The Large Hadron Collider, or LHC, is the largest and most complex machine ever made and is expected to revamp modern physics by smashing together particles in a bid to recreate the conditions of the Big Bang.
Staff in the control room on the border of Switzerland and France clapped as two beams of particles were sent silently first one way and then the other around the LHC's 27km underground chamber.
"Things can go wrong at any time," said project leader Lyn Evans, who wore jeans and running shoes for the LHC's debut.
"But this morning we had a great start."
It will be weeks or months before two particles ever crash together in the giant tube, and even longer before scientists can interpret results, said Jos Engelen, chief scientific officer of CERN, the European Organization for Nuclear Research.
"Anything between a year and four years, depending on how difficult this new physics is to find," Mr Engelen said.
Pyjama-clad scientists calling themselves "Nerds in Nightshirts" partied at the Fermi National Accelerator Laboratory in Batavia, Illinois as they waited late into the night for the first signals from the 10 billion Swiss franc ($11 billion) machine.
The first blip came soon after the LHC was activated at 9.30am CERN time (5.30pm AEST), when it was 1.30am in Batavia, home of the Tevatron – which lays claim to being the most powerful particle collider until the LHC starts smashing protons.
Physicists brushed off suggestions that the LHC, dubbed a "doomsday device" by some, could create tiny black holes that could suck in the planet.
"The worries that scientists had were nothing to do with being swallowed up by black holes and everything to do with technical hitches or electronic failure," said Jim al-Khalili, a physicist at Britain's University of Surrey.
"Now, after a collective sigh of relief, the real fun starts," Mr al-Khalili said.
"No matter what we find, we will be unlocking the secrets of the universe."
The LHC will send beams of subatomic particles called protons whizzing around the tube at just under the speed of light.
The hope is that they will smash into one another and explode in a burst of new and previously unseen types of particles – recreating on a tiny scale the heat and energy of the Big Bang that gave birth to the universe 13.7 billion years ago.
At full speed the LHC will engineer 600 million collisions every second. Data will be transmitted via a network called The Grid to scientists at 170 institutions in 33 countries.
"It is sort of a virtual United Nations," said Michael Tuts, a physics professor at Columbia University in New York and program manager for 400 US physicists working on one LHC project.
The experiments could confirm the existence of the Higgs Boson, a theoretical particle named after Peter Higgs, who first proposed it in 1964.
Also referred to as the "God particle", the Higgs Boson could help explain how matter has mass.
"I think it's pretty likely" that it will be found, Mr Higgs told reporters at the University of Edinburgh, where he is a retired professor of physics.
Scientists halted the particle beam's counter-clockwise spin temporarily last night after problems with the machine's magnets caused its temperature to warm slightly.
CERN officials said such minor glitches were to be expected given the intricacy of the machine, which is cooled to minus 271.3 degrees Celsius.
Large Hadron Collider website – http://www.lhc.ac.uk/
Large Hadron Collider webcams – http://www.lhc.ac.uk/web-cams.html
Large Hadron Collider on Wikipedia – http://en.wikipedia.org/wiki/Large_Hadron_Collider