Sunday, July 08, 2007

Quantum erasure and apple of the tree of knowledge

I skimmed the SciAM article on building a home "quantum eraser", but I didn't appreciate that the 1982 experiment is just one notch weirder than the what I'd read in Gribbin's 1997 book [1]. I realized how ++weird the result is upon reading Greenes description today [2].

As I dimly recall it, the "Aspect experiments" showed that the measured interference pattern that
  • requires passage of a single photon through spatially separated paths simultaneously [3]
  • requires no peeking after the photon has travelled the spatially separated paths
It's the "no-peeking after" part that made the experimental results so delicious. Causality (arrow of time), 1 dimensional time, and 3 dimensional space and "objective" reality all took big hits from the Aspect experiment. The discussion ever since has been which, if any, survived.

My weak understanding of Greenes' non-mathematical interpretation of the "erasure experiment was that the "no peeking after" clause is extended as follows:

  • tricks that allow you to defer "peeking" until after the interference pattern ought to be formed don't work either (no surprise here, this is a variant of the Aspect experiment)
  • if you set up a "deferred peeking" trick, but then undo it before you "peek", then the interference pattern can return. (surprise)
So it's the last bit that adds an additional note of sublime weirdness to the aspect experiment. It makes it just a bit easier to believe that the fundamental rule is really "no peeking". This is just something you can't know about.

It invariably reminds anyone taught by nuns of the "apple of the tree of knowledge" -- "but from the tree of the knowledge of good and evil you shall not eat, for in the day that you eat from it you will surely die.".

The universe seems to be telling us we can't watch some things too closely. Naturally, that only increases our appetite ....

[1] I really don't think Gribbin discussed this experiment, even though it was old when he wrote the book. It doesn't show up here either. I wonder why not ...
[2] Greenes is a cosmologist at heart, and he tends to shy away from the quantum weirdness Gribbin and others embrace, but he did a good job describing erasure.
[3] Paradox puzzles often require one to think carefully about what words mean, so I italicized a few of the interesting ones. Spatial separation, for example, has been shown not to mean what we thought it meant. Two entangled photons a universe apart are, by some measures, not separated at all.

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