![]() You can also curve the paper, and the tracks will curve too. You could draw tracks on a piece of paper to show this. If you image a point in space, the future path of any object which reaches that point will depend upon its current direction. If you want to be elsewhere, you'll have to exert a force to move yourself. If you're 100 miles above the Earth, and not at an orbital speed, "here" will be closer to the Earth's surface in the future. The curvature of spacetime defines what "here" means, and what it will mean in 1 second, or 1 minute, or 1 hour. It’s as if the universe is correcting itself like: “Oh nope you’re supposed to be HERE now-boop.” (Either that or you suppose there is a universal "present time" for everyone, but special relativity says that is impossible.) To resolve it, you have to realize that we're really just extended objects in spacetime that both intersect "the park at noon" (a spacetime point). I saw this confusion in an askphysics post, or maybe another physics subreddit, a while back. Then you go to the park at noon, but by then I'm back home at 4pm, so we miss each other. For example, suppose you're at home at 8am, and I'm at the park at noon. A different "speed" wouldn't even make sense.Īnother way the idea of moving through spacetime can be misleading is that it suggests that two particles that cross the same position at the same time could miss each other because their "motion through spacetime" wasn't synchronized right. The answer is simply that you are considering a spacetime point one second in your future and asking how far in the future it is. For example, the parent post asked why we have to move at one second per second. Sure, you can define that (or your clock example) to be motion, but it's not useful and can be misleading to someone who isn't aware of how tautological it is. That's like pointing at a line and saying the line advances at one centimeter per centimeter. That's just a way of parametrizing your spacetime extent though, and not really any kind of "time" with respect to which motion in spacetime makes sense. ![]() * Someone will inevitably say you can move with respect to your proper time or someone else's time coordinate. The action of gravity now is that in the absence of other forces, particles' worldlines are "straight lines" (geodesics) in curved spacetime. Cross sections of the 4D object give its ordinary 3D spatial shape.įor the individual particles composing the object, they are lines ("worldlines") extending along the time direction. It's better to think of objects like yourself as things with 4-dimensional extent that are really long in the time direction. Movement in spacetime doesn't make sense movement is a change in position with respect to time, but spacetime already includes time, so there's no other "time" with respect to which movement could take place. That's not really an accurate view either. But it’s just so strange to me that when the clock ticks forward I move simply because that’s where I’m supposed to go according to spacetimes geometry.
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