I’m sure I’m not really understanding gravity theory, but I have in mind the illustration of marbles on a bedsheet. If that bedsheet is space-time, why isn’t there some sort of universal up and down as objects relate to each other?
In: Planetary Science
The bedsheets are basically a 2d representation of a 3d phenomenon. Gravity doesn’t push down. It warps space in every direction.
The marble on a bed sheet demo only works if you are viewing the sheet from above. In reality, gravity attracts regardless of which angle you are viewing from.
We visualize gravity using a 2 dimensional model where space-time is a flat sheet that is distorted by objects pressing, as you say, “down” on them.
But space-time is actually 4 dimensional (3 spatial dimensions plus time), and it’s impossible for us to visualize that. “Down” in a 4 dimensional system is incomprehensible. The only way we can really work with it is mathematically.
Think about this: a black hole, i.e. a hole in spacetime, is actually a sphere.
Because it’s pushing in a dimension we can’t see.
The bedsheet analogy represents a 2D universe, and all that flexing is taking place in the 3rd dimension, which you can’t see if you’re in the 2D universe.
There’s no up or down in space, but there is an up and down in that 4th spatial dimension we can’t see, and that’s the direction spacetime flexes in.
Gravity has nothing to do with objects “pushing down” on the fabric of space-time, so that’s where you going wrong.
The “marbles on a bedsheet” image, like this [Earth on a two-dimensional grid](https://miro.medium.com/v2/resize:fit:1200/1*wjG1-Cwv7di8KOf1ShO8TA.png) is misleading. That picture implies a two-dimensional sheet of fabric with a ball sitting on top of it, which makes it look like earth is “pushing down” on that two-dimensional sheet..
But space isn’t a two-dimensional sheet.
Rather *space* is three dimensions, so the gravitational warping of space (just space, haven’t gotten to space-time yet) looks more [like this](https://quantumdust.files.wordpress.com/2013/01/20130113-113906.jpg).
It’s not “pushing down”; it’s warping space in *every* direction.
But it’s not just warping space, it’s also warping *time*.
Time is a fourth dimension and we don’t have a good way to draw four dimensions, so we’re kind of out of luck for a visual depiction. But, basically, when we’re talking about space-time, space is warped kind of like the second image above and time is also warped — the stronger the gravity the more slowly time moves.
So someone on the surface of the Earth experiences time at a different rate than someone orbiting the Earth. (The difference isn’t that much, but it *is* measurable.).
EDIT: I realize I didn’t really answer your up/down question. If you look at that second image, though, you can see that the Earth really isn’t sitting “on top of” space and that space _all around it_ is warped by the Earth’s mass. So there’s not top or bottom or up or down in that image.
So people might suggest that “down” is the direction that gravity pulls you, so if you were orbiting around earth, “down” would be “toward earth.”
But that’s not really accurate, because you’re also orbiting a much bigger thing — the Sun. So “down” is _also_ toward the Sun.
And if you were floating somewhere in space between the Sun and the Earth, there’s actually a point in there where the Sun’s gravity is acting on you more than the Earth’s gravity and the Earth would stop being “down” altogether and your only “down” would be toward the Sun … at least until you were fried to a crisp.
Or imagine you’re floating out in intergalactic space someplace. Again, depending on where, gravity from the Andromeda galaxy might be acting on you more than the Milky Way galaxy. But compared to a few hydrogen atoms floating in intergalactic space near you, *you* might be the most massive nearby object so *you* would be “down” to those atoms!
So, yeah. “Down” in space is entirely relative.
Every object creates a gravity “hug” to anything that gets near enough to it according to how much mass it has. The more mass, the stronger the hug is. The hug is shaped like a ball.
Unfortunately, as a human you won’t be able to visualize the reality of the warping of space-time due to gravity/energy. The only reason we can apply the concept of this kind of warping to tell us useful things is because of Einstein and the metric tensors in general relativity.
To paraphrase Mandalorians, “the tensors are the way” (via the related math) we can calculate and deduce new facts about the universe (if the question is related to GR).
But the main issue is not just that we exist within the 3 dimensions that you want to visualize, we have an issue visualizing how time warps as well. We exist in a reality where time has one dimension (forwards/backwards). But when space-time is warped, that’s no longer true and we don’t have a good way to intuitively visualize that outside of just noticing that time ticks slower in the presence of large gravitational fields. But that ticking rate is a consequence of space-time curving and we can calculate it all with those tensors.
It is hard to model it in 3 dimensional space. Both as a demonstration as well as in your mind.
What is happening to the bed sheet is happening in ALL directions at the same time. Not just the flat plane you see in the demonstration.
[This should help](https://images.app.goo.gl/sXQUzwgwnvS5st7s8)
The bedsheet illusion is meant to let you visualize how different size object will create gravitational pull over distances. And I think it’s better viewed as a kind of rubber sheet, not a cotton one. And… it relies on local gravity to demonstrate universal gravity, which has inaccuracies in the modeling.
Imagine normal objects on a thin (unbreakable) rubber sheet larger than your typical soccer or football field.
A marble makes a really small dent. The dent is deepest next to the marble, but it curves up towards the main plane of the rubber sheet pretty fast. The sheet is slightly stretched. And any small things next to marble will get pulled to marble quickly. Super technically, that small marble’s dent goes all the way to the edges to f the sheet, but it’s practically immeasurable after a few inches.
Now put a bowling ball on the sheet in a different location. Bigger dent, and is noticeable much further. Maybe a few feet. But still hard to measure at the far edges. But, given enough time, the marble will roll to the bowling ball, no matter how far.
Now put a large boulder on another part of the sheet. Huge dent. Measurable from much further. Still barely noticeable at the farthest edge. Anything near its pit will roll toward it.
And if you observe the line between the bowling ball and the boulder, you would see a slight trough, because these two objects are attracted to each other.
All of that is useful to convey a basic understanding of gravity without getting into math.
But here’s the thing: that rubber sheet is relying on the existing gravity of our planet to demonstrate this effect. In actuality, all it represents is distance, in any direction, between points of mass, and how they can attract each other. It is not meant to represent up, down, left, or right.
And it fails to be able to demonstrate what happens when you add objects not on the plane of the sheet. We already have three objects on the sheet, and can somewhat imagine their interaction using this model.
But what happens if we add a fourth boulder that is 5 meters above the sheet of rubber? The first thought is that it will fall onto the sheet, cause a big bounce, and pull everything towards it. But in space, it’s ok to have things not on the same plane. It just means things also get pulled ‘up’ and ‘down’ not just across the flat sheet.
Which comes back to the original question: there is an up and down to the rubber sheet, because of limitations in the modeling. But in space, there is no up and down (or it is only relevant to the observer, or based on agreed social construct).
Here is a very ELI5 visual:
Imagine you are pointing “down” at the same time as someelse on the exact other side of the earth. You are both pointing down, but technically, you are pointing at each other in opposite physical directions. For you to point the same physical direction as them, you’d have to point “up”.
Gravity gives you a point of reference that is pulling you relatively down on the surface of a sphere. In space, you don’t have that single point of reference, but many of varying shapes and sizes.
Another easy visual, tides. From our perspective, the tide comes in and out. We see that as moving “front to back”. But if you could ask the water, it’s just moving down towards the moon. It’s not moving to and fro, but constantly towards the moon (barring the resistance of land and the lack of force from the moon compared to Earth, but this is ELI5, not explain like in in Jr high).
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