Communicating underwater is a tricky business, as any commander of a nuclear submarine will tell you. These guys can remain hidden more or less indefinitely, operating at a depth of 300 meters or so, but communicating is a serious pain in the stern because it can only be done near the sea surface, where submarines are most vulnerable to detection and attack.
That’s because radio waves do not travel well through water. Only extremely low frequency (ELF) waves (with a frequency less than 100Hz) make any headway in water. But they are difficult to produce at high power, and even then, they only allow data rates of around 1 bit per minute.
Instead, submariners have to rely on very low frequency (VLF) waves of a few kilohertz. These allow up to 50 bits per second, but they don’t travel far through water. That means they can only be detected by trailing a long radio antenna close to the surface.
So how to improve matters? One suggestion is to use neutrinos to send information. The problem is that although neutrinos pass easily through water, they also pass through everything else, making them close to impossible to detect. For that reason, neutrino communication has always been thought a nonstarter.
Now a new analysis suggests that submariners may have been too quick to dismiss neutrinos. Patrick Huber, a physicist at Virginia Tech, says that neutrino communication could offer data rates of up to 100 bits per second at any depth. That’s three orders of magnitude better than ELF communication.
So what’s changing to make neutrino communication practical? First, says Huber, is the ability to generate and detect intense beams of neutrinos. Physicists generate beams of neutrinos by accelerating muons to high energy, which then decay, producing neutrinos that, because of the moving reference frame, are tightly collimated. Detecting neutrinos is simply this process in reverse. When the neutrinos interact with matter, they produce muons that can be detected relatively easily.
But how easily can this be done for submarine communication? Huber says that one of the most intense neutrino beams is used in an experiment called MINOS, which sends a beam from the Fermi National Accelerator Laboratory in Chicago to a 5,000-metric-ton muon detector in a mine in northern Minnesota, a distance of more than 700 km.
The trouble is that, in the two years that MINOS has been running, the detector has spotted only 730 muons. “Obviously, an improvement of at least six orders of magnitude is required,” says Huber, with no little understatement.
But he believes this kind of improvement will be possible with the next generation of muon accelerators.
Let’s take his word on that. The question then is how to detect these neutrinos in a submarine. Here, Huber has been a little more creative. He says there are essentially two ways to spot neutrinos. “We would use thin muon-detector modules, which can be used very much like wallpaper to cover the majority of the vessel’s hull,” says Huber. This effectively turns a submarine into a giant, cylindrical muon detector about 10 meters in diameter and 100 meters long.
How would this work? “The muons would enter on one side of the submarine and leave it on the other side,” he says. “The entry and exit points are measured, and thus the muon direction can be reconstructed quite precisely.”
But there’s also another way to detect neutrinos: look for the Cerenkov light radiation produced by fast-moving muons in seawater. That’s clever, because it allows you to create a detector with dimensions that are roughly the distance that light travels in seawater, about four kilometers or so. Of course, there’s no shortage of noise from bioluminescence, sunlight, and moonlight, but Huber seems confident that all of that could be filtered out.
The bottom line is that submariners could one day use this technology to receive messages at data rates of up to 100 bits per second.
There is one drawback, of course. It’s only possible to receive messages in a submarine in this way, not to send them. That’s not something that TV viewers have worried about much. But commanders of nuclear submarines may have a different view.
Ref: arxiv.org/abs/0909.4554: Submarine neutrino communication
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