The Possibility of Paradox-Free Time Travel 421
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."
Time travel leads to Parallel universes that make (Score:5, Insightful)
Time travel leads to Parallel universes that make paradox not happen in the one you left.
And Back to the Future. (Score:5, Insightful)
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/ [smbc-comics.com]
Dress it up! (Score:5, Insightful)
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".
Or... (Score:2, Insightful)
Or, we go with the simple, elegant solution to the problem...it's not possible.
Re:Dress it up! (Score:5, Insightful)
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:And Back to the Future. (Score:5, Insightful)
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. (Score:5, Insightful)
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:Time Cube? (Score:2, Insightful)
"after reading 1 page of that site I NEED SERIOUS MEDICATION"
Call the webmaster, you can probably get a pretty good deal since he's obviously not using his..
Time travel never involves paradoxes (Score:4, Insightful)
The concept of a paradox is entirely a human concept - in other words, it's in the eye of the observer. The universe wouldn't "classify" you going back in time and killing your great-grandparents before you were born as a paradox, simply because the universe is not an observer. It would happen - so what - "It is what it is". That would just be part and parcel of the way the universe works in that particular case.
Attempting to say that this would result in a paradox as far as the universe is concerned is anthropomorphizing the universe to an absolutely unforgivable degree. Sure, it makes for a good time travel story, but the universe won't lose any sleep over it, any more than it does for me writing "The next phrase is false." "The previous phrase is true." "Both the previous phrases are true" "The previous phrase is true" There's no paradox. The universe doesn't suddenly go wonky, and cats mate with dogs, etc.
Pseudoscience or not, it's still interesting (Score:4, Insightful)
I'm completely with the parent.
While I don't think that you can call pseudoscience the exploration of the implications of a theory that are unlikely to be possible in practice, let's assume it's pseudoscience. So what? It was pretty interesting read, at worst it will serve as an inspiration to some science fiction author. It's interesting. You know why? Because it is "fun stuff"! Also, as the parent stated, exploring the theoretical possibilities provides better understanding of the model, allows you to improve the model and might allow you to find the boundaries where the model stops being correct. Also, making advances in the part that don't apply in practise might improve the understanding of the practical part of the model. Infinitesimals don't seem to exist in our universe, but the models that explore their properties closely have been the basis for most of the physics.
Also, I might not understand the article completely, but it, along with another report that was posted to Slashdot less than an year ago, seems to show a method of time "travel" that doesn't allow to send information back in time at all. Seems reasonable, and not against anything that I know about the world I'm living in. It would completely blow out my idea of time -- I'm a firm believer that only the current moment exists, and you can't affect or travel to previous ones, and that other interpretations of time are merely implementation details of the physical models we use -- but these results wouldn't be against any physics I know. Also, even if there is an experiment that confirms that this paper isn't bullshit, and it is empirically proven that this kind of time "travel" is possible, the results won't have a single interpretation. I wouldn't be surprised even if someone builds another model that doesn't involve any time travel that explains the same empirical results.
Yes, someone needs to verify the premises and the conclusions, I don't have good enough knowledge to do that myself on the first read, but I didn't see any mistakes pointed out by the GP, only baseless claims, so I'd rather go with the article and/or the paper. I have a direct question for the GP: We have no idea what time is, OK. Suppose that our current theoretical model allows for time travel (which would seem to be the case unless the article is full of mistakes). Are you denying that testing them would allow us to be closer to understanding what time is?
If (Score:3, Insightful)
If time travel existed at some point in the future, we would have had evidence of its existence in the past...
Re:Time travel never involves paradoxes (Score:2, Insightful)
While the concept of a paradox is indeed about our language and logic, the point is that theories about the universe are also just in our language and logic. The point is that it's possible to say nonsense, and sometimes such nonsense shows up by the statement (or collection of statements) being paradox. So basically time travel paradoxes mean that any description of the universe which involve classical causality and time travel are internally inconsistent, and therefore cannot be used to describe the real world. The question if it is possible to have a theory with time travel, but without paradoxes is the question if any theory which includes time travel (and causality) can be internally consistent and therefore a possible description of the real universe. If there are no consistent theories with time travel, then time travel doesn't exist - not because the universe cares about our language and logic, but because time travel, which itself is a concept which exists only inside our language and logic, and would have to be mapped to our observations of the outer world, would be a meaningless concept. Describing the real world with meaningless concepts is futile, therefore we have to demand that our concepts we use to describe the world are free of internal inconsistencies, not because the world would care about those inconsistencies, but because or description of the world with those concepts would not work.
Or in short: A useful description of reality is free of paradoxes not because nature cares about our descriptions, but because descriptions with paradoxes can't actually describe anything, and therefore especially cannot describe reality.
Re:Caution about ArXiv (Score:5, Insightful)
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.
Re:If (Score:2, Insightful)
Only if time travel allows you to go back into the past arbitrarily far.
YES!! (Score:0, Insightful)
I love these sorts of articles...the ones that allow Slashdot members to pretend they know something about advanced physics, etc. Monday morning quarterback? How about monday morning scientist.
Seriously...half the people offering opinions probably read A Brief History of Time and that's the extent of their knowledge...that won't stop them though from acting like they know something.
"During the week I do desktop support...but in my evenings I comment upon research in advanced theoretical physics. Did I study that at school? Nope...but I stayed at a Holiday Inn once"