Scientists switched on the world's largest atom smasher for the first time since the $US10 billion ($A10.8 billion) machine suffered a spectacular failure more than a year ago, circulating beams of protons in a significant leap forward for the Large Hadron Collider.
The European Organisation for Nuclear Research has taken the restart of the collider step by step to avoid further setbacks as it moves toward new scientific experiments - probably starting in January - regarding the makeup of matter and the universe.
It has undergone a year of repairs after it was heavily damaged by a simple electrical fault.
Progress on restarting the machine, on the border between Switzerland and France, went faster than expected on Friday evening and the first beam circulated in a clockwise direction around the machine at about 10pm local time, said James Gillies, spokesman for the European Organisation for Nuclear Research.
"Some of the scientists had gone home and had to be called back in," Gillies told The Associated Press.
The exact time of the start of the Large Hadron Collider was difficult to predict because it was based on how long it took to perform steps along the way, and in the end it happened about nine hours earlier than expected, Gillies said.
This is an important milestone on the road toward scientific discoveries at the LHC, which are expected in 2010, he said.
About two hours later the scientists circulated another beam in the opposite direction, which was the initial goal in getting the machine going again and moving it toward collisions of protons, CERN said. The LHC also will be used later for colliding lead ions - basically the nucleus of the element that is about 160 times as heavy as a single proton. That should reveal still more scientific secrets.
"It's great to see beam circulating in the LHC again," said CERN Director General Rolf Heuer. "We've still got some way to go before physics can begin, but with this milestone we're well on the way."
With great fanfare, CERN circulated its first beams September 10, 2008. But the machine was sidetracked nine days later when a badly soldered electrical splice overheated and set off a chain of damage to massive superconducting magnets and other parts of the collider, in a 27km circular tunnel under the Swiss-French border.
CERN has $US40 million ($A43.5 million) on repairs and improvements on the machine to avoid a repetition.
"The LHC is a far better understood machine than it was a year ago," said Steve Myers, CERN's director for accelerators. "We've learned from our experience and engineered the technology that allows us to move on. That's how progress is made."
The LHC is expected soon to be running with more energy the world's current most powerful accelerator, the Tevatron at Fermilab near Chicago. It is supposed to keep ramping up to seven times the energy of Fermilab in coming years.
This will allow the collisions between protons on the machine to give insights into dark matter and what gives mass to other particles, and to show what matter was in the microseconds of rapid cooling after the Big Bang that many scientists theorise marked the creation of the universe billions of years ago.
The two parallel tubes the size of fire hoses send billions of protons whizzing around the collider in opposite directions at nearly the speed of light. In rooms the size of cathedrals 300 100 metres below the ground the magnets force them into huge detectors to record what happens.
The beams travelled on Friday night at a relatively low energy level, but Gillies said the LHC was expected soon to start accelerating them soon so that the collisions they make will be more powerful - and revealing - creating as yet unseen insights into nature.
The LHC operates at nearly absolute zero temperature, colder than outer space, which allows the superconducting magnets to guide the protons most efficiently.
Physicists have used smaller, room-temperature colliders for decades to study the atom. They once thought protons and neutrons were the smallest components of the atom's nucleus, but the colliders showed that they are made of quarks and gluons and that there are other forces and particles. And scientists still have other questions about antimatter, dark matter and supersymmetry they want to answer with CERN's new collider.
The Superconducting Super Collider being built in Texas would have been bigger than the LHC, but in 1993 the US Congress cancelled it after costs soared and questions were raised about its scientific value.
"The next important milestone will be low-energy collisions, expected in about a week from now," said Gillies.
These will give the experiments their first collision data, enabling them to calibrate their equipment for the scientific work ahead, eagerly awaited by particle physicists from countries around the world, he said.
Until now all the data they have recorded has comes from cosmic rays from outer space.
Gillies said the LHC should be ramped up to 3.5 trillion electron volts some time next year, which will be three and a half times as powerful as Fermilab. The two laboratories are friendly rivals, working on equipment and sharing scientists.
But each would be delighted to make the discovery of the elusive Higgs boson, the particle or field that theoretical gives mass to other particles. That is widely expected to deserve the Nobel Prize for physics.
More than 8,000 physicists from other labs around the world also have work planned for the LHC. The organisation is run by its 20 European member nations, with support from other countries, including observers Japan, India, Russia and the US that have made big contributions to the LHC.
CERN has received support from around the world in getting the LHC up and running again, the organisation said.
"It's been a Herculean effort to get to where we are today," said Myers. "I'd like to thank all those who have taken part, from CERN and from our partner institutions around the world."
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