They hope the experiment in the Large Hadron Collider (LHC) will unlock the secrets of the universe and shed light on fundamental questions in physics.
Today has been reporting from Cern, with Science correspondent Tom Feilden.
After a tense start, he says, the switch-on went better than anyone could have hoped.
Excited physicists have been congratulating each other, there were cheers in the control room when engineers heard of the successful test and everyone has been celebrating the start of an experiment which - they hope - will deepen our understanding of the Universe.
Since the moment everyone had been waiting for - first beam - things have developed rapidly.
The whole day had been allocated to get the beam round the whole machine once. But it was so successful, the injection of the second beam to be brought forward. The has now completed its journey going in the other direction.
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