A ubiquitous river of invisible particles, zooming mostly undetected through the black void of space at nearly the speed of light, may hold vital clues to everything from black holes to missing matter to the origin of the universe. These particles are nearly massless, carry no electric charge and are thus loath to interact with matter, making them frustratingly difficult to detect.
In fact, we won't even know if these super-high-energy cosmic neutrinos, as they are called, do exist until if and when they are discovered. But finding them will require a really big telescope.
Maybe something as big as the Moon.
Peter Gorham, of NASA's Jet Propulsion Laboratory, isn't waiting around for someone to build such a large detection device. Instead, he and a colleague are employing the Moon itself. Gorham and David Saltzberg, a particle astrophysicist at UCLA, revived and added to a 35-year-old idea first proposed by Soviet scientists.
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Neutrinos everywhere!
All told, more than a billion neutrinos zipped through your body while you read this sentence. At any given instant, hundreds are thought to inhabit every marble-sized patch of space, on average, throughout the universe.
The vast majority are leftovers of the Big Bang or are generated by nearby stars like our Sun. Others are created when cosmic rays slam into Earth's atmosphere. Only these relatively lower energy neutrinos have actually been detected so far.
Knowing that the highest energy neutrinos cannot penetrate matter as well as their less energetic cousins, the researchers borrowed a little time from NASA's Deep Space Network of radio telescopes (an idea first suggested in 1988 by Igor Zheleznykh) to look for signs of neutrinos crashing into the Moon. The researchers expect that as neutrinos enter lunar soil, they release brief pulses of microwave radiation that would be detectable on Earth.
Gorham and Saltzberg describe their idea, and the initial data, in a scientific paper that has not yet been published. So far their observations, limited to just 30 hours in the past year, have not yielded any neutrinos. This fact alone may rule out some of the more exotic cosmological models that other scientists have developed on the assumption that super-high-energy neutrinos exist, Saltzberg said.
In that sense, the longer that the researchers find nothing, the more they learn.
But no one is giving up. In fact, other neutrino experts who pursue their quarry via more conventional means welcome the novel and inexpensive lunar telescope.
"Since it is relatively cheap it is well worth trying," said Trevor C. Weekes a pioneer in high-energy astrophysics at the Harvard-Smithsonian Center for Astrophysics and the Whipple Observatory. Weekes, who is not involved in the project, said that if a detection were made, it will probably "not be unambiguous and will demand confirmation in a more conventional detector."
Seeking the unknown
Neutrinos are among the most abundant and energetic particles in the universe. They are elementary particles which, along with other odd characters like quarks and leptons, make up all matter -- from atoms to molecules, pencils, people and stars.
Low-energy neutrinos can penetrate right through Earth and even through stars. But the higher their energy, the less able neutrinos are to penetrate solid matter, because their energy causes them to interact with the matter. So the most energetic would be the rarest, because most would be absorbed during their space travels.
Researchers estimate that these particles might arrive in our corner of the cosmos at a rate of only one per square kilometer per day. But they are at the heart of the most important cosmological questions -- how the universe formed and evolved, and what sorts of objects are still out there.
"It may actually be that there's some brand new physics that we don't know about," Saltzberg said in a recent telephone interview.
Powerful and puzzling questions
One pressing question that neutrinos might help answer is the infamous missing matter problem -- a sobering realization that about 90 percent of the mass of the universe is unaccounted for, based on observations of how galaxies behave under the effects of unseen gravity.
While neutrinos were once thought to have no mass, a discovery announced in 1998 by researchers using the underground Super-Kamiokande detector in Japan showed that they do weigh something, and in fact might comprise as much mass as all the stars in the sky.
Scientists also expect to learn much about the universe by figuring out where neutrinos come from.
Other observations of distant, high-energy emissions provide indirect clues to possible sources of neutrinos. For example, researchers have measured intense gamma ray bursts and high-energy cosmic rays coming from distant, unknown sources.
The high-energy cosmic rays, in particular, leave even the smartest
. No one can figure out how these cosmic rays, which could only be produced by the most extreme and presumably very distant events, survive long journeys to our galaxy. Because they are of such high energy, they should be absorbed along the way as they crash into stars and sometimes dense interstellar clouds of gas and dust. If instead these emissions are produced more locally, their powerful sources should be easy to find.
"We think we know most of what exists in our local neighborhood," Saltzberg said. "And there isn't anything which is a good candidate for producing these enormously high-energy particles. So it's a bit of a mystery where these are coming from."
Importantly, particle physicists think that whatever is spitting out these cosmic rays must also produce high-energy neutrinos.
"Whatever the source, the machinery which accelerates particles to the higher energies will inevitably also produce neutrinos," said John G. Learned, a particle astrophysicist at the University of Hawaii.
Learned said neutrinos may prove indispensable in understanding gamma ray bursts, flashes of energy that in a moment exceed the energy output of entire galaxies. Increasingly, researchers suspect that gamma ray bursts are related to supernova, which are the explosions that signal the end of a star's normal life.
Supernovae in turn "are central in understanding the evolution of galaxies, star formation and the genesis of heavy elements," Learned pointed out. "All of your atoms have been through a supernova sometime long ago."
In that sense, the search for neutrinos is a search to understand the origin of our universe and ourselves.
Next Page: How to make a telescope out of the Moon
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