The safety of high energy particle collisions was a topic of widespread discussion and topical interest during the time when the Relativistic Heavy Ion Collider (RHIC) and later the Large Hadron Collider (LHC)—currently the world's largest and most powerful particle accelerator—were being constructed and commissioned. Concerns arose that such high energy experiments—designed to produce novel particles and forms of matter—had the potential to create harmful states of matter or even doomsday scenarios. Claims escalated as commissioning of the LHC drew closer, around 2008–2010. The claimed dangers included the production of stable micro black holes and the creation of hypothetical particles called strangelets, and these questions were explored in the media, on the Internet and at times through the courts.

To address these concerns in the context of the LHC, CERN mandated a group of independent scientists to review these scenarios. In a report issued in 2003, they concluded that, like current particle experiments such as the RHIC, the LHC particle collisions pose no conceivable threat.Blaizot JP, Iliopoulos J, Madsen J, Ross GG, Sonderegger P, Specht HJ (2003). Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC. CERN. Geneva. CERN-2003-001. A second review of the evidence commissioned by CERN was released in 2008. The report, prepared by a group of physicists affiliated to CERN but not involved in the LHC experiments, reaffirmed the safety of the LHC collisions in light of further research conducted since the 2003 assessment."The safety of the LHC". CERN 2008 (CERN website). It was reviewed and endorsed by a CERN committee of 20 external scientists and by the executive committee of the Division of Particles & Fields of the American Physical Society,CERN Scientific Policy Committee (2008). SPC Report on LSAG Documents. CERN record."Statement by the Executive Committee of the DPF on the Safety of Collisions at the Large Hadron Collider " issued by the Division of Particles & Fields (DPF) of the American Physical Society (APS) and was later published in the peer-reviewed Journal of Physics G by the UK Institute of Physics, which also endorsed its conclusions."LHC switch-on fears are completely unfounded ". The Institute of Physics. PR 48 (08). 5 September 2008.

The report ruled out any doomsday scenario at the LHC, noting that the physical conditions and collision events which exist in the LHC, RHIC and other experiments occur naturally and routinely in the universe without hazardous consequences, including ultra-high-energy cosmic rays observed to impact Earth with energies far higher than those in any man-made collider.

Background

Particle colliders are a type of particle accelerator used by physicists as a research tool to understand fundamental aspects of the universe. Their operation involves directed beams of particles accelerated to very high kinetic energy and allowed to collide; analysis of the byproducts of these collisions gives scientists good evidence of the structure of the subatomic world and the laws of nature governing it. These may become apparent only at high energies and for tiny periods of time, and therefore may be hard or impossible to study in other ways.

Because of the high energy levels involved, concerns have arisen at times in the public arena as to whether such collisions are safe, or whether they might, by reason of their extreme energy, trigger unforeseen problems or consequences.

Examples of colliders Concerns were noted during the construction of the Large Hadron Collider (LHC), which began operations in 2008, is the world's largest and highest-energy particle accelerator complex, intended to collide opposing beams of either protons or lead nuclei with very high kinetic energy.CERN Communication Group (January 2008). "CERN FAQ — LHC: the guide " (PDF). CERN. Geneva (44p).Achenbach, Joel (1 March 2008). "The God Particle". National Geographic Magazine. It was built by the European Organization for Nuclear Research (CERN) near Geneva, in Switzerland. The LHC's main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. The first particle collisions at the LHC took place shortly after startup in November 2009, at energies up to per beam.CERN press release (2009) "LHC ends 2009 run on a high note." On 30 March 2010, the first planned collisions took place between two 3.5 TeV beams, which set another new world record for the highest energy man-made particle collisions. In 2012 the beam energy was increased to 4 TeV, after upgrades in 2013 and 2014 collisions in 2015 and 2016 happened at an energy of 6.5 TeV per proton.Record breaking collision at 13TeV, CERN Press release

Similar concerns had previously also been raised in the context of the Relativistic Heavy Ion Collider, with Frank Close, professor of physics at the University of Oxford, to comment at the time that "the chance of [strangelet creation] is like you winning the major prize on the lottery 3 weeks in succession; the problem is that people believe it is possible to win the lottery 3 weeks in succession."BBC End Days (Documentary)

Relativistic Heavy Ion Collider

Concerns about possible adverse consequences were raised in connection with the RHIC particle accelerator.Matthews, Robert (28 August 1999). "A Black Hole Ate My Planet". New Scientist.Horizon: End Day. BBC. 2005. After detailed studies, scientists reached such conclusions as "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."

Before the Relativistic Heavy Ion Collider started operation, critics postulated that the extremely high energy could produce catastrophic scenarios, such as creating a black hole, a transition into a different quantum mechanical vacuum (see false vacuum), or the creation of strange matter that is more stable than ordinary matter. These hypotheses are complex, but many predict that the Earth would be destroyed in a time frame from seconds to millennia, depending on the theory considered. However, the fact that objects of the Solar System (e.g., the Moon) have been bombarded with cosmic particles of significantly higher energies than that of RHIC and other man made colliders for billions of years, without any harm to the Solar System, were among the most striking arguments that these hypotheses were unfounded.

The other main controversial issue was a demand by critics for physicists to reasonably exclude the probability for such a catastrophic scenario. Physicists are unable to demonstrate experimental and astrophysical constraints of zero probability of catastrophic events, nor that tomorrow Earth will be struck with a "doomsday" cosmic ray (they can only calculate an upper limit for the likelihood). The result would be the same destructive scenarios described above, although obviously not caused by humans. According to this argument of upper limits, RHIC would still modify the chance for the Earth's survival by an infinitesimal amount.

Concerns were raised in connection with the RHIC particle accelerator, both in the media and in the popular science media. The risk of a doomsday scenario was indicated by Martin Rees, with respect to the RHIC, as being at least a 1 in 50 million chance.Cf. Brookhaven Report mentioned by Rees, Martin (Lord), Our Final Century: Will the Human Race Survive the Twenty-first Century?, U.K., 2003, ; note that the mentioned "1 in 50 million" chance is disputed as being a misleading and played down probability of the serious risks (Aspden, U.K., 2006) With regards to the production of strangelets, Frank Close, professor of physics at the University of Oxford, indicates that "the chance of this happening is like you winning the major prize on the lottery 3 weeks in succession; the problem is that people believe it is possible to win the lottery 3 weeks in succession." After detailed studies, scientists reached such conclusions as "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."

History of discussion The debate started in 1999 with an exchange of letters in Scientific American between Walter L. Wagner and F. Wilczek, in response to a previous article by M. Mukerjee. The media attention unfolded with an article in U.K. Sunday Times of July 18, 1999 by J. Leake,Sunday Times, 18 July 1999. closely followed by articles in the U.S. media.e.g. ABCNEWS.com, from the Internet Archive. The controversy mostly ended with the report of a committee convened by the director of Brookhaven National Laboratory, J. H. Marburger, ostensibly ruling out the catastrophic scenarios depicted. However, the report left open the possibility that relativistic cosmic ray impact products might behave differently while transiting earth compared to "at rest" RHIC products; and the possibility that the qualitative difference between high-E proton collisions with earth or the moon might be different than gold on gold collisions at the RHIC. Wagner tried subsequently to stop full energy collision at RHIC by filing Federal lawsuits in San Francisco and New York City, but without success.e.g. NBC News, June 14, 2000. The New York suit was dismissed on the technicality that the San Francisco suit was the preferred forum. The San Francisco suit was dismissed, but with leave to refile if additional information was developed and presented to the court.United States District Court, Eastern District of New York, Case No. 00CV1672, Walter L. Wagner vs. Brookhaven Science Associates, L.L.C. (2000); United States District Court, Northern District of California, Case No. C99-2226, Walter L. Wagner vs. U.S. Department of Energy, et al. (1999)

On March 17, 2005, the BBC published an articleBBC, 17 March 2005. implying that researcher Horaţiu Năstase believes black holes have been created at RHIC. However, the original papers of H. Năstase and the New Scientist articleE. S. Reich, New Scientist 185:2491, 16 (2005). cited by the BBC state that the correspondence of the hot dense QCD matter created in RHIC to a black hole is only in the sense of a correspondence of QCD scattering in Minkowski space and scattering in the AdS5 × X5 space in AdS/CFT; in other words, it is similar mathematically. Therefore, RHIC collisions might be described by mathematics relevant to theories of quantum gravity within AdS/CFT, but the described physical phenomena are not the same.

Large Hadron Collider

In the run up to the commissioning of the LHC, Walter L. Wagner (an original opponent of the RHIC), Luis Sancho (a Spanish science writer) and Otto Rössler (a German biochemist) expressed concerns over the safety of the LHC, and attempted to halt the beginning of the experiments through petitions to the US and European Courts.Boyle, Alan (2 September 2008). "Courts weigh doomsday claims". Cosmic Log. msnbc.com."Some fear debut of powerful atom-smasher" CNN.com. 30 June 2008.Muir, Hazel. (28 March 2008). "Particle smasher 'not a threat to the Earth'". NewScientist.com.Overbye, Dennis (21 June 2008). "Earth Will Survive After All, Physicists Say". The New York Times.Sancho, Luis (June 2008). "Fear review". Harper's Magazine. These opponents assert that the LHC experiments have the potential to create low velocity micro black holes that could grow in mass or release dangerous radiation leading to doomsday scenarios, such as the destruction of the Earth.Boyle, Alan (19 August 2008). "Twists in the Doomsday debate". Cosmic Log. msnbc.com.Boyle, Alan (27 March 2008). "Doomsday Fears Spark Lawsuit". Cosmic Log. msnbc.com. Other claimed potential risks include the creation of theoretical particles called strangelets, magnetic monopoles, and vacuum bubbles.