How Gravity Makes Things Fall
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by Super User, 7 years ago
1. As a falling object's path goes increasingly in the space (down) direction, it goes a little bit less in the time direction. Gravity is effectively converting some of its travel through time into travel through space. How much time converts into how much space? It works out to be 186,000 miles of space for every second of time  that's the speed of light! The equivalence between a little time and a lot of space has a parallel with Einstein's famous equation E = mc^2, where a little mass is equivalent to a lot of energy  also with the speed of light c as the conversion factor, only squared. It's amazing how all these physics concepts fit together.
2. An object moving up or down at the speed of light, such as a photon, follows a vertical path on our graph. If we could warp the Spacetime Stretcher as much as we wanted, we could make the time axis curve around and reintersect with this vertical line. That would be a black hole: Even a photon moving straight up would get "warped back" to the place where it started.
3. Spacetime warping is usually depicted as going into a higher dimension  in this case, bending toward or away from the camera. But such a device would have been much harder to build. You can think of this model as a twodimensional "projection" or "shadow" of threedimensional curvature, i.e. the flattening of a flaredout surface that curls away from the plane of the graph.
4. In addition to mechanically calculating trajectories of objects falling on the Earth, Mars, or Moon, the Spacetime Stretcher (at a zoomedout calibration scale) can also calculate the lengthcontraction and timedilation observed with a rocket traveling close to the speed of light.
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7 years ago