It may seem as though every new day brings an announcement of a scientific breakthrough of the highest order. Should you freak out about every new record-breaking neutrino? In this week's "Ask a Physicist," we'll find out.
Image: The IceCube detector under Aurora Australis. Courtesy of the IceCube Collaboration.
If you're a regular reader of io9, I presumably don't need to make the case that science is awesome and exciting and that scientific discoveries have repeatedly changed the way we perceive our world. But that said, breakthroughs don't happen every day, which means that almost daily announcements of revolutionary new discoveries are almost all overhyped. If you're paying attention, you could be forgiven for wondering, after each new announcement:
Is this result really a big deal?
For instance, last year there was a genuinely exciting announcement of a pair of high-energy neutrinos discovered at the South Pole. Just a few months ago, this was followed up by a third detection at the same energy, prompting a rash of followup questions asking whether this was really such a big deal. In this case, I think it is, and I'll give you a crash course in neutrinos to explain why.
Why Neutrinos Matter
Just about everything we know about the universe outside of this planet comes to us in the form of light. Sure, every now and again, a rock falls from the sky. We're also constantly being bombarded by high-energy cosmic rays — charged particles emitted both from sun and from places beyond.
But neutrinos are special. For example, in 1987, neutrino detectors allowed us to essentially glimpse inside the workings of a supernova explosion 160,000 light-years away. I've written quite a lot about neutrinos in past columns, but it's worth reminding ourselves of a few key points:
- Neutrinos barely interact with anything else. Take a look at the Standard Model of physics, and you'll neutrinos have the distinction of being the only fundamental particles that only interact via gravity (and then, only barely) and via the aptly named "weak force." Though neutrinos are ubiquitous (the sun makes about as many neutrinos as it does photons), they are very hard to detect. Solar neutrinos, for instance, can travel through a light-year's worth of lead with a fairly small chance of hitting anything.
- Neutrinos are very, very light. For a long time it was assumed that neutrinos had no mass at all, which means that they travel very, very close to the speed of light. This tiny bit of mass (and high end estimates put them at about a millionth the mass of an electron, the next lightest particle) allows the three different "flavors" of neutrino to turn into one another.
- All neutrinos spin the same way, left-handed. Take your left hand and point your thumb in some direction. If a neutrino is moving in that direction, then the curl of your fingers tells you which way the neutrino is spinning. All neutrinos are created this way and honestly, we don't know why.
The combination of these observed facts mean that neutrinos provide tantalizing clues about the next stage in our understanding of the physical world.
Part of the mystery is that the Higgs mechanism (whose eponymous particle was discovered a few years ago) is supposed to give mass to a whole host of ordinary particles: electrons, quarks (the particles that make up protons and neutrons), and carrier particles known as W's and Z's – but not to neutrinos.
Neutrinos shouldn't have mass according to the Standard Model. It all has to do with the relationship between spin and mass. All particles with mass travel at less than the speed of light, which means that in principle, you could fly a spaceship and overtake a neutrino (though you don't actually have to do so for the point to be valid). From the perspective of the spaceship, the neutrino is pointing backwards and thus, from a perfectly valid point of view, a left-handed neutrino becomes a right-handed one. Believe it or not, that sort of reasoning really does rule out the idea that the Higgs can give rise to a massive neutrino. So where does its mass come from?
There's a lot of speculation, not least that there may be as-yet undiscovered "sterile" neutrinos out there. But in order to probe all of this, we need to detect lot more information about neutrinos from a fairly wide range of sources (not an easy task) to start deducing.
The IceCube Experiment
About 10 years ago, the University of Wisconsin began construction of a very strange looking telescope. The IceCube Neutrino Observatory is situated near the south pole and consists of 86 "strings" of detectors, each stretching about a kilometer and a half below the ice. It looks for all the world, both in placement and in construction, like an inverted Fortress of Solitude.
But it does beg the question: why build an experiment at the south pole at all? Sure, the land is cheap, but it's kind of inconvenient.
Courtesy: The IceCube Collaboration.
One advantage of Antarctic observing is all of the ice. The detectors actually use the ice at the South Pole as part of the detection. Neutrinos come flying in from deep space and some few of them will interact with the subatomic particles in the ice. The collisional energy is so high that the recoiling particles (muons, in this case) are moving faster than light*. When this happens, the muons give off "Cherenkov Radiation," and the detectors pick up that signal and the physicists are able to interpret the energy of the original neutrinos.
* Legal disclaimer: Faster than light, in this context, means faster than light could travel through the ice, not faster than light can travel through the vacuum of space. That, so far as we can tell, is impossible.
The South Pole has other advantages as well. For instance, at 1 1/2 km below the surface, pressure has squeezed all of the air bubbles out, making the backgrounds much cleaner than they would be otherwise. And, simply put, there's a lot of ice down there.
While there's a ton of interesting science that IceCube can do – measurements of neutrinos from the atmosphere, searches for dark matter, and much else – I want to focus on the original question, super-energetic neutrinos from deep space.
Ultra-High Energy Neutrinos
At this point, we've been detecting extraterrestrial neutrinos for half a century, at the rate of a few a day at each of a dozen or so sites around the world. So why, you might ask, is it worth the extra effort to build a detector in Antarctica that has recorded only a few dozen high-energy neutrinos? That's thirty-seven (!) individual particles out of an entire vast cosmos.
I'd argue that it is, and that the original hype surrounding these discoveries was well worth it.
In April of last year, the ICECube team announced that they discovered a pair of insanely high energy neutrinos, with a 3rd announced a few months ago. The energy range for these neutrinos was approximately 1 Petaelectronvolt, which to put things in perspective, is roughly 140 times the peak energy of particles in the Large Hadron Collider. It's thousands of times more energy than found in the mass of any particle we've ever yet discovered, which raises the obvious question: where the hell did these things come from?
Image: The highest neutrino event ever detected. Courtesy of the IceCube experiment.
Physicists throw around big numbers all of the time, so maybe it's best to put these energies in perspective. At the very low energy end, huge numbers of neutrinos were created in the early universe – almost as many as the number of photons. But we've never seen these primordial neutrinos, in large part because they have energies 10 quadrillion times smaller than the monsters found at IceCube.
Even the neutrinos created in the sun and the typical ones created in nuclear reactors here are earth are a billion times less energetic than the ones seen at the South Pole. Even the highest energy cosmic rays produce neutrinos only about a tenth as energetic as the ones detected at IceCube.
This essentially means that whatever we're seeing wasn't created in our solar system, and very likely not in our Galaxy at all. Instead, we're seeing relic neutrinos from halfway across the universe. From where? We can't say.
That's the truly exciting part. Scientists speculate about physics "beyond the Standard Model," all the time, but it's much more important when we have experimental results to really push us forward. We don't know what's creating such high energy neutrinos, but since the announcement there have literally dozens of papers purporting to explain them. I wouldn't presume to mediate the melee, but the very fact that we have verifiable and continually repeatable data that isn't explained by our current model should be cause for intense curiosity, at least.
Dave Goldberg is a Physics Professor at Drexel University, your friendly neighborhood "Ask a Physicist" columnist, and, most recently, author of The Universe in the Rearview Mirror: How Hidden Symmetries Shape Reality, which comes out in paperback tomorrow. You should absolutely send him all of your questions about the universe. You can also follow him on facebook.