This image shows one of the highest-energy neutrino events of this study superimposed on a view of the IceCube Lab at the South Pole. When a neutrino interacts in the Antarctic ice, it generates particles that in turn make flashes of light in the ice. In this illustration, the size of each sphere is proportional to the intensity of the light, and the color shows its time, where red is the earliest. Image: IceCube collaboration
January’s column was about snowflakes; this is about an ice cube, but a huge one called – wait for it – IceCube. Each side is 1 kilometer, 1,000 meters, so the volume is one billion cubic meters, and it weighs a billion tons. Calculations are easy in the metric system!
Scientists wanted a massive block of clear ice to detect mysterious neutrinos coming from far away in the universe. These particles interact with matter so rarely that 99.9999% pass right through that block leaving no trace. But one in a million hits a quark, much smaller than a proton, creating a shower of new particles, which make flashes of light in the ice.
The scientists did not have to make a giant ice cube; they found one under the snow at the South Pole. The ice there is more than 2.5 kilometers deep, and they chose a cubic kilometer with no air bubbles and little dust. But they had to get 5,000 light detectors 1.5 to 2.5 kilometers down in the ice and connected to the surface with cables.
The ingenious solution was to drill 60-centimeter-diameter holes with hot water. About 400,000 gallons of oil were used to heat water and melt vertical columns of ice. The water did not refreeze for several days, enough time to lower cables with photomultipliers every 60 meters. Starting in 2004 that took seven years. The detectors will be stuck in the ice for a thousand years – or less if the Antarctic melts faster.
It’s dark down there in the ice, but sometimes a neutrino makes a light flash bright enough to be detected through the clear ice by hundreds of photomultipliers. From the pattern of the signals, their brightness and time, the energy of the neutrino and its direction can be calculated.
Some neutrinos had energies of more than 1,000 trillion electronvolts (or TeV, in the language of scientists). That is 150 times the energy of the protons in CERN’s Large Hadron Collider. Those neutrinos must have come from the deep cosmos.
In September 2017, an ultrahigh-energy neutrino was detected from a gamma-ray source called a “blazar.” Alerts were sent to astronomers who saw bursts of gamma rays and X-rays from the same spot in the sky, confirming its origin. That neutrino had been travelling through space at 186,000 miles per second for 4.6 billion years before its death in a glorious flash of light.
Neutrinos from the center of the sun and from a supernova explosion had been detected before, but this was really the birth of neutrino astronomy.