Moonbeams

THE Large Hadron Collider (LHC) is far and away the most powerful particle accelerator built by the hand of man. Yet it is puny compared to the most powerful particle accelerator of them all: the universe itself. Earth is under constant bombardment from cosmic rays (mostly atomic nuclei travelling at high velocity) that streak in from deep space, smash into the atmosphere and disintegrate in a puff of radiation and subatomic debris.

Though the vast majority of cosmic rays are pale shadows of the things physicists cook up in atom-smashers, a real whopper arrives occasionally. The most energetic of all are millions of times more potent than anything the LHC can generate, packing the energy of a rapidly delivered cricket ball into a single, blisteringly fast atomic nucleus.

Studying such ultra-high-energy rays is difficult, though, because they are so rare. Each square kilometre of Earth is hit, on average, by about one a century. The Pierre Auger Observatory, a facility in Argentina which surveys 3,000 square kilometres of Earth’s atmosphere, picked up around 15 a year between 2005 and 2008. But Justin Bray, an astrophysicist at Southampton University, in Britain, has plans to do much better. With the help of the Square Kilometre Array (SKA), a continent-spanning radio telescope being built in southern Africa and Australia, he proposes to use the Moon as a colossal cosmic-ray detector.

The general idea is not Dr Bray’s—it was first mooted in 1989, and since then several experiments have confirmed that it should work in theory. But only the SKA (itself a sort of astronomical LHC, in that it will be the biggest radio telescope ever built) will be sensitive enough to make it practical.

The telescope would not detect the cosmic rays themselves. Instead, it would listen for the brief radio-frequency spike caused when one ploughs into the lunar regolith. Such collisions would be detectable only if they occurred near the lunar limb, but even so that would provide up to 100 times the area the Pierre Auger Observatory can look at. Dr Bray expects to detect around 165 a year.

Gathering all those data should allow physicists to answer some fairly basic questions, such as where cosmic rays come from. Most originate outside the solar system, but pinning down their exact birthplace is difficult. Because they are electrically charged, their paths are bent by the Milky Way’s magnetic field. But the highest-energy particles have their paths bent the least. If researchers can work out whence they are travelling, they may be able to deduce what is generating them.

Nor will the SKA pick up only cosmic rays from its examination of the Moon. It should also be sensitive to ultra-high-energy neutrinos interacting with the lunar surface. Such neutrinos are generated alongside cosmic rays, but then travel in helpfully straight lines because they are, as their name suggests, electrically neutral. That means where they have come from should be easier to work out.

Whatever is responsible for generating high-energy cosmic rays and their associated neutrinos, it must be one of the most powerful processes in the universe. One candidate is “active galactic nuclei”—gigantic black holes at the centres of galaxies which emit huge quantities of radiation as dust and gas fall into them. Another is extremely violent stellar explosions. But there are more exotic possibilities. Some theorists wonder if ultra-high-energy cosmic rays are a decay product of dark matter, a mysterious substance that makes up 85% of the matter in the universe. Others talk of “topological defects”—kinks in the space-time continuum predicted by some speculative theories of physics. Studying cosmic rays should, in other words, let physicists probe how reality behaves in conditions no Earth-bound experiment is ever likely to be able to replicate.