Post your favorite solution to time-travel paradoxes

[Magic_Man]

I recently read "Time Traveler," a biography of the physicist Ronald Mallett, who vaguely recently discovered a new way to manipulate time and space with lasers. He's actually trying to build a working "time machine" in a research lab. It actually wasn't a great book, but it's a quick read if you're interested in seeing how a physicist progresses through the different stages of academia.

A better book on time travel is "Time Travel In Einstein's Universe," which covers many different ways that time travel is allowed in relativity and our current scientific understanding. There are some ideas for how to build time machines, but some of them are pretty ludicrous.

At any rate, whenever you discuss time travel, the question of paradoxes arises. What happens if you go back and kill your grandfather? Or, slightly more twisted, can you become your own father? A more troubling question is "where are all the time travelers?" If it were possible to travel through time, why do we not see visitors from the future? A number of possible explanations have been offered:

1) Time travel to the past is actually impossible, despite what some physicists have discovered. In other words, there is some aspect of physics that they do not understand which prevents backwards time travel. You can travel forward only (we know at least this must be true because it has actually been observed, using high-speed jets).

2) Stephen Hawkings Chronological Protection Conjecture. This is similar to (1). In this theory, physics somehow prevents time travel from being invented. At the moment the machine is switched on, for example, it is "accidently" destroyed.

3) In a similar idea, physics somehow prevents the paradoxes from occurring. You can't kill your grandfather, because at the last minute he moves out of the way, or your gun jams. I think this explanation is ridiculous, because you're obviously still affecting the past in SOME way.

4) Society is destined to destroy itself before time travel is invented. I think that to believe this, you have to believe in determinism, but I haven't thought too much about it.

5) When you travel backwards, you actually go to an alternate universe, as in the "many worlds" theory of quantum physics. So you can kill your "grandfather", but it doesn't affect YOU, because you are not the actual grandchild of the person you killed.

6) Backwards time travel is possible, but only to the moment that the machine is turned on. So if we invent time travel in 2167, we can only travel back as far as 2167. This explanation is consistent with the math that Ronald Mallet and others have worked out, and seems to me to be the most plausible explanation. It doesn't account for grandfather paradoxes, but it does explain why we don't see time travelers.



There are lots more theories, but this is a good start. What's your favorite "solution" to the paradoxes of time travel? Do you think that we will ever successfully travel backwards in time?

[yukoncpa]

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"where are all the time travelers?"

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I figure at some point in the future, everyone is really hot looking and having wild, orgiastic sex. No need for time travelers to head back any further in time.

[arahant]

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"where are all the time travelers?"

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I figure at some point in the future, everyone is really hot looking and having wild, orgiastic sex. No need for time travelers to head back any further in time.

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Best response possible. I think we may as well just lock this thread now.

[Insp. Clue!So?]

I think the problem is, "the past" doesn't actually exist so there's nothing to go back to. Yes you can see events from the past but those are reflected photons. Statistically it is possible all those events from 1954 or whatever could reoccur if countless trillions of atoms suddenly wooshed over to one side or the other one day in a fit of random madness, but something like that probably wouldn't happen in a billion lifetimes of the universe.

Of course the above hypothesis/half-baked notion depends on a particle concept of the universe, at least above the Planck scale, so if any real physicist would like to come and burst my bubble then by all means please do so.

[TimWillTell]

30 years ago I read "The man who folded himself."
It's a story about a man who invents a time-travelling machine.
Before he tries it, a stranger comes by his house and warns him not to use it.
Off-course he tries the machine and he gets in extreme troubles.
To get out of the trouble, he decides to go back in time to warn him and tell him not to try the machine...

I liked the book very much, the only downside it had, once you have read it, you will never be able to believe that it is possible to travel back in time; something even my physics-teacher wasn't able to do.

(so it somehow degrades movies like Back to the Future, and Terminator; that's the downside.)

[m_the0ry]

Causality is obviously the biggest logical barrier to time travel. Yes multiverse theory theoretically gets around causality, but it can be shown that no meaningful information exchange can exist between universes. Nothing as complex as a human could ever traverse universes without being obliterated.

[Metric]

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2) Stephen Hawkings Chronological Protection Conjecture. This is similar to (1). In this theory, physics somehow prevents time travel from being invented. At the moment the machine is switched on, for example, it is "accidently" destroyed.

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It is arguable that there is some indirect theoretical evidence for this -- however, it's really not much more than a guess so I don't find it terribly convincing.

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3) In a similar idea, physics somehow prevents the paradoxes from occurring. You can't kill your grandfather, because at the last minute he moves out of the way, or your gun jams. I think this explanation is ridiculous, because you're obviously still affecting the past in SOME way.

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Actually, this one seems right to me -- it goes by the name "Novikov self-consistency principle."
http://en.wikipedia.org/wiki/Novikov...ency_principle
This is very consistent with my understanding of covariant quantum information theory -- there is just one "state" of the universe (representing all times), and what we percieve as "now" is just information existing on a "slice" of the full state. Time travel might exist in such a picture, but the history will always be self-consistent, basically by definition. Any "changes" you the time traveller make are not changes at all -- you were already there in the past, and any influences you make were already taken into account before you entered the time machine.

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4) Society is destined to destroy itself before time travel is invented. I think that to believe this, you have to believe in determinism, but I haven't thought too much about it.

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This could be true, but I don't see any reason that it must true.

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5) When you travel backwards, you actually go to an alternate universe, as in the "many worlds" theory of quantum physics. So you can kill your "grandfather", but it doesn't affect YOU, because you are not the actual grandchild of the person you killed.

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This one seems very cheap to me, and I don't know of any physics-based evidence for it at all.

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6) Backwards time travel is possible, but only to the moment that the machine is turned on. So if we invent time travel in 2167, we can only travel back as far as 2167. This explanation is consistent with the math that Ronald Mallet and others have worked out, and seems to me to be the most plausible explanation. It doesn't account for grandfather paradoxes, but it does explain why we don't see time travelers.

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This is one is probably true for any "reasonable" time machine people have yet thought about, but it's independent from the Novikov principle. They can both be true.

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There are lots more theories, but this is a good start. What's your favorite "solution" to the paradoxes of time travel? Do you think that we will ever successfully travel backwards in time?

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The most obvious way to do time travel involves wormholes, which would imply unimaginably advanced technology -- detailed control over macroscopic black holes, or the like. So while it might be possible, the technology required is so far removed that any predictions I could make would be meaningless.

[Quark]

http://video.google.com/googleplayer.swf...erMode=embedded
http://video.google.com/googleplayer.swf...erMode=embedded

Link to 2 awesome time-travel documentary.

[Piers]

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At any rate, whenever you discuss time travel, the question of paradoxes arises. What happens if you go back and kill your grandfather? Or, slightly more twisted, can you become your own father?

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There is only a paradox if you have bought into the illusion of free will. If you go back in time you can only do the things that you did. You can only ‘repeat’ the past not change it.

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1) Time travel to the past is actually impossible, despite what some physicists have discovered. In other words, there is some aspect of physics that they do not understand which prevents backwards time travel. You can travel forward only (we know at least this must be true because it has actually been observed, using high-speed jets).

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What is time travel? Are we talking about an actual object going back in time, or just information?
What if an object identical to an object currently existing spontaneously came into existence a thousand years ago? Can we say the object went back in time? Would it matter if object did not disappear from the current time?
What about if someone a living a thousand years ago would to suddenly have my memory and personality. Could you say I had traveled back in time?
Is there any other sense in which one can say something has gone back in time?

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2) Stephen Hawkings Chronological Protection Conjecture. This is similar to (1). In this theory, physics somehow prevents time travel from being invented. At the moment the machine is switched on, for example, it is "accidently" destroyed.

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Duh!

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3) In a similar idea, physics somehow prevents the paradoxes from occurring. You can't kill your grandfather, because at the last minute he moves out of the way, or your gun jams. I think this explanation is ridiculous, because you're obviously still affecting the past in SOME way.

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You cannot kill you grandfather because you did not kill your gradfather (unless you did of course). As I said before you are getting confused about what free will is.

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4) Society is destined to destroy itself before time travel is invented. I think that to believe this, you have to believe in determinism, but I haven't thought too much about it.

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Duh!

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5) When you travel backwards, you actually go to an alternate universe, as in the "many worlds" theory of quantum physics. So you can kill your "grandfather", but it doesn't affect YOU, because you are not the actual grandchild of the person you killed.

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I guess this is the main way of making all those Sci Fi time travel stories sound plausible..

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6) Backwards time travel is possible, but only to the moment that the machine is turned on. So if we invent time travel in 2167, we can only travel back as far as 2167. This explanation is consistent with the math that Ronald Mallet and others have worked out, and seems to me to be the most plausible explanation. It doesn't account for grandfather paradoxes, but it does explain why we don't see time travelers.

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Kind of I guess.

You could for instance cause someone to go back in time in the following way.
Select a random adult human. Tell him your going to send him into the past and zap him unconscious.
Make a note of everything about the human, genetic fingerprint, image of the exact state of the brain etc.
Then wait a thousand years, and then create a clone of the human. Grow the human in the tank and give him all the same memories and experiences as the earlier human. When he is in the exact state as the earlier human, wake him up, then tell him you are going to send him into the past. Then kill him, destroying the body in the process.

Vola! You have just sent someone a thousand years into the past.

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? Do you think that we will ever successfully travel backwards in time?.

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No.

[MaxWeiss]

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"where are all the time travelers?"

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I figure at some point in the future, everyone is really hot looking and having wild, orgiastic sex. No need for time travelers to head back any further in time.

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Best response possible. I think we may as well just lock this thread now.

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