The man with his finger on the button was Lyn Evans, a Welsh engineer who has devoted 14 years of his life to the machine. The moment came at 8.32am UK time and was broadcast around the world, and via videolink to more than 300 journalists who had descended on the laboratory to witness the event.
The LHC lies 100 metres beneath fields and farmland, where it occupies a 17-mile (27km) circular tunnel carved through rock and sandstone.
When it is working at full speed, it will be the most powerful particle collider on the planet. Inside, it will crash subatomic particles together with enough energy to re-create the intense conditions that existed one trillionth of a second after the big bang.
Yesterday, the scientists' ambitions were more modest. Before the machine can be put to work, its makers had to take it for a test drive.
The goal was to send beams of protons around the collider's hollow ring in both directions, to make sure there were no obstructions and to check that powerful superconducting magnets surrounding the ring can steer the beams with exquisite precision. When Cern attempted this on an older, less powerful collider in 1996, it found two beer bottles stuck inside the ring.
Yesterday, the tests went more smoothly than many scientists dared hope for. At 9.28am UK time, two spots flickered on to a screen in the control room, one spot caused by the beam on the way in, the other as it completed its first lap. Cheering, relieved scientists clapped and slapped each other on the back. The test had taken less than one hour. "My first thought was one of relief. I'm too preoccupied at the moment to have emotions," said Evans, who later confessed to laying a bet with fellow physicist Steve Meyers, head of Cern's accelerator and beam operations, that he could get the beam to circulate within an hour.
By 2pm UK time, the scientists had sent a beam of protons around the machine in the opposite, anticlockwise direction to the first beam.
David Evans, a physicist from the University of Birmingham, who works on one of the machine's giant detectors, said: "It's gone so well I'm optimistic we can probably do low-energy collisions within days. We could be looking at high-energy collisions within weeks." Now the real work begins. Scientists will spend the coming days and weeks fine-tuning the machine and testing the four huge detectors, which will sift through the subatomic debris of the collisions for evidence of new physics.
Tejinder Virdee, a physicist at Imperial College London and head of one of the LHC's detector groups, said: "With the LHC, we will be able to look deeper into matter, and look further back in time than ever before.
"Particle physics is a modern name for the centuries-old effort to understand the laws of nature."
Within weeks, the machine could produce particles of dark matter, a mysterious substance that stretches through the universe and clings around galaxies. The discovery would be profound.
Astronomers know that normal matter, the stuff of stars and planets, makes up only 5% of the observable universe. Dark matter accounts for a further 25%, with the remaining 70% being the even more exotic dark energy, which drives the expansion of the cosmos.
By creating a microcosm of the big bang, scientists hope the machine will explain how the forces of nature became what they are today.
The machine will also hunt the famed Higgs boson, or "God particle". Named after Peter Higgs, an Edinburgh University physicist, the Higgs boson is crucial to understanding the origin of mass.
Cern thought it had caught a glimpse of the Higgs particle before with its previous particle collider in 2000.
It will now race against scientists at the American Fermilab, near Chicago, which is working around the clock to discover the particle first. "This is a unique machine and it will certainly advance the knowledge of mankind. But we also know that pushing technology to the limit always has spinoffs. We don't know what the LHC will bring apart from wonderful science, but we're already working on a far more powerful system than the internet. Where we lead, others will follow," said Evans. Read
Moment of Truth
The cheering began at 8.32, when the first particles were detected snaking around the first three kilometres (1.9 miles) of the 27km (17mile) LHC ring. By 8.55, it was halfway around the track, which will soon be used to smash protons and lead ions against each other at 99.9999991 per cent of the speed of light. At 9.28, only 56 minutes after the start-up, came the champagne moment — the double trace showing that the beam had completed the first of countless trillions of laps that will explain many of the enduring mysteries of the Universe.
Once the clockwise beam was circulating, the anticlockwise stream with which it will ultimately collide was inserted in the afternoon, completing its own tour of duty soon after 2pm. Over the next few days, they will be tuned and “captured” so they fire in neat pulses. Then it will be time for business – the collisions that will generate new physics.
By recreating the environment of the dawn of time, the LHC will detect phenomena that have never before been observed. It should find the Higgs boson, the so-called “God particle” that theory suggests gives matter its mass, but which has never been found. It should also determine whether all particles have a twin, as a theory known as “supersymmetry” suggests, and thus explain the mysterious “dark matter” that pervades the Universe, but which cannot be seen.
The LHC may even find new dimensions, beyond the three of space and one of time with which we are familiar. It promises to unlock great secrets of the cosmos.
“Particle physics is a modern name for the centuries-old effort to understand the laws of nature,” said Professor Tejinder Virdee, who heads the Compact Muon Solenoid detector team. “Humankind has an unquenchable thirst for knowledge and understanding the surroundings in which we live.
“The excitement is of having completed a machine that’s taken 20 years to plan and build. Now we’re looking forward to the really interesting part: even more excitement awaits us as we start doing the science. The LHC is going to look deeper into matter and go back further in time than we’ve been able to go before. It’s the most powerful microscope ever built and at the same time the most powerful telescope ever built.
“We have these theories, and now we’re getting into new territory to put them to the test. We don’t know what we’re going to find – that’s why we do the experiments.”
The first trial collisions, from which researchers will calibrate their detectors, could start as early as next week. The LHC will then start operating at about 70 per cent of maximum energy, before it is ramped up to full power next year. Discoveries about supersymmetry could come quickly, but the hunt for the Higgs boson will take longer, with few results expected before 2010.
We will be looking at what the Universe was made of billionths of a second after the Big Bang 
