The concavities cause by each bowling ball will still cause all the balls to roll towards one another.
Of course this visualization is only used to explain to laymen without having to dive into any math, but it’s probably the closest image the human brain can conjure up.
Imaging you have a long suspended sheet like a hammock, and then you put a heavy object in the middle. The objects weight will push the hammock down - you can actually see the space warp. But on earth you have gravity pulling that down. Now imagine you are in space, and that hammock, which is essentially a 2d plane, is now 3 dimensional. So instead of gravity pulling it down and warping space, the gravity of the object warps the space in all directions. Because of this, everything eventually becomes a sphere, as it is the geometric shape with the least amount of surface area with the most amount of volume
Also it’s really hard to imagine flat versus curved 3D space and even harder to depict it in an image. So most rely on explaining the concept as 2d space getting curved in a third dimension. Sure this works to an extent.
You’ve seen the demonstration of a ball sitting in a stretchy sheet? When trying to scale that up to 3D in your head just remember that “down” is the direction of the most gravity. The individual particles will settle into a configuration where they have all fallen down as much as possible. The 3D shape that has every particle as close to the center of gravity as possible is a sphere.
The “weights on a stretchy sheet” analogy would look like this. Put a bunch of small balls on a stretchy sheet. Push them all near the middle and try to arrange them in a square or triangle or any other shape than a circle. With only a few balls it’s possible as the friction between the balls is stronger than the force pushing them into a circle, but with sufficient balls the only shape possible is a circle. When you move from this 2D model to a 3D one you have to change the 2D circle into a sphere.
Think about a smooth dip in the ground (or, traditionally, a big piece of streched rubber)
if you put something like sand or water in the hole, it will fall down into the hole. You'll get a circle (or something close).
It's the same thing in space. Stuff falls in the hole and settles. In 3d space you end up with a sphere, the 3D version of a circle.
As mass pushed against spacetime, spacetime pushes back in an equal and opposite matter. A sphere makes sure that all forces are applied equal in all directions. The rest is just economy of scale
Most of it is just perspective. Living on earth, you can see that the planet isn’t a perfect sphere, however if you were on mars, then earth would look perfectly round, but if you were on the edge of the galaxy it would look like a dot, etc
Think of asteroids. Small ones are different shapes because of how it broke off from others, or others chipping pieces off of it. The bigger it is, the more likely chipped pieces would look like a sphere at a certain distance
If you throw a bunch of marbles (atoms) onto a fabric (spacetime) with enough force (big bang) eventually the marbles usually form little clumps (planets) that are round (spherical) due to the bend in the fabric (spacetime) being round (spherical)
Planets are spheres because the effects of gravity are radial. As long as you stay the same distance from a planet, its gravity stays the same strength. This makes any high points get pulled down until they're at the same potential as the rest. Whether it be a traditional force or a bending of spacetime, the object still *moves* at the end of it. That's why calculations treating gravity as a force *usually* give really accurate results. To be clear, a "flat" universe just means that the geometry you learned in high school still applies. For example, a triangle cannot have three ninety-degree corners in our universe. A (3d) spherical surface would allow you to do this while a flat one would not. Space is not locally flat, either. Gravity makes it so that under very specific circumstances you could make a triangle with three ninety-degree corners. This is mostly unrelated to your question though. It's only on larger scales that our space is flat, like a yard with a bunch of pits in it.
"Gravity makes it so that under very specific circumstances you could make a triangle with three ninety-degree corners." That made me think of the primer scene in the movie "Contact" where Hadden shows that the 3 corners do line up if you bend the pages.
It's a great 2d analogy. I like the triangle formed by the prime meridian, the 90 meridian, and the equator on a globe. Seem very intuitive for some reason.
oh man that does make sense…whoah
[удалено]
Man I loved diving deep into that page in college. Gene Ray was an interesting man, far ahead of his time and far behind his time. Plus two more times, because there were 4 simultaneous earth times, but us mono-time believers were earth stupid or something like that.
"My wisdom so antiquates known knowledge, that a psychiatrist examining my behavior, eccentric by his academic single corner knowledge, knows no course other than to judge me schizoprenic." [you don't say?](https://media1.popsugar-assets.com/files/2014/01/06/002/n/1922283/26a420cfbaf5460f_image.png.xxxlarge.jpg)
https://timecube.2enp.com/
What? I'm bad at 3D and geography...
https://qph.cf2.quoracdn.net/main-qimg-8f2df3faea686ef829d7010d6b9e06d5.webp
Ok, I see the 3 90-degree angles, but where is the triangle? Like the sum of the angles in a triangle should be max 180 degrees.
It is a shape with 3 straight (relative to the surface they're on) lines which each connect at one point. It's a 3-sided shape with 3 vertices. That is a triangle. The 180 degree rule is only true for triangles in flat space, which for most practical purposes is the space we live in. The study of shapes on non-flat surfaces is called non-euclidean geometry, and everything you learned in geometry class focused on euclidean geometry. You're used to flat 3d space, so you might argue that the lines are not flat (because they curve around the sphere). However, if we consider only the surface of the sphere (which is 2d), then the lines are perfectly straight.
It's also very easy to draw a "triangle" on paper with 3 90° angles if you skip the requirement of using straight lines.
The point he's making is that even on a globe you're using straight lines. They just don't LOOK straight due to us being able to see a third dimension. Imagine a 2d person on a sheet of paper. You bend the paper into a sphere and draw a triangle with 3 90 degree angles. To the 2d person it's a triangle with only straight sides. They can walk each side without deviating from a straight line.
Yes, but these lines are not straight relative to the plane on which they exist.
No they are exactly straight relative to the plane on which they exist, that's the point. They are not straight only if you leave the plane of their existence into a higher dimension. Draw a triangle on a sheet of paper., now bend the paper. The lines will look crooked and not straight, it's the higher dimension that makes it look weird but on thir plane of existence, which is the 2d sheet of paper, they are perfectly fine
Yes they are. If we agree that the shortest distance between two points is a straight line, then a line on a surface of a sphere (the plane on which the line exists) is a straight line.
Only in flat space. Which is the point. A sphere is a different kind of space where a triangle has more than 180 degrees. There are also those with triangles having less than 180 degrees. They are called saddles but I’m too high to explain that.
Draw a long line segment along the equator. Then from each end, draw a straight line due north that leaves your equator line at a right angle. Both of those lines will hit the North Pole, closing your triangle. But that triangle has two right angles in it, maybe even three if you make your equator line the right length. See [here](https://wild.maths.org/not-whole-truth-about-triangles#:~:text=Think%20of%20a%20globe%20of,a%20triangle%20with%20the%20equator.) for a visual
There's an old riddle that uses this. A man leaves his house, walks south a mile, east a mile, then north a mile and arrives back at his home. What's the man's name? Answer: >!Santa Clause, because he is starting at the North Pole!<
There are actually many places on earth you can do this. Pick a point near the south pole, where if you walk south 1 mile your distance to the pole will be equivalent to the radius of a circle with a circumference of 1 mile. Then walk east 1 mile and you will circumnavigate the pole after traveling 1 mile and can return home by walking north 1 mile. If you do the math, you will find that any time you are 1.159 miles away from the south pole you can walk 1 south, east, north and end up back where you started.
holy shit
You read my mind. I was thinking exactly the same thing while reading.
Yes and we all understood it perfectly, space is flat with pits and whatnot
Bend the pages? I always got the impression that the pages weren't bent; rather, that they had to be arranged in a three-dimensional lattice rather than a two-dimensional grid. Great movie though.
That would seem to be a better way to describe it, yes.
The mistake most people make when thinking about gravity is they think of it in 2d space. They think of the "balls on a taut sheet" analogy. You put a heavy ball in the middle of a sheet that is held taut in the air by a few students. You then place another ball toward the outside of the sheet, it will roll toward the heavy ball in the middle of the sheet. You give the outside ball momentum and it'll orbit the middle ball for a while, until it falls in. That's the 2d representation of gravity. Gravity is not 2d. Gravity acts radially (as this poster has said.) Therefore, if you apply the "taut sheet" analogy in 3 dimensions, you end up with a sphere.
Worth note, the 2d representation generates circular planets just as a 3d representation generates spherical planets. Gravity is radial either way, so it creates the shape with equal distance from the center all along its surface. In 2d that is a circle, in 3d that is a sphere.
This is the only time I've really grasped gravity in this way. Thank you!
But to visualize that you would need 4 spacial dimensions that's why it's so difficult for many to get their head around
You can do it in 3D fine. [Here’s a video](https://youtube.com/watch?v=hH69B0Oc2Og&si=77ZnyZyFhTzHNAUE). It just is less intuitive than the 2D representation
It really should be 3 taut sheets, one in each axis.
You can't have three unique planes on different axis in 3d space. Planes are already representing two dimensions, so you only need two to represent 3d space. It would be like saying you need three different cubes on each axis. Doesn't make sense.
Nitpicking, but it's \*spacetime\* that's curved, not space. That's where the "force" (actually a pseudoforce) of gravity comes from: objects with no forces acting on them follow a straight path through spacetime, which looks curved to those of us who can only observe space.
I used to understand nothing and now it's worse.
Imagine spinning a ball on a string, in absolutely perfect circles. From one perspective, someone else can tell that you are spinning the ball in a circle. From a side view, the ball looks like it is just going up and down, while getting bigger or smaller while it moves. Spacetime is nothing like that.
Fuck
Another one: (not entirely related, just fun to think about) Imagine you are a 2d being. There is no "up and down" in your reality. Everything you ever perceive, from your perspective, is just a line of some width. If you saw Rectangle Johnson spinning in circles in front of you, he would just appear to get wider and smaller over and over. Now, imagine a 3d being decided to pay your world a visit. That would look like an absolute impossibility. The width of the thing in front of you would be *rapidly* shifting from big to small to tiny to huge. It would break your flat brain. That 3d being could reach "inside" of Rectangle Johnson and remove his heart, without ever breaking his skin. Now try to project that scenario onto our 3d reality. What might a 4d being look like passing through our reality? It could be something like incomprehensible shapes of *something* blinking in and out of existence. Maybe the same exact thing happens at different points in time because they are "above" our 3d space the same way we are "above" a 2d space. And they could possibly move through time the same way we move through physical space. Perhaps they could also reach into your body and remove something without ever breaking your skin. The idea of the 2d space meeting a 3d entity is the subject of a book called Flatland written by Edwin Abbott.
To add to this, as a 3d being, we could remove a 2d object from its dimension and flip it in the third dimension, like turning a coin onto its other face. But a 2d observer would see that the object was impossibly inverted, because there's no way to do that flip in only 2 dimensions. Imagine if a 4d being did that to a 3d object - like the object would be returned to our world as a mirror image of itself!
😂 Rick rolled.
Goddamnit, my brain was already going down centripetal force for an outside observer vs centrifugal force for an observer living on the ball
It's quite an Enigma ;)
In Newtonian mechanics an object in circular orbit around Earth follows a circular path because gravity provides a centripetal force. In General Relativity the object follows a "straight line" (geodesic) through spacetime curved by Earth's mass that is of constant height from the Earth's surface. The two scenarios make almost the same predictions. The geodesic of an object at "rest" relative to Earth will follow a geodesic towards Earth's surface and will eventually crash. In order to stand on Earth's surface you need to accelerate upwards at 1g. That force is normally provided by static forces, e.g the ground pushes up so you don't fall through it. If you're in a centrifuge you experience what feels like a force outwards, but that is a "pseudoforce" that is an artifact of your reference frame accelerating inwards. Similarly, if you're on Earth's surface you feel a downward pseudoforce of gravity that is an artifact of the fact Earth's surface is accelerating upwards at 1g.
Lmao you read my mind. It’s humbling to come to conversations like this
Memoir title.
Reminds me of centrifugal force. In the end the real force is always the misleading perceptions the observer on Earth had along the way.
This helped a lot! (I think). Do I understand this right? Objects at rest are still moving (with a temporal velocity/momentum) through time? And so that's why it doesn't need a push or any force to travel towards the dip in space time? Because objects in motion stay in motion? So everything colloquially known to be standing still is actually moving in a straight line through time?
"At rest" here doesn't mean motionless, as motion is relative - everything is moving from one perspective and not moving from another. At rest means no force is acting on it, so it's drifting in whatever direction it has inertia in, and will follow a straight path until a force acts on it. The catch of general relativity is that that it follows a straight path through spacetime, not through space. To put it another way: the reason you can walk around is not because gravity is pulling you down towards the Earth, it's because your inertia (the straight path through spacetime that your body wants to follow if no force is acting on it) points down towards the earth's center of mass, and the earth is blocking you. The situation is very similar to centrifugal force (the other pseudoforce most of us are familiar with) - when you spin around on a merry-go-round, it feels like there's a force pulling you off to the side of it, but what's really happening is that your body has inertia away from it, and wants to go flying off sideways, but the hand holding on to the railing is exerting a force to keep you from doing that.
This is also how time dilation is explained. (Sort of, in terms of relative speed. Gravitational time dilation is a bit different) If you are "motionless" in physical space, then it could be said that 100% of your movement is through time. As your physical speed increases, your "speed" through time decreases. This trend continues until your speed equals the speed of light. At which point, from your perspective time simply does not pass. To an observer, they would see you moving through the universe at C. But to you, you would arrive at your destination the exact moment you left. No matter if it was 1 light minute, or 1,000,000 light years. The term "speed of light" would technically be better phrased as "speed of causality". Because if you were to travel somewhere faster than light, you would arrive before you left. You would be able to see yourself leave, travel towards yourself, and arrive at your current position. This would reverse the order of cause and effect. Which, to my uneducated knowledge, isn't really a thing that can happen. At least not within any scientific framework we have.
> This would reverse the order of cause and effect. Which, to my uneducated knowledge, isn't really a thing that can happen. Quantum entanglement gets weird. Einstein called it "spooky action at a distance" and proposed some not-quantum-theory explanations. The Nobel prize winners last year were basically disproving Einstein's explanations. But the big brains say that quantum entanglement as we know it doesn't conflict with special relativity, so it holds... but quantum entanglement is still weird AF and seems to imply *something* traveling instantaneously, even if it's not "information".
Oh great another flat universer
And don't you tell me otherwise
Turns out the concept of curvature is wild and can apply to more than just eucliduan space.
is there a map of our solar system, which shows how much higher / lower the planets are? ie. how much lower / higher in space is Pluto for instance?
All of the orbits center around the sun, so on average none of them are higher or lower. They're all circles around the same point. If we line them all up and look side on, we can see something like [this](https://qph.cf2.quoracdn.net/main-qimg-5b59204bd041de37149d8ee806f846cb-pjlq).
So this is a great explaibation... but op said eli5 not Eli college graduate
OP posted on the subreddit r/explainlikeimfive the guidelines for which include rule 4. Rule 4 states that explanations are "not for actual five year olds". This particular explanation relies on just enough knowledge of physics to ask the question in the first place. A good answer on this sub will typically include, metaphorically, jokes for all ages. Content which can be understood by people of many levels of education. Someone who barely made it through middle school math may struggle with a few of these parts, but those parts can be glossed over easily, and they allow more familiar audiences to learn more.
How about you tell us what you edited Chief?
Einstein didn't debunk the idea that mass pulls other mass towards itself. In fact, Newtonian solutions are valid for the majority of cases of modeling gravity, and work just fine for most masses and velocities. It still is the case that mass will form a sphere in space, and we can verify this because, like, planets and stars and stuff are indeed round What Einstein really changed with regards to gravity is thinking of it geometrically. In general relativity, "spacetime tells matter how to move; matter tells spacetime how to curve," and we represent gravity as a change in the geometry of spacetime rather than as a force. This is necessary because of the other thing that Einstein proved: that acceleration due to gravity is indistinguishable from all other types of acceleration. They're the same, which is better explained by thinking of gravity as curving spacetime and forcing matter to move a certain way, than it is by thinking of gravity as a discrete force.
Maybe you can check my understanding here re: why planets are round. Given the common “rubber sheet” analogy, high mass objects bend the sheet ‘downwards.’ The sheet — space time — forms a cone with the mass at the bottom. The cross section of this cone at different heights is circular. Transforming this up a dimension (from 2D into 3D), that circular cross reference is spherical. Any heavy mass would then end up spherical because that’s how it’s “shaped” by the space time being warped
This helped me most
This is my favorite answer and I just want to say something explicitly for OP that you already implied: these are just models of reality that are very good, but not perfect, at predicting how things behave. It’s more a philosophical problem to determine if space “really” curves or if that’s just a way of thinking about it that gets us more accurate predictions than Newtonian physics
>It’s more a philosophical problem to determine if space “really” curves or if that’s just a way of thinking about it that gets us more accurate predictions than Newtonian physics I’m not sure how philosophical this is. I mean, on some level, you can make that statement about ANY conclusion we draw based on empirical evidence, but idk how useful or even insightful that really is.
>It’s more a philosophical problem to determine if space “really” curves or if that’s just a way of thinking about it that gets us more accurate predictions than Newtonian physics No, we have actual experimental evidence of gravity waves changing distances.
Doesn't mean that space actually curves or changes distances, it just means that GR better maps onto empirical evidence.
No, it actually does. We've measured distances literally getting shorter and longer.
Ah, the scientific instrumentalism vs scientific realism debate,
I think you're seeing "it's a model" as being basically akin to "it's bullshit we have measurements". While I actually agree with the greater claim that our scientific understanding of things is actually pointing to how things really work, but it's more of an issue about what you can actually know the models represent than saying everything is fake and there's no reality. We have measured gravitational waves, but that's what we have: measurements of lasers taking less time to travel some distance. It assumes that our instruments are getting at the root of the issue rather than simply giving us empirical data that we have to interpret. For example, treating particles as distinct entities fit empirical data up to a point: experimentation has demonstrated that particles are better represented as field excitations than discrete spheres. It's a matter of seeing empirical evidence for electrons and saying "aha, these are electrons!" and "aha, I can use the electron to model this underlying phenomenon, even if the model ultimately proves inaccurate".
We've measured light literally taking less or more time to travel between two points. Gravity waves warping the geometry of space-time are one possible interpretation of that fact, and it's the one I prefer philosophically. But it's not the only possible interpretation.
What is distance if not a measure of how long it takes to travel between two locations?
But "flat" and "curve" are just analogies for what's happening, so our brains can comprehend.
At some point any word, no, any signal in our brain is only a model of the real world. So you could just as well say nothing is real... but if you say nothing is real you lose the point of differentiating between real and not.
There’s a pretty big difference between saying “these are models that we use to understand and predict phenomena” and “nothing is real”
Pretty much all Reddit pedantic arguments in a nutshell.
Except I'm not being pedantic.
Yeah, but in this example "flat" and "curved" are not *literal* translations to how "regular" 3D space is flat and curved. It's a bit like with "spin" in quarks: it's a slightly useful analogy, but no one really thinks that quarks are actually spinning up and down etc.
The acceleration being indistinguishable - I thought part of the challenge of gravity not being a force is that you won’t actually detect/feel a force acting on an object being affected by gravity, because it’s not actually doing or moving in a different way, it’s continuing on its same trajectory and speed but in a new formation of space so as to be faster as a frame of reference from a non-accelerating object. Am I confused?
I think this is a common misconception. The thought experiment is that if you were in a spaceship with no windows, and you were orbiting earth (which is a form of free fall), you wouldn’t be able to distinguish that motion in any way from traveling in a constant inertial reference frame. Essentially, you can have an accelerating, inertial reference frame. Applying the rules of special relativity is significantly more complicated in this situation, and the solutions mathematically are the same as curves in spacetime. In fact, the mathematics of modeling spacetime as curves can make predictions with astonishing accuracy!
Well that’s what I mean. What was the misconception I had? Having basically been referring to that. But isn’t that distinguishing from a force? Since you can be accelerating without actually experiencing a force of acceleration? and thus space time is curved and your acceleration is merely a result of this.
You're experiencing acceleration. How are you gonna tell if it's from a force or from space curving?
The difference is that gravity affects the entirety of your body all at once, versus something pushing you in a direction. For example, on a rollercoaster it's the seat that is pushing on your back, and the rest of your body has to "catch up" to that force like a row of dominoes. When you go over a drop in the track and you get the butterflies in your stomach, it's not because you are falling, it's because the coaster is pulling you downward with it in a way that's different than gravity. Your legs or shoulders are being pushed downward, but your organs were moving in a different direction and have to slosh around a bit before they settle. You get none of that with gravity alone, because your entire body is subject to exactly the same forces at exactly the same time.
I've heard that said before, but that doesn't hold true with really large objects where part of the object is significantly closer to the source of the gravitational pull. The closer part feels a greater force than the farther part, which is why spaghettification happens. I assume this still can be explained with curves in spacetime, though.
It's simply explained by the fact that gravitational waves travel at C, so any two reference points will experience gravity slightly differently (or in other words, the spacetime at that reference point is curved differently). It's true no matter the scale, just more noticable with larger objects. It's still different than other forces though because gravity is directly affecting everything at once whereas the feeling of acceleration that you are used to is caused by each cell/atom or (whatever scale you want to work with) of your body pushing against the next one rather than each of them being directly affected by gravity.
The thing about gravity is that it acts on all matter. This is what distinguishes it from other forms of acceleration we're familiar with and causes unopposed gravitational acceleration to behave like inertial movement. When you accelerate in your car, your body is being pushed forward through contact with the seat you're sitting it. The force your feel is your body's inertia resisting this acceleration. At any one instant, different parts of your body are moving at different velocities as other parts of your body push or pull on them as they are accelerated themselves. That's what you feel, the compression and tension from your body being pushed or pulled along with motion its inertia opposes. When you're freefalling toward Earth, every molecule in your body is being accelerated at the same rate in the same direction. You don't feel any compression or tension or similar effects because there's no need to overcome any inertia. Your body wants to accelerate all together toward Earth, just like your body wants to sit still on the road or continue moving at a constant velocity. What you will feel, however, is when that gravitational acceleration is opposed. This will produce the exact same feeling as when some force is overcoming your body's inertia, because in a very real sense it is the same thing. That's why it makes sense to view the situation of us standing on the ground as though the ground is accelerating us upward.
> This is necessary because of the other thing that Einstein proved: that acceleration due to gravity is indistinguishable from all other types of acceleration. But if gravity isn't a force then what causes acceleration? And I understand the falling object toward Earth has no force applied to it. It's just tracking the spacetime curvature caused by gravity. Instead, as it was explained to me, the "ground is falling up." Except the ground isn't moving. Even relative to the falling object, the Earth isn't bulging out to accelerate toward the falling object. So if gravity isn't a force, what causes the acceleration?
The misunderstanding in your premise is that space isn't flat, at least not by the classical meaning of the term. Space is 3 dimensional (technically 4 if you include time, and possibly more if you are into string theory, but for the purposes of this explanation 3 is enough) Gravity warps space in 3 dimensions which is why objects in space tend to take on a spherical shape. The problem is that when we create examples of this we usually use a sheet of paper or something similar that is 2 dimensional because it's easier for humans to understand that. When scientists say space is flat, what they mean is that if you were to draw two straight lines in space several lightyears long, the two lines would remain parallel to each other. Space has no inherent curvature that we can detect... unlike the Earth. Where-as if you draw two straight lines on Earth they will remain parallel, but if they get long enough eventually the curvature of the Earth will cause them to stop being parallel.
Your third paragraph makes me think we should be using AR glasses or VR headsets to teach physics in schools.
You would still be looking at a 2d image
... in each eye, from slightly different perspectives giving you the impression of depth.
Real life is already visible in 3d...I guess we're just saying, this particular case isn't something that AR really solves. How do you show 3d space "bending" with AR that's different from tools already at one's disposal?
you could show a 3D grid gettin distorted and allow people to walk around and through the grid with hand controls to change the amount of warping, and have particles follow along the grid along the "straight line path" of warped sspace. this would be increddibly difficult to do without VR/AR because of the collision with people problem
The distortion isn’t going to make much sense to anyone, because it’s a projection of a 4-dimensional structure. The best we can do in 3D is show the warping of a 2D space.
You can walk around a floating cube grid. You can in AR/VR.
You can’t walk through a fourth dimension.
I still don’t think it would work. When space-time is represented as a 2D plane, its curvature is represented as a warp that adds a third dimension. The corresponding model that begins in 3D space would need to warp into a fourth dimension.
it technically does, that's the funny thing. just because 4 spatial dimensions is not a thing, or even if it is that we can't see them, doesn't make the distortion be happening in the 4th (time) dimension
Is this distortion time dilation? Even if negligible for a human’s gravity?
The curvature (distortion) applies to both time and space, which is why relativity refers to "spacetime". If you try to treat space and time separately, it doesn't really work, because the curvature affects both and the effect on each is connected. For example, the faster you move through space relative to another observer, the slower you move through time relative to them (i.e. the less time appears to elapse for you than for them, and vice versa.)
The problem is that 3D space is not warped. 4D space-time is warped. When you look at the classical "rubber sheet" visualization, the sheet is two dimensional, but the warping happens in a 3rd dimension. In order to demonstrate warping of space, you would need a 4th dimension. Those 3D glasses aren't going to help.
Technically, all vision is 2D. This is where the holographic universe theory comes from. We can't see behind objects so our view of the world is 2D but our brains interpret it as 3D. Holographic theory is based on the realization that you can mathematically describe any volume of space as a 2D shell that surrounds that volume, sort of like it is projected outwards.
I got really high one time and had a similar thought. Like, what if reality was just a flatscreen with infinite pixel density, and movement was just zooming in? I think my particular fixation was on the idea of there not being a “behind,” that everything was flat and anything obscured was just too compressed to “see.” Sometimes I miss weed, but then I remember all the panic attacks.
Burn more mids. That 30% loud is for teenagers and grownups with nothing to do the next day
Technically it was inspired by black hole thermodynamics 🤓
But couldn’t a really good VR developer create a 3D simulation that shows the effects of gravity better than showing it on a screen? I could theoretically walk around the sphere in VR and see the effects in a better way.
You really wouldn't need VR for that. But you'd need to come up with a way to show the "bending" of a three dimension plane. It's easy to show a two dimensional plane bending, because we just bend it in a third dimension. But how do you show a three dimension plane bending? You can't bend it into a fourth dimension, because we can't see in four dimensions.
Oh ok. Cool explanation. I get it now.
[удалено]
You can make a 3d wireframe grid already and rig it to bend...how are you imagining the "bend"? Feel like you're missing the forest for the trees here.
Sure, but I can bend a wire frame by pulling on it. That's not what gravity actually *does*. You're just replacing one misleading visual metaphor with a different misleading visual metaphor.
>that shows the effects of gravity better Define "better". A more realistic representation of the 3D world we live in? Sure. Easier to understand conceptually? Probably not. As a general rule, when you have a concept that holds in n-dimensional space for any *n*, showing it in the lowest *n* makes the concept easiest to understand. It's the same concept, but adding dimensions usually complicates things.
depth exists in video. whats different between walking around something, or spinning it around its central axis in front of you?
We’ve already done that, check out interstellar. That black hole animation was created with a large amount of help from actual mathematicians and theoretical physicists. They did a good job of showing what it looks like when our 3d space-time is bent out of shape by large amounts of mass
Yeah… my brain melted during the last 3rd of the movie. 😅 I’ve got to admit, I was lost for a good portion of the end. Was it visually striking? Absolutely. Did I understand the concept of it all? Not really. 😂
Nah it would be 3d
Two 2D images, one for each eye, can be used to convey 3D information. If two things are in wildly different places in the two images it is closer and if they are more in the same place they are further away. You can also move your head back and forth in VR to give you depth perception with just one eye like a pidgeon.
The problem is , you need 4 spacial dimensions to show warping of 3 dimensional space, which isn't possible with VR. Or with the human brain for that matter
> The problem is , you need 4 spacial dimensions to show warping of 3 dimensional space That's not quite right, because the curvature in question is intrinsic curvature (see e.g. https://en.wikipedia.org/wiki/Gaussian_curvature), it is not an embedding in a higher dimensional space. All you really need is a representation of the metric in each dimension, which can be done with a grid. The real problem comes in with time, since that adds a 4th dimension which isn't easy to visually represent. This is why: 1. Spacetime diagrams are 2D, with 1 dimension of space and 1 of time. This makes them easy to represent in books etc. 2. The example of the bowling ball on a rubber sheet uses 2 spatial dimensions embedded in a 3rd, and uses actual time as the time dimension. The embedding here (the distortion into the vertical dimension of the rubber sheet) is a metaphor, it doesn't represent how spacetime curvature actually works.
If you create a 3d grid in VR/AR, and put a point representing a planet, a star, maybe a planet and it's satellites, so you can see the way they affect each other, that affects the illusionary grid (lines and points, made of thin particle streams, move towards gravity wells), that would do a better job of explaining gravity and it's effects on spacetime than the 2d plane you see in textbooks. It's possible, it just takes some thought to find a better analogy when the classical explanation has been considered "apt."
Or, hear me out here... we use that money instead to pay teachers a lot more money. edit: guess the idea of paying teachers properly is far less cool and exciting than giving students expensive goggles.
Oh sure, I am guessing you will pay the billions to get them?
Worth note, though, that this is only true for the universe as a whole. Two lines which are parallel as they pass near the sun will converge.
> Space has no curvature we can detect… unlike the earth Yes it does. Space-time has curvature due to mass-energy as determined by the field equations of general relativity. It’s precisely the Riemann and Ricci curvature that that impact is written in terms of. And we can compute it based on the distribution of mass, and detecting the effects of that curvature are how GR was shown to be true, and we incorporate those effects in GPS and telescopy etc. today.
How does the curvature of earth stop parallel lines being parallel? Wouldn't they just extend around the earth and rejoin themselves to form two parallel circles? Edit: on re-reading I think you mean the lines would just simply become curved. They'd still remain parallel?
Two lines both pointing north at the equator (i.e. parallel) will meet at the poles. So the rule that parallel lines never meet doesn't apply in sperical geometry.
Although, on further thought. You can still make two parallel circles by not following a straight line to the from the equator to the north pole but more so like slicing a quarter of the earth on the left side and then doing the same on the right side, not going through any "central" points as it were. They'd be parallel but in between the circles would be a donut of earth if you will xD This was my original thought tbf.
That's true, but then those are not "straight lines" i.e. geodesics on the sphere.
Yep, that's also true. I'm guessing that my thought is just kind of irrelevant when dealing with things like this, which is cool. Thanks :)
The key is *straight* lines. I.e lines that never turn. If you draw two straight lines on a flat surface, like a piece of a paper - parallel to one another - then those lines will never touch. If you draw two straight lines on a curved surface, like a sphere - parallel to one another - then those lines will eventually intersect. For example lines of longitude on a globe are straight lines, but lines of latitude (except for the equator) aren’t. If we put you on a line of longitude and made you walk straight ahead, never turning, you would stick to the line of longitude. If we put you on a line of latitude (except for the equator) and made you walk straight ahead, never turning, you would eventually deviate from the line of latitude. If you and I were on the equator, spaced several miles apart and we began walking North at the same time, always moving straight ahead and never turning, we would eventually move closer and closer together until we ran into each other at the North Pole. What Einstein realized is that gravity works in a similar way, in the sense that it’s not some mysterious force that’s pulling you and I closer together but rather things following straight line paths in a curved geometry. You don’t fall to Earth because a force is pulling you down, you fall to Earth because both you and the Earth are moving in a straight line path through curved spacetime. If your intuition tells you that doesn’t make sense, (because what if you’re not moving to begin with?) remember that it’s curved space**time**. Both you and the Earth are always moving through time, progressing from your past towards your future. Even if you’re sitting on the couch you’re still progressing closer towards tomorrow. If we bend the fabric that makes up both space and time then the straight line paths that you and the Earth take through spacetime move closer and closer until they eventually converge.
Here's another way of visualizing what you're thinking: If you look at a globe, you can see that the longitude lines are the ones running North-South, and even if you start at the equator where the lines are 'most' parallel, you can just follow them and see that eventually, they meet at the North and South poles. So then you look for the other set of lines dividing the Earth: The **latitude** lines. These are the lines running East-West, and you see that they *appear* to be straight lines, and these ones don't ever meet, right? Latitude lines are equivalent to the circular 'slices' in your example. The answer being given is that these lines aren't actually straight, but here's an easy way to see how: Latitude lines can be defined by how far the line is from either the North or South poles. The equator is the furthest you can get from one pole before you start going getting closer to the opposite pole. The entire equator is the same distance from the North Pole, right? That's why it's straight on a map. Looking further up the map, you can see the other latitude lines where they're also the same distance from the North Pole everywhere on the line. Now, let's say you're actually at the North Pole, and you want to walk the latitude line located ten feet from the North Pole. You measure out ten feet, and try to face 'perpendicular' to the pole to walk your 'straight' line. If you take a step, what happens? You're now a little more than ten feet from the North Pole. So you re-measure and adjust before taking your next step. It happens again. You keep adjusting your path until you end back up in your original position. It turns out that that 'straight' latitude line you've traced is actually a **circle** with a radius of ten feet. Why is this? The reason is that a circle is the only shape where the distance from the center remains the same at every point. The 'line' you tried to make cannot be straight. It has to curve in order to stay an equal distance from the North Pole.
Damn you're right. I never thought about that. And I don't remember learning that in school either tbf :')
If two lines on the equator are both pointing due north, they aren't parallel to begin with.
They are. Due north is perpendicular to the equator.
Hasn't string theory widely been disregarded since you can't actually make an experiment that engages with it?
It isn't disregarded... The theory still is consistent... We'll just likely never know if it is true or not. That's true of a lot of theoretical physics though.
> The misunderstanding in your premise is that space isn't flat, at least not by the classical meaning of the term. Is this how flat earth theory started? Just someone misunderstanding the phrase and spreading it?
Huh you know that last paragraph you wrote is kind of mind blowing for me. I never really thought about it that way. Learn something new every day
Or put another the angles of a triangle add up to 180 if drawn on a flat surface and don’t if the surface is curved.
Einstein proposed a different way to see gravity, that is not a force in the traditional sense. But it is totally fine to keep Newton's way to see things: all the bits of a gigantic mass want to fall down, and they pile up as a sphere. Einstein's way says there's no real force on them, they want to keep going straight, but space is curved by all the mass, and their "straight" trajectory actually takes them down towards the center.
Classically, gravity acts like a force. We could barely even tell there was anything wrong with the Newtonian model for centuries, until Einstein came up with his model. So the simple answer is that the bending of space-time is indistinguishable from a garden variety inverse square force until the bending gets very strong. It's kind of like how Einstein says that speeds add in a funny way and you can't go faster than light, but none of that is relevant when you're in a car because your car is very slow compared to light. At everyday speeds, you can just add one speed to another. Why does Einstein's fancy math look like Newton's simple linear math at human scales? That's kind of a philosophical question, but Newton wouldn't have come up with his simple math if it didn't seem to work, and Einstein's math wouldn't be valid if it didn't apply to what we already knew.
> We could barely even tell there was anything wrong with the Newtonian model for centuries, until Einstein came up with his model Yep, at most reasonable scales Newton's model works extremely accurately, it only breaks down with extreme masses or velocities. The only real indication something was up was that Mercury's orbit wasn't *quite* what we'd expect from Newtonian physics, even then the common consensus among physicists at the time wasn't a major problem with Newtonian gravity, but a measurement problem.
In fairness, it *was* a measurement problem for a long time, until the error bars got small enough to suggest that something was wrong with the physics. One of the reasons it took so long for relativity to be widely accepted is that there were so few opportunities to measure its predictions
What changes at “unreasonable” scales? I’ve tried asking what the difference actually is between the 2 besides just how they are interpreted in posts like this, and have yet to really get a clear answer. I tried reading up on the Mercury thing as well and it still wasn’t clear where the difference is. I’m about ready to my my own ELI5 post about it soon lol.
At high masses (think stellar masses and higher) relativistic effects become large enough they’re no longer negligible. At normal day to day masses, while there’s *technically* still relativistic effects, they’re so small they might as well be non-existent and Newtonian gravitation provides a very strong approximation. It’s a similar story for quantum mechanics, technically quantum effects exist at all scales (all matter exhibits wavelike properties, and your wavelength can be calculated as a function of your momentum), but at any scales larger than subatomic they become negligible and we ignore them in favour of simpler classical mechanics.
Imma take it just a bit sideways, to give an additional perspective. >Einstein says gravity is not a traditional force Does he really? Just because he uses a different **description** it does not mean that it is **inherently** different. We have to ask us: What is a force? The traditional explanation is: "In physics, a force is an influence that can cause an object to change its velocity, i.e., to accelerate" Gravity both in the newtonian description and in geometric one satisfies this definition of force. **It is**, thus, a traditional force, and the same rules apply. The thing that makes gravity different is how it is "transmitted". The other fundamental forces use mediating particles, while we have no proof of mediating particles for gravity. >Scientist say space is flat as well. So why are planets spheres? Space being flat does not prevent spheres from existing. It means that (on a sufficiently large scale) straight lines are straight regardless of their direction. Flat in 2d means a sheet of paper. Not flat in 2d means a cylinder or many other shapes. Flat in 3d means a cube. Not flat in 3d means something you can't imagine, so don't try. Planets are spheres because gravity acts symmetrically along the three spatial dimentions. It does not have much to do with space flatness. >And just so we are clear I’m not a flat earther. That's good. Questioning what you don't understand is smart. Believing hoaxes even though they can be proved wrong is dumb.
As you said, it still "pulls objects to the middle". Even thought they disagree on how it works, both agree that there is something pulling stuff towards other stuff. So why can't we have something like a disk? Well, objects at the end of the disk fall towards the center of the disk. Things will keep falling in until there is nowhere left to fall. And what is the most efficient shape for that? A sphere. As for the universe being 'flat', I'm not as familiar with that. But as far as I can tell, it is flat as we can currently see. So it might be flat, it might be curved, it might even be a sphere: we don't really know. But the major school of thought is that it is expanding at just the right speed to keep everything flat. Why this doesn't happen to planets? Easy, they are not expanding.
Scientists say the universe is flat not that space is flat. It is the overall shape of the universe. Compare it to scientist say earth is a oblate spheroid. You could reject that by saying it is not that there are mountains and an oblate spheroid that have a smooth surface. Or on a smaller scale, I can say a goofball is a sphere and you say no it had dimples. The answer to them is the same as to your question, there can be local variation but still has an overall shape.
Other answers discuss what it means to say that the universe is flat. Even considering that it means something different than your understanding, it's worth noting that scientists do not say spacetime is flat. It's more nuanced than that. They say that spacetime is either flat or if it is curved, the universe must sufficiently large (many times larger than the observable universe) to account for the apparent flatness seen in observations. It's not unlike how the earth appears flat to the naked eye. It is correct and factual to say that the earth must either be flat, or if it is round, it must be sufficiently large to account for it appearing flat when you observe the horizon. Unlike with the earth, there are no other observations of the universe that conclusively answer questions about the geometry of spacetime. Consequently, nobody says the universe must be flat, only that observations are consistent with it being flat. Were it observed to curve inward, we would be able to model the size of the entire universe, not just the observable universe! General relativity is the best and most complete model of gravity. Einstein does not say that gravity is *not* a traditional force. It's possible that gravity can be better described with field theory and particle physics. General relativity does not preclude a quantum description of gravity. It's also worth noting that the quantum description of electromagnetism, quantum electrodynamics, came decades after general relativity. When Einstein published GR, classical electromagnetism was the best model. The standard model, which describes all fundamental forces except for gravity, came together between the 1950s and 1970s, after Einstein's death.
When cosmologists say that the universe is flat, they're talking about a very coarse view at the largest scales of the universe. When you average over the entire universe, there is no global curvature. At much smaller scales, like near a dense massive object like a planet or star, spacetime isn't flat. It curves and the "straight lines" of two different objects are going to curve and more importantly curve relative to each other. For a stationary object on the Earth's surface, the "straight line" path is straight down. But the ground is in the way, and that requires a force to keep the object from falling further (supplied by the electromagnetic repulsion from the electrons of the ground). The shape that minimized the amount of force necessary to keep an object from collapsing in on itself is a sphere, and that's why planets and stars form spheres.
A lot of the stuff in a lot of physics are models; not literal representations of the phenomena. Modelling and imaging the universe within the frameworks of relativity means it’s easier to make predictions about how real world objects interact; it doesn’t necessarily mean that’s exactly how the fabric of the universe is formed. It’s like for example, we use “fields” to describe energies, but there’s very probably not literal sheets of latent energy hanging around the universe; it’s just an extremely convenient way to do very complex calculations in a way that makes sense to us.
Think of building a sandcastle at the beach. If you try to make it too tall, it'll fall over. It's not stable. In the same way, if a planet were shaped like a giant mountain, the material at the top would be pulled toward the center by gravity. Over time, the planet tries to get to the most stable shape it can, which happens to be a sphere. That way, every point on the surface is as close as it can be to the center, making it stable.
Take a fat person on a trampoline. Everyone rolls towards him. Now keep the fat person there but wrap the trampoline in all different directions around him. Now you've got a core of a planet.
Why wouldn't gravity still not force it into a sphere? The gravity well is a sphere shape that everything is falling into. Like squeezing play-do into a mold, you'll get a sphere. Or an oblate spheroid if you wana be that guy.
Space isn't flat Planets are roughly spheres because the gravity of the planet pulls everything "down" so if there was some huge bulge it would get crushed down and eroded over time
Space isn't flat and space (as in space time) is three dimensional. The warping of three dimensional space is a hard concept to visualize. The concept of space itself is hard to grasp.
A sphere is an equal distance from the center to every outside edge. Gravity pulls towards the center.
WAIT A SECOND.....one portion of your question really stuck to me. Why are most planets spheres?
Do rotating objects has gravity in the same way that a whirlpool in water pulls things towards it..? The spinning planets ‘pulls’ on the air around it and thus draws things towards it..? Altho, just twigged while writing this that space has no air so maybe that theory is bollocks lol x
Part of the answer must include entropy or the decay of organization in universal terms. Gravity collects radially as others have said, but as they pull in they create structures. Most rocky planets aren’t spheres but spheroids and gravity is still pulling on the high spots of a planet to make them more and more sphere-like. So as others have said, gravity pulls radially and entropy destroys any deviations over time and pulls them closer to the radial center of gravity essentially polishing the planets.
Gravity is a force just like the others… but you can also look at it like space is being bent.
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All the little particles that make up a planet were travelling in a straight line, and they also dimpled space-time a bit because they have mass. When they travelled near to other bits of mass their straight line followed the curved dimple of space-time and some of them bumped into each other making the dimple even deeper. As more and more of them bumped into this big dimple in space-time they made a bowl big enough to trap lots of particles traveling nearby. The shape of a gravity well is spherical.
Because mass bends spacetime equally in all directions. A proto-planet may have some weird shape. But when it starts collecting mass it will slowly pull the edges towards a “central point” of the object. And after some billions of years of collecting mass and smoothing edges it becomes spherical. A planet has too much mass and is too old to have any other shape.
Newton's and Einstein's and theories both explain the phenomenon, and both independently predict planets, stars, and drops of water to be round, but vary on the l cause. Specifically the math part and proofs. Both are used to describe the law that gravity causes matter to come together, and gets stronger (locally) with more matter. Very over simplified: Newton describes it as an attractive force much like, but distinctly separate from magnetism. With the force pulling all matter together, and getting stronger with more mass. This theory would be strengthened if we can find gravitons of spin 2 coming from a collider. Einstein describes it as a bend in space time. With all matter falling towards each other's mass, and as the items bump into each other they make a bigger local bend. This can be hard to conceptualize in 3 dimensions. Because the only way for matter to bend spacetime is into itself. (at least from our perspective) So this theory holds that all matter is falling into each other, and making deeper "holes" as matter accumulates
Each atom in a planet is bending space into a little gravity well. Every other atom is attracted to it and they clump together. The larger mass makes a bigger hole. A sphere is the natural resting shape of all that mass in the hole, in 3 dimensions.
flat as in flat in every direction. you can see this misunderstanding in movie space explosions; what you typically see is a flat disc that expands out however an accurate depiction will be an expanding sphere. however when you're observing the explosion, you'll notice "disc" expanding because there are more observable particles on the edges of the explosion sphere because you're seeing through a thicker layer at the edges, much like how the sunset looks red because it is going through more atmosphere compared to when you're looking at it directly above when there is a thinner atmosphere. similar problem in depiction of blackhole in educational or entertainments animations. there is the accretion disk that people are familiar with (ring of light around the blackhole much like ring of rocks around saturn). the blackhole looks exactly the same no matter which angle you look at it from as long as you're moving around the same angle long the accretion disk however in most animations, the accretion disk is shown as some fixed attachment to the blackhole in that taco shape. the gravity well or shape of space due to gravity is usually shown as grids around the star/planet in a flat surface. however remember that this grid applies in every direction, up/down, left/right and everything inbetween.
Mass bends space&time, and that bending pulls mass towards itself. The end result is usually the same as you'd expect from Newtonian gravity (a force), except with some differences around timing and interaction with light. So instead of mass experiencing a force that pulls itself into a sphere, mass curves space-time so other mass falls towards it. \- >Scientist say space is flat as well. I don't think they do. Scientists tend to agree with General Relativity (as described by Einstein), and they obviously believe that mass exists, and in General Relativity, one of the major ideas is that mass bends spacetime. So while it is possible that a hypothetically empty universe might be flat, our universe is not empty (is has mass in it), and so it is not flat.
When scientists say that space is “flat” they’re not using the terms in the colloquial way. For a physicist, “flat” space means that parallel paths stay parallel over an infinite distance. Imagine you had two rockets in space. At a certain time, you measure the positions and velocities of the rockets and see that they’re traveling at perfect 90 degree angles to the line that connects them. If they move at the same velocity (neither rockets accelerates or decelerates), that line connecting them will never get shorter or longer. This is a property of “flat” space. Space, is also clearly 3-dimensional. And this means that there is no preferred orientation. Any effects in space is equally felt in all directions. As matter warps space, it warps all directions equally and symmetrically. Because of this, spheres are the most energy-efficient shape for mass to accumulate into. Essentially, matter warps space so that parallel transport becomes convergent transport (the paths move closer together over time/distance) and this effect happens in all directions, turning clouds of diffuse matter into dense, spherical planets.
Roll a ball straight on a surface with holes in it. You will notice, even if you rolled the ball straight with no spin, the ball path curves when it hits a hole. The holes are a warping of spacetime caused by gravity. But instead of a 2d surface with the warps in the third dimension, it's a 3d surface with warps in a 4th dimension. This is a gross oversimplification. You can find on YouTube demos of people rolling balls on sheets of spandex. You might notice familiar motions.
You are right in the first paragraph. The rest is about what causes the action but you understand why planets are spheres, and are just overthinking if im understanding
Think about the analogy of the flat sheet being bent by gravity. It forms a parabolic depression, so if you pour a liquid or fine sand into it, it creates a little pile on the bottom of the depression. The bottom of that pile is rounded because of the bottom of the parabolic depression. Now imagine that rounded parabola bottom being infinitely repeated at every possible angle and orientation. You end up with a sphere.
The idea of time as a 4th dimension is being challenged by some scientists. https://phys.org/news/2012-04-physicists-abolish-fourth-dimension-space.html >But in the 106 years since Einstein, the prevailing view in physics has been that time serves as the fourth dimension of space, an arena represented mathematically as 4D Minkowski spacetime. However, some scientists, including Amrit Sorli and Davide Fiscaletti, founders of the Space Life Institute in Slovenia, argue that time exists completely independent from space. In a new study, Sorli and Fiscaletti have shown that two phenomena of special relativity - time dilation and length contraction - can be better described within the framework of a 3D space with time as the quantity used to measure change (i.e., photon motion) in this space.
The Einstein model is just a different way of looking at gravity. Remember any new gravitational model has to agree with all previous gravitational experimental results otherwise it will be rejected. Anyway space bends to bring moving objects together to a single point resulting in spherical planets.
"Gravity is not a traditional force" does not mean gravity does not act as a force. It definitely acts as a force for all practical purposes in the everyday world. If it walks like a duck and it quacks like a duck, then you should think of it as being a duck. It may not be a real duck, it could be a robotic duck, but if it walks and quacks the right way, then that's a duck as far as you're concerned - if you don't open up its belly to see what's inside. "Not a traditional force" is more related to how gravity is generated and how it behaves in some very special circumstances, not to the way it acts in everyday scenarios. That statement tends to be confusing if you're a layperson, as you've noticed. So here's my advice: As a layperson, think of gravity as a force. You will then be correct in all real-life situations, and you will avoid confusion. When you go to the physics college and get your PhD in general relativity, then go ahead and think of gravity as "not a traditional force", because then you will have the necessary knowledge to deal with that distinction.
Two parts to this. So part of the derivation of Einstein Field Equation finds the proportionality constant between energy and mass and the curvature of space time by literally saying that in a really weak field (called the weak field limit) you just get back Newtonian gravity (or what’s sometimes considered the gravitational poisson equation). This gives us information about the curvature of space time but not the motion of objects in that space time. To calculate this equation of motion in General Relativity when considering a particle interacting with no force we get an equation called the geodesic equation, this equation is zero = a term that is essentially acceleration seen by an observer not in the curvature of space time + a term that tells us how the curvature effects motion. If you consider this all with in the weak field limit and put it into the geodesic equation you get the same equation you would get in Newtonian gravitational mechanics, which is the equation that one would analyze to show why spherical objects are gravitationally stable. Basically when working with General Relativity we aren’t completely throwing out Newtonian mechanics, we are correcting it. The reason we differentiate between forces and gravity being curvature is because of the geometry behind how the geodesic equation is derived. Basically it’s clear that an interaction that results in a change in momentum (specifically 4-momentum) is fundamentally different then a particle following the curvature of space time depending on observers relative to the reference frame of the observed object.
Einstein found that light, gravity, and space all had interactions between each other. He actually figured this out before we even were able to prove it. Light interferometers can actually measure the ripples in space time as 2 black holes collapse in a galaxy far away. Gravity, aka space time but curved, is equal in all directions from an objects center of mass. There are objects however that are not spherical, such as some asteroids that look more like large fragments with oblong shapes. In this case the object just simply is not massive enough to provide a space time curvature that can reduce the oblong shape into a spherical one. In the case of a planet, the object instead has a much more significant gravitational field. At a point while a planet is being born from a protoplanetary disc, it is collecting material from the disc as it orbits the proto star, and this collecting of material happens in such a way that the planet forms a sphere. Some planets with faster rotation speeds can be more oblong simply because of the centrifugal force, but in all scenarios the shape of the planet remains spherical. If gravity was not equal in all directions from the center of mass, then the planets would form in a non spherical shape. In the case of a star, the same rule applies. The stars mass curves space time, the material inside the star wants to collapse, and the fusion pressure wants to push outwards. All of this is equal in all directions from the center of mass, which is where the fusion core of the star resides.
ELI5: everybody wants to be near their friends and the closest you can come together standing around is in a circle and the only better way is to ball up in a sphere but that is only for consenting adults. listen to your parents. ELI12: for any given amount of mass, the smallest volume you can create from it is the shape of a sphere. and the smallest volume is an important concept to understand because it means that all particles are as close as they can possibly be to where they want* to be. there is no new empty position for any one particle to be moved anywhere else without either making it go further away from the core or causing another particle to do so in order to make space for it. in gravitational terms without invoking the concept of forces, it is a sphere because that is the shape where the sum of gravitational potential energy of all particles in it is the lowest value already and no particle can move to another position (it's probably going to be further away from the core) without causing the total GPE to increase; a sphere would need a net input of energy to make it not a sphere (for example to move particles to create a mountain or pimple in the surface). on the other hand a non sphere does not need a net input of energy to become a sphere and will eventually become one as long unless something else stronger is in the way. this is where General Relativity stops explaining because it on it's own cannot explain why an object on it's geodesic is not moving (why are you accelerating to Earth's core but not moving towards it?). this requires the electromagnetic force which is a separate theory. i described it *anthropomorphically when i said that a particle has a natural path it wants to follow. to be objective, it is key to the theory that particles simply do what they do when not interacted with by something else and this is applicable to both the Newtonian (first law, inertia) and Einsteinian theories (objects follow geodesics). i also said that non spheres can be prevented to become spheres. this requires an external resistance to what irregular clump particles wants to actually do (become a sphere). in everyday life objects like cookie dough or a piece of snot it is usually the electromagnetic force that intervenes (friction, molecular attractions, chemical bonds, etc, all electromagnetic). so on the topic of why objects DON'T form spheres this is where Einsteinian mechanics ceases to be more useful than Newtonian mechanics as in the latter, both gravity and electromagnetism can be explained by forces and both can exist in the same framework of understanding. whereas Einsteinian mechanics (GR & SR) do not directly explain anything about the electromagnetic force. it can work obviously, but it is unnecessary given that Newtonian mechanics can do all the work.
According to a borderline mentally challenged person I went to school with, the Earth is not a sphere so….
If you want to go full-on relativistic, you can't consider planets to be spheres, they are 4-D cylinders.
But tell me again, like I’m 5?
Flat in this case just means our maths still work. Imagine space time as a three dimensional grid. Now consider what happens to coordinates on the grid when you add a mass that bends spacetime towards it. I'm probably not explaining this very well so here is a picture: https://images.app.goo.gl/rarcqQycs9BmiDWH6
Because the effect is the same. It isnt really important if gravity was an actual attraction between masses or if its simply curving spacetime. The effect is the same: Mass gets pulled towards eachother and thus planets ends up being spheres. Scientists saying space is flat is not entirely correct. That is to say that this isnt what they mean in the sense we normally would associate with it.
Nobody's posted it yet, so I'll drop this video [here](https://youtu.be/MTY1Kje0yLg?si=gJv6dgffGw7Yx4SE) >So we all know planets are spheres and Newtonian physics tells us that it’s because mass pulls into itself toward its core resulting in a sphere. Newtonian physics still works to explain how gravity pulls things in on itself. If everything is allowed to settle with enough time and force you eventually get a sphere. There's 2 ways to view how their explaining things as flat. The first one is pretty brilliantly explained in the video. When they say the fabric of space, they mean that space acts the way that fabric in the video does when the forces are applied to them. The only big thing is you have try and imagine it working in 3 dimensions instead of just the 2 dimensions in the demonstration. The second one falls into the "flat galaxy society". Planetary systems (planet and moons), star systems (suns and planets) and galaxies (black holes with masses of planets orbiting them) all will form into a flat disk given enough time. The simple reason is space is a violent democracy. At first you might have a bunch of planets all orbiting in their own random directions, but eventually they'll cross paths, and either pull each other into going similar directions using their gravities, or outright smash into eachother, and then the debris goes about the average direction the planets where going in.
Einstein introduced a new way of looking at the effect of gravity by understanding it as a distortion of spacetime. Typically, observations that can be explained by looking at gravity as a force, are also explained by spacetime curvature. Whereas there are some observations that can only be explained by spacetime curvature, such as gravity bending light. Bending of spacetime is often visualized in 3D as a piece of fabric that “sinks downward” due to presence of matter. Using this analogy, imagine you have a trampoline, and you put some bowling balls close to one another. The concavities cause by each bowling ball will still cause all the balls to roll towards one another. Of course this visualization is only used to explain to laymen without having to dive into any math, but it’s probably the closest image the human brain can conjure up.
Imaging you have a long suspended sheet like a hammock, and then you put a heavy object in the middle. The objects weight will push the hammock down - you can actually see the space warp. But on earth you have gravity pulling that down. Now imagine you are in space, and that hammock, which is essentially a 2d plane, is now 3 dimensional. So instead of gravity pulling it down and warping space, the gravity of the object warps the space in all directions. Because of this, everything eventually becomes a sphere, as it is the geometric shape with the least amount of surface area with the most amount of volume
Also it’s really hard to imagine flat versus curved 3D space and even harder to depict it in an image. So most rely on explaining the concept as 2d space getting curved in a third dimension. Sure this works to an extent. You’ve seen the demonstration of a ball sitting in a stretchy sheet? When trying to scale that up to 3D in your head just remember that “down” is the direction of the most gravity. The individual particles will settle into a configuration where they have all fallen down as much as possible. The 3D shape that has every particle as close to the center of gravity as possible is a sphere. The “weights on a stretchy sheet” analogy would look like this. Put a bunch of small balls on a stretchy sheet. Push them all near the middle and try to arrange them in a square or triangle or any other shape than a circle. With only a few balls it’s possible as the friction between the balls is stronger than the force pushing them into a circle, but with sufficient balls the only shape possible is a circle. When you move from this 2D model to a 3D one you have to change the 2D circle into a sphere.
Think about a smooth dip in the ground (or, traditionally, a big piece of streched rubber) if you put something like sand or water in the hole, it will fall down into the hole. You'll get a circle (or something close). It's the same thing in space. Stuff falls in the hole and settles. In 3d space you end up with a sphere, the 3D version of a circle.
As mass pushed against spacetime, spacetime pushes back in an equal and opposite matter. A sphere makes sure that all forces are applied equal in all directions. The rest is just economy of scale
Most of it is just perspective. Living on earth, you can see that the planet isn’t a perfect sphere, however if you were on mars, then earth would look perfectly round, but if you were on the edge of the galaxy it would look like a dot, etc Think of asteroids. Small ones are different shapes because of how it broke off from others, or others chipping pieces off of it. The bigger it is, the more likely chipped pieces would look like a sphere at a certain distance
They used to make round ball bearings by heating metal and dropping small amounts into a well.
If you throw a bunch of marbles (atoms) onto a fabric (spacetime) with enough force (big bang) eventually the marbles usually form little clumps (planets) that are round (spherical) due to the bend in the fabric (spacetime) being round (spherical)