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I love movies and TV shows about time travel. But I just don't think it's possible at all. If you could travel back in time, that would mean that future and th

I love movies and TV shows about time travel. But I just don't think it's possible at all. If you could travel back in time, that would mean that future and the past must co-exist with the present. If the pasts exists for us, the future exists, which means events have already happened, which means if time travel was to be invented then it would have already happened and someone would have traveled back to the past. But if traveling back to the past, and changing something, then any changes to the past would alter the future, but changing the past could prevent time travel being invented which would mean the person never traveled back to change the past to alter the future to begin with... Unless changing the past splits time into 2 with both outcomes. If the purpose of traveling back in time is to fix a problem that occurs in the future, but the act of changing the past splits time, then the time you knew would not be altered, only the alternative timeline you created. But if the past and future already exist, and time travel is possible, then all possible futures have ready happened and all possible pasts have already happened which would mean you have infinite timelines of every possible outcome of time itself. Because any change to the past in a given timeline only splits time to another outcome for the timeline, we would never know if our future changed or not making the entire event pointless. I only believe it's possible to travel to the future, but not 'skip' in the same way back to the future does. Only freeze ourself and awake in the future which is kind of just pausing and letting time pass by. ^ my opinion.

TremendousJudge 8y ago
>I only believe it's possible to travel to the future Don't want to sound pedantic, but we do this all the time. Also, time travel to the future (in a "when you come back 80 years have passed" sense) is something that happens at relativistic speeds; this is fact as far as physics is concerned.
alexanderdmitri 8y ago
First thing that occurs to me as a work around for this paradox is to think of traveling into the past as recreating a certain state as opposed to actually "reversing" the clock so to speak. Like pulling a commit hash from a while a back and then forking the repo. Maybe we're just not on a fork yet?
- philliphaydon 8y ago
  If you go back in time but don't do anything at all in order to not change any event from occurring the way its meant to then I guess it could be like that. But if we assume there's only 1 timeline and we change the past then the current future changes, you wouldn't know that you changed the future because the timeline you came from is the timeline you altered. But if you altered the timeline preventing you from needing to travel back then you would never travel back to prevent the timeline from occurring. The timeline would shift in such an odd way too, because the past and future co-exist with each other, if you changed the past to fix the future the moment you travel would alter the timeline instantly and possibly prevent the need to ever travel in which case everyone would be shifted all of time from the moment the change in the past occurred... It becomes very confusing, think I need a migraine pill and some sleep now...
  - krapp 8y ago
    There's a simple solution here - all pastward time travel (and by extension all FTL travel, since it's the same) results in a parallel timeline.
larntz 8y ago
Disclaimer, I'm not a mathematician, physicist, or even of notable intelligence... but I too have seen a lot of movies :) I agree it's unlikely to be possible that we could send a person or object back to another time or forward in an movie trope kind of way... What if everything exists at once, but there's an unalterable sequence of events. So, like you say everything has already happened past, present, and future including any kind of time travel. But, let's say time travel is possible, but is limited to small state changes in the building blocks of the sequence of record (dare I say, the blockchain of time). In theory we could make these changes and they would be completely undetected until we understand both how to create the change and know when and where to look for the changes. Now suppose in the future we find a way to do this and collectively agree on some kind of location to start a linked list of information from other "times". Thus begins some kind of technological singularity. Instantly we gain access to all the knowledge anyone at any time in existence that has the means wants to share. Now our problem is how to process that much information in a way to make it useful and trustworthy in a reasonable amount of time. This isn't well thought out and I'm sure there's some kind of paradox in there somewhere, but it was a fun Friday musing. I think this kind of allows for "time travel" to be possible via encoding information instead of killing Hitler while also allowing for those changes to have already happened without altering our currently observable past. If anyone knows of any sci-fi stories or movies that explore a similar concept I'd be totally interested in the recommendation.
antidesitter 8y ago
> But if traveling back to the past, and changing something Therein lies the problem. What makes you think the past can be changed? If the past can’t be changed, there’s no problem with time travel (at least on these grounds). Moreover, if you adopt the block universe view, as general relativity seems to demand, there’s no such thing as change at all. There’s no becoming, only being. This was the conclusion reached by Parmenides in ancient times. Julian Barbour wrote a book on the subject called The End of Time, for those who are interested.
- ASalazarMX 8y ago
  Are you implying traveling to the past doesn't change anything? You could spread antibiotic resistant bacteria, you could walk over a bee that was going to pollinate a flower with a significant mutation, you could breathe their seasonal flu and carry it over to your time before it manifests. The only way time travel consistent with known physics is if past past travel is impossible. You can visit the future, but if you do, you can't travel back. Besides paradoxes, time travel to the past would make perpetual motion machines possible.
  - antidesitter 8y ago
    > Are you implying traveling to the past doesn't change anything? Exactly. The whole timeline already exists and is self-consistent. Whatever you do in the past already happened, and it did so in a way that was consistent with the future you travelled back from. Time travel to the past is not inconsistent with known physics (see the various GR spacetimes with CTCs). There are reasons to think it’s probably impossible in our universe, however.
    - ASalazarMX 8y ago
      Don't be silly. If you dedicated your whole life to invent a time machine, succedded at your 80s, met your younger self and gave him your research so he can build it in his 30s, how is that self-consistent?
      - deleted 8y ago
Varcht 8y ago
Unless changing the past splits time into 2 with both outcomes. Many-worlds [0] [0] https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation
- atq2119 8y ago
  ... which is entirely unrelated. Really, what is it about quantum physics that causes people to make these weird connections all the time? (P.S.: The many-worlds interpretation is better called universal wave function theory. Less sexy, but more accurate.)
  - oarabbus_ 8y ago
    Can you justify your statement? Wavefunction collapse would certainly implies there can be multiple coexisting states. Only one is observed by "your" reality.
    - atq2119 8y ago
      Wavefunction collapse (which is actually not a thing in MWI/universal wavefunction theory) is an event or transition that moves strictly forward in time. Time travel, if possible, would be entirely orthogonal. There is no movement between the "worlds" in "many worlds" (which is part of the reason why "many worlds" is such a misleading label). At best there's interference between them.
c22 8y ago
If all time is flat then perhaps the only time travelers who manage to make it back in time are the ones who don't change anything. Or, to look at it another way, their changes are already "baked in".
garmaine 8y ago
“Primer” is the one exception. It deals with time travel in a physically realistic way (not surprising given it is an independent artsy film school movie by a writer/director with a math background).
saghm 8y ago
> But if the past and future already exist, and time travel is possible, then all possible futures have ready happened and all possible pasts have already happened which would mean you have infinite timelines of every possible outcome of time itself. Because any change to the past in a given timeline only splits time to another outcome for the timeline, we would never know if our future changed or not making the entire event pointless. Not necessarily disagreeing, but does something being pointless mean that it's not actually possible?
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
- 8y ago
kelnos 8y ago
So two possibilities that might solve your objections: 1) If, as you suggest, going back to the past to change something splits time, then everything is fine, albeit in a sort of selfish way: you went back to change something for yourself, but the timeline where the (presumably bad) event happened will still exist, you just won't have to experience it anymore. That is, of course, assuming that when you go back to change time, when you go forward again to return to your own time, you actually move forward on the new timeline, not on the old one. And this also doesn't address what you do with the you that exists in that timeline already. Even when you go back to your original time (just on the new timeline), it's still a problem: when you arrive back at your original time, there should be two of you (because the future you on the new timeline didn't need to go back in time to change something, because it was already changed). 2) The other possibility is that you don't actually need to change anything. If the many-worlds theory is true, then those other possible universes already exist. So if you want to change something, you just use some bit of technology to find the universe that has the change you want, and then transport yourself to it. You still have the problem of what you'll find there: will there be another you that you have to murder and hide the body (leaving your old universe with a never-to-be-solved missing-person report)? Or will you have to send him to your old universe, where he'll be confused about events being different? But then what stops him from just using that same technology to return to his universe to kick you out? Sorta tangential: time travel might not even need to be what we think it is. If many-worlds is true, then it stands to reason that those many possible universes include one where the big bang happened a year later (or any other arbitrary time sooner or later) than it did in our universe. So if you want to go have a do-over for an event that happened a year ago, you just find a universe that's delayed a year, and switch to it. You still have the aforementioned problem of what you do with that universe's you. And in that case your body is also a year older than it should be in your new universe, so you probably wouldn't want to pick a universe so far back such that you look significantly different than the you that belongs in that universe. At any rate, this is all really fun to think about.

TremendousJudge 8y ago

>I only believe it's possible to travel to the future Don't want to sound pedantic, but we do this all the time. Also, time travel to the future (in a "when you come back 80 years have passed" sense) is something that happens at relativistic speeds; this is fact as far as physics is concerned.

alexanderdmitri 8y ago

First thing that occurs to me as a work around for this paradox is to think of traveling into the past as recreating a certain state as opposed to actually "reversing" the clock so to speak. Like pulling a commit hash from a while a back and then forking the repo. Maybe we're just not on a fork yet?

philliphaydon 8y ago
If you go back in time but don't do anything at all in order to not change any event from occurring the way its meant to then I guess it could be like that. But if we assume there's only 1 timeline and we change the past then the current future changes, you wouldn't know that you changed the future because the timeline you came from is the timeline you altered. But if you altered the timeline preventing you from needing to travel back then you would never travel back to prevent the timeline from occurring. The timeline would shift in such an odd way too, because the past and future co-exist with each other, if you changed the past to fix the future the moment you travel would alter the timeline instantly and possibly prevent the need to ever travel in which case everyone would be shifted all of time from the moment the change in the past occurred... It becomes very confusing, think I need a migraine pill and some sleep now...
- krapp 8y ago
  There's a simple solution here - all pastward time travel (and by extension all FTL travel, since it's the same) results in a parallel timeline.

larntz 8y ago

Disclaimer, I'm not a mathematician, physicist, or even of notable intelligence... but I too have seen a lot of movies :) I agree it's unlikely to be possible that we could send a person or object back to another time or forward in an movie trope kind of way... What if everything exists at once, but there's an unalterable sequence of events. So, like you say everything has already happened past, present, and future including any kind of time travel. But, let's say time travel is possible, but is limited to small state changes in the building blocks of the sequence of record (dare I say, the blockchain of time). In theory we could make these changes and they would be completely undetected until we understand both how to create the change and know when and where to look for the changes. Now suppose in the future we find a way to do this and collectively agree on some kind of location to start a linked list of information from other "times". Thus begins some kind of technological singularity. Instantly we gain access to all the knowledge anyone at any time in existence that has the means wants to share. Now our problem is how to process that much information in a way to make it useful and trustworthy in a reasonable amount of time. This isn't well thought out and I'm sure there's some kind of paradox in there somewhere, but it was a fun Friday musing. I think this kind of allows for "time travel" to be possible via encoding information instead of killing Hitler while also allowing for those changes to have already happened without altering our currently observable past. If anyone knows of any sci-fi stories or movies that explore a similar concept I'd be totally interested in the recommendation.

antidesitter 8y ago

> But if traveling back to the past, and changing something Therein lies the problem. What makes you think the past can be changed? If the past can’t be changed, there’s no problem with time travel (at least on these grounds). Moreover, if you adopt the block universe view, as general relativity seems to demand, there’s no such thing as change at all. There’s no becoming, only being. This was the conclusion reached by Parmenides in ancient times. Julian Barbour wrote a book on the subject called The End of Time, for those who are interested.

ASalazarMX 8y ago
Are you implying traveling to the past doesn't change anything? You could spread antibiotic resistant bacteria, you could walk over a bee that was going to pollinate a flower with a significant mutation, you could breathe their seasonal flu and carry it over to your time before it manifests. The only way time travel consistent with known physics is if past past travel is impossible. You can visit the future, but if you do, you can't travel back. Besides paradoxes, time travel to the past would make perpetual motion machines possible.
- antidesitter 8y ago
  > Are you implying traveling to the past doesn't change anything? Exactly. The whole timeline already exists and is self-consistent. Whatever you do in the past already happened, and it did so in a way that was consistent with the future you travelled back from. Time travel to the past is not inconsistent with known physics (see the various GR spacetimes with CTCs). There are reasons to think it’s probably impossible in our universe, however.
  - ASalazarMX 8y ago
    Don't be silly. If you dedicated your whole life to invent a time machine, succedded at your 80s, met your younger self and gave him your research so he can build it in his 30s, how is that self-consistent?
    - deleted 8y ago

Varcht 8y ago

Unless changing the past splits time into 2 with both outcomes. Many-worlds [0] [0] https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation

atq2119 8y ago
... which is entirely unrelated. Really, what is it about quantum physics that causes people to make these weird connections all the time? (P.S.: The many-worlds interpretation is better called universal wave function theory. Less sexy, but more accurate.)
- oarabbus_ 8y ago
  Can you justify your statement? Wavefunction collapse would certainly implies there can be multiple coexisting states. Only one is observed by "your" reality.
  - atq2119 8y ago
    Wavefunction collapse (which is actually not a thing in MWI/universal wavefunction theory) is an event or transition that moves strictly forward in time. Time travel, if possible, would be entirely orthogonal. There is no movement between the "worlds" in "many worlds" (which is part of the reason why "many worlds" is such a misleading label). At best there's interference between them.

c22 8y ago

If all time is flat then perhaps the only time travelers who manage to make it back in time are the ones who don't change anything. Or, to look at it another way, their changes are already "baked in".

garmaine 8y ago

“Primer” is the one exception. It deals with time travel in a physically realistic way (not surprising given it is an independent artsy film school movie by a writer/director with a math background).

saghm 8y ago

> But if the past and future already exist, and time travel is possible, then all possible futures have ready happened and all possible pasts have already happened which would mean you have infinite timelines of every possible outcome of time itself. Because any change to the past in a given timeline only splits time to another outcome for the timeline, we would never know if our future changed or not making the entire event pointless. Not necessarily disagreeing, but does something being pointless mean that it's not actually possible?

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
8y ago

kelnos 8y ago

So two possibilities that might solve your objections: 1) If, as you suggest, going back to the past to change something splits time, then everything is fine, albeit in a sort of selfish way: you went back to change something for yourself, but the timeline where the (presumably bad) event happened will still exist, you just won't have to experience it anymore. That is, of course, assuming that when you go back to change time, when you go forward again to return to your own time, you actually move forward on the new timeline, not on the old one. And this also doesn't address what you do with the you that exists in that timeline already. Even when you go back to your original time (just on the new timeline), it's still a problem: when you arrive back at your original time, there should be two of you (because the future you on the new timeline didn't need to go back in time to change something, because it was already changed). 2) The other possibility is that you don't actually need to change anything. If the many-worlds theory is true, then those other possible universes already exist. So if you want to change something, you just use some bit of technology to find the universe that has the change you want, and then transport yourself to it. You still have the problem of what you'll find there: will there be another you that you have to murder and hide the body (leaving your old universe with a never-to-be-solved missing-person report)? Or will you have to send him to your old universe, where he'll be confused about events being different? But then what stops him from just using that same technology to return to his universe to kick you out? Sorta tangential: time travel might not even need to be what we think it is. If many-worlds is true, then it stands to reason that those many possible universes include one where the big bang happened a year later (or any other arbitrary time sooner or later) than it did in our universe. So if you want to go have a do-over for an event that happened a year ago, you just find a universe that's delayed a year, and switch to it. You still have the aforementioned problem of what you do with that universe's you. And in that case your body is also a year older than it should be in your new universe, so you probably wouldn't want to pick a universe so far back such that you look significantly different than the you that belongs in that universe. At any rate, this is all really fun to think about.