I’ve been watch the science channels lately and they keep talking about how gravity is just a warping of space. They show a bowling ball on a trampoline as a visual aid. I understand the concept, if something get near the depression it falls ‘into’ the mass (bowling ball). Similarly, I understand that being up away from the planet causes you to fall into it’s mass. But if I’m standing on the object that is causing the depression, why am I still affected by gravity?
That stupid bowling ball on a trampoline metaphor is poorly thought out and messes everyone up.
The idea is just meant to be that mass is curvature of space; curvature in the sense that “straight” paths through space can nonetheless approach each other at an accelerating rate. For example, as applied to the “straight” paths through space taken by you and the Earth, this results in you accelerating towards the Earth (i.e., gravity).
But pulling the right things from the bowling ball on a trampoline analogy rather than the wrong things is very difficult; people think “What’s the bowling ball and what’s the trampoline and what keeps them separate?” and “Wait, what explains the gravity pulling the bowling ball into the trampoline?” and all such things, questions which are meaningless except as a way of pointing out how clumsy and inapt the metaphor is.
My naive 2 cents: You are always affected by gravity, the only reason you don’t fall further down is because the earth itself is pushing you back.
For example, if you’re tied to a horizontal spring which is under tension, the spring pulls you towards it. But if you’re standing against a wall, and there’s a small hole in the wall with the spring going through it, the spring is still pulling you. The reason you’re not moving through the wall is because the wall is pushing you back.
I don’t think the bowling ball on a trampoline example is so bad. It certainly explains things in this case.
If you put the bowling ball on the trampoline and then you add a marble, what happens to the marble? It moves into the center until the surface of the bowling provides a force to oppose gravity, at which point the marble stays put. The marble is still trying to go down closer to the center of the trampoline, but the bowling is stopping it.
If you add lots more marbles, eventually the marbles themselves will be providing a force that prevents other marbles from getting closer. (And, at the same time, they’ll be increasing the amount of depression in the trampoline).
Maybe your problem with the image is that you think of yourself standing on top of the bowling ball? In the trampoline image, every object has to be on the trampoline (since it’s a two-dimensional analogy for a three-dimensional space, you just have to make that assumption for it to work).
The simplest way to think about it is to imagine all the mass of the earth concentrated as a point at the earth’s core. That way it affects everything at the surface equally.
The biggest key here is that it’s not just a warping of space; it’s a warping of spacetime. And even when you’re standing still in space, you’re still moving through time. You’re always the marble that’s rolling past, never one that’s just sitting there.
Every individual particle of the earth is pulling on you - the bits on the other side of the earth, thousands of miles away, have a relatively weak pull. The bits under your feet have a relatively strong pull. The matter making up that hill a mile to your left is actually pulling you off to the side, and so on. The net result of all those jillions of tiny particle-particle attractions is a sum total force that acts along the line between your center of mass and the center of mass of the planet, pulling you “down”.
If you go down a few hundred meters into a cave there will be slightly more mass of the earth above you, pulling you in the direction of the surface, and thus there’s slightly less mass below you, pulling you down towards the center of the earth. If you have a very accurate instrument you can measure those changes.
The only way you’d feel “no gravity” would be if you were right at the center of mass of the earth - then you’d have equal amounts of mass on all sides pulling in opposite directions and the net force would be zero.