OK, I can’t help it. I wrote about neutrinos once before, but they’re too cool and I can’t leave them alone.

I’ve been reading about IceCube, a neutrino telescope at (literally, at) the South Pole. What in the world is a neutrino telescope doing there?

Well, it’s using the Earth. The idea is this. Neutrinos hardly interact with matter at all. Even the Earth is hardly a barrier to them. So by staring not up but down, into the incredibly thick and pure ice of the South Pole, what you’re actually doing is staring in the direction of neutrinos that have just passed through the Earth via the North Pole. Anything else, any other particles, would have been absorbed by the Earth long before they reached you, so you’re looking at the northern sky with a filter that only lets neutrinos through.

OK, so great. Neutrinos go through the entire Earth. Surely they’ll go through your little experiment just as easily. The IceCube neutrino telescope is a cubic kilometer, but that’s nothing compared to the Earth.

True, but remember that there’s lots and lots and lots of neutrinos. Almost all make it through the Earth, but that still leaves a huge number that interact with the Earth. And almost all make it through IceCube, too, but a smaller number interact with the ice.

Notice that the neutrinos that interact aren’t somehow weaker or slower than the rest. All these neutrinos are identical (though there are three different types, but there’s a twist there, too! See, aren’t neutrinos cool?). Just because a neutrino “made it” through the Earth doesn’t give it any better or worse chance of making it through IceCube.

So what does that mean, make it through? Or, more to the point, not make it through? What exactly happens to these little neutral ones?

Now the story gets really cool. Amazing, really.

Every once in a great while, a neutrino will slam into a neutron. When this happens, the neutron spits out something. The something depends on which kind of neutrino hit it. An electron neutrino causes an electron to come out (leaving a proton behind). A muon neutrino causes something else, a sort of electron on steroids, to come flying out. It’s called a muon.

OK, I have to tell this story. When the muon was first discovered, a physicist (one of my favorites) named I.I. Rabi, said, “Who ordered that?” The muon didn’t make any sense at the time. It was the wrong weight to be anything predicted. It seemed to have exactly the properties of the electron, except for two. It was much heavier than the electron, and it quickly decayed into (you guessed it) an electron. So what good was it. Who ordered that, indeed?

Later, scientists found that they could use muons from cosmic rays to verify Einstein’s relativistic time dilation, but that’s another story. This is about neutrinos!

Anyway, if the neutrino makes a muon, something amazing happens. The muon comes flying out of the atom at breakneck speed (if muons had necks). It’s going so fast, in fact, that it is actually faster than the speed of light.

Wait a minute, you just mentioned Einstein, and now you’re breaking the one law that everyone knows Einstein proved. Thou shalt not go faster than the speed of light.

Yes, but . . .

No buts, it’s your rule, now you have to obey it.

But wait.

OK, what?

Einstein said nothing can travel faster than the speed of light in a vacuum. Light, it turns out, travels just that fast. In a vacuum. But in ice, light goes a lot slower. And the muon can go faster than light in ice. Einstein is still intact, but the muon still does something remarkable.

Just as an airplane going faster than sound creates a sonic boom, a muon going faster than light creates a luminal boom! That’s right, a sonic boom for light. And it gets better. That luminal boom comes out as light we can see. And . . . ta daa . . . it’s blue!

That’s the blue glow you see around nuclear power plants. It’s really there, and it’s caused by particles moving faster than the speed of light in water. How cool is that?

So now you’ve got this ice, you’ve got these muons made by muon neutrinos, you’ve got this blue glow. The ice below the South Pole is probably the purest and clearest in the world. There’s nothing to compete with this blue light, and it just lights up that ice, traveling a great distance through the crystal clear solid water. And when it comes to a detector (called a DOM for Digital Optical Module), that detector grabs the blue glow and stores it away. You’ve just detected a neutrino!

OK, so what? So you’ve just detected a neutrino. Big deal.

It is a big deal, and here’s why. Neutrinos weigh almost nothing. Almost. We now know that they have a tiny, but real, mass. Why? Because of Einstein again. Any particle with zero mass travels at the speed of light, but any particle with a real mass, no matter how tiny, travels slower. At the speed of light time stops. But at less than the speed of light, time ticks away, however slowly.

Remember I mentioned the other twist about neutrinos? Here it is. Neutrinos can change back and forth, from one type to another. We know that now, but didn’t know it just a few years ago, and that caused a big worry. It seemed the Sun was making far too few neutrinos. Since neutrinos come directly from the Sun’s core, while visible light takes a long, long time to reach the surface, some scientists worried that perhaps the Sun’s core was dying. Instead, the answer is that the Sun is making the right number of neutrinos, but we were only able to detect one of the three kinds coming out. Since the Sun only makes one of the three kinds, it must be the case that the other two kinds (called the muon neutrino and the tau neutrino) pop into existence as the other kind pops out – in other words, the neutrinos turn one into the other.

So what does that have to do with mass? If the neutrinos were massless, then time wouldn’t pass for them, and they’d have no time to change one into the other. The fact that they can and do proves that they have mass.

Again big deal. Right? Wrong.

The big deals are many. First of all, neutrinos don’t weigh much, but there are a lot of them. A lot of them. Suddenly their mass becomes important for lots of things, including supernova explosions.

But there’s more than that. No theory we currently have shows why or how the neutrino should have mass. The mass of the neutrino points toward new physics. It’s like that cloud on the horizon of physics at the end of the 19th century that led to radioactivity, special relativity, quantum mechanics, and the modern world.  The 20th century’s cloud was the neutrino mass, and the more we learn about these amazing, ghostly particles, the closer we will come to seeing what wonders await behind this cloud. I for one can’t wait to see.

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