Amidst the dubious news of neutrinos potentially traveling faster than light, it's easy to lose sight of something even stranger: neutrinos are in a constant identity crisis, oscillating between different types. Why is this? In this week's "Ask a Physicist" we'll find out.
I've been talking a lot about neutrinos lately, what with the prospect of faster than light neutrinos allowing you to potentially telephone the past. Even when they're not ostensibly traveling faster than light, neutrinos are extremely odd. Avid secondary school physicist, Abhishek asks:
I was hoping that you would be able to create a post on why neutrino oscillations occur because I realise that they are happening, but am still not sure as to why this occurs, or what the implications of this are.
Neutrinos are rascally little devils. They are notoriously difficult to measure and, so far as we can tell, basically impossible to confine. They interact so weakly with atoms that they can pass through a light-year's worth of lead with only a 50-50 chance of interacting at all.
But there are far stranger things about neutrinos than their inability to play well with others or the (almost certainly erroneous) claims of faster-than-light travel. As they fly through space at near-light speeds, they seem to be constantly changing form — kind of a subatomic Zan or Jayna. To really understand why shape-changing neutrinos are such a surprise, we need to look back into past a hundred thousand years ago.
A Little Bit of History
About 160,000 years ago there was a supernova explosion in the nearby galaxy, the Large Magellanic Cloud. Since light takes time to reach us, we only saw the explosion in 1987 — and it was one of the most spectacular astronomical events in human history. Along with radiation, lots and lots of neutrinos got released in the explosion as well – enough that huge numbers of them reached the earth, at more or less the same time. Neutrinos travel so close to the speed of light, that if they aren't massless, they are very close.
We can't predict when a supernova is going to go off, so it seems like a bad strategy to simply wait for one in the hopes of capturing its neutrinos. Fortunately, supernovas are not the only neutrino factories out there. Our own sun produces about the same number of neutrinos as it does the more obvious photons in the course of going about its thermonuclear business.
The neutrino detection business has been around for a while. By the 1960's, there was a fair amount of interest in trying to capture the neutrinos from the sun, and so Raymond Davis of Brookhaven National Labs and John Bahcall, then at Cal Tech, led the efforts to build a giant, underground swimming pool. The Homestake Observatory, built in an abandoned gold mine in South Dakota, was essentially a hundred thousand gallon tank filled with cleaning fluid. A neutrino flies in, hits one of the chlorine atoms, turns the chlorine into argon, and the argon decays, giving off light. What could be simpler?
The only problem is that the detectors weren't giving the results that we'd expected. Bahcall predicted about 2 to 3 times as many neutrinos as were actually detected in the Homestake experiment. Somehow, someone was stealing most of the neutrinos! But who?
Credit: Jeff Blomquist.
How unique is a particle, really?
One of the strangest things in our so-called "standard model" of particle physics is that rather than simply have one type of each particle, we tend to have three nearly identical particles, each with increasing mass. For example, the muon and the tau particle are almost exactly the same as an electron only — how can I put this delicately — huskier.
Likewise, there are 3 different kinds of neutrinos – electron, mu and tau. The ones that are created in nuclear fusion are the electron version, and the early neutrino detectors were only able to measure only electron neutrinos, making the other two essentially invisible. So, the reasoning went at the time, maybe the "missing" neutrinos turned from electron neutrinos into something else.
This sounds like one of those "Why not?" kind of questions, and may seem like the sort of explanation that physicists might pull out of their asses, but bear with me and consider the following three statements of quantum fact:
- Unique particles aren't always unique. You may recall a while back I wrote about a property known as spin. If you don't want to go back and read the original article, the gist is that an electron is, in some ways, like a little magnetized top. It can spin one way or the other and depending on which direction it spins, it's going to interact with a magnet differently.
- What we normally think of as the same kinds of particles – a spin-up electron and a spin-down electron, for example – can act like different particles under some circumstances. The converse is also true. Two particles that we normally think of as different can behave the same under some circumstances. If the differences are large enough, we call them two different particles, and if the differences are small (like with the spin-up and spin-down electron), we call them two different states of the same particle.
- Many particles aren't in one particular state or another; they're in a combination of two or more different states. In one of the first of these columns, I talked about the Schroedinger's Cat experiment, in which a cat is confined to a sealed box with a radioactive pellet attached to a poison dispenser.
- As a reminder, a) Schroedinger was making fun of the standard interpretation of quantum mechanics, b) It is only intended to be a thought experiment, and c) It is also extremely messed up, so please don't try this at home. The takeaway is that before you open the box, the standard interpretation says that the cat is neither dead nor alive, but really just a combination of the two. Quantum mechanics is filled with cases of particles doing two (seemingly) mutually exclusive things at the same time.
- Probabilities change with time. Quantum mechanics is all based on the idea of "waves," which essentially means that the probability of measuring something (say a dead cat or an electron with spin up) can change with time, but the probabilities are quite predictable. It turns out that the greater the energy differences between the different states, the faster those probabilities change. Likewise, if there's no energy difference, there's no change at all.
Let's combine all of those ideas and make a totally astounding (but correct) leap of faith: neutrinos of different types turn into one another. This isn't as crazy as it sounds. After all, from the particle's perspective, there's no huge difference between being in two different possible states or being two different entirely different types of particles.
Neutrinos with an identity crisis
Experimentally, we have three different kinds of neutrinos: one that interacts with an electron, one that interacts with a muon, and one that interacts with a tau particle. Just as we can think of an electron as a combination of two different particles: a spin-up electron and a spin-down electron, we can think of neutrinos in the same way. As a twist, we have an entirely different way of sorting the neutrinos: 1, 2, and 3, each with a different mass.
Neutrino #1 is a combination of mostly electron neutrino, combined with a good chunk of mu neutrino, and a smidgen of tau neutrino. Neutrino #2 is a different combination, and Neutrino #3 is a yet different combination still. Whether we call them 3 different particles or 3 different states of the same particle is irrelevant. What is relevant is that the neutrinos are not going to be observed the same way every time. This idea known as neutrino oscillation, since the neutrinos oscillate between identities: electron, mu, or tau.
Now here's the beautiful part: this only works if neutrinos have mass, and different masses at that. The energy of a neutrino is, naturally, E=mc^2, which means that if none of the neutrinos have a mass then the energies for all of them are the same and the neutrinos would never change form.
It's a cool idea, but is it correct?
To test it, you need to generate a whole bunch of a particular type of neutrino, build a giant underground swimming pool full of cleaning fluid to detect that kind of neutrino, and see what fraction of the expected numbers are missing. Mu neutrinos, for example, get created by cosmic rays in the upper atmosphere. Electron neutrinos get created by nuclear reactors.
In 1998, this sort of approach hit pay dirt, and the Super-Kamiokande experiment was the first to detect unequivocal signs of neutrinos oscillations, and hence, that neutrinos have mass. Subsequent experiments confirmed and put tighter constraints on the mass of the neutrinos. As you may remember, the OPERA collaboration — the ones who caused such a commotion with all of this faster-than-light neutrino business — were really looking for neutrino oscillations.
So neutrinos have mass, but, since they're traveling insanely close to the speed of light, not a lot of it. To give you an idea, the most massive neutrino has a mass probably about ten million times smaller than an electron (the next lightest fundamental particle). Even so, since there are so many neutrinos out there (about a billion times as many neutrinos as photons) you might suspect that neutrinos make up the elusive "Dark Matter" in the universe. Probably not, I'm afraid. While it's not technically ruled out, the current best guess on the neutrino masses mean that at most, neutrinos are likely to make up only about 1% of the missing mass. That's not gonna cut it.
There are a few other complications. The first is that these experiments don't just measure the neutrino mass – they also measure the intrinsic mixing between the three types of neutrinos: the amount of mu neutrino in neutrino type #1, for instance. We have no idea — and I mean no idea — why neutrinos should be as mixed up as they are. Hell, we don't even really know what there should be three different kinds of neutrinos in the first place, let alone why they should be intrinsically mixed up with one another. It's just something that we measure, and to a physicist, that's kind of unsatisfying.
And the origin of matter.
I've saved the best part for last. I recently wrote about the origin of matter, and why it's important that matter and anti-matter be different, even if the difference is slight. The problem is that it's incredibly difficult to measure significant differences between matter and anti-matter.
Neutrino oscillation experiments could provide a really good test of the difference between the two. Think of it this way: if mu neutrinos can turn into electron neutrinos, then anti-mu neutrinos can turn into anti-electron neutrinos. After all, the antimatter universe is supposed to be a perfect mirror to our universe. But what if they don't oscillate the same way? In the next few years upgrades to existing experiments will be able to put pretty tight constraints on the differences between neutrinos and anti-neutrinos and with it, give us yet another clue into the ultimate dominance of matter over antimatter in our universe.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University and is currently working on "The Universe in the Rearview Mirror," a new book all about symmetry that will be published by Dutton in 2013. Please send email to firstname.lastname@example.org with any questions about the universe.