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An intriguingly clean and simple explanation - but my knowledge about the Theory of Relativity is not really up to speed to know if this is physically sound. Wh

An intriguingly clean and simple explanation - but my knowledge about the Theory of Relativity is not really up to speed to know if this is physically sound. What this article does not explain, however, is why we cannot travel back in time...

vlovich123 4y ago
People who actually understand this should definitely correct me. My understanding of the analogy is that your velocity through time would have to cross the 0 point (i.e. positive flow through time, 0, negative flow through time). However, 0 is when you're traveling at the speed of light through space and not traveling at all through time. I'm assuming there's either no energy left in the time direction to push you or something about the 4d geometry makes it impossible because a negative time direction would require somehow inverting all the space in the universe (crossing some infinite asymptote) and that's not physically possible.
throw7 4y ago
One thing that helped me along is also the explanation that between two points in space, a straight line is the shortest distance. Take a different path and it will always be longer. Now, a straight line between two points in spacetime is the... longest time, which, at least to me, is counter intuitive. (note: i'm far from an expert in relativity)
yucky 4y ago
My admittedly ignorant take on it is that time moves in one direction, just at varying speed depending on how quickly you move. Much in the same way that distance travelled is always positive, regardless of direction. Traveling back in time would be like traveling a negative distance.
- ml_more 4y ago
  Weirdly, it seems that the equations for time work in either direction. Questions of causality aside it might be possible to get something to move backwards in time. Probably not anything living though. Or rather not anything you’d like to keep living.
  - jfengel 4y ago
    You are correct. The equations do work in either direction. Special relativity does not, in itself, forbid you going backwards in time. But it does tell you what would be involved in going backwards in time that way. They involve things like imaginary mass and imaginary energy -- things that pop out of factor sqrt(1-v^2/c^2) when v>c. As far as we can tell, imaginary mass and imaginary energy are nonphysical. Relativity doesn't say it has to be that way, but nobody has ever seen them and they don't seem to exist. There are hints of it in the Standard Model, which define mass and energy in terms of even powers of real numbers. Again, that doesn't prove that other things can't exist, only that we've never seen them, and would expect the universe to look very different if they did exist. It might have something to do with the configuration of the universe at very early times, the low-entropy state that is the reason that the universe is interesting (and, in particular, capable of having us in it). But that is an open question. Meanwhile, special relativity is sufficient to rule out time travel unless you want to make some assumptions that mass and energy are very different from anything we've ever seen them do.
thinkmcfly 4y ago
I'm not a physicist so someone please correct me if I'm wrong. There is no 'past' to return to if you could reverse the arrow of time. I don't think the universe has frames like a video that you can rewind to. Interactions are probabilistic with time going forwards and they would still be probabilistic were time inverted. Source: I listen to podcasts
BizarroLand 4y ago
The most likely answer is that the past doesn't exist. We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well. If that is the case, when a moment passes it ceases to exist in the same way that a ripple in a puddle of water does. If you wanted to go backwards in time, you would have to take the whole universe with you.

vlovich123 4y ago

People who actually understand this should definitely correct me. My understanding of the analogy is that your velocity through time would have to cross the 0 point (i.e. positive flow through time, 0, negative flow through time). However, 0 is when you're traveling at the speed of light through space and not traveling at all through time. I'm assuming there's either no energy left in the time direction to push you or something about the 4d geometry makes it impossible because a negative time direction would require somehow inverting all the space in the universe (crossing some infinite asymptote) and that's not physically possible.

throw7 4y ago

One thing that helped me along is also the explanation that between two points in space, a straight line is the shortest distance. Take a different path and it will always be longer. Now, a straight line between two points in spacetime is the... longest time, which, at least to me, is counter intuitive. (note: i'm far from an expert in relativity)

yucky 4y ago

My admittedly ignorant take on it is that time moves in one direction, just at varying speed depending on how quickly you move. Much in the same way that distance travelled is always positive, regardless of direction. Traveling back in time would be like traveling a negative distance.

ml_more 4y ago
Weirdly, it seems that the equations for time work in either direction. Questions of causality aside it might be possible to get something to move backwards in time. Probably not anything living though. Or rather not anything you’d like to keep living.
- jfengel 4y ago
  You are correct. The equations do work in either direction. Special relativity does not, in itself, forbid you going backwards in time. But it does tell you what would be involved in going backwards in time that way. They involve things like imaginary mass and imaginary energy -- things that pop out of factor sqrt(1-v^2/c^2) when v>c. As far as we can tell, imaginary mass and imaginary energy are nonphysical. Relativity doesn't say it has to be that way, but nobody has ever seen them and they don't seem to exist. There are hints of it in the Standard Model, which define mass and energy in terms of even powers of real numbers. Again, that doesn't prove that other things can't exist, only that we've never seen them, and would expect the universe to look very different if they did exist. It might have something to do with the configuration of the universe at very early times, the low-entropy state that is the reason that the universe is interesting (and, in particular, capable of having us in it). But that is an open question. Meanwhile, special relativity is sufficient to rule out time travel unless you want to make some assumptions that mass and energy are very different from anything we've ever seen them do.

thinkmcfly 4y ago

I'm not a physicist so someone please correct me if I'm wrong. There is no 'past' to return to if you could reverse the arrow of time. I don't think the universe has frames like a video that you can rewind to. Interactions are probabilistic with time going forwards and they would still be probabilistic were time inverted. Source: I listen to podcasts

BizarroLand 4y ago

The most likely answer is that the past doesn't exist. We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well. If that is the case, when a moment passes it ceases to exist in the same way that a ripple in a puddle of water does. If you wanted to go backwards in time, you would have to take the whole universe with you.