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

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.

[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!

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.