The building blocks

Suppose we were time travellers, and could transmit one key item of modern knowledge to a great intellect of the ancient world - Aristotle, for instance. What would we choose to tell them, a single sentence that would most transform their view of the world? We could tell them the scale of the universe - that the stars are other suns, and that there are billions of them. Or that all species emerged, over billions of years, via natural selection.

But I think what would enlighten them most of all would be the knowledge that all the stuff in the world is made of atoms - not of earth, air, fire and water, as the ancients believed. But what are the atoms themselves made of? Are they like an onion-skin with layer upon layer of structure, or will we soon reach bedrock, in the sense that the stuff of the universe will be fully understood?

It might seem paradoxical that the biggest scientific instruments of all are needed in order to probe the very smallest things in nature. The micro-world is inherently "fuzzy" - the sharper the detail we wish to study, the higher the energy that is required and the bigger the accelerator that is needed.

The Cern laboratory in Geneva was set up in 1955, to bring together European scientists who wished to pursue research into the nuclear and sub-nuclear world. Physicists then had greater clout than other scientists because the memory of their role in the second world war was fresh in people's minds. Through a succession of projects - each too expensive for any single European country to fund - Cern has been at the forefront of endeavour to build ever more powerful accelerators probing ever smaller scales. This culminates in the Large Hadron Collider (LHC). Within its circular tunnel, 27km in circumference, protons hurtle around at 99.99% of the speed of light. The amazing technology combines huge civil engineering with microscopic precision.

Cern is a triumph of European collaboration, but it now has a global ascendancy, and is the premier laboratory in the world for particle physics. When it switches on this summer, the LHC will generate, in a microscopic region where beams of particles collide, a concentration of energy that has never been achieved before - a concentration that mimics, in microcosm, the conditions that prevailed in the universe during the first trillionth of a second after the big bang.

The impacts may generate particles of a novel kind never before detected in a laboratory (and which may even never have existed on the earth before). This possibility is especially interesting, because one of the most perplexing features of our universe is that there is a lot of material which isn't made up of ordinary atoms. It's possible that this "dark matter" consists of particles that are left over from the fiery beginning of the universe. The LHC may allow scientists to create and study these particles.

There are strengthening links between the sciences of the very large and the very small. It's even possible that the LHC might tell us about the nature of space itself. In everyday life we regard space as dull vacuum. But this dismissive attitude is as misleading as it would be for us to believe that invisible clear air is less substantial that the clouds floating in it. Most theorists suspect that space has an intricate structure - that it is "grainy" - but that this structure is on a much finer scale than any known subatomic particle. The structure could be of an exotic kind: extra dimensions, over and above the three that we are used to (up and down, backward and forward, left and right).

A polished surface may seem smooth, but when viewed under a microscope it has bumps and dips in it: likewise our space, viewed on an ultra-fine scale, may have extra dimensions. The favoured view is that these extra dimensions only manifest themselves on scales a trillion trillion times smaller than atoms, and one of the most fascinating outcomes from the LHC could be the first evidence for them.

Whatever comes out of the LHC, the results will be a stimulus to next-generation Einsteins who will achieve the next steps in a quest, which started in ancient times, to understand the building blocks of the natural world.

· Martin Rees is Astronomer Royal and president of the Royal Society