But previous particle colliders are peashooters compared with the LHC.

So what will it discover?

The holy grail is the elusive Higgs boson, named after Edinburgh physicist Professor Peter Higgs, who postulated it in 1964 as a solution to the embarrassingly unanswered question of why matter has mass, and thus exists in a form that allows it to make planets and people, while some phenomena – such as light – do not. Higgs proposed that the universe is pervaded by an invisible field of bosons (the Higgs field) that consist of mass but little else. As particles move through this, bosons effectively stick to some of them, making them more massive, while others, like particles of light (photons), are unaffected. This boson has been sought in every major collider since. To no avail, however: locked securely in the atomic nucleus, it shows itself only at the vast energies which last existed in the moments after the Big Bang. Hence the need for the LHC.

And what happens if they find it?

The Higgs boson is central to modern physics, but has never been found – prompting one Nobel laureate to describe it as the "God particle", and another to dismiss it as the "toilet" into which physics flushes its inconsistencies. Finding it would verify the Standard Model. It would help move scientists closer to a Grand Unified Theory describing all physical phenomena in nature – to "know the mind of God", in Stephen Hawking's phrase. Hawking himself, however, is betting against the discovery (it is conceivable that an even more powerful accelerator might be needed). Higgs himself will be "very puzzled" if it isn't found – "I will no longer understand what I think I understand," he declares. Years' worth of Nobel prizes for physics may be riding on the results.

What else might they find?

The point is they don't really know: they are waiting for the data to speak. But physicists hope to discover a "jewel box" of new particles; to test a whole group of theories, like string theory and supersymmetry, that go beyond the Standard Model; to throw light on the mystery of "dark matter", which seems to make up most of the mass of the universe but is not visible; even, perhaps, to see evidence of further dimensions. They will also be looking for "mini black holes". This has fed fears that the LHC is a doomsday machine, creating black holes that could swallow up the entire Earth. Two amateur physicists, Walter Wagner and Luis Sancho, have even sought a restraining order on the LHC in the US courts. Concerns have also been raised about "strangelets", notional objects made of quarks which could turn all matter into "strange matter", making the world vanish. These worries are regarded as coming from science’s lunatic fringe. Cern promises that any black holes will be small, fleeting and "benign", and that we are "safe from strangelet-initiated catastrophe".

And the practical implications?

The main purpose of the LHC is knowledge for its own sake. On the other hand, as the world's premier blue-skies thinking laboratory, Cern produces a great many practical spinoffs. In 1989, Tim Berners-Lee invented the World Wide Web while working there. Civil engineering and superconductor technologies have been pushed to new boundaries for the LHC; imaging techniques developed for the project are now used in hundreds of hospitals. Cern has also created the world's most powerful computer network, the "Grid", to process the staggering amount of data produced by the LHC. It uses the processing power of research institutes in 33 nations, and the technology is expected to be made publicly available, revolutionising computing in the process.