Neutrino astronomyBalloon with a view
An experiment in Antarctica may solve the mystery of cosmic rays
MEET ANITA. Strictly, ANITA III—for she is the third iteration of the Antarctic Impulsive Transient Antenna. Her job, when she is launched sometime in the next few days, will be to float, suspended from a giant balloon, over Antarctica’s ice, in order to record radio waves which that ice is giving off. These radio waves are generated by neutrinos passing through the ice, making Antarctica the biggest neutrino-detection laboratory in the world.
The particular neutrinos that ANITA seeks are of extremely high energy. Where they come from, no one knows—nor, strictly speaking, is it actually known that they exist, for ANITAs I and II, which were smaller devices, failed to find them. But theory says they should be there, generated in whatever giant explosions also create cosmic rays.
Cosmic rays are high-velocity protons, sprinkled with a smattering of heavier atomic nuclei, that fly through space until they hit something such as Earth’s atmosphere, when they disintegrate into a shower of other particles. They have been known for a century, but their origin remains mysterious because, being electrically charged, their paths are bent by the galaxy’s magnetic field. That means the directions they come from do not point to whatever created them.
Neutrinos, however, are electrically neutral, as their name suggests. Their paths should thus point back towards their origins. Neutrinos do not interact much with other sorts of matter, but when one of ultra-high energy does so, the result is a shower of particles travelling at speeds which exceed that of light in ice. An object travelling faster than light’s speed in the medium through which it is passing will generate electromagnetic waves. These are known, after their discoverer, as Cherenkov radiation. And it is pulses of radio-frequency Cherenkov radiation, the electromagnetic equivalent of a sonic boom, which ANITA is looking for.
Once airborne under her balloon—an object made of cling-film-like plastic that, when fully inflated, will be a fifth of the size of a football stadium—ANITA will take advantage of the polar vortex, a wind in constant revolution around the pole. She will fly at an altitude of 35-40km, which will mean her antennae can see 1.5m km2 of ice. Ultra-high-energy neutrinos travelling through the ice are thought to interact with it and produce Cherenkov radiation about once per century per km2, so an area of this size would be expected to yield about 40 bursts a day. ANITA will complete several laps of the continent, each lasting about 15 days. Then the balloon will be cut loose, and she will deploy a parachute and be guided back to the surface for re-use.
Astrophysicists are not the only people rubbing their mittens together in expectation of the results of this experiment. The neutrinos ANITA is looking for are far more energetic than anything produced by the Large Hadron Collider, the world’s most powerful particle accelerator. That means they may obey hitherto unperceived extensions of the laws of physics. One possibility is that, among the Cherenkov-radiation-generating particles produced when a neutrino collides with the ice, there may be an occasional miniature black hole.
That would be particularly exciting, because such black holes might themselves disintegrate in a characteristic puff of radiation named after another physicist, Stephen Hawking. If Hawking radiation exists, it means black holes are not truly black—a discovery which would almost certainly win Dr Hawking a Nobel prize.
Though it is not designed to search for Hawking radiation, ANITA would probably see it if it were there. And, since Hawking radiation is created, quite literally, out of nothing (the particles it is made from emerge from the vacuum of space and then steal the energy needed to become real from the black hole itself), that would assist understanding of a very strange piece of physics indeed.