Neutrinos are having a moment. They’re speeding across Europe (just how fast is under review), they’re changing flavors in China and, now, they’re carrying rudimentary messages through bedrock in Illinois.
A team of physicists encoded a short string of letters on a beam of neutrinos at Fermi National Accelerator Laboratory in Batavia, Ill., and sent the message to a detector more than a kilometer away. On the journey the neutrinos passed through 240 meters of solid rock, mostly shale. What was the word they transmitted in the preliminary demonstration? “Neutrino.” The experiment is described in a paper posted to the physics preprint server arXiv.org.
Neutrinos have been proposed for a variety of communication scenarios in which radio waves or optical signals fall short. Neutrinos rarely interact with ordinary matter, and they easily pass through solids that would screen out most other particles. So neutrino beams could be used to send messages through the Earth, or to communicate with a planetary rover parked on the far side of Mars, out of radio contact.
But the very slipperiness that makes neutrinos so intriguing for communication also makes them incredibly hard to use. Almost all of the neutrinos in a beam zip right through even the largest detectors. In the Fermilab experiment, the physicists fired a proton beam into a carbon target to produce a shower of particles called pions and kaons that quickly decay into neutrinos. For every pulse of 22.5 trillion protons, the physicists registered an average of 0.81 neutrino with the 170-ton MINERvA detector.
In other words, even with the benefit of a world-class proton accelerator and a mammoth particle detector, neutrino-based communication is far from efficient. By representing their one-word message in bits of ASCII code using a series of on-off pulses to communicate digital 0s and 1s, the physicists achieved a data rate of approximately 0.1 bit per second. At that rate it took more than six minutes to accurately relay the simple message “neutrino,” and that's omitting the extra bits needed to synchronize the signal transmission. Transferring the entire 5.8 petabytes of data stored at the nonprofit Internet Archive would take about 15 billion years—just a shade longer than the age of the universe.