Q&A: the Large Hadron Collider
Updated on 10 September 2008
What is an LHC and why would you want one? Science correspondent Tom Clarke explains.
The LHC? What's That?
It's the Large Hadron Collider, and the biggest scientific experiment of all time. Large, because it is exactly that - a ring 27 kilometres around with particle detectors the size of multi-storey buildings along its length.
Hadron, because it uses protons from the centres of atoms as its raw material (just to be difficult physicists sometimes refer to them as hadrons).
Collider, because it smashes those hadrons together.
Why would you want one?
Physicists' main goal is to understand the nature of the universe: what it's made of, why we're here and how long it will last into the future.
In order to do that they need to look beyond ordinary matter like planets, chairs and fingernails and probe exotic forms of sub-atomic matter from which the ordinary matter is composed.
The LHC smashes things together so hard it's recreates the conditions that existed just a billionth of a second after the Big Bang.
In the minute rubble of the collisions it produces they should find some of these special particles that make up the universe we can't normally see.
Will it create a mini black hole?
The LHC may well create mini black holes. Because protons are very heavy, smashing them together could, for a fraction of a second, make something heavy enough to suck in other matter under its own gravity: a micro-back hole.
However it is almost inconceivable that the black holes could get any bigger. The scientists behind the LHC have gone to great lengths to show that cosmic rays hit our atmosphere with much more energy than the LHC's protons.
The cosmic rays should be making black holes all the time. And, so the argument goes, we're still here. There are a few scientists who are worried about the black hole risk, but as it was recently explained to me, if the laws of nature did allow micro-black holes to grow, they should be the least of our worries.
If the LHC doomsayers are right the machine could also create a theoretical phenomenon called a "vacuum bubble." This would be capable of instantly swallowing the whole universe, making a black hole look a bit of a pussycat really.
How does it work?
By looking at the pieces thrown out of the collisions between the protons. The ring is like a giant Scalectrix track that speeds two bunches of protons in opposite directions using superconducting magnets.
When the beams of protons are going round at just shy of the speed of light (11,000 laps of the 27km ring per second), the beams are crossed and the protons collide.
The vast detectors trace the light and heat and energy tracks of the fragments as they fly outwards. From that scatter, they identify these fragments; some of them new and exotic particles in the own right.
What it will it find?
The LHC was designed specifically with one particle in mind: the Higgs boson. It's a particle proposed by British theorist Peter Higgs that explains why all the matter in the universe has mass.
It's one of the last missing species in the zoo of particles that makes up the ordinary universe. But the LHC could answer other questions about parallel universes, antimatter and most intriguingly dark matter.
Dark matter is everywhere, but invisible. Theory predicts dark matter is made up of invisible "supersymmetric" particles that are twinned with every particle of ordinary matter.
Supersymmetry means they're similar, only a heavier and sort of mirror image of their twin. Think an invisible Arnold Schwarzenegger particle for every Danny DeVito particle.
What happens tomorrow?
Just between you and me, not very much. They try and get the beam up and running doing smooth laps of the track. Sounds easy, but it's actually fiendishly hard; 9600 individual magnets have to be tuned to steer the particles around.
Once they've done that, they'll have a celebration and a breather. Then they need to get a beam going the other way.
Once they have two stable beams, they can start trying to collide. That's harder than it sounds too. One LHC insider told me it's like trying to fire one gun from New York, another from London, and getting the bullets to hit each other mid-Atlantic. If all goes to plan, the first collisions will be in a month or so.
When will it make its first discovery?
If they are lucky, the LHC could start to indicate exotic new things like the Higgs boson within months - though it could take a year or two to have the statistical proof needed to confirm its presence.
For the other discoveries like dark matter, it's more likely to take years of painstaking analysis and sifting through reams of data.
Just how much data will there be?
If you thought the machine was impressive, the numbers it should produce are awe-inspiring. Each year the LHC will churn out 15 petabytes (15 million gigabytes) of data.
That's enough to fill a stack of CDs 12 miles high. To handle the data CERN's IT department have built the biggest computer in the world. Not in one place, but connected in a system similar to the internet called a grid.
It's about 100,000 individual computers in 250 physics labs in 50 countries worldwide. The collision crunching software the computers run has the rather cute name of BOINC.
Is it really worth it?
Big, expensive basic research experiments have a difficult time justifying their existence to taxpayers. Compared to grants for cancer research, or world poverty, the LHC's money won't lead to immediate dividends.
However, the scientists that work there will quickly, and justifiably, point to what physicists fiddling about with matter have achieved before, albeit indirectly.
Work on the atom led to discoveries like the computer chip, nuclear power, radiotherapy for cancer and other, less obvious spin-offs. The internet was invented at CERN to help the scientists communicate with each other.
What if it doesn't work?
It should definitely find the Higgs boson - if it exists in the form that theory predicts. If it doesn't, there's something seriously wrong with physics.
However, of if it only finds the Higgs boson and nothing else, there might mean trouble too. Although this is what the machine was built to achieve, it managed to gather the billions of pounds of funding it requires because of the other discoveries it promises to make.
If these discoveries aren't forthcoming, things like the LHC may start to be frowned upon by the governments who have to pay for them. I am told this is currently what American particle physicists are worrying about.
They are drawing up plans for the next big smasher: the International Linear Collider, to be based in the US. If the LHC doesn't find lots of interesting things, they worry the money men might walk away from the ILC. So it's all eyes on the LHC for the next couple of years.