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BLACK HOLES : The ultimate particle smashers

WHAT will happen to fundamental physics when our descendants 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.

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