Time travel paradoxes

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thoughtfully
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Re: Time travel paradoxes

Postby thoughtfully » Sun Dec 07, 2008 1:28 pm UTC

ddxxdd wrote:If I turned all my protons into anti-protons, all my neutrons into anti-neutrons, and somehow created some form of "anti-physics" that made all the laws of physics inside my body go backwards, then I wouldn't go back in time. I'd just turn into a toddler.

But, time must flow backwards for however many years it takes. This makes as much sense as what one sees in scifi sometimes: some poor schmuck is stranded in a "time bubble" (or something) and is suddenly an old man (or the corpse of an old man) to his pals. Well he can't become an old man without fifty plus years of metabolizing and maturation. Where's all the food/water/oxygen come from? Why doesn't he get gangrenous sores after sitting in one place for more than a few days?

To turn into a baby, the subject would actually have had to "live" those years in reverse, which implies consuming a lot of urine, CO2, solid waste, and dead skin cells, and spitting out O2 and unconsumed nourishment.

A living organism can't survive isolated from the rest of the Universe for long, whichever way time is flowing. It might be possible if the required support systems (say, a planet) and a source of energy is included.

The experience of the subject would be normal with respect to his body and whatever else in within the bubble, but the world outside the bubble would appear to be running in reverse, and vice-versa. This would violate the Second Law, but we're being speculative here :)
ddxxdd wrote:However, if I modified this "anti-physics" machine to make THE REST OF THE UNIVERSE (or at least this planet) go backwards in time, then I would've successfully travelled back in time.

This form of time travelling creates no paradoxes. "Universal Time" doesn't go backwards, only "The-rest-of-the-universe's-time" goes backwards, while your time remains in tact. There will be no copies of you in the "past", and you can kill your grandfather all you like.

Now if only someone else on these fora could come up with other physical methods of time travel, where you can meet your younger self...

In this fora, we obey the Laws of Thermodynamics!
Ok, that's the only theoretical paradox I see. I expect the practical requirements may be unphysical, however :)
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Re: Time travel paradoxes

Postby Tass » Mon Dec 08, 2008 1:48 pm UTC

ThinkerEmeritus wrote:It is difficult to hold a conversation about time travel without having any idea how the time travel would be accomplished. To attack this problem, I will propose general principles for eventually building a time travel machine.

[Tongue-in-cheek mode on] Physics actually contains a mechanism for travelling backwards in time, as first pointed out by Feynman. It does not allow the sci-fi version of jumping to a particular time, but it does allow reversing the direction you are moving in time. So you could in principle go backwards in time, but you would move at the same rate that you would otherwise move forward. Clearly this will limit the willingness of volunteers to remain in motion, but that helps explain the absence of observed time travellers.

If the energy of a particle is reversed in sign, it automatically moves backward in time. This reversal doesn't happen spontaneously, but it can be arranged if you can dispose of the large amount of energy that results. The particle moving backward in time is called an antiparticle, and examples are well-known. Once we have adequate knowledge of the human body, it should be possible to move all the particles in the body together into corresponding negative-energy states, and that body will procede to go back in time. I assume that since the human body generates only classical information it is essentially a classical collection, and if this is not true building the machine will be a great deal harder if not impossible. Moreover, the volunteer must be protected from collisions with all positive-energy matter, which would otherwise fall into the negative-energy states and corrode the volunteer. So if the volunteer is to accomplish anything useful, he/she [hereafter he for brevity] must be returned to a forward-moving body at some time in the past. Hence useful time-travel cannot extend to times before the invention of the time machine, which further explains why time travellers are not yet seen. All the mechanical laws of physics are essentially the same when time is reversed, so there should be no anomalies during the trip. It is not possible to predict how thermodynamics will work during the time travel, but we will assume that since we know the condition of the body at the later time and not at the earlier time, thermodynamics will also reverse for the time-travelling body.

To a body travelling forward in time, like us, the time-traveller will appear to be a body made of antimatter travelling with us in a positive-time direction. The return to forward time-travel will appear to be a person-antiperson pair creation event. I have not explored the possiblities very carefully, but I think that this fact will prevent any serious paradoxes.

So all that is left is for some reader to accumulate the knowledge necessary to build such a machine. In the meantime we should be able to determine in principle if the process is safe, i.e. if there are indeed no possible paradoxes.

If anyone thinks that the machine has already been made secretly, and that we should try to find out about it, I suggest a careful examination of the biographies of all twins.

Have fun. In particular, is the volunteer moving backward still the same person or just a copy? If not the same person, is participating in the experiment, which necessarily involves the disappearance of the original, forward-moving version, a case of suicide? [/tongue-in-cheek mode]

EDIT: I have spent some time thinking about the billiard-ball paradox upthread with this mechanism for time travel. I haven't spotted a rule that would forbid the paradox. Interesting. Could it be that the mechanism works only in quantum mechanics?



That would be like this: I gather a lot of energy, enough to make two older copies of myself, one of anti matter, another of matter. The matter one lives on and continues to age, the antimatter one lives backwards and gets younger and younger. Once he is a perfect copy of me, we anihilate.

Feynman diagram:

Code: Select all

             Energy
Me            )
  \           (
    \        /\
      \    /    \
        \/        \
         )          \
        (             \
         )            Me
      Energy

Trouble is that I would need the information of how my complete state would be in in the older form. The second law is turned upside down for the anti-me part. He rembers being me, he remembers the anihilation istant, and the anxiousness he (I?) felt rigth before it, and he remembers setting the whole thing up. The older matter-me remembers the whole thing including two rather similar anihilations.

But how do I produce an anti-me destined to "become" the exact counterpart to what I will become? I can't. I can't reverse the second law. It is possible that this sort of thing could just happen, that it was non of my doing, but it is roughly as likely as me tunneling through the door, since there is no reason this blob of anti-particles would have to be a me.

With wormholes and so, spacetime could be warped in a way to require that two different spacetime are dependend on each other in this way. Then the equations of motion would have to be solved self-consistently. This should certainly be possible. Trouble is that there would be no 2nd law in such a spacetime, in shorth such a space time would have to be a thermal equilibrium. Therefore I believe that we will never experience it, because anything in thermal equilibrium is not alive. All our paradoxes (paradoxi??) arise because we try to consider time travel while still having life, intelligence, memory, metabolism, "free will",... in short: The second law.

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Re: Time travel paradoxes

Postby ThinkerEmeritus » Tue Dec 09, 2008 4:39 pm UTC

Tass wrote:
That would be like this: I gather a lot of energy, enough to make two older copies of myself, one of anti matter, another of matter. The matter one lives on and continues to age, the antimatter one lives backwards and gets younger and younger. Once he is a perfect copy of me, we anihilate.

Feynman diagram:

Code: Select all

             Energy
Me            )
  \           (
    \        /\
      \    /    \
        \/        \
         )          \
        (             \
         )            Me
      Energy

Trouble is that I would need the information of how my complete state would be in in the older form. The second law is turned upside down for the anti-me part. He rembers being me, he remembers the anihilation istant, and the anxiousness he (I?) felt rigth before it, and he remembers setting the whole thing up. The older matter-me remembers the whole thing including two rather similar anihilations.

But how do I produce an anti-me destined to "become" the exact counterpart to what I will become? I can't. I can't reverse the second law. It is possible that this sort of thing could just happen, that it was non of my doing, but it is roughly as likely as me tunneling through the door, since there is no reason this blob of anti-particles would have to be a me.

....


1. I really like your ASCII Feynman diagram. Perfectly clear and not terribly difficult to draw.

2. What I had in mind is to assume that the second law works forward on the negative-energy state and hence backward in collections of antimatter. Granted that assumption is a stretch, but it isn't impossible. After all, the laws of physics are to high approximation time-reversal invarient (even if not exactly so) and no one has observed a large number of interacting antiparticles to see which way their entropy goes. Thus you make a backward-travelling you and the equipment required to turn you back to forward travelling, so the information needed to duplicate you is available whenever needed. This would have a similar effect as ddxxdd's idea of changing the whole universe except for himself into antimatter, so he is travelling in the opposite time direction as the universe with the second law working in his direction. However, neither seems to take care of the paradoxes, unless I my quick analysis of the billiard-ball paradox was mistaken.
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Re: Time travel paradoxes

Postby Berk and Hair » Tue Dec 09, 2008 5:30 pm UTC

I have a problem with the self-consistency thing. I can understand the idea that you can't change the past because it has already happened, but if you tried to kill your grandfather what would make you fail? The self-consistency principle seems to say that the statement "you can't affect the past in such a way as to make it inconsistant with the present" is a physical law. But the idea of a physical law that works on such a macro* level , that affects a complex series of events (making the decision to kill, firing the gun, your grandfather dying) as a single entity seems wrong to me. Or does the physical law only apply to the actual moment of your grandfathers death (or to the moment he was too severely wounded to ever successfully reproduce)? And if that's the case does that mean you could shoot him ten times, douse him in petrol and set him alight without killing or neutering him?


* for lack of a less wanky word.

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Re: Time travel paradoxes

Postby thoughtfully » Tue Dec 09, 2008 6:26 pm UTC

The point is that the past cannot be changed. If you travel back in time, whatever actions happen in your present also occurred in your past. There is no distinction. Time loops around on itself.
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Re: Time travel paradoxes

Postby ddxxdd » Tue Dec 09, 2008 8:38 pm UTC

Berk and Hair wrote:I have a problem with the self-consistency thing. I can understand the idea that you can't change the past because it has already happened, but if you tried to kill your grandfather what would make you fail?
thoughtfully wrote:The point is that the past cannot be changed. If you travel back in time, whatever actions happen in your present also occurred in your past. There is no distinction. Time loops around on itself.


"The past" is a word that's very loosely thrown around in time travel debates, so let's break down the definitions a little more:

Imagine three scenarios.

Scenario A:
1. Your grandfather is born.
2. You are born
#1 is the past, #2 is the present. The past cannot be changed.

Scenario B:
1. Your grandfather is killed in a brutal fire.
2. You are never born
#1 is the past, #2 is the present. The past cannot be changed.

Scenario C:
1. Your grandfather is born.
2. You are born.
3. One of Randall's comics turns your monitor into a time portal.
4. You go into the time portal, and Earth 2008 becomes Earth 1958.
5. You light a cigarette, drop it, and light your pa's barn on fire.
6. Your grandfather is killed in a brutal fire.
7. You are never born

If you consider your "past" to be the previous numbers in that succession, then you never changed your past. And the past cannot change. Unless the past DID change because you consider event #1 and event #5 to both be in the past, in which case you did change the past because the 2nd past is actually your future, and #4 turns the future into the past, and the past past disappears into nowhere because it was never the past...

The point is, time travel discussions rely heavily on your definition of the "past".

As for whether or not Scenario C results in you living indefinitely in the new past, disappearing, and/or creating a time-loop paradox: it depends highly on the mechanism of time travel.

thoughtfully wrote:A living organism can't survive isolated from the rest of the Universe for long, whichever way time is flowing. It might be possible if the required support systems (say, a planet) and a source of energy is included.

The experience of the subject would be normal with respect to his body and whatever else in within the bubble, but the world outside the bubble would appear to be running in reverse, and vice-versa. This would violate the Second Law, but we're being speculative here :)


"Thoughtfully", what would it take for my own time to go forward at a normal pace while the universe's time went backwards?
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Re: Time travel paradoxes

Postby thoughtfully » Wed Dec 10, 2008 2:10 am UTC

The simplest way would be for the Universe to do its own thing while your time went in reverse. There really isn't any way to distinguish the two. Maybe the Second Law allows this, like our friend who is actually qualified in this subject has suggested :)
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Re: Time travel paradoxes

Postby Berk and Hair » Wed Dec 10, 2008 7:49 am UTC

But suppose it was possible to build a time machine. Suppose you use it to send 100 people back in time to do something to prevent their own birth. You can say they are all going to fail because if they hadn't already failed they wouldn't be there to go back in time. But what actually makes them fail? What physical process (is going to)/(already has) caused them to fail? Saying inconsistencies cannot occur does not answer this question.

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Re: Time travel paradoxes

Postby EdgarJPublius » Wed Dec 10, 2008 8:26 am UTC

the probability of an event that would produce a non-self-consistent paradox is zero.

Say you go back in time to shoot your grandfather. The gun you use will jam or misfire, or you will miss, or get beat up by your own grandfather before you can pull the trigger if you aren't stopped even earlier by any number of things.

Inversely, the probability of events that would stop you from being able to kill your grandfather increase until the probability of you not being able to kill your grandfather is one.

at least, that's what some theories say.
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Re: Time travel paradoxes

Postby Sunsnail » Wed Dec 10, 2008 9:36 am UTC

EdgarJPublius wrote:the probability of an event that would produce a non-self-consistent paradox is zero.

Say you go back in time to shoot your grandfather. The gun you use will jam or misfire, or you will miss, or get beat up by your own grandfather before you can pull the trigger if you aren't stopped even earlier by any number of things.

Inversely, the probability of events that would stop you from being able to kill your grandfather increase until the probability of you not being able to kill your grandfather is one.

at least, that's what some theories say.


That really seems silly. It's almost as if the universe were a magical fairy world where there are intelligent forces preventing you from doing certain things.

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Re: Time travel paradoxes

Postby danpilon54 » Wed Dec 10, 2008 12:43 pm UTC

Its because you have already failed. Causality goes in the forward time direction. Since before you went back in time, you had not killed your grandfather. Even if there is some story of him almost getting killed by somebody looking suspiciously like you, he didnt die. So when you go back, he wont die. Free will is just something people want to be true, like hidden variables in quantum mechanics.
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Re: Time travel paradoxes

Postby ThinkerEmeritus » Wed Dec 10, 2008 5:38 pm UTC

danpilon54 wrote:Its because you have already failed. Causality goes in the forward time direction. Since before you went back in time, you had not killed your grandfather. Even if there is some story of him almost getting killed by somebody looking suspiciously like you, he didnt die. So when you go back, he wont die. Free will is just something people want to be true, like hidden variables in quantum mechanics.


Of course, there is no evidence of travel back in time. When you propose a theory, if someone finds a paradox [in fancy language, a self-inconsistency], the theory must be modified or abandoned. For the theory that time travel is possible, perhaps abandoning the theory is the only choice. After all, "Free will Time travel is just something people want to be true, like hidden variables in quantum mechanics."

[Sorry about the misquote. I couldn't resist. Please don't go back and kill my grandfather. That wouldn't be fair to my brother.]
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Re: Time travel paradoxes

Postby Berk and Hair » Wed Dec 10, 2008 6:34 pm UTC

Sunsnail wrote:That really seems silly. It's almost as if the universe were a magical fairy world where there are intelligent forces preventing you from doing certain things.


'Whenever people agree with me I always feel I must be wrong.' - Oscar Wilde

Meaning no disrespect to Sunsnail, his comment, which is in agreement with my own, makes me question my own reasoning. The fact that something is silly is not a valid reason for assuming it to be false. Just look at the platypus. Perhaps my example (100 guys trying to kill their grandfathers and failing) is what Daniel Dennett would call an 'intuition pump'.

But there still needs to be a physical law or mechanism by which the attempt would fail and the impossibilty of time travel is the only thing I can see which would fulfill this need.* As I said in my first post, the statement "you can't affect the past in such a way as to make it inconsistant with the present" seems too 'macro' to be a physical law.

danpilon54 wrote:Its because you have already failed. Causality goes in the forward time direction. Since before you went back in time, you had not killed your grandfather. Even if there is some story of him almost getting killed by somebody looking suspiciously like you, he didnt die. So when you go back, he wont die. Free will is just something people want to be true, like hidden variables in quantum mechanics.


I do not believe in free will (if physical determinism is true then no free will. if physical determinism is not true then it is because of sub-atomic randomness. unless somebody seriously makes the claim that the mind can influence sub-atomic randomness then this doesn't offer the possibility of free will either) so you can't exactly dismiss my arguement as being driven by pro free will prejudice. There still needs to be a mechanism to stop the contradiction occuring.


*Discussing the consequences of time travel if time travel is impossible is as daft as discussing what a circle would look like if pi = 42.

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Re: Time travel paradoxes

Postby thoughtfully » Wed Dec 10, 2008 7:38 pm UTC

Berk and Hair wrote:*Discussing the consequences of time travel if time travel is impossible is as daft as discussing what a circle would look like if pi = 42.

Would it be awfully cruel of me to mention hyperbolic geometries?
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Re: Time travel paradoxes

Postby Berk and Hair » Wed Dec 10, 2008 8:28 pm UTC

thoughtfully wrote:
Berk and Hair wrote:*Discussing the consequences of time travel if time travel is impossible is as daft as discussing what a circle would look like if pi = 42.

Would it be awfully cruel of me to mention hyperbolic geometries?


If god had intended us to study non-euclidean geometry he'd have made the world round.

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Re: Time travel paradoxes

Postby danpilon54 » Thu Dec 11, 2008 1:23 am UTC

ThinkerEmeritus wrote:
danpilon54 wrote:Its because you have already failed. Causality goes in the forward time direction. Since before you went back in time, you had not killed your grandfather. Even if there is some story of him almost getting killed by somebody looking suspiciously like you, he didnt die. So when you go back, he wont die. Free will is just something people want to be true, like hidden variables in quantum mechanics.


Of course, there is no evidence of travel back in time. When you propose a theory, if someone finds a paradox [in fancy language, a self-inconsistency], the theory must be modified or abandoned. For the theory that time travel is possible, perhaps abandoning the theory is the only choice. After all, "Free will Time travel is just something people want to be true, like hidden variables in quantum mechanics."

[Sorry about the misquote. I couldn't resist. Please don't go back and kill my grandfather. That wouldn't be fair to my brother.]


I was assuming that time travel was possible, not suggesting it. If you just assume it's not possible then why are we posting in this thread?
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Re: Time travel paradoxes

Postby swik » Thu Dec 11, 2008 8:07 am UTC

If you traveled back in time...and everything was just how it existed at that moment, save for the addition of you, wouldn't that be a violation of the conservation of energy or whatever?

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Re: Time travel paradoxes

Postby jaap » Thu Dec 11, 2008 9:06 am UTC

swik wrote:If you traveled back in time...and everything was just how it existed at that moment, save for the addition of you, wouldn't that be a violation of the conservation of energy or whatever?


But what if time travel wasn't like an instant transportation across time, but that you had to pass through all the intermediate moments as well? Then you'd have your old self, your new self, and your self that is travelling back in time. The latter would have negative energy/mass as it were to compensate for your forward-travelling new self.

It is not only the time dimension we should be worried about. Although this leads to mass/energy conservation, it still in effect will allow mass to be instantly moved from one place to another, which could lead to some interesting gravity effects.

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Re: Time travel paradoxes

Postby Ashbash » Thu Dec 11, 2008 3:11 pm UTC

I've always thought that if a universe can prevent its inhabitants from travelling at faster than light speed, and violating mass/energy conservation, then it might not be that far of a stretch to theorise that a universe can prevent time travel paradoxes.

Also, if 'flipping' to antimatter can get a human to travel at -1 second per second, then all you'd need to do is build an antimatter spaceship and travel at really high velocities rel earth in order to get to the preferred point in time.


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