The Large Hadron Collider is located at CERN, the European Organization for Nuclear Research, near Geneva, Switzerland. This is CERN's Globe of Science and Innovation, which hosts a small museum about particle physics inside. The ATLAS experiment is housed underground nearby.
The Large Hadron Collider is located at CERN, the European Organization for Nuclear Research, near Geneva, Switzerland. This is CERN's Globe of Science and Innovation, which hosts a small museum about particle physics inside. The ATLAS experiment is housed underground nearby.
Elizabeth Landau/CNN
The Higgs boson, the elusive particle that scientists had hoped to find for decades, was detected by two general-purpose experiments at the Large Hadron Collider, as scientists announced in 2012. The Compact Muon Solenoid (CMS) experiment, pictured, is one of them.
The Higgs boson, the elusive particle that scientists had hoped to find for decades, was detected by two general-purpose experiments at the Large Hadron Collider, as scientists announced in 2012. The Compact Muon Solenoid (CMS) experiment, pictured, is one of them.
Ben Brumfield/CNN
The ATLAS experiment, seen here in 2011, also detected the Higgs boson, a particle that helps explain why matter has mass. It has been called the "God particle" because of a book by that title, but scientists hate the name.
The ATLAS experiment, seen here in 2011, also detected the Higgs boson, a particle that helps explain why matter has mass. It has been called the "God particle" because of a book by that title, but scientists hate the name.
CERN
Much of three stories of electronics at CMS are involved in making split-second decisions about what data to keep and what to discard. This is one of those areas.
Much of three stories of electronics at CMS are involved in making split-second decisions about what data to keep and what to discard. This is one of those areas.
Ben Brumfield/CNN
A technician works on the CMS experiment. Technicians are adding new cooling lines for CMS for a system that will be put in place in two or three years.
A technician works on the CMS experiment. Technicians are adding new cooling lines for CMS for a system that will be put in place in two or three years.
Ben Brumfield/CNN
CMS is adding this layer for the next run of particle collisions to improve the detection of muons, which are fundamental particles.
CMS is adding this layer for the next run of particle collisions to improve the detection of muons, which are fundamental particles.
Ben Brumfield/CNN
Physicists work in the CMS control room.
Physicists work in the CMS control room.
Ben Brumfield/CNN
The nearly 14,000 tons of machinery can all collapse together, or separate, when high-pressure air is pumped in. This is one of the pads to help slide it all around.
The nearly 14,000 tons of machinery can all collapse together, or separate, when high-pressure air is pumped in. This is one of the pads to help slide it all around.
Ben Brumfield/CNN
CMS has 76,000 lead-tungstate crystals that shatter electrons and photons, allowing scientists to observe particles such as the Higgs boson that exist for only an instant. Some of those crystals are in the endcap.
CMS has 76,000 lead-tungstate crystals that shatter electrons and photons, allowing scientists to observe particles such as the Higgs boson that exist for only an instant. Some of those crystals are in the endcap.
Ben Brumfield/CNN
Evaldas Juska, an engineer, is working on computers involved with CMS.
Evaldas Juska, an engineer, is working on computers involved with CMS.
Ben Brumfield/CNN
CMS was constructed at ground level, then pieces of it were lowered through this hole in the cavern.
CMS was constructed at ground level, then pieces of it were lowered through this hole in the cavern.
Elizabeth Landau/CNN
This is the CERN Computing Center. Tim Berners-Lee invented the World Wide Web at CERN.
This is the CERN Computing Center. Tim Berners-Lee invented the World Wide Web at CERN.
Elizabeth Landau/CNN
One of the world's first web servers, a NeXT computer from 1991, is seen at CERN. The handwritten note indicates, "This machine is a server. DO NOT POWER DOWN!" On the right is an old Ethernet cable, which can handle only 10 Mb/second, and was largely replaced by the mid-'90s.
One of the world's first web servers, a NeXT computer from 1991, is seen at CERN. The handwritten note indicates, "This machine is a server. DO NOT POWER DOWN!" On the right is an old Ethernet cable, which can handle only 10 Mb/second, and was largely replaced by the mid-'90s.
Ben Brumfield/CNN
While they take their work seriously, that doesn't mean the scientists at CERN don't have a sense of humor. Here we see CERN's "Animal Shelter for Computer Mice," where used and unwanted computer mice have a place to call home.
While they take their work seriously, that doesn't mean the scientists at CERN don't have a sense of humor. Here we see CERN's "Animal Shelter for Computer Mice," where used and unwanted computer mice have a place to call home.
Ben Brumfield/CNN
A sculpture garden featuring artwork made from pieces of old experiments decorates the grounds at CERN.
A sculpture garden featuring artwork made from pieces of old experiments decorates the grounds at CERN.
Elizabeth Landau/CNN
A collection of empty relics from the celebrations of different milestones of the CMS experiment.
A collection of empty relics from the celebrations of different milestones of the CMS experiment.
Ben Brumfield/CNN
"Don't feed the physicists" marks a box of coins where CMS scientists deposit change to pay for coffee.
"Don't feed the physicists" marks a box of coins where CMS scientists deposit change to pay for coffee.
Ben Brumfield/CNN
Joe Incandela, the spokesperson for CMS, says that about 4,000 scientists work there.
Joe Incandela, the spokesperson for CMS, says that about 4,000 scientists work there.
Ben Brumfield/CNN
Exploring the universe at CERN

Story highlights

The Large Hadron Collider (LHC) begins again after a two-year shutdown

The restart was delayed in March

CNN  — 

The world’s biggest and most powerful physics experiment is taking place as you read this.

The Large Hadron Collider (LHC), a particle accelerator and the largest machine in the world, is ready for action following a two-year shutdown.

After problems that delayed the restart in March, scientists at the European Organization for Nuclear Research (CERN) completed final tests, enabling the first beams to start circulating Sunday inside the LHC’s 17 mile (27 km) ring.

“Operating accelerators for the benefit of the physics community is what CERN’s here for,” CERN Director-General Rolf Heuer said on the organization’s website. “Today, CERN’s heart beats once more to the rhythm of the LHC.”

The LHC generates up to 600 million particles per second, with a beam circulating for 10 hours, traveling more than 6 billion miles (more than 10 billion kilometers) – the distance from Earth to Neptune and back again. At near light-speed, a proton in the LHC makes 11,245 circuits per second.

Why does it matter?

It took thousands of scientists, engineers and technicians decades to devise and build the particle accelerator, housed in a tunnel between Lake Geneva and the Jura mountain range.

The purpose of the lengthy project is to recreate the conditions that existed moments after the “Big Bang” – the scientific theory said to explain the creation of the universe. By replicating the energy density and temperature, scientists hope to uncover how the universe evolved.

Our current, limited, knowledge is based on what’s called The Standard Model of particle physics. “But we know that this model is not complete,” Dr. Mike Lamont, operations group leader at the LHC, told CNN in March.

The burning questions that remain include the origin of mass and why some particles are very heavy, while others have no mass at all; a unified description of all the fundamental forces such as gravity; and uncovering dark matter and dark energy, since visible matter accounts for only 4 percent of the universe.

The LHC could also question the idea that the universe is only made of matter, despite the theory that antimatter must have been produced in the same amounts at the time of the Big Bang.

CERN says the energies achievable by the LHC have only ever been found in nature.

The machine alone costs approximately three billion euros (about $3.3 billion), paid for by member countries of CERN and contributions by non-member nations.

The organization also asserts that its guidelines for the protection of the environment and personnel comply with standards set by Swiss and French laws and a European Council Directive.

Scientists and physics enthusiasts will be waiting with bated breath as the LHC ventures into the great unknown.

“After two years of effort, the LHC is in great shape,” said CERN Director for Accelerators and Technology, Frédérick Bordry. “But the most important step is still to come when we increase the energy of the beams to new record levels.”

Peter Shadbolt contributed to this report.