The Large Hadron Collider is the world's most powerful particle accelerator. Straddling the border between France and Switzerland at the CERN laboratory, the LHC is designed to answer some of the most profound questions about the universe:
What is the origin of mass? Why are we made of matter and not antimatter? What is dark matter made of?
It could also provide important new clues about conditions in the very early universe, when the four forces of nature were rolled into one giant superforce.
Particle smasher
To find out, the LHC will set protons travelling at 99.9999991% of the speed of light around a circular tunnel. It will then smash them together at four points on the ring, each of which are surrounded by huge experiments.
The collision energy produced is 14 teraelectronvolts (TeV), seven times greater than its nearest counterpart � the Tevatron at Fermilab in Batavia, Illinois.
In everyday terms, this energy isn't so great � a flying mosquito has about 1 TeV of kinetic energy. What makes the LHC so special is that this energy is concentrated in a region a thousand billion times smaller than a speck of dust.
The LHC is the latest in a long tradition of particle accelerators used to explore the building blocks of matter and the forces that act between them. Nearly 100 years ago, New Zealand physicist Ernest Rutherford revealed the structure of the atom by firing alpha particles at a thin gold foil.
Similarly, in the 1930s physicists used electromagnetic fields to accelerate protons to high energies inside long vacuum tubes. At very high energies the protons were smashed apart, only for the fragments and collision energy to be transformed into other particles.
Exotic output
So great is the concentration of energy at the LHC that it recreates conditions similar to those 10-25 seconds after the big bang, soon after the particles and forces that shape our universe came into being. With so much energy available, the LHC should be able to create certain massive particles for the first time in the lab.
Among them, physicists hope, will be the Higgs boson, the particle that gives others their masses. They will also be looking for signs of a theoretical supersymmetry that might give us clues about how the forces looked in the early universe.
Supersymmetry predicts that every particle we know has a heavy supersymmetric partner. The lightest supersymmetric particle is also a promising candidate for dark matter, the invisible entity thought to amount to 95% of the universe's mass.
The Higgs and supersymmetry are on firm theoretical footing. Some theorists speculate about more outlandish scenarios for the LHC, including the production of extra dimensions, mini black holes, new forces, and particles smaller than quarks and electrons. A test for time travel has also been proposed.
Building the LHC
The LHC was conceived back in 1979. It is housed 100 metres underground in a circular tunnel once filled by another machine called the Large Electron Positron Collider, which switched off in 2000. Protons are injected into the LHC from a chain of smaller accelerators that whips them up to higher energies.
Every second the protons will make 11,245 laps of the 27-kilometre ring and, at four points on the ring, they are made to collide head-on with protons travelling in the opposite direction.
Surrounding each of these collision points are four giant detectors called ATLAS, CMS, LHCb and ALICE. ATLAS and CMS are designed to look for all kinds of particles created in the collisions, while LHCb and ALICE are specialised detectors.
Results galore
LHCb is designed to make exquisite measurements of particles called B mesons that could reveal subtle differences between matter and antimatter. B mesons live for a short while before decaying, and this process could be influenced by the Higgs, supersymmetric particles, or some other new physics.
ALICE will come into its own starting in November 2009, when the LHC will swap its colliding protons for lead ions for a few weeks. These collisions should create temperatures 100,000 times hotter than the centre of the sun, hot enough to reveal a novel state of matter called the quark gluon plasma. By studying this, physicists hope to understand how the quarks and gluons from the big bang fireball condensed into the protons and neutrons we see today.
In addition there are two smaller experiments called LHCf and Totem, for testing theories of ultra-high-energy cosmic rays and measuring the size of protons. Over 6000 scientists work on the LHC and its experiments, which cost $10 billion.
No worries
Many people are worried by theorists' predictions that the LHC could create microscopic black holes that could inflate and swallow the Earth.
CERN physicists have assessed the risk to be as close to zero as scientists dare to say. If black holes are created at the LHC, they will evaporate within 10-26 seconds, via a process first described by British physicist Stephen Hawking.
Even if Hawking is wrong and black holes do not evaporate, physicists have experimental reasons for feeling safe. Cosmic rays from outer space have far greater energies than the LHC will produce and have been colliding with the solar system's planets for billions of years without problems.
What's more, there are far more cosmic ray collisions than LHC collisions. No black holes have swallowed Jupiter or Saturn.
Accelerator physicists plan to send the first protons around the entire LHC ring on 10 September. If all goes well, collisions will start a few weeks later at a reduced energy of 10 TeV.
