The God machine

What on Earth is this machine?

It is the Large Hadron Collider (LHC), the world's largest particle accelerator. Built at the European Organisation for Nuclear Research (Cern) near Geneva and housed within a 27km underground circular tunnel – 5km longer than the London Underground Circle Line – it is designed to reproduce the incredibly high energies found in the first trillionth of a second after the "Big Bang", which brought our universe into existence. After its official switch-on on 10 September, it will fire two beams of protons (tiny subatomic particles) in opposite directions around the tunnel before making them collide at 99.9999991 per cent of the speed of light.

Why are scientists doing this?

"We simply want to understand what the world is made of, and how," says Jos Engelen, Cern’s chief scientific officer. Over the past century, physicists have gone a long way towards identifying the basic building blocks of the universe. First came the discovery that each atom has a heavy nucleus, consisting of protons and neutrons (collectively known as hadrons), orbited by a matching number of light electrons. But these hadrons were found to be made up of yet smaller particles: quarks, glued together by gluons. In the 1970s, the "Standard Model", a sort of kits part for the subatomic world, was developed. It lists the categories of fundamental matter particles – quarks and leptons, each coming in six "flavours" – as well as things called bosons (some of which, such as gluons, carry the forces that bind other subatomic particles). The Standard Model has very successfully predicted subatomic interactions, but it is incomplete: it doesn't explain how gravity works, and its explanation of mass is as yet untested.

Where does the Large Hadron Collider come into this?

All these discoveries have been made using devices which smash up atoms and examine the resulting subatomic debris. In 1932, the Cambridge scientists John Cockcroft and Ernest Walton first split the atom, using a particle accelerator to fire protons into lithium atoms, producing helium. Since then, bigger and better atom-smashers have confirmed the existence of a whole world of mysterious, and mysteriously-named particles, such as the W and Z boson, the charm quark and the top quark.