13 things that do not make sense: #3

3 Ultra-energetic cosmic rays

FOR more than a decade, physicists in Japan have been seeing cosmic
rays that should not exist. Cosmic rays are particles - mostly protons
but sometimes heavy atomic nuclei - that travel through the universe
at close to the speed of light. Some cosmic rays detected on Earth are
produced in violent events such as supernovae, but we still don't know
the origins of the highest-energy particles, which are the most
energetic particles ever seen in nature. But that's not the real
mystery.

As cosmic-ray particles travel through space, they lose energy in
collisions with the low-energy photons that pervade the universe, such
as those of the cosmic microwave background radiation. Einstein's
special theory of relativity dictates that any cosmic rays reaching
Earth from a source outside our galaxy will have suffered so many
energy-shedding collisions that their maximum possible energy is 5 ×
1019 electronvolts. This is known as the Greisen-Zatsepin-Kuzmin
limit.

Over the past decade, however, the University of Tokyo's Akeno Giant
Air Shower Array - 111 particle detectors spread out over 100 square
kilometres - has detected several cosmic rays above the GZK limit. In
theory, they can only have come from within our galaxy, avoiding an
energy-sapping journey across the cosmos. However, astronomers can
find no source for these cosmic rays in our galaxy. So what is going
on?

One possibility is that there is something wrong with the Akeno
results. Another is that Einstein was wrong. His special theory of
relativity says that space is the same in all directions, but what if
particles found it easier to move in certain directions? Then the
cosmic rays could retain more of their energy, allowing them to beat
the GZK limit.

Physicists at the Pierre Auger experiment in Mendoza, Argentina, are
now working on this problem. Using 1600 detectors spread over 3000
square kilometres, Auger should be able to determine the energies of
incoming cosmic rays and shed more light on the Akeno results.

Alan Watson, an astronomer at the University of Leeds, UK, and
spokesman for the Pierre Auger project, is already convinced there is
something worth following up here. "I have no doubts that events above
1020 electronvolts exist. There are sufficient examples to convince
me," he says. The question now is, what are they? How many of these
particles are coming in, and what direction are they coming from?
Until we get that information, there's no telling how exotic the true
explanation could be.

"One possibility is that there is something wrong with the Akeno
results. Another is that Einstein was wrong"