Faster than light neutrinos: what is going on? Part I

The OPERA neutrino detector in Gran Sasso, Italy.

I was going to post this all at once, but when I realize I was way over 1100 words and how much I had yet to say, I decided to split this post in two. Here is the first part, which is mostly putting things into context. The second part is here.

You may have seen this story in Nature News yesterday. A very significant anomaly have been observed in the travel time of neutrinos going from CERN, Geneva, Switzerland to Gran Sasso, Italy. Let me say this first: tremendous science is being done here folks. This is not the usual extraordinary claim, as extraordinary evidence is coming along with it. However, the claim is so extraordinary that it will require evidence even more extraordinary. But first, let me explain why the claim that any particle could travel faster than the speed of light is outrageous to any self-respecting physicist.

Why breaking the speed of light is troubling

I think everybody has some conception of the speed of light being an unbreakable barrier. We always have this fact at the back of our mind while watching sci-fi movies and a part of us goes “if only…”. What I really want to express here is how solid and fundamental is this statement.

First of all, the concept of a constant velocity of light really took hold in physics with the advent of the special relativity of Einstein. To be fair, it has been one of the consequences of the unification of electricity and magnetism by Maxwell some time before, but Einstein was the first to generalize the concept of the speed of light as a limiting velocity to all matter. The consequences were mind-numbing. Suddenly, time had to stretch and space had to contract in order to keep the speed of light constant in all possible reference frames. The way that space and time could “rotate” into one another was precisely predicted, and confirmed experimentally over and over again. In each collision that is being done at the LHC right now, relativistic effects are observed.

So I pointed out that the constancy of the speed of light is a fundamental assumption in special relativity. It also turns out to be a fundamental assumption of general relativity. General relativity forms the theoretical framework upon which the cosmological standard model is based (the famous lambda-CDM model). Additionally, special relativity is a fundamental assumption in quantum field theory, which is the theoretical framework upon which the standard model of particle physics is based. Take these two standard models together, and you have the whole of our most fundamental understanding of nature. If you demonstrate that you cannot assume the constancy of the speed of light, it is exactly like chopping off one leg of the Eiffel tower. It is almost as bad as saying that energy is not conserved, or that the second law of thermodynamics doesn’t hold.

Of course, some people may point out that the speed of light is only constant in vacuum, and that would be perfectly true. The thing is that light can only be slowed down by travelling through various material, and it is because of the funny way it interacts with matter. Light is still going at its constant vacuum speed inside the material when it is traversing the empty space between the atoms of the material. The change in the speed of light in materials is a macroscopic effect, and it can be explained with known physics. It is not the concern of fundamental physics.

So the basic idea is that things can only travel at the speed of light and below. If the thing in question has mass, it has no shot at ever attaining the speed of light (although it can get very, very close). Light is composed of massless particles, which is why they always travel at the limiting speed. Neutrinos are not massless. It has been determined that they have tiny masses, so observing them travelling faster than light is even more troubling.

OPERA and neutrinos

The OPERA experiment is a neutrino detector 1400 m underground designed to observe a specific kind of neutrinos. There are three species of neutrinos which are associated with the electron, the muon and the tau. It has been known for a little while that neutrinos can change from one kind to another, which is also why we know that they are not massless. Neutrinos interact with ordinary matter very, very little. You may have heard that in order to be sure to stop a neutrino in its tracks, it would be necessary to put it through several light-years of lead. It passes through everything. We know they exist because they are also very easy to produce, and when we produce them, we can tell that something that we couldn’t detect got away.

We can also directly detect them if we are lucky. As it turns out, the rarity of interactions is compensated by how many of these things are floating around. The sun emits incredible quantities of the stuff as billions pass through your thumb each second. Every once in a while, one of them collide with ordinary matter and the collision results in the creation of stuff that we can actually measure like light, electrons, muons and taus.

OPERA was designed to observe neutrinos of the muon kind transforming into the tau kind. It is fairly easy to create muon neutrinos if you have an accelerator complex. Get a pion beam after slamming a proton beam into a fixed target, and wait until they decay. When pions decay, they go into a muon and a muon neutrino. You can measure the production position and time of the neutrino by measuring carefully the outgoing muon, and by knowing where the pion was going.

Then, the muons neutrinos traverse 732 km of Earth crust before reaching the OPERA detector. According to neutrino oscillations, some of them should arrive as tau neutrinos. OPERA is still waiting for its tau neutrino signal, and in the meantime they have been performing timing and distance measurements: the ingredients going into the recipe to measure the speed.

In the next part, we will get in the meat of the topic, looking at how exactly they did the measurement, and all the things they considered could go wrong.

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