Neutrinos Travel Faster than Light?

The news media and physics community are abuzz with two reports suggesting that neutrinos might travel faster than the speed of light (from the MINOS and OPERA experiments). The claim is that neutrinos travel faster than light by 1 part in 40,000. The results are significant at the vaunted six-sigma level, barring any currently-unknown error in the experiment.

This has physicists excited, because anything traveling faster than the speed of light would violate Einstein’s theory of Special Relativity, which, along with quantum mechanics, underlies all of modern particle physics. Particle physicists are looking for something like this, because they are deeply dissatisfied with the current theory. The Standard Model contains a number of parameters that have to be made up to match fundamental measurements (although once you do so, it makes a vast array of validated predictions). Something that violates Special Relativity in a way that is barely noticeable would be consistent with all of the tests that have so far said that Einstein was spot-on, but leave room for tweaking the underlying theory.

Of course, the most excited are those who have developed models that predict that neutrinos should travel faster than light, as is evident by some recent edits to the Wikipedia page on neutrinos.

Astronomers, however, are deeply skeptical that neutrinos travel faster than light, because they already have measured the speed that neutrinos travel to an accuracy 10,000 times higher than either particle physics experiment. In 1987, a supernova occurred 168,000 light years from Earth in a nearby galaxy, the Large Magellenic Cloud. Two neutrino detectors were operating, and together they detected 24 neutrinos above the expected background that appeared nearly simultaneously at Earth with the light (photons) from the supernova. The coincidence between the photons and neutrinos arriving at Earth was used to determine the speed with which neutrinos traveled, and it was found to match that of light by 1 part in 450,000,000.

Most particle physicists are probably aware of this, but they have several reasons to give credence to the OPERA and MINOS results:

1) All of neutrino astrophysics involves ~10 MeV neutrinos, whereas the particle physicists were studying neutrinos 1000 times more energetic. Perhaps more energetic neutrinos travel faster than less energetic ones.

2) For 1987A, astrophysicists presumed neutrinos would travel at the speed of light, and focused their search around the time of the event. With the couple dozen events they found, they may not have been able to test for time delays or distributions.

3) For their experiment, the particle physicists simply had to understand the emission time of the neutrinos, the distance to the detector, and the physics of detecting the neutrinos (via muons, I
believe). Astrophysicists have to understand the interior of the Sun to study neutrino oscillations, or of a collapsing star to understand their travel time. Astrophysicists definitely do not understand all the physics of a supernova – most of the models to date still don’t produce an explosion from a collapsing star.

4) Neutrino oscillations required the biggest tweak to the Standard Model (giving them mass) of any recent result in physics.

5) All of physics points to the need for new underlying theories: our theory for gravity (General Relativity) and quantum mechanics can’t be reconciled; the Standard Model has a lot of free parameters, and we haven’t yet found the Higgs Boson; 75% of the energy of the universe is made up of stuff we haven’t been able to measure (darkmatter and dark energy), and only have hypotheses to explain.

If I were to guess as to what would set off a paradigm shift in physics, at the moment, neutrinos seem the most likely candidate. So, yes, I am cautiously excited.

  1. I’m a little late to update this. Suffice it to say, nothing to see here.