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

Primer

[kylben]

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

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: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.

Re:idea of time travel

[PagosaSam]

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