LHC Questions, Answered
The twelve questions people actually ask about the Large Hadron Collider, its shutdown, and the 2030 restart. Short answers here, full guides one click deeper.
When does the LHC restart?
CERN targets June 2030 for first beams in the upgraded High Luminosity LHC. The exact day has not been announced yet; our countdown tracks mid June and will switch to the precise date when CERN names one. Physics collisions for Run 4 follow first beams after a commissioning period.
Why is the LHC shut down right now?
The collider entered Long Shutdown 3 in early July 2026, a planned four year stop to install the High Luminosity upgrade. About 1.2 kilometres of the machine around the ATLAS and CMS collision points is being rebuilt with new magnets, crab cavities and detector systems.
When did Run 3 end?
The final protons of Run 3 circulated on 27 June 2026, closing a run that began in July 2022 and delivered more collision data than the first two runs combined, at a record energy of 13.6 TeV.
What is the High Luminosity LHC?
An upgraded LHC designed to produce collision data roughly ten times faster over its lifetime, targeting about 3,000 inverse femtobarns by around 2041. Collision energy rises only slightly; the transformation is in the collision rate.
Is the LHC dangerous?
No. Cosmic rays strike Earth's atmosphere at energies far beyond the LHC's and have for billions of years without harm, and CERN's independently reviewed safety assessments concluded the collider presents no danger. Nearly two decades of operation have confirmed it.
How big is the LHC?
The ring is 26.659 kilometres around, buried about 100 metres beneath the border of France and Switzerland near Geneva. It uses 1,232 superconducting dipole magnets, each 15 metres long, kept at 1.9 kelvin.
How fast do the protons go?
At full energy, protons travel at 99.999999 percent of the speed of light, completing about 11,245 laps of the 27 kilometre ring every second.
How cold is the LHC?
The superconducting magnets operate at 1.9 kelvin, about minus 271 degrees Celsius, colder than the 2.7 kelvin background temperature of outer space. Cooling the 36,000 tonne magnet system back down is one reason the shutdown takes years.
What has the LHC discovered?
The Higgs boson in 2012 is the headline, completing the Standard Model's particle roster. Beyond it, the experiments have discovered dozens of new composite particles, observed ultra rare decays, measured matter antimatter differences, and mapped the quark gluon plasma.
Will the upgraded LHC create a black hole?
No. Standard physics puts black hole production absurdly far beyond LHC energies, and even in speculative theories any microscopic black hole would evaporate instantly. Cosmic ray collisions of far higher energy have never produced a dangerous one in billions of years.
What comes after the HL-LHC?
The high luminosity programme runs to about 2041. CERN and the global community are studying successor machines, most prominently the Future Circular Collider concept, a roughly 91 kilometre ring. No successor has final approval yet; that decision belongs to the coming years.
Can I visit CERN during the shutdown?
Yes. CERN's Science Gateway exhibition centre in Geneva is open to the public with free entry, and guided tours of surface facilities continue through the shutdown. Underground visits are rare and depend on works; check CERN's official visits page for current availability.
Deeper answers: the restart schedule, the full safety case, and what the High Luminosity upgrade changes.