The safety of the LHC

The Large Hadron Collider can only reproduce phenomena that already occur naturally. Cosmic rays — particles produced in outer space — collide with particles in the Earth's atmosphere at energies far higher than those of the LHC. Over the lifetime of the Earth, nature has already conducted the equivalent of the entire LHC experimental programme about a hundred thousand times, and the planet still exists. The Sun, the Moon, and other dense astronomical bodies have likewise been bombarded by such cosmic rays for billions of years. Whatever the LHC can produce, the universe has been producing all along without harmful consequence.

Because of this, the LHC Safety Assessment Group concluded that **LHC collisions present no danger and that there are no reasons for concern.** This conclusion was reviewed and endorsed by a panel of external scientists and unanimously approved by CERN's Scientific Policy Committee. The American Physical Society's Division of Particles and Fields, the UK Institute of Physics, and the German Physical Society have all endorsed the conclusion that there is nothing to fear from particles created at the LHC.

According to the Standard Model of particle physics, the LHC's energies are far too low to create black holes at all. Some speculative theories that posit extra spatial dimensions suggest that microscopic black holes could be produced — at a rate of perhaps one per second. Even in those theories, the black holes would be harmless: they would disintegrate almost immediately through Hawking radiation.

Hawking radiation is thermal radiation that black holes are predicted to emit because of quantum effects. The smaller a black hole is, the more strongly it radiates, so a microscopic black hole would lose mass faster than it could gain it and would evaporate essentially instantly. There is broad consensus among physicists on the reality of Hawking radiation, but so far no experiment has had the sensitivity required to find direct evidence for it.

Even on the extremely conservative assumption that some microscopic black holes turned out to be stable, they would still pose no danger. A stable microscopic black hole would be far too small — on the order of the Planck length — to accrete ordinary matter at any meaningful rate. And the decisive argument is again astrophysical: if collisions like those at the LHC could produce dangerous stable black holes, then cosmic-ray collisions would already have produced them throughout the universe. Such black holes, produced in collisions with dense bodies like white dwarfs and neutron stars, would be brought to rest inside them. The continued existence of white dwarfs and neutron stars — and of the Earth itself — rules out the possibility that the LHC could produce a dangerous black hole.

**Strangelets** are a hypothetical form of matter made of up, down, and strange quarks. The fear is that a strangelet might convert ordinary matter it touches into more strangelets in a runaway process. But the probability of producing a strangelet *decreases* as collision energy increases, so the high-energy LHC is even less likely to produce one than lower-energy machines that ran for years without incident. Strangelets are only expected to be stable at low energies, and a hot collision environment strongly disfavours forming a cold condensate — about as likely as forming an ice cube in a furnace.

**Vacuum bubbles.** There is a speculative idea that the universe is not in its most stable configuration and that a collision could tip a region into a more stable "true vacuum." Cosmic rays have been colliding throughout the universe at much higher energies for billions of years; if such a transition were possible, it would already have been triggered.

**Magnetic monopoles** are hypothetical particles carrying a single magnetic charge. If they are light enough to be produced at the LHC, cosmic rays striking the Earth's atmosphere would already have produced them, and they cannot catalyse proton decay in a way that threatens the Earth.