Friday, June 03, 2016

Friday Science: Quantum Crazy

I'm now almost half way through Brian Greene's, The Fabric of the Cosmos. I hope I'm not wasting my time.

My first six summaries were:

a. Overview
b. Spinning Space Buckets
c. Relativity and the Absolute
d. Particles Separated at Birth
e. Does time flow?
f. Does time have an arrow?

1. This chapter reviews some quantum craziness. The bulk of the chapter reviews the often rehearsed fact that if you let photons, electrons, etc randomly go through slits or splitters and then hit a screen, they will show an interference pattern, like ripples of waves interfering with each other. But if you put a detector by one or another of the slits or splitters, you won't get the pattern. You'll only get individual dots as if the photons are particles rather than waves.

Greene goes to great lengths to show how bizarre this dynamic is. Let's say you put the detectors light years away from the slit or splitter. If you measure where the particle is, you will get a particle. If you don't measure it, you'll get a wave. I'm not expressing the paradox well. Basically, it's as if you determine which path a particle takes by whether you measure it at some later point, even if you measure it long after it would have already had to have "taken" a path.

Don't measure it--it takes all paths. Measure it at any point along the way and it's as if you force it to have taken a specific path in the past. This is the quantum measurement problem.

Make it so you can measure it in the future, then unmake it so you can't measure it? It will end up as if it has taken all paths. Make it so you can measure it in the future, then measure it? Then it will turn out to have taken a specific path.

2. Quantum theory cannot as yet account for the "collapse" of the wavefunction that takes place when you measure something. It is not accounted for in the original equation put out by Schroedinger in 1926. Physicists still recognize experimentally that there is a "stage one/stage two" dynamic to measurement. Stage one is fully accounted for by Schoedinger's equation. But his equation does not account for the specific location a particle takes when measured.

3. Greene mentions five solutions to this conundrum that have been suggested:
  • Niels Bohr in the twenties basically said the question wasn't allowed. You're trying to come up with an explanation that can't be observed, which is nonsense. Only what can be measured matters. There is no theory answering, "why."
  • Werner Heisenberg in the twenties suggested something slightly different. The wavefunction isn't real in the first place. It's not an objective feature of reality, just a tool we use to predict stuff.
  • Hugh Everett in the fifties gave a "many worlds interpretations." The wavefunction doesn't really collapse. It's only that we only live in one of the universes where it is played out. There are an infinite number of other worlds where the wavefunction plays out in all the other ways.
  • David Bohm revived Louis de Broglie's pilot wave theory in the 1950s. This is the one I like and will no doubt spend my retirement working on. :-) de Broglie suggested that particles have waves. They are thus both particle and wave. They are not truly indeterminate. We just can't see every aspect of them.
  • Ghirardi, Rimini, and Weber (Italians) slightly modified Schroedinger's equations to fit the larger world. They have thus accounted for the collapse of the wavefunction in the formula itself.
Apparently, all of these approaches fit with the experimental data, so there is not currently any real way to settle the dispute.

4. Finally, the chapter discusses "decoherence." The basic idea, if I understand it, is that because we don't have isolated photons in reality, but countless stuff interacting with countless stuff, everything causes the wavefunctions of everything else to collapse. Thus, while an isolated photon might be here or there, in the real world we are all located because everything else causes all our wavefunctions to collapse into a position.

None of this, in Greene's opinion, yet answers the question why time has an arrow. Apparently, only the Ghirardi, Rimini, Weber theory accounts for time's directionality. So the book continues...

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