Saturday, June 18, 2005

no more grandfather paradox

Quantum physicists have decided that the rules of quantum theory make the grandfather paradox go away. This time-travel paradox asks what would happen if you went back in time and killed your grandfather (quantum physicists are always killing things in their thought-experiments... poor Schrödinger's cat) before he met your grandmother (apparently quantum physicists draw the line at killing grandmothers)... therefore your parents and you yourself would never be born, so how could you have gone back in time?

The current theory matches what Heinlein said all along: nature abhors a paradox, and you obviously didn't go back and kill your grandfather, because you're here. Now they're saying that you couldn't if you tried - that the collapse of probability functions does something irreversible to the spectrum of possibilities. In Feynman's sum over histories approach, the history that actually happened is so much stronger than all of the other possibilities that it over-rules any possible changes.

If you think about Cramer's Transactional Interpretation of Quantum Mechanics this makes a lot of sense as well (more so than most things in quantum theory). It's mixing metaphors a bit, but in essence this interpretation allows a back-channel for information from the present to guide the collapse of probabilities in the past, and in the sum-over-histories view the 'future history' in which the waveform has collapsed has a much greater amplitude than any of the others. You can't meddle in the past, because you didn't meddle in the past. This interpretation strongly supports the idea of a forward-pointing arrow of time, but ironically it relies on waves going backwards in time to do so... I'll leave that one for the philosophers.


Blogger Tom said...

But . . . doesn't this invalidate the Terminator movies? *sigh* Hard to accept.

Blogger Foobario said...

When I was in college I took a course in semiconductor physics, and I was deeply disappointed to find that contrary to Scotty's claims, transparent aluminum just isn't going to happen. Now this. Plus no flying cars. Where are the frickin flying cars?

I think we're heading for a future that looks more like Logan's Run than Blade Runner.
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