Subatomic particles found in mile-deep ice are of interstellar origins

Physicists working with the particle detector IceCube, buried near the South Pole, have detected neutrinos of high enough energies to suggest origins in the cataclysms at the Milky Way's fringes, or perhaps even past its doorstep.

Sven Lidstrom/IceCube/NSF
At the IceCube neutrino observatory, buried deep in Antarctic ice, scientists have for the first time ever detected neutrinos of high enough energies to suggest origins far out in interstellar space.

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.”

You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

QR Code to Subatomic particles found in mile-deep ice are of interstellar origins
Read this article in
https://www.csmonitor.com/Science/2013/1121/Subatomic-particles-found-in-mile-deep-ice-are-of-interstellar-origins
QR Code to Subscription page
Start your subscription today
https://www.csmonitor.com/subscribe