That neutrinos play such a key role in advancing physics would have surprised scientists of a few generations ago. For them the neutrino was a figment of imagination, a theoretical necessity, but one that seemed impossible to detect because of its ethereal nature—a ghost of a particle. The story of the neutrino begins in the late 1920s. Physicists had been puzzling over something called radioactive beta decay, in which one kind of atom changes into another. For instance, carbon-14 has eight neutrons and six protons. During beta decay, one of these neutrons decays into a proton and emits an electron. The new nucleus, now with seven protons and seven neutrons, is transformed into nitrogen-14. But during this process, some energy seemed to go missing. It was the Austrian-born physicist and Nobel laureate Wolfgang Pauli who theorized that beta decay must emit an as yet undiscovered neutral particle. A few years later, the physicist Enrico Fermi jokingly named the particle a neutrino, Italian for “little neutral one,” and the name stuck.
For decades the neutrino remained a theoretical construct, a useful particle that helped physicists save their theories from embarrassment. Nobody had seen one. Nobody even knew how to find one—until Frederick Reines, a researcher working at Los Alamos during the 1950s, realized that a nuclear bomb would be a significant source of neutrinos. Reines and his colleague Clyde L. Cowan Jr. thought a nuclear power plant would also be a source. They calculated that a detector near a nuclear reactor would encounter nearly 1013 neutrinos per square centimeter per second. There was just one small problem: Since neutrinos are electrically neutral, they could be detected only if they directly hit the nucleus of an atom. Reines and Cowan would have to look for the signature of such a collision. And they found it.
By the 1960s, physicists following up on Reines’s work had started building neutrino detectors inside mines, using the ground as a natural shield from cosmic rays, which can swamp the signal from neutrinos. (Neutrinos can pass through the thick walls of the mines, but cosmic rays cannot.) In 1968 Raymond Davis and his colleagues from Brookhaven National Laboratory completed an experiment inside the Homestake Gold Mine in Lead, South Dakota. They used a tank containing 100,000 gallons of tetrachloroethylene, a common dry-cleaning agent. When a neutrino smashed into an atom of chlorine, the atom was transformed into one of radioactive argon. By counting the number of argon atoms that were produced, the physicists could calculate the flux of neutrinos coming from the sun. Then in the early 1980s, researchers around the world built detectors using thousands of tons of water in underground tanks lined with photomultiplier tubes (PMTs). The PMTs look for light emitted when a neutrino smashes into water. Normally the neutrino will pass right through water without any interaction. But on the rare occasions when one does hit a nucleus of hydrogen or oxygen, the collision can spit out another subatomic particle, a muon. The charged muon interacts with the water electromagnetically, and because it is moving faster than the speed of light in water, it leaves in its wake a cone of blue light. This is called a Cherenkov cone, after the Russian physicist who first described the phenomenon. It is analogous to the sonic boom caused by an aircraft traveling faster than the speed of sound.
It was another Russian researcher, Moisey Alexandrovich Markov, a “poet” of astroparticle physics, who suggested using natural bodies of water as neutrino detectors. Instead of building tanks of water inside mines, why not use lakes or even oceans? Just submerge long strings of photomultiplier tubes into the water and watch for the Cherenkov light left behind by neutrino-generated muons. The idea was enticing, but there were huge practical difficulties. For one thing, without rock above to protect it, a detector would be exposed to cosmic rays that could drown out signals from neutrinos. More to the point, sunlight (not a problem inside mines) would blot out the Cherenkov emission.
The solution was to go deep, where the sun’s rays could not reach. The physicists realized that they could use the Earth itself as a shield. While many muons can make it through a mile of water, a similar stretch of rock will stop them cold. So a neutrino detector can sit deep underwater, near the lake bed, looking downward for muons created by neutrinos that come from below. None of the muons created by cosmic rays in the atmosphere on the other side of the Earth can penetrate the planet. Neutrinos, however, zip right through, and occasionally one will hit a nucleus in the water or in the lake bed itself. Such a collision generates a muon, which then shoots up toward the surface. Catch an upward-moving muon and you have essentially detected a neutrino that came from the other side of the Earth. All that was needed was a suitable body of water. By the mid-1980s, the Russians realized that they had a massive tank of pure water in their own backyard: Lake Baikal.
On my first morning in Siberia, we drove across the lake toward the telescope. The frozen white lake spread for miles around us in every direction except to the northwest, where we were relatively close to shore. When we stopped to rest, men milled around the vehicles. The subzero temperature seemed to affect everyone differently. Some stood bareheaded; others had woolen caps rolled down to the tips of their ears. And then there was Ralf Wischnewski, in his enormous Russian fur cap that looked like a fluffed-up rabbit. A German neutrino physicist who had been working with the Russians at Lake Baikal for 20 years, Wischnewski was the reason I was here. I had met this ruddy-faced man six months earlier in London, outside the Tate Modern museum on the south bank of the Thames. We walked over to a Greek pub and discussed the Baikal expedition over chilled lager. It was he who had alerted me to the tradition of bringing spirits to share with the Russians during the winter evenings.
And here we were, except that it was still morning. The Russians had planned a welcome drink for Heinze. Kolja Budnev bounded out of our jeep with a bottle of vodka. Someone sliced a sausage into circular pieces. Bright yellow, blue, and red plastic cups were set up on the jeep’s expansive hood, and soon everyone had a vodka-filled cup in hand. Budnev dipped a finger into his and flicked a few drops onto the ice—an offering to the great spirit of Lake Baikal.
Soon we got back into our vehicles and headed toward the neutrino telescope, a contraption made of 11 strings of photomultiplier tubes, each with a large buoy at the top and a counterweight at the bottom. Smaller buoys attached to the strings float about 30 feet below the surface. All year round, a total of 228 PMTs watch for the Cherenkov cones. Each winter the team has to locate the telescope, the upper part of which drifts slightly over the course of the year. The team has two months to carry out any routine maintenance, put the strings back in the water, and get out before the ice cracks.
The term “experimental physics” took on new meaning in this biting cold, which at times dropped to –4 degrees Fahrenheit. Most of the physicists lived in 10-by-20-foot cabins, two to a cabin. Others slept in bunk beds at the shore station, amid workbenches cluttered with computers, electronics, wires, and cables. They worked long hours, from early in the morning to sometimes well past midnight. There was no running water, which meant no showers for two months. Toilets were wooden cabins with pits in the ground. The extreme cold helped control the stench, but it still wafted up when warm urine hit the pit. There was one luxury: the banya, a traditional Russian sauna. Naked men sat in an outbuilding, chucked water on hot stones to raise steam, and beat one another with leafy twigs and branches of birch.
A wicked wind kicked up one evening. It was time for everyone to leave the open ice and head back to the shore station. Once there, I gratefully sat down for a cup of tea, and a can of sweet, syrupy condensed milk materialized. One scientist looked at the can wistfully. Condensed milk had been his dream as a child growing up in the Siberian city of Tomsk. “They had this in Moscow,” he said, “but not in Tomsk.”
Later that evening, I had to head back out and traverse part of the icy lake to reach the canteen for dinner. It wasn’t going to be easy. I had turned up on a frozen lake in the depths of a Siberian winter in “European summer shoes,” as Wischnewski put it, disbelief in his voice. On the lake I found walking nearly impossible, my smooth-soled shoes slipping the entire way. After a few days, I learned to find fresh snow for my shoes to grip, but that night, fear nearly paralyzed me. Fortunately, a jeep pulled up beside me, and Wischnewski, having noticed my plight, asked the driver—Igor Belolaptikov, a tall, mustached physicist from the Joint Institute of Nuclear Research in Dubna, near Moscow—to take me to the canteen. I sat with Belolaptikov at dinner and happily accepted a ride back to his small cabin for a chat about neutrinos.
“My business is the reconstruction of muons and neutrinos,” Belolaptikov said, laughing with a childlike joy as he made this disclosure. That reconstruction is tricky business. Hundreds of photomultiplier tubes watch for the flashes of Cherenkov light at the bottom of Lake Baikal. As a neutrino-induced muon races through the water, the light from its Cherenkov cone reaches different tubes at slightly different times. The skill lies in collecting all the information and sifting through it to reconstruct the path of the upward-moving muon. This can then be used to calculate the path of the original neutrino. It is this ability to figure out where a neutrino comes from that differentiates a neutrino telescope from a mere neutrino detector. A telescope must identify the source of neutrinos in the sky, and the Lake Baikal instrument can do so with an angular resolution of about 2.5 degrees, meaning that it can distinguish neutrinos coming from points in the sky separated by a distance of five full moons. So far the Baikal telescope has seen only atmospheric neutrinos, secondary particles created by cosmic rays crashing into atoms in the air. Everyone here is waiting for the day when a high-energy neutrino from outer space makes its presence felt in their little corner of the lake.
Belolaptikov recalled his first neutrino—indeed, the Baikal detector’s first—from 1993. “It was great,” he said. “Here, you can see.” He leaned over his bunk bed and removed a piece of paper pinned to the wall above. It was a printout of the path of an upward muon, reconstructed from the detection of its Cherenkov cone: the first-ever neutrino seen by humans using a natural body of water as a detector. Belolaptikov and colleagues had done the reconstruction and put the Lake Baikal detector on the map.
The next two days slid by, but even in this short time a rhythm was established. A trip down to the lake in the mornings to get a bucketful of drinking water from a hole in the ice. Then back to the cabin for coffee with condensed milk and honey, making sure to plug the hole in the can of milk with paper to prevent “little animals” (as Wischnewski calls insects) from getting in. From my cabin I could see clear across the lake, and I had to remind myself that it had more water than America’s five Great Lakes put together and a surface area larger than Belgium. Eighty percent of Russia’s freshwater was here. Even at great depths, the lake is well oxygenated, making it one of the most hospitable waters for life. Because of the voracious crustaceans that live at all depths, nothing dead or dying lasts more than a few days in this lake. If fishermen leave their catch in the nets too long, the crustaceans invade the fish through their mouths and gills, eating them from the inside out. These critters keep the lake free of dead matter, leaving it unimaginably clear, especially deep down. Murky waters would make watching for muons nearly impossible. “It is a very, very kind water,” Budnev said.