![]() It is true that a massive object distorts the static space in its vicinity, but that's only half the picture. It is a little harder to visualize, which is why we usually retreat to the simpler demo. ![]() There is no "down" in real space-time, and the curving is happening in four dimensions, not two. That demo is perfectly fine as a first, kindergarten introduction, but we're well past kindergarten now. Rolling other balls on the sheet reveals the "influence" of gravity: They try to follow straight lines, but their paths get deflected by the underlying curvature. A heavy ball, representing a planet or star or black hole or whatever, is placed at the center, pulling the fabric down. You've probably seen the science museum demo or graphic accompanying an article on general relativity showing what looks like a stretched rubber sheet. TThe "time" in space-time is very important to the full theory. In other words, that gravity is the geometry of space-time. The equivalence principle tells you that the paths of falling junk directly inform you about the nature of gravity, and those same paths reveal a complicated geometry of the underlying space-time. These changing paths are exactly what the mathematicians use the language of "curvature" to describe, and that is the language of geometry.ĭing, ding, ding. In both cases, paths start out as perfectly parallel or uniform but change character s. ![]() In other cases, we get diverging, spreading trajectories. In some cases, we get converging, narrowing trails. So, as the pieces of debris continued their descent, they would slowly diverge in their vertical line. But the lucky one at the front of the line would fall slightly faster, due to its slightly closer proximity to the Earth, with the last in line lagging slightly behind. Other bits of junk might start off in a perfectly vertical line aimed toward Earth, evenly spaced apart from each other. If they could pass through the solid Earth, they would eventually collide at the very center. If you watch them closely, you'll see that as they head downward, they'll gradually converge. But as the objects fall to Earth, they follow straight lines heading right for the center of the globe. Some, by pure chance, may start off in a perfectly horizontal line. Watch what happens to the junk you tossed into space. That's something unique to the force of gravity, thanks to the E.P. You're doing a science experiment, and the equivalence principle guarantees that all these bits of debris, no matter their shape or mass, will perfectly trace the effects of the Earth's gravity, without the need for any other calculations. But upon further reflection, your mind eases. Sure, you've now created a cloud of potentially hazardous debris that poses a major risk to satellites and future missions. As the bits of debris float away from you, you ponder the ramifications of what you just did.
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