Think making two bullets collide is difficult? Then try doing it at the subatomic level
After 20 years of planning, building and research, the ‘big bang machine’ – the Hadron Collider at Cern – has begun delivering its promised high energy collisions . So what now? asks DICK AHLSTROM
IMAGINE TWO rifles pointing at each other, a mile apart, and then firing and hitting each other head on. Well that would be easy by comparison with the challenge of getting subatomic particles to strike one another at almost the speed of light in the world’s newest, most powerful collider.
Yet this challenge is just what will occupies physicists and engineers over the coming years at the massive research complex at Cern, Europe’s nuclear research centre. In the process, its LargeB Hadron Collider (LHC) may begin to help solve some of the biggest mysteries of science. The excitement is just settling down in Cern after the momentous news on Tuesday that after 20 years of planning, building and a €4 billion investment, the LHC began delivering high energy collisions.
The four main detectors positioned around the LHC’s 27km-long ring began coping with the first real surge of data from the LHC, recording some 500,000 collision events in the few hours that the machine ran, according to experimenters in the complex.
Cern refers to this start-up phase of the LHC’s operation as its “first physics”, the first delivery of real scientific data. The LHC began circulating beams of particles, protons (hydrogen atoms stripped of their electrons), last autumn, but this represented no more than testing and confirmation that the collider’s new safety systems were fully operable. Tuesday delivered real data, the first in what scientists at Cern expect to be the start of 20 to 25 years of operation.
The stunning technological achievement of building and operating such a device is sometimes overlooked, but is definitely worth remembering as the data begins to flow in the coming weeks. The protons whizz around the 27km closed ring – installed 100m underground in a tunnel under the French/Swiss border – at close to the speed of light, completing more than 11,000 circuits of the ring per second.
Their movement is controlled by thousands of huge electromagnets which steer the particles around the ring and prevent them from touching the sides or each other. These are superconducting magnets, bathed in a surrounding bath of liquid helium that keeps them at a chilly minus 193 degrees. The protons are injected into the ring in little bundles, but this belies what a bundle represents. There are a thousand billion protons clustered together in that bundle, the number so high because they are so impossibly small. Even at those numbers the few millimetres of space they occupy is virtually empty.
The LHC is designed to control two “beams” of protons travelling simultaneously in the ring in opposite directions. Last Tuesday’s run involved injecting two bundles of protons into each of the two beams, so it might seem the space would be fairly crowded. Yet the LHC, when running at design capacity in the coming years, will handle 2,800 bundles of protons per beam, with the protons – numbering about 10 to the power of 11 according to experimenters – yielding about 44 million collisions per second.
We already have accelerators that can smash particles. What the LHC has that others don’t is its colossal collision energies that are multiples of the highest achieved up until this week.
These energies are measured in electron volts, the energy picked up by a single electron when accelerated by a one volt electric field. This is too small to imagine but in the LHC the collisions will arrive at a million million electron volts, 10 to the power of 12 or tera electron volts (TeV). The old collider energy record set in the US was just under 2TeV, but yesterday the LHC’s twin beams each running at 3.5 TeV collided to achieve a combined energy of 7 TeV. And its designed running energy, likely to be reached in its third year of operation, will touch 14 TeV.
Amazing as they are, those are just numbers. What is important is the flood of important new data that will arrive. The collisions smash the proton asunder, revealing its constituent parts. These fundamental particles tell scientists much about the nature of matter and how it all fits together, whether we are talking about a single atom or a galaxy.
With data already flowing, Cern director general Rolf Heuer believes that scientists may soon be able to prove a theory called “supersymmetry”, that there is an inherent physical linkage between pairs of fundamental particles.
The LHC will also allow deeper insight into the mysterious “dark matter”, a substance that we know must exist and makes up near 25 per cent of the entire universe, but that we cannot yet see or understand despite its massive content.
Then there is the famous Higgs Boson. This is an elusive fundamental particle theorised to exist by Peter Higgs but yet to be found. The LHC has the energies needed to reveal the Higgs to us, something at will help complete the jigsaw puzzle of known fundamental particles.