One day our descendants will reach the limit of particle
accelerator technology. We'll surely run out of space and money
long before the smallest building blocks of the universe can be
probed with machines, because of the massive energies
required.
One saviour may be the universe's own particle smashers - black
holes. If two particles are accelerating towards a rotating black
hole with a certain velocity then they should collide with energies
higher than anything we could hope to achieve on Earth.
The singularity at the centre of a black hole is so dense that
any matter and light that reaches the black hole's point of no
return, or event horizon, gets sucked in due to the extreme
gravitational attraction. The closer that particles get to the
black hole, the greater the energy they have.
So could these particles smash together and perhaps reveal
evidence of "new physics"? That's what physicists Stephen West of
Royal Holloway, University of London, Max Bañados of the Pontifical
Catholic University of Chile in Santiago and Joseph Silk of the
University of Oxford decided to investigate.
The particles they chose for their natural accelerator model
were of dark matter. These weakly interacting particles are thought
to collect in a dense core around middle-weight black holes as they
form out of ordinary matter. They wouldn't be the only particles
around, but the team figured the collision results would be more
exciting than other more mundane forms of matter, such as
intergalactic dust.
Particles accelerated by a simple, non-rotating black hole would
become parallel as they approached the horizon but never collide.
But the team's calculations show that if the black hole were
rotating at high speed, and a particle approached at a certain
angle, it would be able to collide with another at extremely high
energy. The results will appear in Physical Review Letters.
After the collision, a significant proportion of the daughter
products would be sucked into the black hole. Yet the authors
suspect that some would have enough energy to escape its clutches,
and they are trying to work out just how large this proportion is.
Some may even have enough energy to be detected by experiments like
ICECUBE in Antarctica, or detectors on satellites.
Some of the products of a black-hole particle collision would have
enough energy to reach Earth
Energies at the Large Hadron Collider are likely to peak at 14
teraelectronvolts. In contrast, the energies around a black hole
would theoretically be limitless, says West. However, you needn't
go beyond the so-called "Planck energy" - the point at which our
mathematical understanding of particle interactions, in particular
gravity, breaks down at the quantum level. This energy is in the
order of 1018 gigaelectronvolts - 100 trillion times more energetic
than the LHC.
"With black hole collisions you really can recreate the
beginnings of the universe," says West. The team's "Planck
accelerator" could potentially probe particles involved in grand
unified theories, the energy scale where the four fundamental
forces merge.
David Ballantyne of the Georgia Institute of Technology in
Atlanta likes the idea. In the past, he has explored whether
particle collisions at the black hole at the centre of the Milky
Way could be responsible for mysterious gamma-ray emissions.
"The idea is interesting enough to continue pursuing," he says.
"I would be very interested to see their predictions for the flux
and energy distribution of the particles following collision." For
one, such particles could tell us a lot about the nature of dark
matter and the structure of space-time around a black hole, he
says.