Will the LHC Destroy Earth?
On March 21, 2008, a suit was filed in Federal District Court in Hawaii asking for a temporary restraining order prohibiting the European Center for Nuclear Research (CERN) in Geneva from turning on the world’s largest particle accelerator, the Large Hadron Collider (LHC), this summer.
The suit contends that the collider could produce a tiny black hole or an exotic object called a “strangelet,” either of which might swallow up Earth and perhaps more.
By the time you read this, a hearing scheduled for June 16 may have settled the suit. In any case, let me give some background on what is an interesting scientific and moral question. The same issue was raised before the Relativistic Heavy Ion Collider (RHIC) was turned on at Brookhaven National Laboratory on Long Island last year, which took place without consequence.
Let’s take a look at the processes involved. First, any black hole that might be formed in an LHC single collision will be tiny. Since the total energy of the two beams is E=14 trillion electron-volts, using m=E/c2 we calculate the mass of the black hole to be 2.4 x 10-23 kilograms, equivalent to the mass of about 15,000 hydrogen atoms. This is far less than the theoretical minimum mass of a black hole, the Planck mass, which is 2.2 x 10-8 kilograms.
Nevertheless, suppose such a black hole is possible. Stephen Hawking has proven that a black hole is unstable with a mean lifetime that depends on the cube of its mass. While the mean lifetime of astronomical black holes is many times the age of the universe, the LHC black hole would survive only 2 x 10-84 seconds before disintegrating into Hawking radiation. Needless to say, this is hardly enough time to swallow up Earth.
So, there should be no problem—unless Stephen Hawking and the rest of the physics community are wrong. No one has ever seen a black hole decay, so as the creationists like to say, this is “theory and not fact.”
Strange matter is more problematic. This is a hypothetical form of matter composed not only of the usual “up” (u) and “down” (d) quarks that compose the nuclei of familiar atomic matter, but also of strange (s) quarks. The proton is udu and the neutron is udd. The Lambda hyperon (Λ), which was first seen in cosmic rays in 1947, is uds, making it an example of “strange matter.” However, because the s quark is much heavier than the u and d, the Λ is unstable and only has a mean lifetime of 2.6 x 10-10 seconds.
It has been conjectured that strange matter may become stable when a sufficient number of quarks are brought together. This could happen because the Pauli exclusion principle favors three distinguishable quarks over protons and neutrons, each of which has two identical quarks. Estimates I have seen indicate that thousands of quarks are needed for stability, but these calculations are highly uncertain.
Where would all these quarks come from? In a high-energy collision between nuclei, a quark-gluon plasma is formed, which at the LHC energy can contain thousands of quarks and massless gluons. This may then condense into strange matter.
A strangelet is a chunk of stable strange matter. The ominous scenario is that any negatively charged strangelet coming into contact with an ordinary nucleus might convert it to strange matter, setting up a chain reaction in which Earth would eventually become a hot lump of strange matter.
The best argument against the suggested catastrophe is that cosmic ray protons of much higher energies than will be produced by the LHC have been hitting Earth and every other object in the universe for thirteen billion years, and nothing to our knowledge has converted to strange matter, or even black holes. The cosmic ray with the highest observed energy has an output of about 1020 electron-volts. Assuming it is a proton, the center-of-mass energy resulting from it colliding with another proton at rest, on the Moon for example, is 4.5 x 1014 electron-volts—higher than the LHC collision energy. They have not destroyed the moon or any objects that we know about.
If strangelets are being produced, we would expect all neutron stars in the universe to be strange stars, and we have observational reasons to think they are not. On the other hand, there are a few anomalous objects that are too dense to be neutron stars but not dense enough to be black holes. Perhaps these are strangelets.
Prominent physicists, including Nobel Prize winner Frank Wilczek, were charged by the director of Brookhaven National Laboratory to analyze the RHIC disaster scenario. In their report published in 1999, they argued that black hole formation from RHIC is highly unlikely. Further, they pointed out that in the five-billion-year life of the moon, approximately 1028 collisions of the RHIC type have occurred. The number of collisions that will occur in ten years of running RHIC, 2 x 1011, is fewer than happen each day on the moon. Their conclusion: “The candidate mechanisms for catastrophe scenarios at RHIC are firmly excluded by existing empirical evidence, compelling theoretical arguments or both.”
However, reputable physicist Adrian Kent has questioned whether a proper risk assessment was done before proceeding with RHIC. The empirical data alone imply a catastrophic probability of about 10-17.
Only by relying on theory does the risk approach zero. As Kent puts it, “When the destruction of the Earth is in question . . . it would be preferable not to have to rely on theoretical expectations alone.”
Still, I’m taking bets that it won’t happen.