This week, the 2011 Nobel Prizes will be announced. Like many scientists, I have mixed feelings about the Nobels: they often do acknowledge important research, but the selection process is not transparent and the decisions can often seem arbitrary when so much excellent work is being done. (I’m focusing on the science prizes; the literature and peace prizes are a different animal, and I haven’t really pondered them as much.) If the Thomson Reuters prediction for the physics prize is correct, I’ll definitely have something to say about it on Wednesday — quantum entanglement is a topic I’ve intended to write about for some time, and I do hope the experimenters who have done great work in that field get the honor. Update: the Nobel Prize obviously wasn’t on entanglement, but something closer to my own heart.
On the other hand, the Nobel Prizes do help perpetuate the idea that there are a few Great Scientists worthy of honor, and while the awards are intended to support people actively doing work, the prizes select those who have already established themselves. The problem is that science is complex, and it may be many years before the full significance of a particular piece of research can be recognized — by which time the researcher may be dead (Nobels are generally not given out posthumously, though there are exceptions) or moved on to other projects. (An odd example of this is Brian Josephson, who predicted the superconducting junctions that now bear his name; by the time he received his Nobel Prize, he was devoting his life to pseudoscientific research on parapsychology, telepathy, and the like.) I guess I would prefer to see more prestigious prizes given out to younger researchers in their early careers, but the Nobels are what they are. Funny how nobody consults me on these matters.
The emergence of importance in scientific research brings us back to the possibly faster-than-light neutrino results from OPERA. As more and more scientists weigh in on the possibilities, the idea that the neutrinos are actually moving faster than light seems less plausible. Andrew Cohen and Sheldon Glashow (the latter a Nobel Laureate) pointed out that neutrinos moving faster than the speed of light in a material would produce radiation; the energy of the radiation has to come from the kinetic energy of the neutrinos, so they slow down. For charged particles such as electrons, a similar phenomenon is known as Čerenkov (pronounced “chairENkoff”) radiation, but the lack of electromagnetic interaction in neutrinos in some ways makes the effect simpler.
According to quantum field theory, the vacuum is full of possible particles; for neutrinos, traveling through a medium like the crust of the Earth (as in the OPERA experiment) doesn’t actually change the situation much because the neutrinos interact so weakly with matter. Under ordinary circumstances, these virtual particles don’t ever become more than possibilities, but when something moving faster than the speed of light in that medium passes through, the shock wave is enough to make them real. The most interesting possibility discussed by Cohen and Glashow is pair-production: an electron-positron pair.
A mu neutrino moving faster than light creates shock waves in the quantum vacuum; these shock waves are above the threshold energy to make an electron and a positron, via Einstein’s famous equation E = m c2. You can’t just make an electron or a positron in isolation, since you need to conserve a number of physical quantities, including electric charge and spin, so the energies involved have to be very large. (Neutrino-antineutrino pairs have a lower energy threshold, but since the neutrino flavors are expected to move at more-or-less the same velocity, it’s not an important process for the discussion at hand.) Transferring that much energy from the neutrino to the electron-positron pair slows it down a lot, so even if the neutrino initially had that much velocity, it would quickly drop below light speed.
As I mentioned in the earlier post, neutrinos aren’t tachyons (hypothetical particles that always move faster than light): that seems evident from Supernova 1987a and other observations. The Cohen-Glashow calculation I think shows pretty clearly why neutrinos aren’t slower-than-light particles that occasionally violate the speed limit set by special relativity. (If that ever occurs, it would be a Lorentz violation, and very few reputable theories allow for this kind of behavior. The name is from physicist Hendrik Lorentz worked out a lot of the math behind relativity before Einstein’s final version came out in 1905.) That leaves two possibilities still: new physics that leaves special relativity in place for nearly every phenomenon we know of, or there is something wrong with the OPERA analysis.
Note that Einstein’s name popped up a couple of times, but it was almost incidental. I think it’s pretty clear from history that relativity would have existed without Einstein, and certainly he didn’t work out all the implications of his own theories. In other words, it’s a big mistake to think of experiments such as this as being a battle with the old guy. So many headlines over the last 10 days have framed the whole issue as “Einstein vs. OPERA”, asking “Was Einstein Wrong?”, but that’s completely the wrong question to ask. Einstein was wrong about many things, and “right” about others, for a certain value of “right”. Over a period of centuries, we’re all going to turn out to be wrong about most of what we think. Newton was wrong in many ways: absolute nature of time and space, the need to “rewind” the mechanism of the Solar System, etc., and that doesn’t even get into his alchemy and numerology. Yet, much of Newtonian physics is useful, and essential for handling everyday problems.
The right question is whether the OPERA results are consistent with what our theories predict, and if not, why not. Relativity has held up under every test so far, so any exception we find is going to be on the hairy edge of what we can do experimentally. It’s not an all-or-nothing proposition, as the analyses I’ve written about and linked to show. Maybe there are exceptions to Einstein’s theories; maybe there aren’t (though that seems unlikely based on history). Whatever the ultimate outcome to the OPERA neutrino experiment turns out to be, it’s a good illustration of how science actually works: scientists eliminating options, figuring out others, testing propositions, and coming out with a clearer picture of our wonderful universe.