Neutrino Beam Sends Message Through Solid Rock

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Fermilab’s Tevatron may have ceased operation, but the various other experiments on-site are still going strong. Case in point: a Fermilab team have made use of a pulsed neutrino beam source at the accelerator’s Main Injector to transmit a coded digital message to a detector located about a kilometer away (0.6 miles).

Think of it as a high-tech telegraph, using neutrinos instead of electromagnetism.

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While radio communications are pretty much ubiquitous, there are challenges. If a spacecraft slips behind the moon or a planet, the radio usually falls silent because of the interference. And submarines frequently lose radio contact in deep waters, because only extremely low frequency waves can easily travel through water. Even then, the data transmission rate is a pokey 1 bit per minute.

Back in 2009, Virginia Tech physicist Paul Huber suggested that using neutrino beams for submarine communication could overcome those challenges and boost data transmission rates as high as 100 bits per second.

Neutrinos are subatomic particles that travel very near the speed of light. They have no charge and, until quite recently, physicists believed they had no mass. They’re extremely difficult to detect, because they very rarely interact with any type of matter. Only one out of every 1,000 billion solar neutrinos would collide with an atom on its journey through the Earth.

How does that help submarine or space communication? Well, neutrino beams are produced by accelerating muons to extremely high speeds. Those muons then decay into neutrinos bunched together into a focused beam. To detect them, you just reverse the process. When neutrinos interact with matter — say, a chlorine molecule in water — they emit muons, which are easily detected.

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Back then, Huber told Technology Review that he envisioned two possible scenarios for using neutrino beams for submarine communications. First, the entire surface area of the vessel could be coated in thin modules for detecting muons, like high-tech wallpaper. Second, one could use a detector to pick up the telltale blue flashes of light that result when muons move through salt water (Cerenkov radiation).

Huber isn’t the first to come up with such a scheme. Back in the 1980s, electrical engineer Dan Stancil was working at Carnegie Mellon University, and broached the possibility of using axions — theoretical particles predicted by some dark matter theories — for communication applications, precisely because they were so weakly interacting.

Fast forward to 2009, and one of his former students, Jim Downey, told Stanceil about the 170-ton MINERvA neutrino detector. That got Stancil thinking about using neutrinos for communication instead, since MINERvA would be the perfect receptor in the system. Even better, its location is close to an intense neutrino beam source Fermilab’s NuMi experiment at the Main Injector — that is also pulsed (a requirement for any “neutrino telegraph”).

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In the end, it took just two hours of beam time to manipulate the pulse to send the word “neutrino” to the waiting detector, written in a 7-bit ASCII code, with pulses of neutrinos representing the 1s and the absence of pulses representing the 0s. MINERvA managed to decode the signal with 99% accuracy after just two repetitions — even though the neutrino beams had to penetrate solid rock for a good 240 meters.

Naturally, there are caveats. The data rate was just 0.1 bits per second, a far cry from the 100 bits per second that might ideally be possible.

Also, this is really just proof of principle. Even neutrino beams will start to spread out like radio waves as they move further and further away from the signal source, hence the need for even greater intensity in the beams — not to mention more powerful detectors.

As for the applications, submarines might be able to receive coded messages via neutrino beam some day, but they would still lack the technological means to send a coded reply. Ditto for interstellar missions. But as the technology continues to advance, who knows? Pulsed neutrino beams might be the radio waves of the future.

Image: The MINERvA detector. Credit: Fermilab