Making sound into light

Energy tends to spread out.  That’s the essence of the Second Law of Thermodynamics, the law of entropy.

Temperature is the amount of energy per particle.  If you have a lot of particles with a little energy each, that’s a low temperature.  That same energy concentrated in just a few particles produces a high temperature.

Suppose you had a red-hot poker.  You could dunk it in a pot of water, and the pot might only be 1 degree hotter than room temperature.  All the energy is there, but it’s spread through the large pot.  There’s no way to extract and concentrate the energy in the water so you can make a red-hot poker.

So it’s a one-way street.  Left to its own devices, energy spreads out, but if you want to take spread out energy and concentrate it, it would take a lot of work, and could only be done with very low efficiency.

Sound is low energy, light is high energy.  A single particle of light, called a photon, has energy about a billion times greater than a single particle of sound, called a phonon.  So it’s easy to turn light energy into sound energy, but impossible to turn sound energy into light energy.

But now that I’ve convinced you it’s impossible, I’ll tell you that it happens, and the discovery of sonoluminescence (1934) was a huge surprise to physicists.  How can the energy of a billion phonons be extracted and funneled into a single photon?  And why doesn’t this violate the Second Law?

I don’t know the answer, so I can’t explain it.  I know it has something to do with the fact that the sound waves are coherent, like a laser, all pointed in the same direction and acting together they can do things they would never be able to do if they were random sound waves going every which way.

When I was an undergraduate, I learned quantum physics from Julian Schwinger.  (Today is his 101st birthday, and that provides me an excuse for writing this column.)  Schwinger was a true scholar, not just a phenomenal mathematician and profound scientific thinker, a cultured and thoughtful person who made far-flung connections in his conversation and his scholarship. His career was jump-started when he did a PhD dissertation under J Robert Oppenheimer at age 21.  He left his mark on 20th Century physics as much as any of the great names who are better known (e.g., Einstein, Schrodinger, Feynman, with whom he shared the 1965 Nobel prize).

Late in his life, Schwinger came up with an esoteric explanation for sonoluminescence, by analogy with Hawking’s account of evaporating black holes.

Around this time was the front-page news of cold fusion in a Utah elecrochemistry lab. It was a flash in the pan.  Cold fusion was soon dismissed as an experimental error.

Correcting this error has required decades; cold fusion is real.

What Schwinger realized was that cold fusion was the same story as sonoluminescence. Both phenomena occur when dispersed energy somehow manages to focus itself and accumulate so that within a low-temperature environment, a few very hot particles can appear.

Twenty-five years have passed since Schwinger theorized about cold fusion. We still don’t know if he was right.

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