Quickie: Time Travel and P=NP

Following on the Quickies on P vs. NP and the Novikov Principle, if time travel is possible, then P=NP--and the encryption we rely on is hosed!

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[michaelmccarty]

I still don't understand. I think I need to take a semester of encryption first.

[emperorpicard]

I slightly disagree with this.

You can think of types of time travel that are still self consistent, one obvious one is the branching timeline solution (many worlds).

Another is putting some constraint on time travel, such as to travel back in time you also have to travel in space, so for every 1 day you travel back you have to travel 1 light day in space. This would make it impossible to effect local events in the past even with time travel.

Another solution is that you HAVE TO put the same drawing on the paper, i.e. no free will. I know we are all uncomfortable with that but it is a solution, the good news is that the constrains on free will would only apply to the past.

Novikov himself supports this point of view, noting that physics already restricts your free will every day.

Basically although your scenario is impossible, that does not mean that time travel in general is impossible, only constraint.

[SuperAtheist]

I don't think time has a rate. That's why the units cancel.

[theatheistpaladin]

Time travel would undo itself if it affects only one universe and doesn't spawn multiverses. If you go back in time to prevent event X, then preventing X would mean you would not travel back in time, therefore event X would happen... there is no way to resolve that logically without invoking a predestination paradox. Then it becomes an unanswerable question of how that was bootstraped. If there is a multiverse, then when you go back to your own time either you will be unable to do so or return to a world where event X still happened.

[masterofmuppets]

Jeez what did you study to become Super Brain.

[NowisEvollovetion]

Time travel is in fact impossible. And why is that?

Well, time does not in fact exist other than as a concept in mans mind.

I mean, what time was it before the very first clock was invented?

I will tell you. It was (is) now then (now), and it is still now, now.