Monday, September 26, 2011

Neutrinos and the Speed of Light

Last week saw two phenomena that seem to challenge a central postulate of the theory of relativity. First, researchers at CERN reported that a stream of neutrinos was determined to have traveled faster than the speed of light. Following that announcement, my inbox was filled at an even faster rate with messages from friends, family, colleagues, former students, and Playground readers alerting me to this fact. On both grounds, it warrants some background discussion.

The Special Theory of Relativity

First, what does Einstein's theory say? Einstein proposed two theories of relativity -- the special theory in 1905 and the general theory in 1916. This result concerns the first of them. The theory of special relativity is based on two postulates, assumptions about the nature of the world and of the laws that govern its behavior. The first is called the light postulate and states that the speed of light is constant for all observers regardless of the state of motion of the source or the observer. If you shine a flashlight in my eyes, I see the light coming at me at the speed of light. If you run towards me or away from me while keeping the light in my eyes, the speed I see the light coming at exactly the same speed no matter what. This seems peculiar and violates what Newton said about motion, but is a result of our best theory of electricity, magnetism, and optics, what are called Maxwell's equations, named after British physicist James Clerk Maxwell who discovered none of them.

By combining these two postulates, Einstein derived a number of odd results. One of them is that the speed of light becomes a limiting velocity. Objects with no mass must travel at the speed of light, whereas objects with a positive mass must travel less than the speed of light. As an object moves faster, its mass increases. More mass means you need to put in more energy to speed it up. As an object approaches the speed of light, its mass increases towards infinity in such a way that it would take an infinite amount of energy to get it across the threshold to or past the speed of light. This is why it is, according to the special theory of relativity, an impossibility for a massive object to exceed the speed of light. It is not an engineering problem -- it isn't that we just haven't figured out how to do it. Rather, it should not be able to happen. That is why this result, if correct, would be so interesting.


A fascinating little particle, "neutrino" translates to little tiny neutral thing, and so it is. We all know that atoms are made up of a heavy nucleus with positively charged protons and electrically neutral neutrons surrounded by very light, negatively charged electrons. Positive and negative charges attract, so what would happen if you put a proton and an electron close together? They would pull on each other and join into one particle, the positive and negative charges offsetting and the masses combining. What has no electrical charge and a mass equal to a proton and electron together? A neutron.

When Wolfgang Pauli, known as the most brilliant, anal retentive, conversationally abusive jerk in the history of physics, examined the situation, he realized that if neutrons were indeed a proton and an electron combined, that there would have to be a third component to guarantee that certain physical quantities are conserved -- that the entire physical balance sheet turned out just right -- and so he proposed the neutrino, a particle with no mass and no charge.

Needless to say, a particle with no mass or charge would be difficult to detect since it would not interact with most things in a normal fashion. Indeed, it wasn't until over a quarter century later that experimentalists actually found the little guys. As particle physics has progressed, we've realized that neutrinos do have a tiny bit of mass and so, by Einstein's work, should be limited by the speed of light. Additionally, we've also figured out that they should come in three different flavors and that they are capable of switching between them. Truly nifty little dudes.


So, if correct, the CERN result would mean that one of the pillars of modern physics would have a problem. Does it mean we would have to reject it wholesale? Probably not, but it would mean something revolutionary and interesting would have to happen at the conceptual heart of our physics.

But is it right? First of all, these sorts of stories do come out every year or so and usually they fade away for good reason. They were false alarms. Science is like soccer, you have long stretches of what Thomas Kuhn called "normal science" in which well-defined problems are solved using standard means, giving rise to the usual sorts of conclusions which excite only technicians. But every once in a while out of nowhere, there is a run at the goal, the ball gets in the box and a player is streaking in from the wing. But, like in soccer, usually nothing comes of it.

The reason for this is that when we talk of "observations" really what we mean is something other than what we usually think of. We picture scientists in lab coats actually observing things. We think of our own labs in high school and college where we used instruments to measure things. That's not how it works in particle physics. You have computers receiving input from a physical system and software programs doing all sorts of data manipulations based on certain parameters that are presumed. We never actually see anything.

Kent Staley has done amazing work documenting the discovery of the top quark and his discussion makes beautifully clear how complex "observations" are in science today, how many people and how many computer programs with slightly different approaches are required to create what we "see." This is not to say that we shouldn't trust the results once confirmed, but rather to say that often things seem to be swelling when ultimately, they tend to be a part of a more complex conversation that may end up going nowhere.

But, to return to the soccer metaphor, every once in a while someone does score and when it happens great fanfare is warranted. Even the most firmly entrenched theories in science are always open to disconfirmation. That is what makes science so exciting. Is this a shot that will find the top corner in the aim and goal of advancing science? Probably not. But maybe...