Imagine you are now floating in space. There is no gravity, no air, and nothing there except, perhaps, a screen in front of you. You shine a flashlight onto this screen. You can probably guess what will happen: the light will follow a straight path to the screen and you can now make shadow puppets. Now a slightly different but surprisingly similar case. You are in a glass elevator, and again, you have the shadow puppet screen in front of you and your trusty flashlight in hand. Suddenly, the elevator drops and you feel yourself become weightless just like you were in space. You don’t panic, no, you decide to make some more shadow puppets. Shining your flashlight you notice no difference in the shadow puppets. The light (from your perspective) travels in a straight line to the screen.
Here’s the kicker. The light takes some time after leaving the flashlight to reach the screen. A very short time, yes, but some time none the less. Let me remind you that the elevator is falling, so some time after the light has left the flashlight, the screen has moved downwards a slight amount. But you don’t see the light fly upwards because you are falling down, no, to you it travels to the screen like it should. Someone on the ground, however, will see you falling and making shadow puppets, but this peeping tom will see you and the light falling towards the earth.
If you have ever thrown a ball into the air, then you know that it follows a curved path as it falls. The light beam must do the same thing by our earlier logic. But light always takes the path of shortest distance (aka geodesic). But now you say, “Wait. The shortest distance between two points is a straight line! The falling light must not be taking the path of shortest distance!” No. Actually it seems, the shortest distance between two points is not a “straight line”. How, you ask? Because space and time are curved by the earth.
So what does space “look like”? Are we living on some kind of five dimensional sphere? Maybe and maybe not. There is no requirement, physically, for a “fifth dimension” to have space and time curve into. People always like to give the image of a bowling ball on a bed sheet and relate that to the earth curving space and time but you need to be careful with this image. This bed sheet is actually only a two dimensional space (excluding time), when really we live in a three dimensional space, but it’s impossible to imagine a three dimensional space curving. Here’s a nice picture of what I’m talking about.
This bed sheet analogy also leaves out the image of time curving. What does that look like? I have no idea. I don’t know what time “looks like” in that sense. One result of the curvature of time, though, is that someone on a high mountain (a really high mountain) looking down on you will see you travel more slowly and they will see you travel more quickly (or do whatever it is you are doing, more quickly). This effect actually makes a difference for GPS satellites, who’s internal clocks travel more quickly than ours do. (The engineers take this into account, of course, because GPS works well.)
So what is gravity? It’s just a result of the curvature of space. Baseballs, cats, and that bit of coffee you spilled earlier this morning, all fall towards the earth because they all must follow paths of shortest distance, which happen to be what we call: down. We must constantly fall towards the earth because we must move through time. As we move through time, we must take the shortest paths. So it’s not that gravity is pulling us down, it’s the earth that is pushing up on us, preventing us from traveling on the paths of shortest distance. That is what we feel as gravity.
