Saturday, June 18, 2011

Quantum macro

Living in a Quantum World: Scientific American by Vlatko Vedral is the headline article for SciAm's June 2010 issue. It's behind SciAm's remarkably successful paywall, but for the moment you can find a PDF in Taiwan [1].

It's worth a read for my fellow lay fans of Quantum Mechanics. It captures the excitement of the field, where the theoretically incomprehensible is now becoming the materially incomprehensible. Some highlights of note, recognizing that this article is one physicist's personal view ...

  • Until recently many popular presentations of QM, even very fine ones, confined QM to the micro realm. Decoherence, arising from "information leakage" was supposed to flip from a bizarre "binary" quantum world of entanglement to a bizarre "analog" world of gravity and black holes. Vedral and others says it's all QM from the bottom to the top. There's no "flip" between quantum and classic.
  • In one experiment 10^20 atoms of lithium fluoride behaved as though, at some level, they were all entangled
  • There are claims, with some evidence, that quantum effects are leveraged by navigating birds and phyotsynthesis.
  • If entanglement is truly fundamental, then space and time (arrow of time) may in time be seen as side-effects of entanglement (which, I suppose, would make "spooky action at a distance" oddly easier to understand)
  • Even more speculative -- gravity is not fundamental, but is emerges as a side-effect of the three (not four) fundamental forces (weak, strong, electromagnetic). Supposedly "proper" quantum treatment of those forces will yield gravity, which would explain why it's been so hard to quantize gravity.

Perhaps the most interesting bit of the article was a somewhat frustrating description of a Schrodinger Cat variant Bob and Alice thought experiment from 1961 and 1986. I've never heard this one, and I can't find it described properly on the web, so I wonder if this is partly a modern interpretation focusing on how information leakage leads to decoherence [2]. Briefly, it goes like this

  • Bob, the cat, the cat poison and cesium atom are in a room. Alice is outside. The cat poison is released if the atom decays. Bob can see the cat. Alice can't.
  • The cesium atom is "quantum". It is in an indeterminate state of decay or integrity. That's "rock solid" quantum physics.
  • Alice puts a piece of paper under the door. She asks Bob if he can see that the cat is dead or alive, but not what state the cat is in. Just that he can tell.
  • Bob writes he can tell.

So at this point the the cat is dead or alive - at least for Bob.

Here's where I don't get it at all. According to quantum theory this is all reversible. Allegedly Alice can "undo" the observation, but retain the piece of paper. If Bob remembers seeing a dead cat, but Alice makes the poison inert, he'll remember seeing a live cat. So Bob, the Cat, the poison and the Cesium ion are all entangled and indeterminate for Alice, but for Bob they're all determinate. Smells a bit like frames of reference in special and general relativity.

Unfortunately the sidebar doesn't explain how Alice can undo the observation without a bit of time travel. So I suspect the explanation has been a bit butchered, but I'll keep an eye out for a better one (Google is no help today).  Supposedly the equivalent experiment has actually been done by teams led by Blatt and Wineland, and they've shown measurement reversal in the real world (did you just feel the  universe hiccup)?

[1] If you Google on a few unique words in an article, you can usually find one copy somewhere on earth.
[2] Much of the lay physics I read these days uses an information theoretic perspective; much of physics is expressed in the language of information. Reminds me of some of my favorite mind expanding science fiction,  particularly Greg Egan's Permutation City. In that book sentient natives of a simulation with inescapably absurd physics are designed to realize that their universe must be a simulation. Except they're so brilliant they come up with a plausible "natural" explanation, and so disrupt the simulation itself ...

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