The safety of the LHC

The Large Hadron Collider (LHC) can achieve an energy that no other particle accelerators have reached before, but Nature routinely produces higher energies in cosmic-ray collisions. Concerns about the safety of whatever may be created in such high-energy particle collisions have been addressed for many years. In the light of new experimental data and theoretical understanding, the LHC Safety Assessment Group (LSAG) has updated a review of the analysis made in 2003 by the LHC Safety Study Group, a group of independent scientists.

LSAG reaffirms and extends the conclusions of the 2003 report that LHC collisions present no danger and that there are no reasons for concern. Whatever the LHC will do, Nature has already done many times over during the lifetime of the Earth and other astronomical bodies. The LSAG report has been reviewed and endorsed by CERN’s Scientific Policy Committee, a group of external scientists that advises CERN’s governing body, its Council.

The following summarizes the main arguments given in the LSAG report. Anyone interested in more details is encouraged to consult it directly, and the technical scientific papers to which it refers.

Cosmic rays

The LHC, like other particle accelerators, recreates the natural phenomena of cosmic rays under controlled laboratory conditions, enabling them to be studied in more detail. Cosmic rays are particles produced in outer space, some of which are accelerated to energies far exceeding those of the LHC. The energy and the rate at which they reach the Earth’s atmosphere have been measured in experiments for some 70 years. Over the past billions of years, Nature has already generated on Earth as many collisions as about a million LHC experiments – and the planet still exists. Astronomers observe an enormous number of larger astronomical bodies throughout the Universe, all of which are also struck by cosmic rays. The Universe as a whole conducts more than 10 million million LHC-like experiments per second. The possibility of any dangerous consequences contradicts what astronomers see - stars and galaxies still exist.

Microscopic black holes

Nature forms black holes when certain stars, much larger than our Sun, collapse on themselves at the end of their lives. They concentrate a very large amount of matter in a very small space. Speculations about microscopic black holes at the LHC refer to particles produced in the collisions of pairs of protons, each of which has an energy comparable to that of a mosquito in flight. Astronomical black holes are much heavier than anything that could be produced at the LHC.

According to the well-established properties of gravity, described by Einstein’s relativity, it is impossible for microscopic black holes to be produced at the LHC. There are, however, some speculative theories that predict the production of such particles at the LHC. All these theories predict that these particles would disintegrate immediately. Black holes, therefore, would have no time to start accreting matter and to cause macroscopic effects.

Although theory predicts that microscopic black holes decay rapidly, even hypothetical stable black holes can be shown to be harmless by studying the consequences of their production by cosmic rays. Whilst collisions at the LHC differ from cosmic-ray collisions with astronomical bodies like the Earth in that new particles produced in LHC collisions tend to move more slowly than those produced by cosmic rays, one can still demonstrate their safety. The specific reasons for this depend whether the black holes are electrically charged, or neutral. Many stable black holes would be expected to be electrically charged, since they are created by charged particles. In this case they would interact with ordinary matter and be stopped while traversing the Earth or Sun, whether produced by cosmic rays or the LHC. The fact that the Earth and Sun are still here rules out the possibility that cosmic rays or the LHC could produce dangerous charged microscopic black holes. If stable microscopic black holes had no electric charge, their interactions with the Earth would be very weak. Those produced by cosmic rays would pass harmlessly through the Earth into space, whereas those produced by the LHC could remain on Earth. However, there are much larger and denser astronomical bodies than the Earth in the Universe. 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

Strangelet is the term given to a hypothetical microscopic lump of ‘strange matter’ containing almost equal numbers of particles called up, down and strange quarks. According to most theoretical work, strangelets should change to ordinary matter within a thousand-millionth of a second. But could strangelets coalesce with ordinary matter and change it to strange matter? This question was first raised before the start up of the Relativistic Heavy Ion Collider, RHIC, in 2000 in the United States. A study at the time showed that there was no cause for concern, and RHIC has now run for eight years, searching for strangelets without detecting any. At times, the LHC will run with beams of heavy nuclei, just as RHIC does. The LHC’s beams will have more energy than RHIC, but this makes it even less likely that strangelets could form. It is difficult for strange matter to stick together in the high temperatures produced by such colliders, rather as ice does not form in hot water. In addition, quarks will be more dilute at the LHC than at RHIC, making it more difficult to assemble strange matter. Strangelet production at the LHC is therefore less likely than at RHIC, and experience there has already validated the arguments that strangelets cannot be produced.

The analysis of the first LHC data from heavy ion collisions has now confirmed the key ingredients used in the LSAG report to evaluate the upper limit on the production of hypothetical strangelets. For more details see this addendum to the LSAG report: Implications of LHC heavy ion data for multi-strange baryon production (2011)

Vacuum bubbles

There have been speculations that the Universe is not in its most stable configuration, and that perturbations caused by the LHC could tip it into a more stable state, called a vacuum bubble, in which we could not exist. If the LHC could do this, then so could cosmic-ray collisions. Since such vacuum bubbles have not been produced anywhere in the visible Universe, they will not be made by the LHC.

Magnetic monopoles

Magnetic monopoles are hypothetical particles with a single magnetic charge, either a north pole or a south pole. Some speculative theories suggest that, if they do exist, magnetic monopoles could cause protons to decay. These theories also say that such monopoles would be too heavy to be produced at the LHC. Nevertheless, if the magnetic monopoles were light enough to appear at the LHC, cosmic rays striking the Earth’s atmosphere would already be making them, and the Earth would very effectively stop and trap them. The continued existence of the Earth and other astronomical bodies therefore rules out dangerous proton-eating magnetic monopoles light enough to be produced at the LHC.

Other aspects of LHC safety:

Concern has recently been expressed that a 'runaway fusion reaction' might be created in the LHC carbon beam dump. The safety of the LHC beam dump had previously been reviewed by the relevant regulatory authorities of the CERN host states, France and Switzerland. The specific concerns expressed more recently have been addressed in a technical memorandum by Assmann et al. As they point out, fusion reactions can be maintained only in material compressed by some external pressure, such as that provided by gravity inside a star, a fission explosion in a thermonuclear device, a magnetic field in a Tokamak, or by continuing isotropic laser or particle beams in the case of inertial fusion. In the case of the LHC beam dump, it is struck once by the beam coming from a single direction. There is no countervailing pressure, so the dump material is not compressed, and no fusion is possible.

Concern has been expressed that a 'runaway fusion reaction' might be created in a nitrogen tank inside the LHC tunnel. There are no such nitrogen tanks. Moreover, the arguments in the previous paragraph prove that no fusion would be possible even if there were.

Finally, concern has also been expressed that the LHC beam might somehow trigger a 'Bose-Nova' in the liquid helium used to cool the LHC magnets. A study by Fairbairn and McElrath has clearly shown there is no possibility of the LHC beam triggering a fusion reaction in helium.

We recall that 'Bose-Novae' are known to be related to chemical reactions that release an infinitesimal amount of energy by nuclear standards. We also recall that helium is one of the most stable elements known, and that liquid helium has been used in many previous particle accelerators without mishap. The facts that helium is chemically inert and has no nuclear spin imply that no 'Bose-Nova' can be triggered in the superfluid helium used in the LHC.

Comments on the papers by Giddings and Mangano, and by LSAG

The papers by Giddings and Mangano and LSAG demonstrating the safety of the LHC have been studied, reviewed and endorsed by leading experts from the CERN Member States, Japan, Russia and the United States, working in astrophysics, cosmology, general relativity, mathematics, particle physics and risk analysis, including several Nobel Laureates in Physics. They all agree that the LHC is safe.

The paper by Giddings and Mangano has been peer-reviewed by anonymous experts in astrophysics and particle physics and published in the professional scientific journal Physical Review D. The American Physical Society chose to highlight this as one of the most significant papers it has published recently, commissioning a commentary by Prof. Peskin from the Stanford Linear Accelerator Laboratory in which he endorses its conclusions. The Executive Committee of the Division of Particles and Fields of the American Physical Society has issued a statement endorsing the LSAG report.

The LSAG report has been published by the UK Institute of Physics in its publication Journal of Physics G. The conclusions of the LSAG report were endorsed in a press release that announced this publication.

The conclusions of LSAG have also been endorsed by the Particle and Nuclear Physics Section (KET) of the German Physical Society. A translation into German of the complete LSAG report may be found on the KET website, as well as here. (A translation into French of the complete LSAG report is also available.)

Thus, the conclusion that LHC collisions are completely safe has been endorsed by the three respected professional societies of physicists that have reviewed it, which rank among the most highly respected professional societies in the world.

World-renowned experts in astrophysics, cosmology, general relativity, mathematics, particle physics and risk analysis, including several Nobel Laureates in Physics, have also expressed clear individual opinions that LHC collisions are not dangerous:

"To think that LHC particle collisions at high energies can lead to dangerous black holes is rubbish. Such rumors were spread by unqualified people seeking sensation or publicity."

Academician Vitaly Ginzburg, Nobel Laureate in Physics, Lebedev Institute, Moscow, and Russian Academy of Sciences

"The operation of the LHC is safe, not only in the old sense of that word, but in the more general sense that our most qualified scientists have thoroughly considered and analyzed the risks involved in the operation of the LHC. [Any concerns] are merely hypothetical and speculative, and contradicted by much evidence and scientific analysis."

Prof. Sheldon Glashow, Nobel Laureate in Physics, Boston University,

Prof. Frank Wilczek, Nobel Laureate in Physics, Massachusetts Institute of Technology,

Prof. Richard Wilson, Mallinckrodt Professor of Physics, Harvard University

"The world will not come to an end when the LHC turns on. The LHC is absolutely safe. ... Collisions releasing greater energy occur millions of times a day in the earth's atmosphere and nothing terrible happens."

Prof. Steven Hawking, Lucasian Professor of Mathematics, Cambridge University

"Nature has already done this experiment. ... Cosmic rays have hit the moon with more energy and have not produced a black hole that has swallowed up the moon. The universe doesn't go around popping off huge black holes."

Prof. Edward Kolb, Astrophysicist, University of Chicago

"I certainly have no worries at all about the purported possibility of LHC producing microscopic black holes capable of eating up the Earth. There is no scientific basis whatever for such wild speculations."

Prof. Sir Roger Penrose, Former Rouse Ball Professor of Mathematics, Oxford University

"There is no risk [in LHC collisions, and] the LSAG report is excellent."

Prof. Lord Martin Rees, UK Astronomer Royal and President of the Royal Society of London

"Those who have doubts about LHC safety should read LSAG report where all possible risks were considered. We can be sure that particle collisions at the LHC  cannot lead to a catastrophic consequences."

Academician V.A. Rubakov, Institute for Nuclear Research, Moscow, and Russian Academy of Sciences

"We fully endorse the conclusions of the LSAG report: there is no basis for any concerns about the consequences of new particles or forms of matter that could possibly be produced at the LHC."

R. Aleksan et al., the 20 external members of the CERN Scientific Policy Committee, including Prof. Gerard 't Hooft, Nobel Laureate in Physics.

The overwhelming majority of physicists agree that microscopic black holes would be unstable, as predicted by basic principles of quantum mechanics. As discussed in the LSAG report, if microscopic black holes can be produced by the collisions of quarks and/or gluons inside protons, they must also be able to decay back into quarks and/or gluons. Moreover, quantum mechanics predicts specifically that they should decay via Hawking radiation.

Nevertheless, a few papers have suggested that microscopic black holes might be stable. The paper by Giddings and Mangano and the LSAG report analyzed very conservatively the hypothetical case of stable microscopic black holes and concluded that even in this case there would be no conceivable danger. Another analysis with similar conclusions has been documented by Dr. Koch, Prof. Bleicher and Prof. Stoecker of Frankfurt University and GSI, Darmstadt, who conclude:

"We discussed the logically possible black hole evolution paths. Then we discussed every single outcome of those paths and showed that none of the physically sensible paths can lead to a black hole disaster at the LHC."

Professor Roessler (who has a medical degree and was formerly a chaos theorist in Tuebingen) also raised doubts on the existence of Hawking radiation. His ideas have been refuted by Profs. Nicolai (Director at the Max Planck Institute for Gravitational Physics - Albert-Einstein-Institut - in Potsdam) and Giulini, whose report (see here for the English translation, and here for further statements) point to his failure to understand general relativity and the Schwarzschild metric, and his reliance on an alternative theory of gravity that was disproven in 1915. Their verdict:

"[Roessler's] argument is not valid; the argument is not self-consistent."

The paper of Prof. Roessler has also been criticized by Prof. Bruhn of the Darmstadt University of Technology, who concludes that:

"Roessler's misinterpretation of the Schwarzschild metric [renders] his further considerations ... null and void. These are not papers that could be taken into account when problems of black holes are discussed."

A hypothetical scenario for possibly dangerous metastable black holes has recently been proposed by Dr. Plaga. The conclusions of this work have been shown to be inconsistent in a second paper by Giddings and Mangano, where it is also stated that the safety of this class of metastable black hole scenarios is already established by their original work.

 

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