Is Gravity acting upwards or downwards?

[Alex K]

This may be totally wrong and just confuse things further, but I'm throwing it out there anyway. Someone smarter than me can correct me if I'm wrong.

An object's gravitational field is less a function of matter and more a function of space-time. Objects with mass warp space-time just by being in it.

I'm not claiming to be smarter than you. I just had some classes on relativity

According to the Einstein field equation, the presence of mass and energy of an object warps the spacetime. So in a sense both is true, it is a function of matter, and it warps the space-time

This "slants" ALL of space-time toward the object in such a way that other objects will be inclined to "roll" toward it.

You could say that, yes

If all objects in the Universe suddenly lost their motion and momentum, they would immediately start rolling toward each other until everything kind of crashed in on itself somewhere in the middle.

Now, the cosmological expansion of the universe which I think you are alluding to, is (according to its description via general relativity) not so much the movement of objects through space (they could be at rest, even), but the rescaling of space itself between the objects (the shrinking of all measuring rods, if you will).

There's an experiment on YouTube I saw a science class doing where they stretch out a piece of lycra and place various round objects on it to see the patterns they make as they roll. With the right combination of weights and objects, they could create complex orbit patterns where two marbles (for instance) would orbit each other closely while simultaneously orbiting a much larger object. It's a pretty cool demonstration of the principle I'm describing. Basically, gravity isn't exactly pushing or pulling one way or another. It's more like we're resting on a table where EVERY object on the table tilts the whole table in toward itself. Only we're not resting, we're hurtling outward at high speeds.

You probably mean something like that, no?


It's a pretty demonstration, and a little bit analogous to spacetime, but not really - the demo itself needs gravity to act downwards, so it's not really a demonstration of how the warping of spacetime generates gravity, but circular and therefore a very incomplete one at best: This type of demo can't really work because it has no way of showing the warping of time, but only space, and the warping of time and spice (*) jointly into each other is the essential effect which gives you gravitational acceleration (getting movement over time from gravity is due to space and time mixing, if that makes sense).

Here is a relatively (pun intended) accurate illustration of how gravitational acceleration is generated from space-time-curvature

https://www.youtube.com/watch?v=DdC0QN6f3G4

I have to stress that this is my limited, layman's understanding of how this happens. It probably makes no sense to anyone but me.

(*) unintended Dune joke