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
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.
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