Monday, November 21, 2011


Saturday I had my first day of work up at the Vis. It was kind of strange -- I've been volunteering there for over two years now, so it was a little odd being staff instead. Everything went swimmingly, however, and I enjoyed it quite a bit (although I was exhausted by the end of the day! Working 14 hours in a row is a bit tiring).

I also had some time during the day when it wasn't too busy to mull over the report of superluminal neutrinos from back in September. By chance, a report came out the next day (today) by a group of scientists from Italy that puts forward a possible proof that the neutrinos are not traveling faster than c. According to the paper, which builds on work from two American physicists, neutrinos traveling faster than light should emit gamma rays and electron-positron pairs, in a sort of weak-force analog to Cherenkov radiation.

(For those who don't know, Cherenkov radiation is produced when particles with electric charge move faster than the local speed of light in medium. For instance, light travels only about 75% as fast in water as it does in vacuum, so it's quite possible for a particle to move faster through water than light can. When one does, however, it emits a special kind of radiation known as Cherenkov radiation [assuming that the particle is electrically charged, such as an electron]. This effect is visible as the characteristic blue glow of nuclear reactors.)

Neutrinos are not electrically charged, so they don't produce Cherenkov radiation. In fact, they interact through only two of the four fundamental forces, and they happen to be the weakest two: the weak nuclear force, and gravity (this is why they're so hard to detect). However, the paper argues that in analogy with electromagnetism, uncharged, superluminal neutrinos should emit gamma rays and electron-positron pairs through weak interactions.

Now this is all well and good theoretically, but it has practical implications too: by emitting this sort of radiation the particle's own energy is drastically modified (in fact, calculations suggest that each emission would remove more than 3/4 of the neutrinos' energy). This ought to be dramatically visible in a graph of the energy of the arriving neutrinos. And to make a long story short, it's not. The neutrinos' power spectrum looks unaffected, making it virtually impossible for them to have exceeded the speed of light by the amount claimed.

This experimental test is brilliant, because instead of trying to better measure the distance or time (both of which are fraught with difficulty) it attacks the problem from another direction, that of energy. I suggested just such an experiment to measure the neutrinos' energy in one of my first posts on this subject back in October, although I admit I wasn't thinking about this particular test (I wish I'd thought of the weak-force analog to Cherenkov radiation, because it's a neat little idea). Anyway, it's nice to see the scientific method in action here, and I'll try to keep you up to date on this topic in the future.

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