Deductive Logic on the subject of Time Travel.

Main Question or Discussion Point

When I say "Time Travel" in the title, I'm referring to traveling into the past. I just wanted to see if I could establish deductive validity for my assumption that it is not just impossible, but non-sensical, to travel backward in time.

("P" stands for "Proposition" and "C" stands for "Conclusion")

Here we go:
P1: I did not exist in 1776.
P2: I exist now in a reality that includes P1 as being true.
P3: I go back in time (using whatever means) to 1776.
C: I did exist in 1776! ...

But, then, I have invalidated P1 and P2, and even (by extension) P3, so my conclusion violates all the propositions...is this logically sound?

Any and all helpful criticism is appreciated.

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MathematicalPhysicist
Gold Member
Originally posted by Mentat
When I say "Time Travel" in the title, I'm referring to traveling into the past. I just wanted to see if I could establish deductive validity for my assumption that it is not just impossible, but non-sensical, to travel backward in time.

("P" stands for "Proposition" and "C" stands for "Conclusion")

Here we go:
P1: I did not exist in 1776.
P2: I exist now in a reality that includes P1 as being true.
P3: I go back in time (using whatever means) to 1776.
C: I did exist in 1776! ...

But, then, I have invalidated P1 and P2, and even (by extension) P3, so my conclusion violates all the propositions...is this logically sound?

Any and all helpful criticism is appreciated.
from c it can be concluded that you no more life in the present time therefor p1 isnt right anymore and so is p2, instead of them you can propse the premises which come from the conclusion (the first conclusion becomes a premise):
p1:I exist in 1776
p2: i dont exist in 2003.
c: i must have gone back in time (-;

Originally posted by loop quantum gravity
from c it can be concluded that you no more life in the present time therefor p1 isnt right anymore and so is p2, instead of them you can propse the premises which come from the conclusion (the first conclusion becomes a premise):
p1:I exist in 1776
p2: i dont exist in 2003.
c: i must have gone back in time (-;
I'm sure you already know that this doesn't follow, lqg. Ben Franklin existed in 1776, but he didn't go back in time.

No, I seriously need a criticism of my reasoning on the matter, since it is supposed to be "possible" to travel back in time.

BTW, I always "exist in the present time" (as you put it), since the very term "exist" is in the present tense. I don't think P1 can be invalidated.

MathematicalPhysicist
Gold Member

Originally posted by Mentat
I'm sure you already know that this doesn't follow, lqg. Ben Franklin existed in 1776, but he didn't go back in time.

No, I seriously need a criticism of my reasoning on the matter, since it is supposed to be "possible" to travel back in time.

BTW, I always "exist in the present time" (as you put it), since the very term "exist" is in the present tense. I don't think P1 can be invalidated.
but the present time is changing for you when youre travelling in time, if you travell in time the present is ofcourse subjective the past becomes your present and the present becomes your future which you already know it (so you can say the future is forseeable).

Originally posted by loop quantum gravity
but the present time is changing for you when youre travelling in time, if you travell in time the present is ofcourse subjective the past becomes your present and the present becomes your future which you already know it (so you can say the future is forseeable).
Yes, this is all true, and it is the reason why P2 is phrased as it is. I exist now, in a reality that is contingent with P1.

Thanks for the support, I can't seem to find a flaw either...I hope someone else can.

hypnagogue
Staff Emeritus
Gold Member
Originally posted by Mentat
When I say "Time Travel" in the title, I'm referring to traveling into the past. I just wanted to see if I could establish deductive validity for my assumption that it is not just impossible, but non-sensical, to travel backward in time.

("P" stands for "Proposition" and "C" stands for "Conclusion")

Here we go:
P1: I did not exist in 1776.
P2: I exist now in a reality that includes P1 as being true.
P3: I go back in time (using whatever means) to 1776.
C: I did exist in 1776! ...

But, then, I have invalidated P1 and P2, and even (by extension) P3, so my conclusion violates all the propositions...is this logically sound?

Any and all helpful criticism is appreciated.
We can circumvent this contradiction if we assume parallel universes (which is the standard way to get around most time travel paradoxes, such as the matricide paradox). If we assume parallel universes, then we get something like this:

P1: I did not exist in 1776 in universe A.
P2: I exist now in a reality that includes P1 as being true (although I think this one is a little redundant, if we are already assuming P1 to be true).
P3: I go back in time (using whatever means) to 1776.
C: It is logically inconsistent for me to still be in universe A; therefore I must be in a different universe, call it B, with a history/timeline very similar to A's.

You guys ever heard of this theory ----> Present conditions affects past

Sounds illogical isn't it? Its always a forward motion of ------> the present conditions affect the future.

Originally posted by hypnagogue
We can circumvent this contradiction if we assume parallel universes (which is the standard way to get around most time travel paradoxes, such as the matricide paradox). If we assume parallel universes, then we get something like this:

P1: I did not exist in 1776 in universe A.
P2: I exist now in a reality that includes P1 as being true (although I think this one is a little redundant, if we are already assuming P1 to be true).
P3: I go back in time (using whatever means) to 1776.
C: It is logically inconsistent for me to still be in universe A; therefore I must be in a different universe, call it B, with a history/timeline very similar to A's.
Hmm...I suppose this could work, but wouldn't adding an extra Universe violate Occam's Razor?

Also, Stephen Jay Gould (along with many others) is in favor of the idea (the name of which I've forgotten...temporary brain fart) that changing even the smallest thing in history can dramatically change the series of events that happen afterward. He believes that the course of history is occurance acting on the otherwise probabilistic/chance-governed nature of reality. I (currently) agree with him on this point, since the alternative seems like determinism.

Oh yeah, the phrasing of P2 could seem redundant, I suppose, but I couldn't think of a better way to phrase it at the time.

Originally posted by diverz
You guys ever heard of this theory ----> Present conditions affects past

Sounds illogical isn't it? Its always a forward motion of ------> the present conditions affect the future.

I guess it's not unscientific, but I do think it may be illogical...my first attempt at a proof:

P1: I exist in the present (slightly redundant in itself, since "exist" is in the present tense).
P2: P1 is true only because of the previous occurances which lead up to my having been born and living as I've lived.
P3: Because of P1 and P2, any change in the past, caused by me, stops me from existing, which stops me from changing the past, which allows me to exist again...which will go on in circles.

I find time travel an indeed very stupid idea
because, wouldn't your memory of what you have perceived
now and in the past flow back to the
when you travel to the past?!
So you would not even know that you had travelled
back in time becasue you can't bring
your present consciousness to the past.

Sounds logical right?

For example:
Imagine that you are living in negative time zone
(equivilant to travelling back in time)
and you are gliding at a speed of 10m/s for -10s
(because you are in negative space-time).

So the distance travelled= (10m/s)*(-10s)= -100m
Conclusion: If you are travelling back in time,
everything that you did or possess would act
in the reverse manner! (including your brain!)

Originally posted by physicskid
I find time travel an indeed very stupid idea
because, wouldn't your memory of what you have perceived
now and in the past flow back to the
when you travel to the past?!
So you would not even know that you had travelled
back in time becasue you can't bring
your present consciousness to the past.

Sounds logical right?

For example:
Imagine that you are living in negative time zone
(equivilant to travelling back in time)
and you are gliding at a speed of 10m/s for -10s
(because you are in negative space-time).

So the distance travelled= (10m/s)*(-10s)= -100m
Conclusion: If you are travelling back in time,
everything that you did or possess would act
in the reverse manner! (including your brain!)
Ok, so you're talking about regressive backward time travel. Yeah, I agree, it is completely illogical.

Originally posted by Mentat
When I say "Time Travel" in the title, I'm referring to traveling into the past. I just wanted to see if I could establish deductive validity for my assumption that it is not just impossible, but non-sensical, to travel backward in time.

("P" stands for "Proposition" and "C" stands for "Conclusion")

Here we go:
P1: I did not exist in 1776.
P2: I exist now in a reality that includes P1 as being true.
P3: I go back in time (using whatever means) to 1776.
C: I did exist in 1776! ...

But, then, I have invalidated P1 and P2, and even (by extension) P3, so my conclusion violates all the propositions...is this logically sound?

Any and all helpful criticism is appreciated.
Isn't this where I came in, Mentat?

If you went back in time to 1776 (P3) then that would invalidate P1. The rest would still be valid except the inclusion in P2 that P1 is true. It would also be true that in the present time of P2 the fact that you went back to 1776 (P3); (C) would already be in your history which would again make P1 invalid. P1 can never be true if you ever go back in time to 1776. P1 and P2 are true only if you never go back in time to 1776. It isn't a paradox as P1 and P3 are mutally exclusive. One or the other but not both can be true.

Originally posted by physicskid
I find time travel an indeed very stupid idea
because, wouldn't your memory of what you have perceived
now and in the past flow back to the
when you travel to the past?!
So you would not even know that you had travelled
back in time becasue you can't bring
your present consciousness to the past.

Sounds logical right?

For example:
Imagine that you are living in negative time zone
(equivilant to travelling back in time)
and you are gliding at a speed of 10m/s for -10s
(because you are in negative space-time).

So the distance travelled= (10m/s)*(-10s)= -100m
Conclusion: If you are travelling back in time,
everything that you did or possess would act
in the reverse manner! (including your brain!)
Okay im only 15 but i question your idea. Because isn't your memory just chemicals in your brain which your brain reconize as memorys? if thats right when you travel back in time then those chemicals would be the same therefore your memory would be the same? right? Anyways maybe we shouldn't go back in time unless the idea of multiple (and i mean lots!) realitys or timeflow(brain just went dead) right? or we might do one of those weird paradox things where we **** up the universe:D

hypnagogue
Staff Emeritus
Gold Member

Originally posted by Mentat
Hmm...I suppose this could work, but wouldn't adding an extra Universe violate Occam's Razor?
Maybe, maybe not. From where we currently stand, it seems to violate Occam's Razor. But suppose one day we find that time travel into the past is possible. Well, if this occurs, we know that the traditional paradoxes that are associated with time travel must be false, otherwise we would have a logically inconsistent system (much like the one you created). At this point, then, it would be more reasonable to assume parallel universes to explain how time travel into the past is possible without raising all those paradoxes.

Also, Stephen Jay Gould (along with many others) is in favor of the idea (the name of which I've forgotten...temporary brain fart) that changing even the smallest thing in history can dramatically change the series of events that happen afterward. He believes that the course of history is occurance acting on the otherwise probabilistic/chance-governed nature of reality. I (currently) agree with him on this point, since the alternative seems like determinism.
That sounds more like he is describing history as chaotic, as opposed to non-deterministic. In any case, I agree that history probably works like that. No doubt that going back to 1776 would greatly change the future course of history. But the important idea is that that greatly changed future happens to another universe, and thus is not in contradiction with the history we have observed in our own universe.

Hypna,
That line of thought ignores the fact that if some one were to go back in time and do something that would alter history and thus the present and future history, it would already be in our history that that had happened.
Going back in time may or not be possible in one universe, I think not; but, changing past history is logically impossible. As I said, any change would already be part of our history and not a change at all. What has happened in the past happened and that would include someone going back and effecting what was then happening.
Multiverses is another thing. If some one where to go back and do something that would start another universe in which his doing whatever he did would already be in its history. That doesn't make any sense at all to me; but then I don't believe in multi-universes.
Here I agree with Mentat, that that would violate Occam's Razor. Not tha Occam's Razor is an inviolate Law; but, that it is not necessary, elegant or beautiful but messy, redundant and over complicated.

hypnagogue
Staff Emeritus
Gold Member
Originally posted by Royce
That line of thought ignores the fact that if some one were to go back in time and do something that would alter history and thus the present and future history, it would already be in our history that that had happened.
Well, I see what you are saying about the "already would have happened" part. I agree with that. Sometimes it is easier to say "change," however, to draw attention to how things in our respective universes are similar (or identical) up until the point when a time traveler from one enters the other at a certain point in time.

But say someone traveled back to 1776. It would not have already happened in our universe, since in our universe A no one did go back to 1776. That happened in universe B.

Here I agree with Mentat, that that would violate Occam's Razor. Not tha Occam's Razor is an inviolate Law; but, that it is not necessary, elegant or beautiful but messy, redundant and over complicated.
Then again, Occam's Razor is not really relevant here. We aren't talking about whether time travel is actually possible, only if it could be logically possible. Thus, it suffices to create a logically consistent scenario where time travel is logically possible. Whether that logical possibility is then a physical actuality is a question of empirical investigation, which is a separate issue.

Originally posted by hypnagogue

But say someone traveled back to 1776. It would not have already happened in our universe, since in our universe A no one did go back to 1776. That happened in universe B.
Then again "logically" speaking in a multiverse scenario, it would have already happened in B's history. We can't get around it. No matter which history or which unverse's past we go to, It had to have already happened, been recorded and experienced in that universe's history. It is either that or a split occurs in which one universe's past was not visited and changed and ther other was visited and changed but no one would know it because that universe's history would already contain and record the visit and "change" but for them it would not be a change in there history because their history already includes the "change". Only some supra spatial being outside of space and time would be able to see the change by noticing the split.
Simply put, logic will not allow the future to effect any change in the past as it would have already been included in its history and thus is not a change. No way around it. I hate to be a bubble buster because I have always loved sci-fi time travel stories but logically it can't happen.
We can't go back and save Lincoln or assassinate Hitler because if we do it would already be part of our history and it would be imperitive
that we do go back and save or assassinate. This would make the universe's present determined by it past, eliminating free will.
So instead of going back and killing your grandfather to change history all you have to do is not go back and save lincoln in a universe where you did go back and save lincoln; but in that universe you would not have that option or ability. No matter what you tried or tried not to do you would be forced to go back and save lincoln because that is already your history. And on and on and on ad nausium.

Then again, Occam's Razor is not really relevant here. We aren't talking about whether time travel is actually possible, only if it could be logically possible. Thus, it suffices to create a logically consistent scenario where time travel is logically possible. Whether that logical possibility is then a physical actuality is a question of empirical investigation, which is a separate issue.
Okay, I'll go along with that but then I have shown how it is logically impossible. Logic will not only not allow a paradox but it won't allow a messy, ugly, redundant multiverse either.
It is not a Law of logic that can't be violated as a physical law but that if it were violated then it wouldn't be logical. Our universe has so far as we know is logical. It is cosmos not chaos.

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Originally posted by Royce
Isn't this where I came in, Mentat?

If you went back in time to 1776 (P3) then that would invalidate P1. The rest would still be valid except the inclusion in P2 that P1 is true. It would also be true that in the present time of P2 the fact that you went back to 1776 (P3); (C) would already be in your history which would again make P1 invalid. P1 can never be true if you ever go back in time to 1776. P1 and P2 are true only if you never go back in time to 1776. It isn't a paradox as P1 and P3 are mutally exclusive. One or the other but not both can be true.
I don't understand what you are saying. If P3 is true, then both P1 and P2 also have to be true (since my existence is kind of required for "me" to travel back in time ).

Besides, my point is that I can establish P1 and P2 as being true (by scientific and other such proofs) but they would invalidate P3, and thus I can never travel back in time...does that mean that it is illogical to postulate time travel?

Originally posted by hypnagogue
Maybe, maybe not. From where we currently stand, it seems to violate Occam's Razor. But suppose one day we find that time travel into the past is possible. Well, if this occurs, we know that the traditional paradoxes that are associated with time travel must be false, otherwise we would have a logically inconsistent system (much like the one you created). At this point, then, it would be more reasonable to assume parallel universes to explain how time travel into the past is possible without raising all those paradoxes.
"At that point it would be reasonable"? Does that mean that at this point it is not reasonable? Are the "extra" postulates really "extra", since there is no indication (now) that other Universes exist?

That sounds more like he is describing history as chaotic, as opposed to non-deterministic. In any case, I agree that history probably works like that. No doubt that going back to 1776 would greatly change the future course of history. But the important idea is that that greatly changed future happens to another universe, and thus is not in contradiction with the history we have observed in our own universe.
Here's a question for you, with regard to that "extra Universe" idea: How can one ever get to another Universe? This is not an irrelevant question of possibility in practice, I'm talking about possibility in principle. If there is another Universe*, then there is nothing seperating that Universe from ours (not space, not time, not anything), and thus there is (logically) also nothing connecting us, which precludes our ever traveling there, doesn't it?

*In these contexts, I will deign to refer to "Universe" in the common sense of what astronomers refer to as "local universes", and not in the "set of all that exists" sense.

Originally posted by hypnagogue
Then again, Occam's Razor is not really relevant here. We aren't talking about whether time travel is actually possible, only if it could be logically possible. Thus, it suffices to create a logically consistent scenario where time travel is logically possible. Whether that logical possibility is then a physical actuality is a question of empirical investigation, which is a separate issue.
On the one hand, I agree with you that you are just constructing a possible-in-principle framework, in response to my request for all possibilities to be presented.

On the other hand, the problem with Occam's Razor is a real one, in this case, since yours could (note: "could", not necessarily "will") become an ad hoc argument, since you would first add the postulate of an extra Universe. Then postulates of how (even in principle) a connection could be established between Universes, and so on.

Originally posted by Anti-Jesus
Okay im only 15 but i question your idea. Because isn't your memory just chemicals in your brain which your brain reconize as memorys? if thats right when you travel back in time then those chemicals would be the same therefore your memory would be the same? right? Anyways maybe we shouldn't go back in time unless the idea of multiple (and i mean lots!) realitys or timeflow(brain just went dead) right? or we might do one of those weird paradox things where we **** up the universe:D
Welcome to the PFs, Anti-Jesus.

I, too, am 15 years old. It makes me feel good to know that there are others, around my age, that are interested in science or philosophy as I am.

Anyway, I don't think your counter-argument works, because physicskid was referring to regressive time-travel (wherein everything starts going backward, until the point in time that is your destination). As it is, everything going backward requires that you too go backward, since you are part of "everything". Ergo, you would get younger and younger, until you ceased to exist.

hypnagogue
Staff Emeritus
Gold Member
Originally posted by Royce
Then again "logically" speaking in a multiverse scenario, it would have already happened in B's history. We can't get around it.
Yes, I acknowledged this in my previous post.

Okay, I'll go along with that but then I have shown how it is logically impossible. Logic will not only not allow a paradox but it won't allow a messy, ugly, redundant multiverse either.
It is not a Law of logic that can't be violated as a physical law but that if it were violated then it wouldn't be logical. Our universe has so far as we know is logical. It is cosmos not chaos.
You are confusing aesthetics with logic. For something to be logically possible, it does not have to be beautiful or elegant or orderly. It just has to not contradict itself.

hypnagogue
Staff Emeritus
Gold Member

Originally posted by Mentat
"At that point it would be reasonable"? Does that mean that at this point it is not reasonable? Are the "extra" postulates really "extra", since there is no indication (now) that other Universes exist?
Well, you have raised questions as to its reasonableness due to OR, which I can accept. There is indeed no indication of multiple universes as of yet. It is just a theory, but again, that is all that this thread has asked for.

Here's a question for you, with regard to that "extra Universe" idea: How can one ever get to another Universe? This is not an irrelevant question of possibility in practice, I'm talking about possibility in principle. If there is another Universe*, then there is nothing seperating that Universe from ours (not space, not time, not anything), and thus there is (logically) also nothing connecting us, which precludes our ever traveling there, doesn't it?
The traditional answer to a question like this is simply "wormholes." I believe it is consistent with current theory that a region of spacetime could become "pinched off" from our own universe and be connected via a wormhole. How to create or travel through such a wormhole is obviously problematic, but I don't think it has yet been shown to be logically impossible.

In order for your logic to work, P1 P2 and C must all exist at the same time.

When you leave the present time, travel back, the present time does not disappear to everyone else, hence it would be required that both Now, and 1776, exist at the same time, all the time!

That would infer that all time exists in all space all of the time, which invalidates the very notion of time itself.

It is not wrong to postulate time travel but prefferable that you do not become obsessed with something that cannot exist, as time itself does not 'exist', substantively, merely idealogically.

P1: I did not exist in 1776.
P2: I exist now in a reality that includes P1 as being true.
P3: I go back in time (using whatever means) to 1776.
C: I did exist in 1776! ...
P1 is true only if P3 is false; because, if P3 is true at any time in your life, that event would always be true in your history. The only way that P1 can ever be true is if P3 is always false.

P2 is a conditional statement, i.e. it requires that P1 be true for P2 to be true thus it is not a proposition. However if P1 is true then P2 is true if and only if P3 is false.

If P3 ever becomes true then it makes P1 false. P2 is also false then because of the inclusion of P1 and also because you cannot exist in both states at the same time. Your are either in the present or in 1776.

C is only true if P3 is true and P1 and P2 are false.

In other words your entire set of statements is illogical reguardless of whether time travel is logical or not.