Is Time Travel logically possible?

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Is Time Travel logically possible?

#1  Postby andrewk » Nov 13, 2011 12:20 am

Let's talk about whether time travel is logically possible! Assume for the present that it's scientifically possible – I'll come back to that. But does anything forbid it as a logical possibility?

Fiction in which time travel is discussed likes to consider the paradoxes that could arise. What happens if you have a cruel father and you want him not to be your father so you go back in time and frame him for a crime that lands him in jail for all your mother's child-bearing years so that she never meets him? Do you then cease to exist because your parents never conceived you? But if you do then you can't have gone back in time and stopped them from meeting, so you can exist. This is insoluble in the same way as the liar's paradox.

Until recently I thought that this type of paradox may be a reason that time travel is not even logically possible. But now I'm not so sure. I think the paradox can be resolved as follows:
If you want to change history and prevent event x from happening, where x is something that has happened at time t2, and with that aim, you enter a time machine at time t3 (>t2), and try to go back to time t1 (<t2) to prevent x from happening you will fail, because the fact that you have observed that x occurred at t2 means that no action occurred which prevented it.

The reason why you will fail doesn't have to be some weird, paradoxical, highly abstract reason. You might program the time machine wrong and fail to get to the crucial time where you could have acted to prevent x, or you might go back to the right time but the wrong place, you might suffer a heart attack and die before you can prevent x, you might do the action that will prevent x but somebody else undoes your action, or you might press the blue button which you thought would prevent x, but in fact you should have pressed the red button.

We can't predict the reason why you will fail, but we can be absolutely certain that you will fail.

Hence the Time Lords' cardinal rule, that you must not retrospectively change history, which is expressed as a Prescriptive Rule, is actually a Descriptive Rule: that if you seek to retrospectively change history you will fail.

Robert Heinlein wrote a short story “All you zombies”, in which a person who was born an apparently female hermaphrodite is taken as a young woman by a mysterious stranger back to the time a couple of years before her birth, where she meets a man, conceives a child and gives birth to the child, who is herself. Then the mysterious stranger turns up again, whisks her back to the future, where her hermaphroditic status becomes manifest and she undergoes an operation to become fully male. A few years later the mysterious stranger turns up again and takes the protagonist back to ten months before his/her birth, where he meets the mother (himself) and conceives the child with her. The mysterious stranger then reappears and whisks the male protagonist back to the future, where he tells him what has happened – ie that he is his own father and mother - and recruits him into a corps of time patrollers who manage cross-temporal events using time machines. He grows older and becomes that mysterious stranger who facilitated the whole series of transactions.

This whole scenario seems impossible, not to mention immensely complicated and mind bending. I drew a diagram of the time lines and ended up with a very messy piece of paper. However, I don't think there is anything logically impossible about it. There are no contradictions that I can see.

In the third Harry Potter book, Hermione is exhorted to ensure that she does not meet herself when she goes back in time. But there is no reason why such a meeting could not occur, nor would it create any paradoxes.

The Heinlein and Harry Potter scenarios do not involve retrospectively changing history. In the Heinlein story, the person goes back in time to make something happen that they know does eventually happen. In Harry Potter, they go back in time to prevent something from happening that would otherwise happen after the time from which they went back in time. Both of these appear to be logically admissible, whereas going back in time to prevent from happening something that we know has already happened appears not to be.

The Heinlein and HP scenarios involve effects that occur earlier than their causes, which is not what we are used to, but there is no evidence that causes occurring after effects is logically impossible.

Finally, is time travel scientifically possible? Well, who knows, but if, as it appears, there is no logical barrier, then any barrier must be physical. Many Science Fiction stories involve travelling through 'hyper-space' to get to far-away places much sooner than they could arrive travelling at the speed of light. The analogy used is to imagine we are two-dimensional creatures living on a two-dimensional curvy sheet. That distant galaxy may be ten light-years away by the most direct route along the surface of the sheet. But if the sheet is sufficiently curved in 3D space, the distant galaxy may be only one metre away in 3D. If I could only travel through that third dimension, I could hop across that one metre gap to the distant galaxy, rather than having to travel for ten light-years along the surface of the sheet.

But if we could do that with spatial travel, we could do the same thing with time travel. We live on, and appear to be confined to, a four-dimensional space-time manifold. But if that manifold is embedded in a higher dimensional space and is highly curved, it may be that we could hop across a small gap via the extra dimensions (ie 'through hyperspace/time') to get to a point that is 'a long time ago in a galaxy far away'.

Anyway, I'm interested in what others think. It would be especially good to hear from The Doctor or DrWho, as they have first-hand experience of these issues.
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Re: Is Time Travel logically possible?

#2  Postby Gila Guerilla » Nov 13, 2011 12:50 am

Depending on how you view the universe, you cannot go back from time t3, to t1 in order to change what happened at time t2, because at time t3, all of the events which happened at time t2 have already happened at the time you enter the time machine at t3. So for example, you cannot go back and kill your father before he impregnated your mother, because, since you are alive, we already know that the event x at time t2 "your father impregnates your mother', has happened.

However, if you accept the many worlds view of the universe, you might be able to stop your own conception, setting off a new universe, in which you do not get born, while in parallel is the universe which leads to time t3 in which you have been born..
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Re: Is Time Travel logically possible?

#3  Postby jamest » Nov 13, 2011 1:06 am

andrewk wrote:
If you want to change history and prevent event x from happening, where x is something that has happened at time t2, and with that aim, you enter a time machine at time t3 (>t2), and try to go back to time t1 (<t2) to prevent x from happening you will fail, because the fact that you have observed that x occurred at t2 means that no action occurred which prevented it.

Technically, it should be called spacetime travel, as we would have to occupy a previous position in both space & time. However, then it becomes logically obvious that we cannot go back and be privy to an exact event in history, as our very presence would displace/disrupt/alter the very constitutive elements of that particular scene. An extra fish in the fishbowl displaces the water, makes it rise, and would alter the position of all the other fish relative to one another... if you get my drift. Therefore, everything is altered the moment you enter a scene, irrespective of your subsequent influence upon any event.

I think that what you say above acknowledges this. I just wanted to enhance what you said.

The only possibility I see for spacetime travel would be to enter the experience of another entity already 'there', thereby not materially altering the locale. One might become Henry VIII, for example. Of course, thereafter, events are destined to be altered as you wouldn't make the same choices as the big man himself, but for a brief moment you would experience everything just as it was, from a specific vantage point.

... Ironically though, this wouldn't be so much a case of travelling through spacetime, as travelling through one's own mind to the point where you experienced yourself as being somebody else, somewhere else. As bizarre as it all sounds, it's the only possibility I see for spacetime travel.
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Re: Is Time Travel logically possible?

#4  Postby andrewk » Nov 13, 2011 1:32 am

jamest wrote:However, then it becomes logically obvious that we cannot go back and be privy to an exact event in history, as our very presence would displace/disrupt/alter the very constitutive elements of that particular scene. An extra fish in the fishbowl displaces the water, makes it rise, and would alter the position of all the other fish relative to one another... if you get my drift. Therefore, everything is altered the moment you enter a scene, irrespective of your subsequent influence upon any event.
Yes, that is a puzzle!

When I try to think about this and resolve it, I get to a position a little like I do about free will, as follows:

If I go back in time to watch the Battle of Hastings (from a safe distance, using binoculars!) then, although my presence there has impacts - by displacing air and radiating body heat for instance - I don't change history because I was always there at the Battle of Hastings, it's just that that fact wasn't recorded in history, and I didn't know it to be the case myself until I decided to go back there. That is, there was never a Battle of Hastings that didn't have me hiding behind a bush 300 metres away looking through my binoculars (I'm not sure if I'm putting this very well). This has some similarities to a determinist (or is it compatibilist?) view of free will, under which I was always going to say the word 'elephant' at 12:26 pm AEDST on 13 Nov 2011, even though I didn't know I was going to say it until just before, and I feel as though I could have not said it had I wished not to.

I suspect the whole thing only makes sense under a B theory of time.
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Re: Is Time Travel logically possible?

#5  Postby jamest » Nov 13, 2011 1:43 am

andrewk wrote:
jamest wrote:However, then it becomes logically obvious that we cannot go back and be privy to an exact event in history, as our very presence would displace/disrupt/alter the very constitutive elements of that particular scene. An extra fish in the fishbowl displaces the water, makes it rise, and would alter the position of all the other fish relative to one another... if you get my drift. Therefore, everything is altered the moment you enter a scene, irrespective of your subsequent influence upon any event.
Yes, that is a puzzle!

When I try to think about this and resolve it, I get to a position a little like I do about free will, as follows:

If I go back in time to watch the Battle of Hastings (from a safe distance, using binoculars!) then, although my presence there has impacts - by displacing air and radiating body heat for instance - I don't change history because I was always there at the Battle of Hastings, it's just that that fact wasn't recorded in history, and I didn't know it to be the case myself until I decided to go back there.

That doesn't make sense, because it necessitates that you existed before you were born [in the 20th century]. Otherwise, you'd first have to have travelled forward in time, from the 11th century to the 20th century (to the point where you were born), before returning back to the 11th century in the 21st century.
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Re: Is Time Travel logically possible?

#6  Postby surreptitious57 » Nov 13, 2011 3:21 am

Time travel is a theoretical possibilty but
only forward and not backward. The reason
for this is because of General Relativity which
is flat space. Possible however to travel forwads
because of Special Relativity which is curved space

As Einstein referenced light speed is a constant
[ in a vacuum ] so the variable must therefore apply
to time. If time travel were a physical possibility then if
travelling at relative light speed would mean time slowed down
relative to anyone at a fixed point where it would speed up instead
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Re: Is Time Travel logically possible?

#7  Postby andrewk » Nov 13, 2011 3:25 am

Imagine Bob is born in 2250 and invents and constructs a time machine in 2280. It has to be a long way in the future as we know we have no technologies at present that are anywhere near time travel. The 30-year old Bob travels back to 1066 and watches the battle for an hour, then returns to 2280, say five minutes after he left. When we say that Bob was always there at the battle, we just mean that if anybody back in 1066 had looked behind the bush where he was hiding, they would have seen him. There is only one version of the battle, not a pre-Bob (and hence Bob-less) version and a post-Bob (and hence Bob-full) version. And the only version that there is, contains a 30-year old Bob, who appears in his tardis at, say noon, stays to watch some of the battle behind the bush and then gets into his tardis, which disappears, at about 1pm.

Certainly it is the case that Bob exists - as a 30-year old - in a time earlier than the time at which he was born, but that's in the world's time line, not Bob's. One of the consequences of time travel is that people's time-lines, which are all in lockstep without time travel, can diverge with it. In Doctor Who this is used to confusing effect with the Doctor and Professor Riversong repeatedly bumping into each other as they travel time, but in reverse order in relation to their time-lines. Hence the Doctor's first meeting with Riversong's is her last, and vice versa. So there's no need for Bob to make two trips from 1066 to 2280, only one - the return trip.

The most convincing (to me) non-physical argument against the possibility of time travel is that - if people discovered time travel in the future they would probably have visited our past, but we haven't seen them, so we presume they don't (won't? didn't?). A counterargument to that is that, by the time our descendants discover time travel, they may be so sophisticated that they realise how important it is not to upset their ancestors, so they deliberately disguise themselves to fit in, and don't let anybody know they're visiting from the future.
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Re: Is Time Travel logically possible?

#8  Postby james1v » Nov 13, 2011 3:27 am

No.
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Re: Is Time Travel logically possible?

#9  Postby surreptitious57 » Nov 13, 2011 5:14 am

andrewk wrote:
Imagine Bob is born in 2250 and invents and constructs a time machine in 2280. It has to be a long way in the future as we know we have no technologies at present that are anywhere near time travel. The 30-year old Bob travels back to 1066 and watches the battle for an hour, then returns to 2280, say five minutes after he left. When we say that Bob was always there at the battle, we just mean that if anybody back in 1066 had looked behind the bush where he was hiding, they would have seen him. There is only one version of the battle, not a pre-Bob (and hence Bob-less) version and a post-Bob (and hence Bob-full) version. And the only version that there is, contains a 30-year old Bob, who appears in his tardis at, say noon, stays to watch some of the battle behind the bush and then gets into his tardis, which disappears, at about 1pm

Certainly it is the case that Bob exists - as a 30-year old - in a time earlier than the time at which he was born, but that's in the world's time line, not Bob's. One of the consequences of time travel is that people's time-lines, which are all in lockstep without time travel, can diverge with it. In Doctor Who this is used to confusing effect with the Doctor and Professor Riversong repeatedly bumping into each other as they travel time, but in reverse order in relation to their time-lines. Hence the Doctor's first meeting with Riversong's is her last, and vice versa. So there's no need for Bob to make two trips from 1066 to 2280, only one - the return trip

The most convincing (to me) non-physical argument against the possibility of time travel is that - if people discovered time travel in the future they would probably have visited our past, but we haven't seen them, so we presume they don't (won't? didn't?). A counterargument to that is that, by the time our descendants discover time travel, they may be so sophisticated that they realise how important it is not to upset their ancestors, so they deliberately disguise themselves to fit in, and don't let anybody know they're visiting from the future


For this to be possible you would have to disprove the General Theory Of Relativity
As you cannot travel back in time because spacetime is flat only forward as spacetime
is curved. So your particular scenario is only plausible from a science fiction perspective
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Re: Is Time Travel logically possible?

#10  Postby surreptitious57 » Nov 13, 2011 5:53 am

surreptitious57 wrote:
Time travel is a theoretical possibilty but
only forward and not backward. The reason
for this is because of General Relativity which
is flat space. Possible however to travel forwads
because of Special Relativity which is curved space

As Einstein referenced light speed is a constant
[ in a vacuum ] so the variable must therefore apply
to time. If time travel were a physical possibility then if
travelling at relative light speed would mean time slowed down
relative to anyone at a fixed point where it would speed up instead


That should read as Special Relativity which is flat
space and General Relativity which is curved and not
vice versa. You cannot trave back in time but can save
time instead by travelling forward at relative light speed

If you were to travel to Andromeda from Earth at 99. 99999999
per cent light speed which is 2. 5 million light years aways and back
again it would only take 100 years. But for everyone here 5, 000, 000
years would have passed. So you would have saved 4, 999, 900 years and
by arriving earlier would to all intents and purposes have arrived in the past
since you would be at an earlier point time wise than everyone else even though
you both started off from the same position. So possible for the future not the past
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Re: Is Time Travel logically possible?

#11  Postby Preno » Nov 13, 2011 12:57 pm

andrewk wrote:Until recently I thought that this type of paradox may be a reason that time travel is not even logically possible. But now I'm not so sure. I think the paradox can be resolved as follows:
If you want to change history and prevent event x from happening, where x is something that has happened at time t2, and with that aim, you enter a time machine at time t3 (>t2), and try to go back to time t1 (<t2) to prevent x from happening you will fail, because the fact that you have observed that x occurred at t2 means that no action occurred which prevented it.

The reason why you will fail doesn't have to be some weird, paradoxical, highly abstract reason. You might program the time machine wrong and fail to get to the crucial time where you could have acted to prevent x, or you might go back to the right time but the wrong place, you might suffer a heart attack and die before you can prevent x, you might do the action that will prevent x but somebody else undoes your action, or you might press the blue button which you thought would prevent x, but in fact you should have pressed the red button.

We can't predict the reason why you will fail, but we can be absolutely certain that you will fail.
That is basically correct. If you're interested in this topic, I suggest you read Dummett's papers "Can an Effect Precede its Cause" and "Bringing About the Past" (both in "Truth and Other Enigmas").
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Re: Is Time Travel logically possible?

#12  Postby SpeedOfSound » Nov 13, 2011 1:11 pm

Like jamest said. Look at time like a spatial dimension and instead talk about 'space travel' but not the usual kind. A chunk of space actually has to move to another position in the array so we have some torsion. Think on what happens to the area of the array where the thing came from and the area where it went to. The intuitive problem is that we keep wanting to bring causality and time back into our thinking. This leads to some thought about what we think time travel would accomplish in the first place.
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Re: Is Time Travel logically possible?

#13  Postby Doubtdispelled » Nov 13, 2011 1:27 pm

:coffee:
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Re: Is Time Travel logically possible?

#14  Postby hackenslash » Nov 13, 2011 1:39 pm

I'd read Brian Greene's excellent Fabric of the Cosmos for more on this topic.
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Re: Is Time Travel logically possible?

#15  Postby Zwaarddijk » Nov 13, 2011 1:53 pm

Gila Guerilla wrote:Depending on how you view the universe, you cannot go back from time t3, to t1 in order to change what happened at time t2, because at time t3, all of the events which happened at time t2 have already happened at the time you enter the time machine at t3. So for example, you cannot go back and kill your father before he impregnated your mother, because, since you are alive, we already know that the event x at time t2 "your father impregnates your mother', has happened.

However, if you accept the many worlds view of the universe, you might be able to stop your own conception, setting off a new universe, in which you do not get born, while in parallel is the universe which leads to time t3 in which you have been born..

Uhm, there are models that do permit such travel - and we don't, ultimately, know which model is the right one yet!

There's at least one model according to which closed time-like curves will instantly converge on stable states (altho' what a stable state is isn't necessarily obvious). Scott Aaronson has shown that if this model is right, any computational problem in PSPACE could be solved the instant a closed time-like curve circuit for that problem was constructed ( here's the relevant paper.
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Re: Is Time Travel logically possible?

#16  Postby Ibrahim500 » Jul 13, 2019 8:36 pm

@ andrewk, if a person goes to the future(t3) and he wants to prevent x from happening at time t2 he or she can’t obviously prevent it since he is at time t3( the future) and that event happened at time t2, but why can’t he/she prevent x from happening at time t2 if he goes back to time t1( I am thinking, if he goes back to the past, and he wants to prevent x from happening at time t2(the “future”, which is supposed to be the present, he/she can prevent that event from happening since he/she can act on preventing something that he/she knows will happen in the future).
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Re: Is Time Travel logically possible?

#17  Postby Spearthrower » Jul 14, 2019 5:08 am

Time travel of a sort has, thereby, just occurred. A conversation from 2011 with a poster who's not been here in over 4 years! ;)
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Re: Is Time Travel logically possible?

#18  Postby BlackBart » Jul 14, 2019 5:11 am

And we didn't even need a DeLorean! :awesome:
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Re: Is Time Travel logically possible?

#19  Postby The_Piper » Jul 14, 2019 8:49 am

Great SCOTT! :shock:
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Re: Is Time Travel logically possible?

#20  Postby scott1328 » Jul 15, 2019 2:53 pm

The_Piper wrote:Great SCOTT! :shock:


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