We’re figuring out how to see the cosmos in a new way. For centuries, humans learned about the universe with just their eyes, observing in visible light. We eventually developed technology to see in other wavelengths of light — infrared, X-ray, gamma ray — to discover even more about the universe. Now, thanks to two papers in Science in July, astronomers can see with neutrinos, too.
Neutrinos are thought of as ghostly particles because they interact so little with matter. Trillions of the subatomic things pass through your body and the rest of the planet every second, without disturbing any atoms. This indifference to anything in their way makes them ideal for astronomers hoping to understand the extreme environments that can produce them and other interesting but easily blocked particles, like cosmic rays and extremely energetic light.
“Light is whatever’s on the outside of an object,” says University of Maryland physicist Erik Blaufuss, a co-author on both papers. The sun’s light, for example, emerges at its surface, whereas solar neutrinos are created deep within the star. “Neutrinos really are deeper probes on the internal workings of the object.”
The hard part is actually catching a neutrino, especially one from a distant source, but that’s what the IceCube observatory in Antarctica did in September 2017. A particularly energetic neutrino got caught in the cubic kilometer of ice that serves as the observatory’s detector. Astronomers soon realized they could trace its trajectory back to a specific type of black hole, known as a blazar, in a galaxy 3.7 billion light-years away.
That alignment could have just been coincidence, but then other observatories reported a surge in energetic gamma rays coming from the same black hole around the same time as the neutrino’s arrival. The overwhelming likelihood is that the black hole spewed out both the gamma rays and massive amounts of neutrinos, including the one IceCube caught.