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It is already implied by relativity that time doesn't flow. You cannot choose a definition of "now" that some other observer who is moving relatively to you wou

by tbabb 8y ago

It is already implied by relativity that time doesn't flow. You cannot choose a definition of "now" that some other observer who is moving relatively to you would agree with. Under relativity, observers will disagree about which events are happening simultaneously. So if there are two observers in the universe, the notion of a "global now" is incompatible with relativity. If some particular observer's definition of "now" could be determined to be the "right" one by some experiment, that would reject the equivalence principle, which is the foundation of relativity. In other words, not likely to happen. Using this principle one can build a conceptual "chain of simultaneity" between observers that extends arbitrarily far into the future or past. It is no surprise, then, that physics doesn't have a concept of "now". In all physical equations, time is a free parameter, which we can set to any value, and those equations will tell us what we should expect to observe at that time. Physical equations do not reify any particular "t". The fact that time seems to "flow" subjectively for us is an artifact of the fact that information can't (that we know of) flow from the future to the past, therefore at any given slice of time for a given observer, the past appears "set" (the observer has information/measurements about it that confine its possible states), but the future appears undetermined (there are no available direct measurements of it). Why information can't flow from the future to the past is an extremely interesting question that is not terribly well answered, especially given that the laws of the universe are time-symmetrical. I think most physicists would posit that it has something to do with the perplexingly low entropy of the big bang, statistically mandating that later states must have higher entropy, thus birthing the second law of thermodynamics and giving rise to the "arrow of time". Why specifically the Second Law would prevent the flow of even a single bit of information travelling into the past is not clear to me. As the article points out, Deutsch 1991 showed us that grandfather paradoxes don't exist once you factor in quantum mechanics. This is a question that has captivated me for many years.

darepublic 8y ago
I thought that time was essentially an illusion? Isn't time just things changing from one state into the next. So time travel isn't really logical, it would entail changing the entire state of the universe at once from its current state to one it had in the past
kiliancs 8y ago
I enjoyed this PBS Space Time video explaining this and more with examples: https://www.youtube.com/watch?v=HUMGc8hEkpc https://www.youtube.com/watch?v=HUMGc8hEkpc
deleted 8y ago
[deleted]
lisper 8y ago
> Why information can't flow from the future to the past is an extremely interesting question that is not terribly well answered. I took a whack at it a while back: http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
- tbabb 8y ago
  Great article, thanks for sharing! I actually found your Google talk some years ago and found it fascinating. I agree with basically all of your article. It certainly explains the dichotomy of physical laws allowing a spacetime to be "played backwards", while our experience must always be that of it being "played forwards"-- the time reversal of learning something is forgetting it, and this would invert the topological sort of knowledge dependency that places slices of our experience in the "future" or the "past". As such an observer in an isolated part of the universe which was being "played backwards" would label our future as their past. If we buy into the Second Law, it makes sense to me that there would be some direction where there are more messy entanglements between all the parts of the universe, and in the other direction there would be fewer, and we would name the first direction "the future" and the second direction "the past", since there would be information embedded in all those mutual entanglements that would over-constrain states in the past and under-constrain states in the future. What I don't get is what prevents me from setting up a circumstance where a measurement I make in the present is correlated with the result of (say) a coin flip in the future. That is easy to do if the coin flip is in the past, but impossible the other way around. I agree that by your logic if I were getting more information on the whole from the future, then I would almost definition label that "the past". But why not even one bit from the future? I can make a meaningful mark on a particle at time t0 and read it at t1, but not if t0 > t1. How come?
  - lisper 8y ago
    Thanks for the kind words. > there would be some direction where there are more messy entanglements between all the parts of the universe Yes, that's exactly right, except for the word "messy". It's not "more messy", it's just "more". > what prevents me from setting up a circumstance where a measurement I make in the present is correlated with the result of (say) a coin flip in the future Nothing. If you flip a coin with enough precision you can make it come up reliably on one side or the other. ("But that's cheating!" you say. "I want the flip to be random." Well, you can't have it both ways: if the flip is random, then by definition it's not going to be correlated with anything in the past!) > why not even one bit from the future Because then it wouldn't be the future. Think about this: how would you distinguish "receiving a bit from the future", which is apparently not possible, and "making a reliable prediction about the future", which is possible in many cases? > I can make a meaningful mark on a particle at time t0 and read it at t1, but not if t0 > t1. How come? You can't actually "make a mark" on a particle the way you can on a classical object. You can prepare a particle in a particular quantum state, but that's not the same as putting a mark on it.
chr1 8y ago
It is worth to note that systems with preferred system of coordinates and "flowing" time can have Lorentz symmetry (for instance see https://physics.stackexchange.com/questions/887/relativistic-cellular-automata https://physics.stackexchange.com/questions/887/relativistic...) so relativity doesn't tell us about time travel as much as we might hope

by tbabb 8y ago

darepublic 8y ago

I thought that time was essentially an illusion? Isn't time just things changing from one state into the next. So time travel isn't really logical, it would entail changing the entire state of the universe at once from its current state to one it had in the past

kiliancs 8y ago

I enjoyed this PBS Space Time video explaining this and more with examples: https://www.youtube.com/watch?v=HUMGc8hEkpc https://www.youtube.com/watch?v=HUMGc8hEkpc

deleted 8y ago

[deleted]

lisper 8y ago

> Why information can't flow from the future to the past is an extremely interesting question that is not terribly well answered. I took a whack at it a while back: http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...

tbabb 8y ago
Great article, thanks for sharing! I actually found your Google talk some years ago and found it fascinating. I agree with basically all of your article. It certainly explains the dichotomy of physical laws allowing a spacetime to be "played backwards", while our experience must always be that of it being "played forwards"-- the time reversal of learning something is forgetting it, and this would invert the topological sort of knowledge dependency that places slices of our experience in the "future" or the "past". As such an observer in an isolated part of the universe which was being "played backwards" would label our future as their past. If we buy into the Second Law, it makes sense to me that there would be some direction where there are more messy entanglements between all the parts of the universe, and in the other direction there would be fewer, and we would name the first direction "the future" and the second direction "the past", since there would be information embedded in all those mutual entanglements that would over-constrain states in the past and under-constrain states in the future. What I don't get is what prevents me from setting up a circumstance where a measurement I make in the present is correlated with the result of (say) a coin flip in the future. That is easy to do if the coin flip is in the past, but impossible the other way around. I agree that by your logic if I were getting more information on the whole from the future, then I would almost definition label that "the past". But why not even one bit from the future? I can make a meaningful mark on a particle at time t0 and read it at t1, but not if t0 > t1. How come?
- lisper 8y ago
  Thanks for the kind words. > there would be some direction where there are more messy entanglements between all the parts of the universe Yes, that's exactly right, except for the word "messy". It's not "more messy", it's just "more". > what prevents me from setting up a circumstance where a measurement I make in the present is correlated with the result of (say) a coin flip in the future Nothing. If you flip a coin with enough precision you can make it come up reliably on one side or the other. ("But that's cheating!" you say. "I want the flip to be random." Well, you can't have it both ways: if the flip is random, then by definition it's not going to be correlated with anything in the past!) > why not even one bit from the future Because then it wouldn't be the future. Think about this: how would you distinguish "receiving a bit from the future", which is apparently not possible, and "making a reliable prediction about the future", which is possible in many cases? > I can make a meaningful mark on a particle at time t0 and read it at t1, but not if t0 > t1. How come? You can't actually "make a mark" on a particle the way you can on a classical object. You can prepare a particle in a particular quantum state, but that's not the same as putting a mark on it.

chr1 8y ago

It is worth to note that systems with preferred system of coordinates and "flowing" time can have Lorentz symmetry (for instance see https://physics.stackexchange.com/questions/887/relativistic-cellular-automata https://physics.stackexchange.com/questions/887/relativistic...) so relativity doesn't tell us about time travel as much as we might hope