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Ch. 4 - Alternative Histories 
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Post Ch. 4 - Alternative Histories
Ch. 4 - Alternative Histories

Please use this thread for discussing Ch. 4 - Alternative Histories.



Sat Oct 09, 2010 11:43 pm
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Post Re: Ch. 4 - Alternative Histories
Question about Heisenberg uncertainty, esp. as described on p.80. The idea that in order to make an observation, you have to interact with the object such as emitting a photon and therefore affect the measurement, doesn't seem all that mysterious when put that way. But then the leap to saying the electron therefore exists simultaneously in many locations based on probabilities does seem more mysterious. I'm sure I'm leaving out some complications, but my question is whether the inherent inability to measure is essentially the only thing leading to this interpretation of simultaneous position and paths (i.e. the discussion on Feynman's "alternative histories" approach beginning on p. 74).

Given Hawking's view of model-dependent realism, aren't we just saying that it is mathematically convenient to say that particles take all possible paths with certain probabilities, but we can't say this is any way corresponds to a picture of reality. And then he is extending this idea to the universe as a whole, so that all possible paths (histories) can be said to exist.



Sat Nov 06, 2010 10:46 am
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Post Re: Ch. 4 - Alternative Histories
And if we say that in the double-slit experiment that particles take all possible paths, which include "paths that visit the restaurant that serves that great curried shrimp" (p. 75), presumably with a probability of essentially zero, what are we really saying? That anything that can happen does happen? Might happen? Are we making stuff up to try to describe the math?

On p. 79, he says that for large objects "the only destination that has a probability effectively greater than zero is the destination predicted by Newtonian theory."



Sat Nov 06, 2010 11:19 am
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Post Re: Ch. 4 - Alternative Histories
Dexter wrote:
But then the leap to saying the electron therefore exists simultaneously in many locations based on probabilities does seem more mysterious. I'm sure I'm leaving out some complications, but my question is whether the inherent inability to measure is essentially the only thing leading to this interpretation of simultaneous position and paths (i.e. the discussion on Feynman's "alternative histories" approach beginning on p. 74).

You're basically wondering about the possibility of a hidden-variable theory of quantum mechanics, in which something more concrete underlies the mathematics of probabilities generally used to describe quantum mechanics. A finding called Bell's theorem shows that a hidden-variable theory won't work. This Wikipedia article provides more details, but it's very difficult for a non-physicist to understand.



Sun Nov 07, 2010 1:29 am
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Post Re: Ch. 4 - Alternative Histories
JulianTheApostate wrote:
You're basically wondering about the possibility of a hidden-variable theory of quantum mechanics, in which something more concrete underlies the mathematics of probabilities generally used to describe quantum mechanics. A finding called Bell's theorem shows that a hidden-variable theory won't work. This Wikipedia article provides more details, but it's very difficult for a non-physicist to understand.


Yes, I remembered reading something about that, but only a superficial version of it -- that's Einstein's "spooky action at a distance" right? That's where QM gets really mind-blowing. I've read several popular books on physics -- I recall Brian Greene's book had a description of it -- but I need to read more on that, at least as far as I'm able to. Any good popular or semi-popular sources? I even have the (perhaps too optimistic) idea that one day I will learn at least some of the math to go a little further. I still have a decent if a bit rusty knowledge of calculus, so I'm always looking for good book beyond the standard popular treatment but less than a textbook.



Sun Nov 07, 2010 5:03 am
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Post Re: Ch. 4 - Alternative Histories
Are you referring to parallel universes, where each one has its own timeline and if you're still alive in a particular universe, you're doing something different in each one? Each time you have a choice, there blooms a new universe, one for each possible choice. Makes for an awful lot of universes (universi?)



Sun Nov 07, 2010 11:29 am
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