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Saturday, September 4, 2010

Listening to quantum mechanics

The Right Coast blog takes a break from Law, Politics and Culture to weigh in on the interpretation of quantum mechanics. My comment is, basically, that we should see what QM has to say first.

One of the lessons I learned from Albert Einstein is, "Believe the theory."

So what does QM say about this? What do we believe when we believe the theory? Basically, Dan Simon's Choice 3:

QM says that probability is preserved. If you start with a universe with total probability 1.0, you will end with a total probability 1.0. Therefore, there is no creation of parallel universes. (I make a distinction between "creating parallel universes" and the existence of a superposition of states. For one thing,as I discuss below, we already accept the second, but not the first.)

QM says that if you make a measurement with two outcomes, state A and state B, with probabilities pA and pB respectively, you will end up with a state (pA*A+pB*B), with some relative phase difference between the pA and pB substates. This includes the experimenter being in a superposition of (pA*"I measured A!" + pB*"I measured B!"), also with the phase.

What you have instead of the creation of parallel universes, is the subdivision of the original universe into substates that coexist, and in fact can interact, even though the substates are logically inconsistent. Quantum computing is totally based on this principle.

This does not introduce a new concept to QM. The Heisenberg Uncertainty Principle says that all possible quantum states subdivide the universe into incompatible states that coexist and interact. If you put a particle into a state with a specified spread of momenta, it will be subdivided into a collection of incompatible position states such that dPdX >= h/pi is satisfied.

So you're just going from a state that satisfies Heisenberg to a different state that satisfies Heisenberg. Not too startling.

I don't claim that anything above is "true." I do claim that it is what QM says. "Interpretations" like the Copenhagen interpretation assume that QM is lying at some point.

Personally, I think it would be good to follow QM and see where it takes us. For some reason, almost no one agrees. I suspect it's the part where the experimenter suffers splitting like a common electron, but I don't know.

So the state where you're in the shower with Hillary Clinton is not a whole parallel universe. It is a part of the singular universe, with a (for most of us) infinitesimal probability. So is the one where you're in the shower with Kate Beckinsale. Or Brad Pitt. And they interfere with each other. That's life in the quantum universe.

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