Cold Case: IceCube is on the verge of solving one of the oldest mysteries in physics

My new book is about IceCube, a unique telescope at the South Pole that is designed to detect not light, but a strange elementary particle called the neutrino. The way the production process for books works, I had my last chance to change anything editorially about three months ago, in late July. Amazingly (and this is the way science works), the IceCube scientists have already taken a dramatic step forward.

This telescope is made out of ice: a cubic kilometer of diamond clear ice, a mile and more beneath the polar surface, outfitted with a grid of more than 5,000 light sensors. The ice itself is the basic detector: when a neutrino collides with an atomic nucleus in the ice in or near the sensor grid, or even in the bedrock below it, the neutrino will die and give birth to a charged particle that travels in the same direction as the neutrino, dragging a cone of a pale blue light along with it. By tracking this streaking particle as it passes through the grid, the physicists can determine the direction of the parent neutrino. In 2013, the IceCube collaboration announced that they had detected the very first high-energy neutrinos coming from outer space and thus gave birth to the field of neutrino astronomy.

They can also do particle physics with this detector. In fact, IceCube is the largest particle physics detector ever built—by a long shot. It weighs about a billion tons.

In the wide, wide world that is physics, the most comfortable home for this experiment is cosmic ray physics, a wonderful combination of astrophysics and particle physics that has an allusive, artistic flavor to it, because cosmic rays are so hard to pin down. A large and venerable field, predating particle physics, nuclear physics, and even Einstein’s theory of general relativity, cosmic ray physics was born in 1912, when the Austrian physicist Victor Hess made the first measurements, at 17,400 feet in a hydrogen balloon, that gave proof to a pervasive “ionizing radiation,” constantly streaming into the atmosphere—and through it, into our bodies and our planet—from space. Hess’s “rays” are now known to consist mostly of protons and larger atomic nuclei—and now that we know how to detect them, neutrinos as well. When they reach our planet, they usually collide with the atmosphere to generate showers of secondary particles—which makes understanding primary cosmic rays all that more difficult.

It is not possible to do astronomy with a charged cosmic ray particle, since interstellar magnetic fields will bend its trajectory as it flies through space, so that its incoming direction gives no clue as to the place it was born. Since the neutrino is uncharged, on the other hand, it travels in a straight line, like light, and can be used for astronomy.

The astrophysical objects that produce cosmic rays are the scenes of the most violent events in the universe: supernovae, active galactic nuclei, supernova remnants, gamma ray bursters, colliding galaxies, and other strange beasts, some not yet imagined. And all of these creatures are basically huge particle accelerators, operating by the same basic principles as the manmade variety here on Earth. The incredibly high electromagnetic fields generated by these violent events accelerate the charged particles in their neighborhoods to extremely high energies and hurl them off into interstellar and intergalactic space, where some will eventually reach our planet. Cosmic ray physics in fact gave birth to particle physics, for until the 1950s when manmade accelerators came on line, most new particles were discovered in cosmic ray air showers.

In the 1960s, physicists began to realize that cosmic rays can reach astounding energies. The record-holder thus far is the so-called Oh-My-God particle, which was observed by an instrument called the Fly’s Eye in a Utah desert in 1991. This single subnuclear spec packed as much punch as a baseball going about sixty miles an hour—about three hundred thousand times the capability of the most powerful accelerator mankind has ever built, the Large Hadron Collider at CERN. When the Oh-My-God particle hit the atmosphere it gave birth to a shower of about two hundred billion secondary particles and decay products. No one is sure what kind of particle it was.

Since most cosmic ray particles are charged and we can’t know where they come from, exactly which type of cosmic object emits these astoundingly energetic particles has been a mystery now for about sixty years. This is probably the most important question in cosmic ray physics. It has always been one of the main items on the menu for neutrino astronomy, and indeed, a little more than a month ago, IceCube, the world’s first working neutrino telescope, may have provided the answer.

On September 22nd, the blue streak from a muon, born from a neutrino, zoomed through the grid of ice-bound light sensors at the South Pole. IceCube detects about three hundred neutrinos a day, so this alone was not particularly special. What was special was the muon’s energy. It was almost ten times as energetic as any particle that could possibly be produced at the Large Hadron Collider. This meant there was a high probability that the neutrino that created this muon came from outer space.

IceCube sent out an automated, public “alert.” About four hours later, the two IceCube scientists who kept track of such things, Claudio Kopper at the University of Alberta and Erik Blaufuss at the University of Maryland, issued a second alert, encouraging “ground and space-based instruments” to look in the direction the neutrino came from, in order “to help identify a possible astrophysical source for the candidate neutrino.”

For the first time ever, optical telescopes found the likely source of a cosmic neutrino. They identified a blazar, some billions of light years away, within 250 arc-seconds—six-one hundredths of a degree—of the spot where the neutrino was pointing. This is a bullseye.

Blazars are among the brightest, most violent, and least understood creatures in the astronomical zoo. They’re thought to be giant elliptical galaxies with rapidly spinning, supermassive black holes at their cores, which scavenge up the material around them in a sort of cosmic earthquake. What distinguishes them from quasars and other forms of radio galaxy is that they send out high-energy “jets” of light and elementary particles, kind of like laser beams, from their “north and south poles”: up and down along their axes of rotation. The reason they appear so bright is that these jets sometimes aim straight at Earth. And blazars are unruly beasts. For reasons that are not understood, they occasionally flare: they become brighter by factors or ten or more, for period of hours, days, or months. The blazar that was identified with the September 22nd neutrino happened to be flaring.

The first thing to be said is that this was the second indication in the space of about one month that we have now entered a new era in astronomy: multi-messenger astronomy. On August 22nd, three huge gravitational wave instruments had detected a wave from the merger of two neutron stars, they had told optical astronomers where to look, and dozens of telescopes had observed the resulting “gamma ray burst.” Physicists had long suspected that some gamma ray bursts were generated by merging neutron stars, but this was the first proof, and it could only have been provided by this new gravitational messenger. One physicist observed that it was the “first time we have a 3D IMAX view of an astronomical event.” IceCube has now added neutrinos to the palette. Now that we can study the incredible variety of astrophysical phenomena with all wavelengths of light, gravity waves, and neutrinos, it will easier to figure out what makes them tick.

As in the case of the gravity wave, IceCube can do some science with the September 22nd neutrino as well.

It is reasonable to suspect that the objects that emit high-energy cosmic rays, like this neutrino, might also emit high-energy photons, particles of light. So it is especially intriguing that two of the optical telescopes that found the September blazar were gamma ray telescopes: NASA’s Fermi satellite telescope, and MAGIC, the Major Atmospheric Gamma Imaging Cherenkov telescope in the Canary Islands. Gamma rays are the most energetic form of photon.

Fermi has been in low Earth orbit for almost ten years and has made several major discoveries. One is that most of the high-energy gamma rays reaching us from outer space—more than 80 percent—seem to be coming from blazars. So, in recent years, quite a few cosmic ray physicists have been rooting for blazars as the mysterious source of ultrahigh-energy cosmic rays. The simultaneous detection of a high-energy neutrino and gamma rays from a flaring blazar is the best evidence so far that this may be the case.

But one neutrino isn’t all that convincing, is it?

In fact, this same sort of thing, though never quite this convincing, has happened three times before.

About fifteen months ago, on July 31, 2016, IceCube sent out a similar alert. A year later, this past July, the AGILE collaboration (Astro‐Rivelatore Gamma a Immagini Leggero) which operates an orbiting X-ray and gamma ray telescope launched by the Italian Space Agency, announced that they had found an object that looked awfully like a blazar in the direction the neutrino was pointing, and that that blazar had flared for about a day, just one day before the neutrino was detected. Less convincing, as I say, but still there.

In April 2016, the Fermi satellite collaboration announced a similar coincidence between a neutrino that IceCube had detected in 2012, before the days of automatic alerts, and a flaring blazar about 10 billion light years away (which means the blazar flared and the neutrino was born about 10 billion years ago). “Big Bird,” as this neutrino is known, is among the most energetic neutrinos ever detected. (In the early days, the students in IceCube named the most interesting neutrinos they found after Sesame Street characters.) For esoteric reasons that it is not necessary to go into here, it was harder to tell exactly where Big Bird was pointing, so again, this wasn’t entirely convincing.

Perhaps the most intriguing of the four blazar/neutrino connections is the earliest, which occurred fifteen years ago. At that point, IceCube hadn’t been built yet. Its predecessor or prototype, the smaller and much less sensitive Antarctic Muon and Neutrino Detector Array (AMANDA) detected three neutrinos coming from the direction of a flaring blazar in 2002.

Now, none these events alone is enough to claim a discovery. Together, however, I think I can go out on a limb and say that they probably add up to one.

To say that the thousand or so physicists who work on IceCube, Fermi, and MAGIC are excited would be an understatement. They seem to be very close to answering the longest-standing and arguably most fundamental question in cosmic ray physics. They’re doing what physicists do, arguing strenuously among themselves about the statistical significance of what they have found and what sort of public claims they might eventually make. And they’re combing feverishly through their historical data to see what they can see about blazars and cosmic neutrinos … .

That is quite a treasure trove. There is almost certainly more to come.