21 June 2005

The Hubris Of The Time Travel Paradox

From BBC:

Researchers speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is “complementary” to the present.

In other words, you can pop back in time and have a look around, but you cannot do anything that will alter the present you left behind.

The new model, which uses the laws of quantum mechanics, gets rid of the famous paradox surrounding time travel.


The main headache stems from the idea that if you went back in time you could, theoretically, do something to change the present; and that possibility messes up the whole theory of time travel.


According to Einstein, space-time can curve back on itself, theoretically allowing travellers to double back and meet younger versions of themselves.

And now a team of physicists from the US and Austria says this situation can only be the case if there are physical constraints acting to protect the present from changes in the past.


“Quantum mechanics distinguishes between something that might happen and something that did happen,” Professor Dan Greenberger, of the City University of New York, US, told the BBC News website.

“If we don't know your father is alive right now - if there is only a 90% chance that he is alive right now, then there is a chance that you can go back and kill him.

“But if you know he is alive, there is no chance you can kill him.”

To my knowledge, the ‘time-travel paradox’ has always been cast like this in science fiction—the laws of causality forbid you from altering ‘something’ in the past that alters the progression of human (or alien) history otherwise you might not ever go back in time to make the change resulting in the necessity of the change simultaneously both happening and not happening. Rather mind-bending. Even, well... paradoxical. :-P

Taking a step back from this human-centered view of the universe, in my mind the principles described in the article make an argument against time travel in any form. They posit that the space-time curvature/intersection necessary for Einsteinian time travel “can only be the case if there are physical constraints acting to protect the present from changes in the past.” But, one of the most fundamental tenets of science is that measuring a system inevitably alters the state of that system. If a theoretical time traveler looked at something in the past, his eyes would absorb photons that would otherwise strike an object in that past. The simple act of ‘entering’ the past would disturb air molecules from the trajectories they traveled in his absence. Thus, any time travel event would cause changes, and thus by the logic presented in the article, no time travel is thus possible. Only our prideful assumptions permit us to assume that the only changes that matter in our hypothetical mucking about in the past are those that perceptibly affect human events.

But, I'll give you... SF would be a lot more boring if everybody held to a strictly fundamental position on this. ;-)

More information on quantum measurement here; feel free to skip the heavy mathy stuff and start here. The Copenhagen interpretation is one (still rather imperfect) attempt at a resolution for all of these quantum questions. And this is something I ran into while I was Googling around that's even more technical and paradoxical and confusing than the other stuff, but at the same time is really neat.

(Hat tip: Strife)

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