Physicists working with the Antarctic particle detector IceCube have detected neutrinos of high enough energies to suggest origins at, or even beyond, our galaxy’s doorstep. The discovery of these proverbial poltergeists from the Milky Way’s fringes, after more than 50 years of hunting for them, is the first step in physicists’ goal of pinpointing these particles’ still-mysterious origins, a feat that would yield answers to big questions about the universe very far away, and very long ago.
“Neutrinos are the most direct probes we have into the most violent events in the universe,” says Naoko Neilson, an IceCube researcher and a co-author on the paper, published in Science, “and the most violent events in the universe are not well understood.”
The existence of neutrinos was first proposed in a 1930 letter from physicist Wolfgang Pauli to a group of fellow physicists. In the letter, Pauli suggested the subatomic particles as an explanation for the curious point that, in apparent violation of a fundamental law of nature, energy seemed to vanish during the decay of radioactive elements. Perhaps, he theorized, a crook of a particle, unknown to science, was carting off that missing energy. It was a befuddling proposition – outlandish, even. Still, as he put it in the letter, “those who wager win.”
In 1934, Enrico Fermi, another physicist, called the still hypothetical particles neutrinos, after their perplexing, frustrating nothingness (Pauli had initially called them neutrons, but this name became attached to a different, much bigger particle in 1932). The neutrino has a negligible mass and a nil charge. It also moves at the speed of light. It’s possible for the slight particle to pass through an Earth-sized block of lead and interact with nothing at all. A 2004 BBC documentary called the decades long hunt for the particles that ensued from their theoretical prediction “Project Poltergeist.” Fixing the problem of energy loss in radioactive decay had landed physicists the vexing problem of ghost hunting.
As in any good haunting, neutrinos are everywhere: after photons, they are the most common particles in the universe, the everyman poltergeist that inhabits the cosmos and the household toaster all the same.
About ten trillion neutrinos pass through you each second. Most of them were produced during the Big Bang. These “primordial” neutrinos, once hot, have had some 14 billion years to cool, and are very low-energy particles. Even now, low-energy neutrinos are still produced all the time, both on Earth and in our solar system, zipping out of the sun’s pops and sizzles, or out of nuclear reactors, or even out of cat litter, which is just a little bit radioactive. Even your own body emits the occasional neutrino, especially if you eat a lot of potassium-rich foods, such as bananas.
Since the first-ever detection of a neutrino – found in a nuclear reactor in 1956 – enormous particle detectors have been erected all over the world to detect something much, much smaller than a quark. Over the last few decades, low-energy neutrinos produced on or near Earth have been detected in celebrated drips and drabs. Once, in 1987, scientists detected multiple neutrinos of unusually high-energies: these were the first neutrinos found from outside the solar system, from a local supernova about 168,000 light years afield from us, and won their finders the Nobel Prize in physics in 2002.
But there has remained one group of neutrinos missing from scientists’ smallest of the small portrait of the universe, one that physicists have been after for decades: extremely high-energy neutrinos, called astrophysical neutrinos, from the outermost regions of the Milky Way, or perhaps from other galaxies. These neutrinos are brewed in traumatic rumblings that are unlike anything happening in our somnolent local cosmos – “there are not enough violent things in our solar system to emit these high-energy neutrinos,” says Dr. Neilson. By the time astrophysical neutrinos reach our planet, they are the ghostly messengers of ancient cataclysms, bringers of bad news from millions, or even billions, of years ago.
Just what these particles are telling us, though, is still an enigma, says Neilson. Physicists have proposed that the particles could be fleeing a supernova. Or it could be a gamma ray burst. Or something else. Physicists are hoping to find out what it is, and, precisely, where it is.
In theory, that’s possible. Since neutrinos do not have a charge, and because they are so preposterously bantam, they interact little with matter as they pass through it – “they’re very interaction phobic,” as Neilson puts it. That means that physicists can trace the particles in a straight line back to the scene of the celestial crime.
“Neutrinos are messengers of these violent forces in the universe,” says Neilson.
Enter the IceCube Neutrino Observatory. Completed in 2010 after seven years of construction and at a cost of $279 million, it is the world’s largest neutrino detector. It measures about one quarter of a cubic mile, or about the volume of 500 Amiens cathedrals or 546 Giants stadiums. It is also buried at the South Pole, almost a mile deep in ice, which happens to be a good place to study things that happened millions-of-light-years-deep in the space.
At the depth at which IceCube is buried, the pressure from above has squeezed all the bubbles out of the ice, so neutrino interactions can be detected with little dissonance. It is also dark, so that the blue light-emitting particle, called a muon, produced in interactions between neutrinos atoms in the ice is detectable.
In April 2012, IceCube recorded two neutrinos notable for energies much higher than those of the some 100,000 atmospheric neutrinos found in the detector each year. The neutrinos, reported in the scientific journal Physical Review Letters, were called “Bert” and “Ernie.” Their energies, at more than 1,000 tetraelectronvolts, were high enough to suggest interstellar space origins (a flying mosquito has a kinetic energy of about one tetraelectronvolt).
Now, in the latest paper, the IceCube team reports that on returning to their data from May 2010 to May 2012, IceCube in fact detected 26 neutrinos of lower energies than Bert and Ernie, but still suggestive of interstellar origins: all of these neutrinos measured energy levels of more than 30 tetraelectronvolts.
Still, the team does not have enough data to pinpoint from where, precisely, the far-from-home neutrinos are originating, though preliminary work suggests that at least some of them are coming from outside the Milky Way, says Neilson. It could be about a decade before Ice Cube has amassed enough data to attach a precise home address to the particles, she says, and, even then, doing so will depend on how well the data groups into an identifiable cluster. Right now, the data presents an un-telling collection of vagabond neutrinos hailing from all around the skies, she says.
“We’d like to be very sure before making a statement about discovering the source,” says Neilson, “and we just don’t know yet.”