Are these predictions about the Pacific coastline true or just a coincidence?

[moving finger]

Time travel can occur in general relativistic models in which one has closed time-like curves (CTC's). A time like curve is simply a space-time trajectory such that the speed of light is never equalled or exceeded along this trajectory. Time-like curves thus represent the possible trajectories of ordinary objects. If there were time-like curves which were closed (formed a loop), then traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited.

There are a number of solutions to Einstein's field equations which permit the theoretical existence of such CTC's (eg Goedel's rotating universe).

Does this mean that time travel is at least a theoretical possibility?

If yes, how do we address the infamous paradoxes that arise (eg I could go back in time and, God forbid, kill my own mother before I was born)?

Or does the emergence of CTC solutions within GR point to some basic problem with the internal consistency of GR that has remained hidden until now?

see http://arxiv.org/PS_cache/gr-qc/pdf/0211/0211051.pdf

Interested to hear your thoughts!

MF

 

[Garth]

I go with "basic problem with the internal consistency of GR that has remained hidden until now?"

Garth

 

[SpaceTiger]

For some serious academic discussion on the issue, do a search on Richard Gott. Here's an example.

It's not my area at all, so I can't say much about it other than the fact that it is being seriously considered.

 

[JesseM]

If yes, how do we address the infamous paradoxes that arise (eg I could go back in time and, God forbid, kill my own mother before I was born)?

One possible way of addressing paradoxes that scientists have explored is simply to postulate that only self-consistent histories are allowed, meaning you don't really have the free will to do whatever you want when you reach the past. Human actions are hard to model, so instead they like to look at simpler models involving inanimate objects. For example, if you have two mouths of a wormhole that function as a time machine, so if you go in the right mouth, you will exit the left mouth before you entered the first, then you can imagine shooting a billiard ball into the right mouth, and trying to create a paradox by aiming it on a trajectory such that when it comes out of the left mouth, it will hit its earlier self and knock it away from the wormhole entirely. This is known as "Polchinski's paradox" for the physicist who thought it up. But what was found was that no matter what initial position and velocity you give the billiard ball, there will be at least some self-consistent solutions--in the case of Polchinski's paradox, what can happen is that the ball's future self will come out at an angle to give a glancing blow to its earlier self, which instead of completely knocking it away from the wormhole just alters its angle of approach slightly, and this altered angle is exactly the right one to insure that it will come out the other mouth at the right angle to give its earlier self the same glancing blow! There was still a problem, though--it was found there could actually be multiple self-consistent outcomes for the same set of initial conditions (an infinite number of possible outcomes, if you allow the ball to loop through the wormhole multiple times), yet this was supposed to be a purely deterministic classical model, so how would nature decide which outcome would happen in a given case? Later they were able to apply quantum mechanics to the problem and actually calculate the probability of different outcomes, though, so this seemed to be the solution. Of course this does not show that GR is correct in predicting CTCs, but it shows that such a prediction doesn't necessarily have to lead to paradoxes. All this is discussed in detail on pp. 508-515 of physicist Kip Thorne's book Black Holes and Time Warps.

 

[moving finger]

One possible way of addressing paradoxes that scientists have explored is simply to postulate that only self-consistent histories are allowed, meaning you don't really have the free will to do whatever you want when you reach the past.
...
All this is discussed in detail on pp. 508-515 of physicist Kip Thorne's book Black Holes and Time Warps.

yep, and if you don't have the book then check out the following very readable link for a similar explanation :
http://plato.stanford.edu/entries/time-travel-phys/

If you follow all this, then you should come to the conclusion that some kind of self-consistent histories mechanism could remove the paradoxes of time travel - but at what price?

It's easy to model self-consistent histories for billiard balls and quantum objects, but what happens when humans are involved?

The implication is that if CTC's and time travel is allowed, and only self-consistent histories are allowed, then it would be possible for me to go back in time and visit my mother, but somehow (becasue of the self-consistency constraint) it would be impossible for me to prevent my own birth, no matter what I do... is this feasible?
What exactly would prevent me from putting a gun to my mother's head?
If I would be so prevented, what implications does it have for our free will?

MF

 

[JesseM]

The implication is that if CTC's and time travel is allowed, and only self-consistent histories are allowed, then it would be possible for me to go back in time and visit my mother, but somehow (becasue of the self-consistency constraint) it would be impossible for me to prevent my own birth, no matter what I do... is this feasible?

Sure, why not? Your brain is just a large collection of particles whose individual behavior is not so much more complicated than a billiard ball's. I'd think that the quantum principles that were used to calculate the probabilities of different billiard ball outcomes could in principle be used to calculate the probabilities of different outcomes for an arbitrarily complicated system of particles, including a whole planet and all the human brains on it.

What exactly would prevent me from putting a gun to my mother's head?

Maybe you would change your mind, or you'd get hit by a truck on your way to do so, or humans would never reach the point where they are able to build time machines in the first place.

If I would be so prevented, what implications does it have for our free will?

Doesn't every theory that treats the mind as the behavior of a physical brain, as opposed to some supernatural entity that manipulates the brain from the outside, pretty much negate the idea of "free will"? Any deterministic theory of the brain would seem to be incompatible with free will, and quantum randomness is just randomness, not "freedom" (if you feed the output from a random number generator into a deterministic A.I. computer program to give it unpredictability, that's not really the same as 'free will', is it?) Personally I don't think the concept of free will even makes sense as a philosophical idea, regardless of the how the laws of nature work, but that's another issue.

 

[Chronos]

That whole concept is just wrong. You can spiral around in spacetime, but never break the time barrier. Even if you did [which you can't], you would be in imaginary space. That is an unphysical solution.

 

[JesseM]

That whole concept is just wrong. You can spiral around in spacetime, but never break the time barrier. Even if you did [which you can't], you would be in imaginary space. That is an unphysical solution.

What does "break the time barrier" mean? In GR there are solutions where you can travel into the past while never locally exceeding the speed of light or moving backwards in time. I don't think there's any disagreement among physicists that GR does allow for closed timelike curves in certain situations, the open question is whether these would still be possible in a theory of quantum gravity (most physicists would probably bet the answer is no).

 

[moving finger]

Maybe you would change your mind, or you'd get hit by a truck on your way to do so, or humans would never reach the point where they are able to build time machines in the first place. .

OK, time for a Gedanken experiment.
We have a time machine, I go back in time and put a loaded gun to my mother's head... so far nothing wrong, I have not violated any self-consistency constraint. As long as I do not pull the trigger, the universe is OK, and this should all (in theory) be possible - do you agree?

So there I am, standing with a loaded gun to my mother's head.

Are you suggesting that it will be physically impossible for me to pull the trigger?

If so, what exactly (in a physical sense) will stop me from pulling the trigger?

Doesn't every theory that treats the mind as the behavior of a physical brain, as opposed to some supernatural entity that manipulates the brain from the outside, pretty much negate the idea of "free will"? Any deterministic theory of the brain would seem to be incompatible with free will, and quantum randomness is just randomness, not "freedom" (if you feed the output from a random number generator into a deterministic A.I. computer program to give it unpredictability, that's not really the same as 'free will', is it?) Personally I don't think the concept of free will even makes sense as a philosophical idea, regardless of the how the laws of nature work, but that's another issue.

I agree that's another issue, and whether free will exists or not depends very much on how you define free will. Humans have a "feeling" of free will, and in a certain sense they are free even in a deterministic universe - what we need to do is to encapsulate that "feeling" of free will in a definition of free will which is compatible with determinism.
No matter what you think of free will, the fact is that I am free to take a loaded gun and put it to my mother's head and pull the trigger in the present day if I so choose... what exactly would constrain me from choosing to do exactly the same thing in the past?

MF

 

[JesseM]

OK, time for a Gedanken experiment.
We have a time machine, I go back in time and put a loaded gun to my mother's head... so far nothing wrong, I have not violated any self-consistency constraint. As long as I do not pull the trigger, the universe is OK, and this should all (in theory) be possible - do you agree?

So there I am, standing with a loaded gun to my mother's head.

Are you suggesting that it will be physically impossible for me to pull the trigger?

If so, what exactly (in a physical sense) will stop me from pulling the trigger?

What will stop you? The fact that only self-consistent histories are allowed, and a history where you kill your mother before you are born would not be self-consistent. Perhaps the conceptual problem is that you are thinking of time flowing forward, with the universe deciding "what happens next" on the fly, based on past conditions. A better way to think about it is simply generating a whole bunch of random histories, and then throwing out all the ones that don't fit with the laws of physics.

Here's an analogy. Imagine you want to write a computer program to generate a possible chess game. One way is to start with the pieces in their starting configuration, then have the program generate each successive configuration on the next turn from the configuration on the previous turn, using only legal chess moves. But here's another, more elaborate way to do it. have the computer generate an entire series of configurations at once, completely randomly, so it just picks randomly which pieces to put in which positions on which turn. It is very unlikely that the resulting series will look like a legal chess game--a piece might randomly be on a particular square on one turn, but then the next turn randomly be on some totally different square that it shouldn't be able to get to in one move by the rules of chess. But suppose you have access to an idealized computer with nearly infinite speed and memory, and you have it generate a gigantic number of random series this way--if your number is large enough, chances are at least some of the series would just happen to satisfy the rules of a legal chess game. So you could specify that the computer should throw out all series which violate the rules of chess, and be left only with series that represent legal chess games. But since you are dealing with an entire series at once, you could also place other constraints on them, like "throw out all series where white wins", or "show me only series where the black rook checkmates the king in 25 moves", whatever you want. For sufficiently detailed conditions, it might be very hard to generate a chess game that matched them in the traditional way of starting from the beginning and basing each new configuration of pieces on the configuration of the previous turn, but using this brute-force method of generating a near-infinite number of entire histories, and throwing out all but the ones that satisfy your constraints, it's easy to get a game that satisfies any conditions you like without even having to think about it or plan the details of the game.

Similarly, suppose you were using this incredibly powerful computer to generate a simulation of an entire universe--instead of picking some initial conditions and then letting it evolve forward according to some set of laws of physics, you could again specify your "laws" in terms of constraints on entire histories, with the computer generating a huge number of random histories and then throwing out all the ones that don't satisfy the conditions. If the "laws of physics" you pick happen to allow time travel, then obviously any universe that respects the laws of physics locally at every point in spacetime must be globally self-consistent, and the computer will find some histories satisfying this condition. But the computer does not need to have any intelligence to do this, it's just randomly generating a huge number of possibilities until it finds one that satisfies the constraints. From the point of view of a time traveler in this universe, it might seem like the universe was cleverly finding ways to "outsmart" them and thwart their plans, but it would actually be the result of a fairly simple rule, just not a dynamic rule.

No matter what you think of free will, the fact is that I am free to take a loaded gun and put it to my mother's head and pull the trigger in the present day if I so choose...

Do you? If you think in terms of the block time view as opposed to the "moving present" view, then there is "already" a timeless truth about what you will do in the future...so if the spacetime we are living in does not include you shooting your mother, then in a sense you don't have the "free will" to do so. But perhaps what you mean is just that there are possible spacetimes consistent with the laws of physics where you do kill her, and other possible spacetimes where you don't, so in this sense you do have the "freedom" to do so even if in this particular spacetime it happens to be true that you will not.

 

[moving finger]

What will stop you? The fact that only self-consistent histories are allowed, and a history where you kill your mother before you are born would not be self-consistent. Perhaps the conceptual problem is that you are thinking of time flowing forward, with the universe deciding "what happens next" on the fly, based on past conditions. A better way to think about it is simply generating a whole bunch of random histories, and then throwing out all the ones that don't fit with the laws of physics.

...

From the point of view of a time traveler in this universe, it might seem like the universe was cleverly finding ways to "outsmart" them and thwart their plans, but it would actually be the result of a fairly simple rule, just not a dynamic rule.

Wow, my head hurts! But thankyou for taking the time to explain. I understand the concept (I think), but I'm still having trouble translating that into what it would mean in reality.

For example, I consider myself free and if I take it into my head to do so then I can jump about all over the place but no matter how hard I try, the laws of physics do not allow me to jump from here to the moon... I accept that and I can rationalise it in terms of what I understand of the constraints of the laws of physics.

But where I have trouble is translating this into the time-travel scenario. There would seem to be no constraint preventing me from going back in time and putting a gun to my mother's head, in fact no constraint that would prevent me from pulling the trigger (the gun might be loaded with blanks after all). But somehow, if the gun is loaded with real ammo, some "cosmic censor" manages to step in at the last moment and prevent me from pulling the trigger?

Maybe I'm being very dense, or stupid, or both, but I just do not see how it can work, what physical process would suddenly step in at the critical moment to prevent an action or event which violates self-consistency?

If you think in terms of the block time view as opposed to the "moving present" view, then there is "already" a timeless truth about what you will do in the future...so if the spacetime we are living in does not include you shooting your mother, then in a sense you don't have the "free will" to do so.

But... and here is the crux of it...if you tell me that I do not have the free will to shoot my mother in the present day, I can if I so choose prove you wrong by doing just that, without violating any laws of physics... whereas what you are saying is that this does not apply in the time travel case, that somehow in that case there IS a law of physics which will prevent me from shooting my mother, regardless of whether you tell me in advance that I can shoot my mother or not, and regardless of whether I want to prove you wrong or not.

But perhaps what you mean is just that there are possible spacetimes consistent with the laws of physics where you do kill her, and other possible spacetimes where you don't, so in this sense you do have the "freedom" to do so even if in this particular spacetime it happens to be true that you will not.

The multiverse theory, is that what you mean here? - that if I kill my mother before I was born I am simply causing another version of reality, that there is a timeline existing, in a "parallel universe" where my mother is killed and I am not born? Personally I'm not comfortable with that (but that's my problem!)

MF

 

[JesseM]

Wow, my head hurts! But thankyou for taking the time to explain. I understand the concept (I think), but I'm still having trouble translating that into what it would mean in reality.

For example, I consider myself free and if I take it into my head to do so then I can jump about all over the place but no matter how hard I try, the laws of physics do not allow me to jump from here to the moon... I accept that and I can rationalise it in terms of what I understand of the constraints of the laws of physics.

But where I have trouble is translating this into the time-travel scenario. There would seem to be no constraint preventing me from going back in time and putting a gun to my mother's head, in fact no constraint that would prevent me from pulling the trigger (the gun might be loaded with blanks after all). But somehow, if the gun is loaded with real ammo, some "cosmic censor" manages to step in at the last moment and prevent me from pulling the trigger?

Maybe I'm being very dense, or stupid, or both, but I just do not see how it can work, what physical process would suddenly step in at the critical moment to prevent an action or event which violates self-consistency?

Again, just think in terms of my thought-experiment earlier where some mega-computer generates entire histories at random (randomly curved spacetime manifolds with random lines drawn on representing worldlines of particles, perhaps) and then checks each one to see if it obeys the laws of physics at every point in spacetime, and throws out all the ones that don't, leaving only the tiny fraction that do to return as output to the user. If this computer program generated a particular history for you to examine, then just by knowing the rules the computer used to do so, you could know with 100% certainty that any time travel would be perfectly self-consistent, right? So suppose you examine it, and see that somewhere in spacetime intelligent life evolved, and one of the simulated beings was asking the same questions you're asking. Since the history is already complete you can't actually interact with this simulated being, but if you wanted to think up an appropriate response in your mind, what would it be?

One problem with the way you're thinking about this is saying that the thing that would prevent you from killing your mother would only intervene at the last minute. But if you think in terms of generating entire histories and then throwing out all the ones that aren't consistent with the laws of physics at every point, then it's probably much more likely that inconsistencies would be avoided in a more "subtle" way that involved events long before the critical moment of the time traveler meeting his mother, like no time travelers would decide to try to kill their mothers in the first place, or no one would even invent time travel even though it's allowed by the laws of physics. As an analogy, suppose you program a computer to generate 10 random numbers from 1-100, and then throw out all the strings of numbers that don't contain three eighteens. It would be wrong to expect that the first seven numbers would simply obey the normal laws of probability, and then if (as would be more likely than not) none of those first seven was an 18, you would "miraculously" see three 18s in a row at the end. In fact, if you use this rule you are likely to see the three 18s distributed at random throughout the string of ten numbers--do you agree?

But... and here is the crux of it...if you tell me that I do not have the free will to shoot my mother in the present day, I can if I so choose prove you wrong by doing just that, without violating any laws of physics...

But what do you mean "can if I so choose"? Again, it seems to me you're not thinking in terms of the "block time" view, where the future is just as set as the past. If you imagine a God's-eye-view of spacetime as a whole, like unrolling a movie film so you can see every frame at once, from this perspective you can timelessly see whether or not whether someone shoots their mother...if you can see that the answer is no, then is that person correct in saying he "can shoot her if he chooses to"?

The multiverse theory, is that what you mean here?

No, I didn't really mean that. I just meant that if there is a possible universe where you shoot your mother, in some sense maybe that means you "can" do it, even if in the real universe it is already set that you never will. Again, I'm trying to clarify what you mean by "can", from the block-universe perspective.

 

[moving finger]

Another thought :
Causal loops are also possible with CTCs. Though not strictly a paradox, they do pose challenging questions to our intuitions. For example, with a time machine I could send the answer to "Fermat's last theorem" back to a mathematician in the past, he/she could publish it as his/her theorem, and it becomes part of the accepted literature, which (one day) I then copy and transmit back to a mathematician in the past...

This is entirely possible within GR, CTCs, Block Time and violates no self-consistency rules... but the problem is : Where did the original solution come from?

MF

"There's the King's Messenger. He's in prison now, being punished: and the trial doesn't even begin till next Wednesday: and of course the crime comes last of all."
"Suppose he never commits the crime?" said Alice.
"That would be better, wouldn't it?"

 

[moving finger]

So suppose you examine it, and see that somewhere in spacetime intelligent life evolved, and one of the simulated beings was asking the same questions you're asking. Since the history is already complete you can't actually interact with this simulated being, but if you wanted to think up an appropriate response in your mind, what would it be?.

But that's exactly the problem, I don't know!
Being : "Can I travel back in time to visit my mother before I was born?"
Consistency-Cop : "Yes"
Being : "Can I then put a loaded gun to her head?"
Consistency-Cop : "Yes"
Being : "Can I then pull the trigger and kill her?"
Consistency-Cop : "No"
Being : "What exactly prevents me from doing this final act?"
Consistency-Cop : "The law of self-consistency"

It's an "answer", yes... but I'm sorry it's not an answer that I can make any rational sense out of!

One problem with the way you're thinking about this is saying that the thing that would prevent you from killing your mother would only intervene at the last minute. But if you think in terms of generating entire histories and then throwing out all the ones that aren't consistent with the laws of physics at every point, then it's probably much more likely that inconsistencies would be avoided in a more "subtle" way that involved events long before the critical moment of the time traveler meeting his mother, like no time travelers would decide to try to kill their mothers in the first place, or no one would even invent time travel even though it's allowed by the laws of physics.

Likely, maybe, but not essential. I see no reason why there should not be a self-consistent solution which includes me putting a loaded gun to my mother's head... but just not pulling the trigger. If that is a self-consistent solution then it is allowed. I hope you agree with this?

If so, if I am standing there with the loaded gun at my Mother's head (God bless her!), I simply cannot see what it is that "forces" me not to pull the trigger?

As an analogy, suppose you program a computer to generate 10 random numbers from 1-100, and then throw out all the strings of numbers that don't contain three eighteens. It would be wrong to expect that the first seven numbers would simply obey the normal laws of probability, and then if (as would be more likely than not) none of those first seven was an 18, you would "miraculously" see three 18s in a row at the end. In fact, if you use this rule you are likely to see the three 18s distributed at random throughout the string of ten numbers--do you agree?.

From the point of view of pure probability yes I agree, but the fact is there is an acceptable solution which contains 3 x 18's as the last three numbers in the sequence (in fact there are many such solutions). You are not saying that such a solution is forbidden, are you? We already know that we do not live in a very "likely" universe (just look at how low entropy is at our present time, compared to how high it could be).

But what do you mean "can if I so choose"? Again, it seems to me you're not thinking in terms of the "block time" view, where the future is just as set as the past. If you imagine a God's-eye-view of spacetime as a whole, like unrolling a movie film so you can see every frame at once, from this perspective you can timelessly see whether or not whether someone shoots their mother...if you can see that the answer is no, then is that person correct in saying he "can shoot her if he chooses to"?

Yes, my statement is still correct - I can shoot my mother if I so choose. From God's perspective, He sees that the person never chooses to shoot his mother. But the universe could equally well be that the person does shoot their mother, in which case God will see that this person does choose to shoot their mother. There is no "law" either way, constraining me to either shoot or not shoot my mother. But in the time travel case, you are saying there IS a law, the self-consistent history law, which would prevent me from shooting my mother no matter what I choose to do. There is a subtle difference.

No, I didn't really mean that. I just meant that if there is a possible universe where you shoot your mother, in some sense maybe that means you "can" do it, even if in the real universe it is already set that you never will. Again, I'm trying to clarify what you mean by "can", from the block-universe perspective.

I'm lost here. How can there be a universe (if we exclude the multiverse approach) where I shoot my mother before I was born, if this is ruled out by the self-consistent history law?

MF

Alice laughed, "There's no use trying," she said, "one can't believe impossible things."
"I daresay you haven't had much practice," said the Queen. "When I was your age, I always did it for half-an-hour a day. Why, sometimes I've believed as many as six impossible things before breakfast."

 

[JesseM]

But that's exactly the problem, I don't know!
Being : "Can I travel back in time to visit my mother before I was born?"
Consistency-Cop : "Yes"
Being : "Can I then put a loaded gun to her head?"
Consistency-Cop : "Yes"
Being : "Can I then pull the trigger and kill her?"
Consistency-Cop : "No"
Being : "What exactly prevents me from doing this final act?"
Consistency-Cop : "The law of self-consistency"

It's an "answer", yes... but I'm sorry it's not an answer that I can make any rational sense out of!

But would you agree that in the computer simulation analogy, there's no "force" that prevents a simulated being from killing his mother? In this case, it's just that any of the randomly-generated histories that contained discontinuous points where the laws of physics don't hold (as would probably have to be the case in any spacetime which contained both the events of the being killing his mother before he was born and of the same being around to make the trip back in time years later) would have been thrown out by the computer program. The only ones remaining would be the ones that just happened to contain no inconsistencies.

As another analogy, suppose someone goes through a huge amount of footage or convenience store robberies and throws out all the ones where someone is shot, leaving only the ones where no one is. He then gives you some of the footage he has selected. Knowing how the footage was selected, you know that the robbers will never shoot anyone, even if you see them holding a gun to the clerk's head and looking like they could shoot at any moment...is there some "force" that's preventing them from doing so?

Likely, maybe, but not essential. I see no reason why there should not be a self-consistent solution which includes me putting a loaded gun to my mother's head... but just not pulling the trigger. If that is a self-consistent solution then it is allowed. I hope you agree with this?

If so, if I am standing there with the loaded gun at my Mother's head (God bless her!), I simply cannot see what it is that "forces" me not to pull the trigger?

In the "block time" view, all your future actions are set anyway, so again, think of the analogy of watching footage of a robbery that's already happened, knowing in advance that the person who gave you the footage rejected all the clips where anyone actually got shot, only leaving the ones where no one did. As you watch a particular clip, would you ask "what is it that forces the robber not to pull the trigger?"

Yes, my statement is still correct - I can shoot my mother if I so choose. From God's perspective, He sees that the person never chooses to shoot his mother. But the universe could equally well be that the person does shoot their mother, in which case God will see that this person does choose to shoot their mother.

OK, that's what I meant in the earlier posts when I was talking about the meaning of "can" in terms of there being a possible universe where you shoot your mother.

I'm lost here. How can there be a universe (if we exclude the multiverse approach) where I shoot my mother before I was born, if this is ruled out by the self-consistent history law?

On this particular tangent about possible worlds, we weren't talking about time travel, we were talking about the question of whether you "can" shoot your mother now, after you've already been born.

 

[moving finger]

But would you agree that in the computer simulation analogy, there's no "force" that prevents a simulated being from killing his mother? In this case, it's just that any of the randomly-generated histories that contained discontinuous points where the laws of physics don't hold (as would probably have to be the case in any spacetime which contained both the events of the being killing his mother before he was born and of the same being around to make the trip back in time years later) would have been thrown out by the computer program. The only ones remaining would be the ones that just happened to contain no inconsistencies.

Yes, I agree there is no force, but there is an "active selection of possible histories", and I think this points to the problem. In the present time, the only constraints on my free will to do as I choose seem to be physical laws which "force" me to do only those things which are physically possible, and I can rationalise this. But in the time travel scenario, there is no force involved... that is why I cannot rationalise exactly what it is that would prevent me from killing my own mother (if I so choose). I believe you would agree that I can have the intent, the wish, to kill my own mother in both the present and the time-travel scenario? And in the present I could carry out that wish. But in the time-travel scenario I could not carry out that wish, even though I could go all the way up to putting a gun to her head and even pull the trigger... its just that she cannot die!

As another analogy, suppose someone goes through a huge amount of footage or convenience store robberies and throws out all the ones where someone is shot, leaving only the ones where no one is. He then gives you some of the footage he has selected. Knowing how the footage was selected, you know that the robbers will never shoot anyone, even if you see them holding a gun to the clerk's head and looking like they could shoot at any moment...is there some "force" that's preventing them from doing so?

Yes, I can see this. But also, if there are enough videotapes, there will be some tapes which contain scenes where the robbers have the intent to kill, where they put the gun against the victim's head, and they pull the trigger... but for some strange reason the victim never dies. I can see this is logically plausible, but it is still very hard to rationalise it.

In the "block time" view, all your future actions are set anyway, so again, think of the analogy of watching footage of a robbery that's already happened, knowing in advance that the person who gave you the footage rejected all the clips where anyone actually got shot, only leaving the ones where no one did. As you watch a particular clip, would you ask "what is it that forces the robber not to pull the trigger?"

Yes, I would ask that. At least, I would ask : "there have been so many times in these clips that robbers have tried to shoot their victims, how come none of them ever succeeded?"

It's almost like there is a cosmic censor, who eliminates all the non-consistent histories? I know you will say that there does not need to be a supreme being or controlling entity which carries out this censorship, that the "censor" is simply the laws of logic and self-consistency...hmmm.

BTW - I'm interested to know what you think of causal loops (post #13 in this thread)?

Thank you.

MF

"And what does it live on?"
"Weak tea with cream in it."
A new difficulty came into Alice's head, "Supposing it couldn't find any?" she suggested.
"Then it would die, of course."
"But that must happen very often," Alice remarked thoughtfully.
"It always happens," said the Gnat.

 

[Crosson]

This is a great thread discussion, I never thought about how the nonexistence of free will solves the classic time travel paradoxes. I have nothing more to add to this simple but elegant idea, but I would like to make a point about free will.

the only constraints on my free will

ha! You are assuming that you have free will. This is not very likely, because you are made of matter and matter moves according to deterministic laws. ( Please do not object using QM, because the probabilities are deterministic and so can hardly be called free).

In this excerpt from an essay I wrote, I use the term "Metaphysical Freedom" to mean the genuine type of freedom . This as opposed to common usage where a gun to your back or a prison cell effects your freedom.

Theorem: Metaphysical Freedom is a force.

Proof: In order for a decision to be acted upon, an electrical current stimulates various parts of the nervous system. If there is to be a correspondence between the physical actions of the body and the decision of the mind, the decision must be related in a one to one way with this electrical current. This one to one relationship is precisely what we think of as cause and effect, and the cause of electrical current is a force. Therefore, by the definition of a force as the cause of a deviation in smooth motion, the decision is a force.

Heuristic Argument: Clearly, Metaphysical Freedom cannot be a Newtonian Force, because it does not depend on the position or energy of any other particles in the universe. The MF force is a very peculiar force indeed. Actually, it is at least 6 billion different forces (depending on how you feel about animals) because my MF force cannot interact with the matter in your body, or anyone else’s. Adding to the strangeness is that our brains seem to be made of ordinary matter, with no additional properties (of the likes of charge or mass) to affect the interaction with our minds. Also, my MF force did not exist in the universe before 1985 and will not exist after my death, in the sense that it won’t be able to interact with any matter in the universe.

Summary: It is easier for me to believe that Metaphysical Freedom is an illusion rather than believing in a hopelessly complex, highly non-universal, peculiar force. Occam’s Razor is very clear about this issue, simplicity itself should be used when deciding between two theories which cannot be distinguished.

 

[moving finger]

This is a great thread discussion

Thank you

ha! You are assuming that you have free will

ha! You make the presumption that your definition of free will is the same as mine!

MF

 

[moving finger]

I think we have a deep problem with epistemology here.

From the discussion so far, it would seem that the self-consistent histories (SCH) hypothesis does provide at least a possible theoretical solution to the causal paradoxes associated with time travel in GR.

The SCH hypothesis simply put : Only self-consistent histories are allowed, such that any possible history which included, for example, me going back in time and killing my own mother before I am born, would be eliminated by SCH.

The SCH hypothesis seems to solve the problem?

But let us examine this SCH hypothesis in greater detail.

The SCH hypothesis does not prevent me from going back in time and meeting my mother before I was born, therefore the statement (A) :

(A) "I go back in time to meet my mother before I was born, and I pass on the knowledge to her that I am her future son"

is allowed to be true under SCH.

On the surface, what the SCH hypothesis appears to be saying (and this is what we have been debating so far) is that, given (A), then the following statement (B) is also necessarily true :

(B) "I cannot kill my mother before I am born"

BUT the SCH hypothesis is actually much stronger than the above suggests. What the SCH hypothesis is actually telling is that, given (A), then the following more general statement (C) is necessarily true :

(C) "my mother CANNOT DIE before I am born".

(Why is (C) necessarily true? Because from the moment of our meeting due to (A), my mother knows that the universe is necessarily such that I will be born as her son, therefore she knows that she will not die before I am born)

This is a very different proposition altogether.

What this means is that NO MATTER WHAT MY MOTHER DOES from the moment of our meeting until my birth, she will not die.

She can jump off a building, starve herself, drink poison, do absolutely anything, but the SCH hypothesis will not allow her to die...

Is this credible?

I don't think so. There is obviously still something very dubious about either the SCH hypothesis, or about time travel in GR, or both.

MF

"That's the effect of living backwards," the Queen said kindly: "it always makes one a little giddy at first-----"

 

[moving finger]

ha! You are assuming that you have free will.

We can discuss the definition of free will if you wish (I would love that!), but in the meantime, if it helps, replace the words "free will" in the post you refer to by the word "ability".

Therefore instead of the phrase :

the only constraints on my free will to do as I choose

my phrase becomes :

the only constraints on my ability to do as I choose

happy now?

MF

"When I use a word," Humpty Dumpty said in a rather scornful tone," it means just what I choose it to mean -- neither more nor less."
"The question is," said Alice, "whether you can make words mean so many things."
"The question is," said Humpty Dumpty, "which is to be master -- that's all."

 

[JesseM]

What this means is that NO MATTER WHAT MY MOTHER DOES from the moment of our meeting until my birth, she will not die.

She can jump off a building, starve herself, drink poison, do absolutely anything, but the SCH hypothesis will not allow her to die...

Is this credible?

I don't think so. There is obviously still something very dubious about either the SCH hypothesis, or about time travel in GR, or both.

Why is this not credible to you? Do you agree that if we had a sufficiently powerful computer that could generate a near-infinite number of random histories and then throw out all the ones in which the laws of physics are not obeyed at every point in spacetime, then it would output a bunch of histories in which any time traveler who goes back to talk to his mother will find that this is true? If you do agree, how can something that's credible in a perfect simulation of a universe obeying our own laws of physics not be credible in real life?

And again, you seem to assume that because your mother could imagine choosing to try to kill herself, that she would actually go through with it in a non-negligible number of histories, but that probably wouldn't be true (the assumption that the feeling of freedom translates to actual freedom of action)--more likely, in the vast majority of histories where you interacted with her, she would happen to choose not to attempt to kill herself. But yes, in the tiny fraction where she does, somehow she will fail, like she jumps out of a plane but happens to land on a truck transporting 10,000 pillows or something.

I'll get back to you on your other post soon...

 

[moving finger]

Why is this not credible to you? Do you agree that if we had a sufficiently powerful computer that could generate a near-infinite number of random histories and then throw out all the ones in which the laws of physics are not obeyed at every point in spacetime, then it would output a bunch of histories in which any time traveler who goes back to talk to his mother will find that this is true? If you do agree, how can something that's credible in a perfect simulation of a universe obeying our own laws of physics not be credible in real life?

And again, you seem to assume that because your mother could imagine choosing to try to kill herself, that she would actually go through with it in a non-negligible number of histories, but that probably wouldn't be true (the assumption that the feeling of freedom translates to actual freedom of action)--more likely, in the vast majority of histories where you interacted with her, she would happen to choose not to attempt to kill herself. But yes, in the tiny fraction where she does, somehow she will fail, like she jumps out of a plane but happens to land on a truck transporting 10,000 pillows or something.

I'll get back to you on your other post soon...

But just imagine what her situation would be like.
She would feel like she has supernatural powers; she would be able to do absolutely anything and still she knows she will survive. She could test this to the limits, using everything and anything possible to try to bring about her demise... but no matter what she does, she will survive.

Houdini would be very envious

Sorry, it just seems totally preposterous to me.

I have a feeling that the problem hinges on the assumption that we can ever have "certain" knowledge of the future - if in the present we have certain knowledge of a future event then the implication is that no matter what we do, that event must come to pass.

That works fine for things that we can hardly affect like "the sun will rise tomorrow"... but when it comes down to more mundane things like "I will eat two eggs for breakfast tomorrow" then (armed with this apparent foreknowledge) I cannot see how the SCH hypothesis can intervene to prevent me from eating porridge (if I so choose) instead of two eggs for breakfast tomorrow...

In other words - if you predict what I am going to do at some future date - I believe I can easily prove you wrong.

The SCH hypothesis would imply, on the other hand, that (if your prediction is based on actual knowledge of the future) then I cannot prove you wrong.

MF

 

[JesseM]

But just imagine what her situation would be like.
She would feel like she has supernatural powers; she would be able to do absolutely anything and still she knows she will survive. She could test this to the limits, using everything and anything possible to try to bring about her demise... but no matter what she does, she will survive.

If believing she couldn't die would make her more likely to risk her life, then in most universes maybe she just won't believe it in the first place.

Houdini would be very envious

Sorry, it just seems totally preposterous to me.

You never really answered my question about the computer simulation:

Do you agree that if we had a sufficiently powerful computer that could generate a near-infinite number of random histories and then throw out all the ones in which the laws of physics are not obeyed at every point in spacetime, then it would output a bunch of histories in which any time traveler who goes back to talk to his mother will find that this is true? If you do agree, how can something that's credible in a perfect simulation of a universe obeying our own laws of physics not be credible in real life?

What are your answers to this?

I have a feeling that the problem hinges on the assumption that we can ever have "certain" knowledge of the future - if in the present we have certain knowledge of a future event then the implication is that no matter what we do, that event must come to pass.

Do you agree that in the computer simulation thought-experiment, simulated beings in histories the computer spits out could in certain cases have this sort of certain knowledge of the future?

That works fine for things that we can hardly affect like "the sun will rise tomorrow"... but when it comes down to more mundane things like "I will eat two eggs for breakfast tomorrow" then (armed with this apparent foreknowledge) I cannot see how the SCH hypothesis can intervene to prevent me from eating porridge (if I so choose) instead of two eggs for breakfast tomorrow...

In other words - if you predict what I am going to do at some future date - I believe I can easily prove you wrong.

What would be your response to a being in such a simulated universe who made the same arguments? Do you think he'd have the freedom to "prove wrong" anything a time traveler told him he was going to do?

 

[moving finger]

We seem to be stuck on : "the SCH hypothesis explains why there is no paradox" vs "the SCH hypothesis doesn't seem to make sense".

Instead of each side simply re-stating positions, it might be more productive if I re-phrase the problem as I see it?

Re-Phrasing The Problem

The fundamental problem (to me) seems to hinge on the possibility of Epistemic Determinism - ie that infallible foreknowledge can somehow constrain the history of the universe.

If someone tells you that he/she has infallible foreknowledge and he/she KNOWS that you will eat two eggs for breakfast tomorrow, then we have several possible alternatives, which I summarise as A B and C below :

A : Infallible foreknowledge is possible, and no matter what you do, you cannot avoid eating two eggs for breakfast tomorrow. This is a variation on the "I can't kill my own mother before I am born" time-travel paradox, and requires some kind of Self-Consistent Histories (SCH) hypothesis to be correct (ie SCH somehow eliminates all non-consistent histories).

B : Infallible foreknowledge is possible, but armed with this information you can still avoid eating two eggs for breakfast tomorrow if the Multiverse hypothesis is correct, ie there are multiple possible futures.

C : Infallible foreknowledge is not possible, in which case you can easily avoid eating two eggs for breakfast tomorrow, thus proving him/her wrong.

Thus it seems we must adopt one or other of the following scenarios is true :

A : Infallible Foreknowledge combined with the SCH hypothesis.
B : Infallible Foreknowledge combined with the Multiverse hypothesis.
C : Infallible Foreknowledge is not possible.

Now there seem to be solutions to GR which do allow time travel, hence it would appear on the surface that infallible foreknowledge could be possible in GR. In this case, it would appear that either the SCH hypothesis or the Multiverse hypothesis is also required to ensure full consistency.

My problem with (A) is that I simply cannot conceive of a situation where someone tells me that I will eat two eggs for breakfast tomorrow, and I simply find it impossible, no matter what I do, to prove that person wrong.

An alternative solution is that there is something wrong with GR, that time travel is in fact not possible, hence (C) would be correct.

MF

 

[moving finger]

What would be your response to a being in such a simulated universe who made the same arguments? Do you think he'd have the freedom to "prove wrong" anything a time traveler told him he was going to do?

Jesse

Thank you for your contributions - they have been very stimulating!

I owe you a much more detailed reply, I know that, and I will provide it, but I am unfortunately short of time right now - please bear with me and I'll be back in the next couple of days.

Meanwhile - a little something to ponder on :

The problem here I believe is one of infinite self-referential loops.

I grant you that the simulated Block universe may have a certain configuration prior to my intervention, and this configuration is fixed.

If I now interact with that universe by inputting information at a certain point (ie I tell one of the simulated beings what his/her future is going to be) then my simple act of inputting that information CHANGES THE UNIVERSE, so that the outcome is no longer constrained to be what it was prior to my intervention, in other words the infallible foreknowledge of the simulated being's future that I imparted is no longer infallible, because the simple act of me imparting that knowledge changes the universe and hence allows the future to change.

I believe if we follow this through, then we will find that infallible foreknowledge is in general not possible, for this very reason.

I'm going to mull over it for the next day or so...

MF

 

[chronon]

Being : "Can I travel back in time to visit my mother before I was born?"
Consistency-Cop : "Yes"
Being : "Can I then put a loaded gun to her head?"
Consistency-Cop : "Yes"
Being : "Can I then pull the trigger and kill her?"
Consistency-Cop : "No"
Being : "What exactly prevents me from doing this final act?"
Consistency-Cop : "The law of self-consistency"

This assumes that the consistency-cop steps in at the last minute to ensure consistency. This isn't how it would work at all. The consistency cop just ensures something happen somewhere in the chain of events to prevent the final outcome. In the wormhole/billiard ball situation the consistency-cop doesn't swerve the ball at the last minute, but rather makes sure the whole situation is consistent. I've written a Java applet simulating the wormhole+billiard ball at http://www.chronon.org/Applets/bouncer.html.

In the case of shooting your mother, maybe a passer-by is traumatised by the experience of seeing someone having a gun fired at her head, and as a result goes round secretly replacing live ammo with blanks in the future, some of which ends up in your gun. Or maybe there is some other path which results in the same effect.

 

[moving finger]

Jesse

Just posting ideas & issues as they occur to me...

Asymmetry in the Block Time view

If you think in terms of the block time view as opposed to the "moving present" view, then there is "already" a timeless truth about what you will do in the future...

Imagine the computer simulated universe that you have suggested a couple of times.

This might seem to be a good example of a "Block Time" universe. There is perhaps just one history consistent with a particular set of space-time conditions (which, as long as information is not detroyed, I think it does not matter if these are initial conditions or final conditions).

Looking at this simulation from outside, we "hyperbeings" can see the worldline of every simulated being in this universe. For any simulated being at time "t", we see their entire past and future mapped out, and fixed. We see that these simulated beings have no genuine "free will", they are constrained to do exactly what the deterministic program makes/tells them to do. Their future is just as determined as their past, there seems to be no asymmetry to time in their universe.

I think you will agree with me so far.

These simulated beings, as part of the program, "process information" and they act on the information they have available at the present time - purely deterministically - such that for any time "t", the "future" for each being is dependent on the information available at time "t". Now IF the information at time "t" is produced entirely self-consistently within the simulation (ie there is no interaction with the outside), then it is also true (assuming no information is lost or destroyed) that there should be only one unique set of "past" states which is consistent with the information at time "t". This is simply another way of saying the model is completely time-symmetrically deterministic.

What happens if we hyperbeings now interact with this Block Universe?

In other words, what happens if we "inject" additional information into the simulation at a particular time "t"?

It is then possible for that information to change the history/worldlines of the beings in that simulation at all future times >"t", but it is not possible for it to change the simulation for any past times <"t". In this sense, time now becomes asymmetric within the simulation. We haved created a discontinuity at time "t" where the new information has been introduced, such that the future states (after "t") are no longer simply determined from the past states (prior to "t")

If some hyperbeing therefore intervenes and provides information (eg "infallible foreknowledge") to a simulated being (about that simulated being's future and past) at time "t", then this very input of information can change the future but not the past, hence the "infallible foreknowledge" is not in fact infallible.

Still have to develop this further...

Just mulling things over...

MF

 

[moving finger]

This assumes that the consistency-cop steps in at the last minute to ensure consistency. This isn't how it would work at all.

How do you know that?
There is nothing inconsistent with me putting a loaded gun to my mother's head, therefore the SCH hypothesis should not forbid this, therefore there should exist self-consistent worlds where this will happen.
The only inconsistency is if she dies, and this and only this is what the SCH hypothesis forbids.

The problem is more general in that not only can I not kill my mother before I am born, there is simply no way she can die before I am born. Therefore if she knows infallibly that she will give birth to me on some future date, then (like Superman) she knows also that she is effectively "unkillable" until that future date - better than Superman, she's not even vulnerable to Green Kryptonite - she simply cannot die (the SCH hypothesis will not allow her to die).

The idea that a person can be, or even think themselves to be, "unkillable" seems silly and wrong, which points to an error in our assumptions somewhere - maybe the SCH hypothesis is wrong, or maybe the assumption that we can ever have infallible foreknowledge is wrong (which would also imply that our idea of time travel is flawed). I suspect the latter.

MH

 

[chronon]

Therefore if she knows infallibly that she will give birth to me on some future date, then (like Superman) she knows also that she is effectively "unkillable" until that future date

In the case of going back in time to change the past to be inconsistent with the present, the self-consistency hypothesis says that somehow your actions will result in a consistent world. You are looking at the idea of an oracle which can tell the future infallibly. Such oracles have appeared in fiction, and the moral is invariably that although what they say is true, it is also highly misleading.

 

[Crosson]

Moving Finger, please understand this point:

When I say you don't have free will, I mean that you don't have the ability to choose the actions of your body.

Untill you understand that this is where some of the other posters are coming from, you will continue to argue on the grounds that:

There is nothing inconsistent with me putting a loaded gun to my mother's head

Correct, this action would be consistent. But you can't choose to do this action! (you do not have the "ability")

You can't choose to do anything ever; all we do is go around riding rails (having deterministic collisions in our brain which make our body act certain ways) like robots, thinking that we have the ability to choose (right before my arm goes up, I get a sensation of thinking about raising my arm).

It may be impossible for you to believe that you never have the ability to choose in any situation, but feel free to reread the argument for this position that I originally posted.

 

[JesseM]

Jesse

Thank you for your contributions - they have been very stimulating!

I owe you a much more detailed reply, I know that, and I will provide it, but I am unfortunately short of time right now - please bear with me and I'll be back in the next couple of days.

No problem--I also still have to reply to some of your earlier posts, like the one about information-loops.

Meanwhile - a little something to ponder on :

The problem here I believe is one of infinite self-referential loops.

I grant you that the simulated Block universe may have a certain configuration prior to my intervention, and this configuration is fixed.

If I now interact with that universe by inputting information at a certain point (ie I tell one of the simulated beings what his/her future is going to be) then my simple act of inputting that information CHANGES THE UNIVERSE, so that the outcome is no longer constrained to be what it was prior to my intervention, in other words the infallible foreknowledge of the simulated being's future that I imparted is no longer infallible, because the simple act of me imparting that knowledge changes the universe and hence allows the future to change.

Actually, I wasn't imagining that you had the ability to interact with the universe--the computer will just split out complete histories, all you can do is view them. But in these histories, there may be simulated intelligent beings who learn to time travel, and a time traveler may meet another being and tell him in advance what he was going to do. So if this being asks the time traveler, "but what will prevent me from doing something different?", I'm asking you what you think the best answer to him would be if you were to imagine responding to him in your head, you can't actually tell him the answer since you are not part of the simulation.

 

[chingkui]

Another thought :
Causal loops are also possible with CTCs. Though not strictly a paradox, they do pose challenging questions to our intuitions. For example, with a time machine I could send the answer to "Fermat's last theorem" back to a mathematician in the past, he/she could publish it as his/her theorem, and it becomes part of the accepted literature, which (one day) I then copy and transmit back to a mathematician in the past...

This is entirely possible within GR, CTCs, Block Time and violates no self-consistency rules... but the problem is : Where did the original solution come from?

I think I am more interested in this type of "spontaneous generation of information" paradox...
I begin to like this idea: A crazy scientist who clone himself bring his clone to a time machine and send him back 50 years, and when the clone grow up, he become the scientist who clone himself...

Anyone know anything about this type of paradox? thanks.

 

[Garth]

I think Star Trek did something similar with 'transparent aluminium' if I recall.

Garth

 

[Alkatran]

Don't you see the problem with your logic?

1: Yes, universes where you can travel back in time would be permitted
2: Universes where you stuck a gun to your mother's head would be permitted, but ONLY THOSE IN WHICH YOU DIDN'T SHOOT.

For example, you go up to your mother, put the gun to her head, and a police officer shoots you. Or she starts to cry and you decide you can't do it. Or you just decide you don't want to do it. ETC

If you want to justify time travel in a very science-fictiony way, think of events as progressing through time. IE you're 30 when you shoot your mother the day before you were born, so for the 30 years, until that 'event' reaches the time when you went back in tiem, your mother dies. Then the time line switches back because no on is going back in time and 30 years later, when you grow up, you go back... etc. That would also explain the 'solve theorem, send back' problem. Someone solves it originally, sends it back in time, and THEN the loop starts.

 

[moving finger]

Hi Crosson

When I say you don't have free will, I mean that you don't have the ability to choose the actions of your body.

With respect, Crosson, I think this depends on one's definition of "you" and "your body".

When it comes to "choice", I believe that even a simple machine can "choose", in the sense of taking two or more inputs and producing one output. The ability to choose says nothing about "free will" (whatever that may be - and there are many ways to define it).

Correct, this action would be consistent. But you can't choose to do this action! (you do not have the "ability").

Please explain what it is precisely that prevents me from having the "ability" to put a loaded gun to my mother's head (before I am born), since you already agree there is nothing in the SCH hypothesis which prevents this?

You can't choose to do anything ever; all we do is go around riding rails (having deterministic collisions in our brain which make our body act certain ways) like robots, thinking that we have the ability to choose (right before my arm goes up, I get a sensation of thinking about raising my arm).

The simple fact is that I do choose. A choice is simply "taking 2 or more inputs and producing 1 output" - even a machine can "choose" in this sense. Please understand that I am not suggesting there is any free will involved in the choice. Even if there is no "free will" involved (please define free will?), agents still make choices about whether to do (A) or (B). Even a simple machine bereft of free will makes choices. I grant you it may be a deterministic process, but that does not mean that there is no choice involved. It is simply a choice governed by determinism, that's all. Even a choice governed by determinism is still a choice.

MF