Safety of high-energy particle collision experiments

Similar concerns had previously been raised about the Relativistic Heavy Ion Collider (RHIC). Before RHIC started, critics postulated catastrophic scenarios including black holes, transitions into a different quantum-mechanical vacuum (false vacuum), or creation of strange matter more stable than ordinary matter. A striking counterargument was that objects like the Moon have been bombarded by cosmic particles of significantly higher energy than RHIC for billions of years without harm. Scientists concluded that "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet" and that there is "powerful empirical evidence against the possibility of dangerous strangelet production." Physicists noted they cannot demonstrate a zero probability for catastrophic events — they can only calculate upper limits. Martin Rees indicated the risk of a RHIC doomsday scenario as being at most a 1 in 50 million chance.

In the run-up to LHC commissioning, Walter L. Wagner (the original RHIC opponent), Luis Sancho (a Spanish science writer), and Otto Rössler (a German biochemist) expressed safety concerns and attempted to halt experiments through petitions to U.S. and European courts. These opponents asserted that LHC experiments could create low-velocity micro black holes that might grow in mass or release dangerous radiation, leading to doomsday scenarios. Other claimed potential risks included creation of strangelets, magnetic monopoles, and vacuum bubbles. After his federal lawsuit was dismissed, Wagner told a Daily Show correspondent that he believed the chance of the LHC destroying Earth to be 50%, "since it will either happen or it won't."

The mainstream scientific response was emphatic. Brian Greene, writing in The New York Times, reassured readers: "If a black hole is produced under Geneva, might it swallow Switzerland and continue on a ravenous rampage until the Earth is devoured? It's a reasonable question with a definite answer: no." The BBC reported that "the scientific consensus appears to be on the side of CERN's theorists," who say the LHC poses "no conceivable danger."

Although the Standard Model predicts that LHC energies are far too low to create black holes, some extensions of the Standard Model posit extra spatial dimensions in which it would be possible to create micro black holes at the LHC at a rate of approximately one per second. According to standard calculations these are harmless because they would quickly decay by Hawking radiation — thermal radiation predicted to be emitted by black holes due to quantum effects. Smaller black holes are predicted to be larger net emitters of radiation and to shrink and dissipate almost instantly. The LHC Safety Assessment Group (LSAG) notes that "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."

According to the LSAG, even if micro black holes were produced and were stable, they would be unable to accrete matter dangerously because of their extremely small size, on the order of the Planck length. Such black holes would also have been produced by cosmic rays and stopped in neutron stars and white dwarfs; the continued existence of these dense bodies rules out the possibility that the LHC could produce a dangerous black hole. As the LSAG put it: "Black holes produced in cosmic-ray collisions with bodies such as neutron stars and white dwarf stars would be brought to rest. The continued existence of such dense bodies, as well as the Earth, rules out the possibility of the LHC producing any dangerous black holes."

Strangelets are hypothetical small fragments of strange matter containing roughly equal numbers of up, down, and strange quarks. If they exist and were produced at the LHC, they could conceivably initiate a runaway process converting ordinary matter to strange matter. However, the probability of strangelet creation *decreases* at higher energies, so the LHC is less likely to produce them than its lower-energy predecessors. Strangelets are bound only at low energies (1–10 MeV), while LHC collisions release 7–14 TeV; the second law of thermodynamics very strongly disfavours forming a cold condensate in a hot medium — about as likely as producing an ice cube in a furnace.