The Possibility of Paradox-Free Time Travel

relliker writes in with word of a paper up on the ArXiv by Seth Lloyd and co-workers, exploring the possibility that "postselection" effects in non-linear quantum mechanics might allow paradox-free time travel. "Lloyd's time machine gets around [the grandfather paradox] because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway... Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. ... Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear."

Comments

Time Cube?

[Lythrdskynrd]

http://timecube.com/ ... obviously.

Re:Time Cube?

[hvm2hvm]

fuck, it looks like a neural network with a few weeks of training on conspiracy theory material has been put on repeat

Caution about ArXiv

[vsage3]

I have neither the capacity nor the will to vet the paper, but it should be noted that ArXiv is not peer reviewed. While experimentalists use it as a place to publish pre-prints of their papers and will typically only put them up after the papers have been accepted, but theorists use the medium as a substitute for publishing and so many wacky and untrue claims get put up there.

Re:Caution about ArXiv

[martin-boundary]

Nonsense. Appeal to authority is one of the standard logical fallacies.

The problem with this in the case of theoretical papers is that theory, quite often, is complicated. So complicated that there will be bugs in the statements of the proofs, and ambiguities or inaccuracies in the statements of the theorems. These are picked up by peers when they review a paper formally, but usually skipped when they peruse the paper cursorily.

What's worse, when a paper is coauthored by a well known researcher and one of his students, it's highly likely that the student did all the work and was merely pointed in the right direction by the other. Then the authoritative paper you read on xxx is no better in quality than a paper written by an unknown.

It's ok to feel that appeal to authority works surprisingly well... if your standards are so low. But some people have higher standards and make a clear distinction between formal peer review and unpublished dissemination.

Time travel leads to Parallel universes that make

[Joe The Dragon]

Time travel leads to Parallel universes that make paradox not happen in the one you left.

It's the Dark Side

[greyworld]

Am I the only person who has noticed the Authors name? Seth Lloyd = Sith Lord I think we should be very cautious of these findings.

And Back to the Future.

[khasim]

The problem with all of those approaches is that they assume a "meta-time" (even if not stated as such) that will alter the PRESENT based upon changes in the FUTURE.

That's how a photograph that you have right (taken in the future) now will change based upon events that have not happened yet.

Once you get past that, you understand that there is no "grandfather paradox". If it exists in the current time then it exists in the current time. The future will not reach back and "clean up" the present to make it more acceptable to the future.

Obligatory cartoon linkage:
http://www.smbc-comics.com/

Re:And Back to the Future.

[Schadrach]

So, what you are suggesting in the case of the grandfather paradox is that you kill your grandfather, are never born, and yet continue to exist unscathed? Essentially being a causeless effect, or rather an effect that causes it's cause never to have occurred?

Essentially the opposite of a closed temporal loop where something is it's own cause.

Of course the SMBC leaves out the third possibility: You go back in time and only change things that were unintended, causing you to not notice any changes because they "were always like that" as of the moment you made said changes. But that of course assumes that changing something that would effect you in some way actually does effect you, and not cause you to live without ever being born (for example).

Re:And Back to the Future.

[Cylix]

The post selection method would be...

You killed the man you thought to be your grand father, but it turns out you are from an illicit relationship. Your grand mother quickly remarries and the man assumed all the roles the other fellow would have done.

That version of the grand father post selection paradox can go soap opera silly really fast. It would get really strange if everyone you kept killing in your family tree resulted in discovering that each generation was conceived in a series of illicit relationships. Take that days of our lives!

Re:And Back to the Future.

[tius]

I think the problem is that everyone assumes that time is essential to processes within the universe. There's nothing that I've ever read or studied that suggests reasonably that this assumed correlation is actually real.

Consider that if time is real, and not fundamental to processes (i.e. the correlation is not causal) then it is entirely possible to travel back in time and kill your grandfather without self destructing. This is because in the stream of processing your existence, all the important stuff has already occurred in the process dimension.

But looking around this idea it seems more likely to me that time is an illusion as far a real dimension is concerned. It strikes me more as an analog to temperature; i.e. it's a statistical like reference. Do we really experience the passing of time or do we really just have a sense of passing of process?

How do we measure time? By change in processes. You can pull in relativity and still see that the effects on time are purely due to the real effects on the spacial dimensions. The relative dimensional changes in space lead to changes in the processes used to measure time.

I'm not sure if this is true, but if it were it eliminates the absurdity of the grandfather paradox.

Dress it up!

[Beelzebud]

You can dress up pseudoscience with a bunch of equations, but tell me how this is based on any type of actual science. If this is science, then Deepak Chopra must be an actual genius...

I'm not an expert but it does seem like a lot of physicists are just lost in their own little worlds. I realize science is a process, but spending valuable time "researching" time travel, before we can even explain what time or even gravity is, seems like skipping over the hard work to spend time on "fun stuff".

Re:Dress it up!

[Lifyre]

Having been part of the physics community for a few years (got a BS in it for some reason) I can say those little worlds often result in some useful science. Usually when someone not in the fantasy land looks in sees that one or two things that guy is working on might have merit and looks into it. Who knows if one of these guys working on time travel might actually figure out what time is? If we don't know what time or gravity even is who is to say that this work might not be instrumental in figuring it out? As long as it is a minority working on the fantastical, science will still make progress with a few boosts here and there by some crazy idea that actually works.

Re:First Post?

[maxwell demon]

Has nobody anything snarky to say?

They are too busy trying to build postselection time machines. Expect to lose your first post status as soon as one of them succeeds.

Primer

[kylben]

The movie "Primer" had an interesting take on avoiding paradoxes. http://video.google.com/videoplay?docid=3909854615539675694# (entire movie online)

Re:Sign me up!

[ThePangolino]

But if I'm going BACK in time, I'm taking some aspirin, toothpaste, deodorant, and toilet paper with me. I hope the machine is big enough.

Don't forget your towel you insensitive clod!

Fact

[Charliemopps]

“The Encyclopedia Galactica has much to say on the theory and practice of time travel, most of which is incomprehensible to anyone who hasn’t spent at least four lifetimes studying advanced hypermathematics, and since it was impossible to do this before time travel was invented, there is a certain amount of confusion as to how the idea was arrived at in the first place. One rationalization of this problem states that time travel was, by its very nature, discovered simultaneously at all periods of history, but this is clearly bunk. The trouble is that a lot of history is now quite clearly bunk as well.”

Satruday Morning Breakfast cereal Anticipated this

[goombah99]

Yesterdays SMBC had a good point:

You can not have any motivation or objective if you are going to travel otherwise the act of time travel is a paradox. SMBC put it thusly: if you are travelling to change some outcome, and you succeeded, you would not have had the motivation to time travel to make that change.

SMBC's conclusion was that only nitwits have the capacity to time travel and the fact that there seem to be so many confirms that time travel must be going on right now.

But another way to say this is, you can only choose objectives that either already happened in your past or are inevitable no matter what you do.

For example, You could however travel with the objective of sinking the Titanic, but not the objective of preventing the sinking. If you saved the titanic, it would never occur to you to try to save the titanic.

For example, If your objective was to save Abe Lincoln and you succeeded, then it never would have occurred to your pre-travel self that you needed to go back and save abe lincoln.

What all this adds up to I think is that time travel is still forbidden but observational time travel-- gathering information-- is not forbidden.

THere is an interesting proof regarding the computability of any proposition by David Wolpert that shows time travel is forbidden unless the information you gain by doing so is probabilistic or faulty. That is he proves rigorously that it is not possible to answer any arbitrary true/false question about the past with perfect fidelity. Thus time travel that preserves information with fidelity is forbidden. Error prone time travel is however allowed.

Re:idea of time travel

[PagosaSam]

You know, that a funny thing about infinities. You have room for everything. ;-)