AS the European Centre for Nuclear Research (CERN) was preparing itself to switch on the first proton beam in the Large Hadron Collider (LHC), the media and the Internet were agog with reports about how experiments with the accelerator could lead to a catastrophic end to the world. Indian television channels were particularly guilty of playing up this unfounded threat to mankind. These reports were based on views expressed by a few – on the basis of an incomplete understanding of physics related to the phenomena that physicists are trying to probe with new particle accelerators.

Gigantic experimental devices such as the LHC are pushing the frontiers of energy to recreate in the laboratory the conditions that existed in the early universe. As new energy domains open up, surely new phenomena will be revealed and it is natural to wonder – even for scientists – if any of these would be potentially “dangerous”. Indeed, such concerns were expressed for the first time before the commissioning of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Labs (BNL), United States. In 1999, a committee set up to look into these “disaster scenarios” concluded: “Candidate mechanisms for catastrophic scenarios are firmly excluded either by existing empirical evidence, compelling theoretical arguments, or both.”

As the LHC was nearing completion, various law suits were filed to stall its commissioning. Since the LHC will reach even higher energies, CERN’s former director-general, Luciano Maiani, ordered a new comprehensive study of the issue in 2003 taking into account new developments since the BNL report. This study too dismissed doomsday speculations.

Early this year, a five-member LHC Safety Assessment Group, led by John Ellis of CERN, updated this study. This has now been published as a paper in the September 5 issue of Journal of Physics G: Nuclear and Particle Physics. It not only revisited the disaster scenarios considered by the BNL report but also new ones, which began to be aired following some new speculative theoretical models that have been put forward in recent years. The scenarios included:

• Production of microscopic black holes that will accrete all matter around it and, therefore, the earth itself;

• Production of “strangelets”, the hypothetical microscopic lump of a state of matter that is different from the familiar normal matter around us, that will keep growing by coalescing with ordinary matter and turning it also into strange matter through nuclear interaction;

• Production of a small “vacuum bubble”, a more stable configuration than the present one in which the universe exists, which would expand and suck not only the earth but the entire universe into it.

• Production of magnetic monopoles, which will cause protons and neutrons that make up ordinary matter to decay and eventually destroy all matter.

The study reaffirmed the earlier conclusions that particle collisions in the LHC present no danger. An international team of physicists comprising Peter Braun-Manzinger, Merreo Cavalli-Sforze, Gerard ‘tHooft, Bryan Webber and Fabio Zwirner reviewed the report.

The essence of the report’s argument is that if the LHC could lead to any of the above catastrophes, we would not even exist today to run an LHC. For, whatever LHC will do, nature has already done so many times with cosmic rays during the lifetimes of astronomical bodies such as the earth and the sun and these have withstood the onslaught of cosmic rays for billions of years. Although the LHC will achieve an energy (of 14 x 10¹² eV) that no other particle accelerator has achieved before, cosmic rays, which produce collisions, with far greater energies, the highest energies being of the order of 10²⁰ eV, are all-pervasive.

The report estimates that the universe replicates the number of collisions in the LHC over 10 trillion times a second and has thus carried out 1031 LHC experiments since the origin of the universe. By calculating the flux of cosmic rays that has struck the earth since its creation with energies greater than the LHC energy, the report says: “Nature has already conducted the equivalent of a hundred thousand LHC experiments on the earth, and the planet still exists.” Similarly, collisions of cosmic rays with the sun are equivalent to one billion LHC experiments, and the sun still exists.

Scenario (i): The gravitational pull of a black hole is related to the amount of matter it contains – the less there is, the weaker the pull. Even if microscopic black holes are created in the LHC collisions, since they result from colliding particles with energies of the order of a flying mosquito, these black holes cannot generate enough gravitational pull to accrete surrounding matter, the report points out. The report adds that, if the LHC can produce black holes, cosmic rays would have produced many more and these have caused no harm. Also, black holes lose energy through a quantum mechanical process called Hawking Radiation. Any black hole that cannot attract matter will thus shrink and disappear through this radiation. That is, even if the LHC produced them, these would exist only for a fleeting moment.

Scenario (ii): The existence of ‘strange’ matter – which contains ‘strange’ quarks unlike ordinary matter which contains only the ‘up’ and ‘down’ quarks that make up the protons and the neutrons – has never been proven. Even if they exist, they would be extremely unstable, argues the report. Furthermore, their electric charge would repel normal matter and would gradually decay. Thus, if strangelets exist and were to be produced at the LHC, they would be harmless.

Scenario (iii): If the LHC could produce vacuum bubbles, the report argues, cosmic rays would have produced them and these bubbles of the new stable vacuum would have already expanded to consume large parts of the visible universe several billion years ago.

Scenario (iv): Monopoles are particles carrying isolated or free north or south magnetic poles only. There is no evidence so far of their existence but these are theoretically possible. In some Grand Unified Theories (GUTs), monopoles can cause protons and neutrons to decay into electrons or positrons and unstable mesons. But these monopoles are extremely heavy, with mass of the order of 10²⁴ eV and, therefore, will not be produced by the LHC. Even if the LHC could produce monopoles, only a microgram of matter would be destroyed in this manner before the monopole exited the earth, according to one study. Independent of this observation, the report notes, cosmic rays would have produced them in large numbers when striking the earth or astronomical bodies. Therefore, if monopoles are produced at the LHC, they would be harmless.

R. Ramachandran