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 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).
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