By Justin Raizes
A Brief History
Over the past several decades, the desire to explore particle physics has motivated the construction of higher and higher energy particle accelerators. As these accelerators have been built, concerns over the safety of the experiments have arisen.
In 1999, during the construction of the Relativistic Heavy Ion Collider (RHIC), an article in The Scientific American titled “A Little Big Bang” spurred several entries in the Letters to the Editor section of The Scientific American about the safety of the new collider. One of the submitters, Michael Cogill, was concerned in general about “somehow [altering] the underlying nature of things such that it cannot be restored”, while the other, Walter Wagner, had more specific concerns about the possibility of creating a miniature black hole. Frank Wilczek of the Institute for Advanced Study in Princeton, N.J. responded to the letters with reassurance that such a disaster was very unlikely. Nevertheless, the media quickly latched onto the concept and trumpeted it with alarming titles such as “A Black Hole Ate My Planet” and “A ‘big bang’ machine”.
In response, Brookhaven National Laboratories, the commissioners of the RHIC, asked a panel of scientists to review the speculative disaster scenarios and assess the safety of the project. The panel ultimately found the project to have a high safety margin, and it proceeded.
These issues rose again in 2008 prior to the first run of the Large Hadron Collider (LHC). Again, media published articles with alarming titles such as “The Final Countdown”, and there was even a lawsuit filed against the project seeking a restraining order. CERN, the commissioners of the LHC, asked a panel of scientists to review the results of the 2003 study into the safety of the LHC. Again, the panel ultimately found the project to have a high safety margin, and it proceeded.
Overview of Disaster Scenarios Considered
The concerns which arose generally fell into one of three categories:
- Formation of a miniature black hole or other gravitational singularity which absorbs matter.
- Triggering of a vacuum instability.
- Formation of “strangelets” which absorb matter.
Miniature Black Holes
The black hole is a widely recognized phenomenon, even if it is not well understood by the average layman. A black hole consists of extraordinarily dense matter, to the point where space-time begins to warp and it absorbs surrounding matter.
If a miniature black hole were to form on Earth, it would begin to eat away at the surrounding matter, eventually consuming the Earth. However, as shown by Giddings and Mangano in 2008, this would occur at an extremely slow rate. In fact, the formation of a miniature black hole would not significantly reduce the lifespan of the Earth. Furthermore, other effects, such as thermal impact, would also not significantly change the condition of the Earth. With respect to direct human impact of a miniature black hole, we are reassured by Peter Fisher’s statement that “a fast moving black hole with the mass of the moon (radius of a proton) will go right through you with no damage.”
Of course, for any of this to happen, a miniature black hole would actually have to be formed on Earth. The RHIC safety panel considered both classical and quantum gravitaty. They determined that the masses and distances involved in the RHIC are much too small and large (respectively) to create any sort of black hole, and that the probability of emitting a graviton at the RHIC was on the order of 10^-34. Additionally, cosmic rays regularly collide with much more energy than that present at the RHIC, and we have observed no formation of a black hole within our solar system’s vicinity.
Vacuum Instability
Contrary to how a layman thinks of empty space, empty space is actually highly structured, and can exist in various states. In quantum mechanics, a vacuum is the state of lowest possible energy. It has been theorized that our current vacuum is only a false vacuum, having a locally, but not globally, minimal energy. If this is true, then a sufficiently violent disturbance might trigger a decay into a different state. If such a decay occurred, it would spread throughout the universe at the speed of light, and be “catastrophic”.
The 1999 panel investigating the safety of the RHIC claimed that “theory strongly suggests that any possibility for triggering vacuum instability requires substantially larger energy densities than RHIC will provide”. However, rather than simply rely on this, they also brought up the point that cosmic ray collisions have been occurring throughout the history of the universe, and concluded that “if such a transition were possible it would have
been triggered long ago.” For this point, they cited Hut and Rees’ 1983 work detailing the number of cosmic ray collisions whose effects we have observed and examining the probability of these past cosmic ray collisions triggering an observable vacuum phase transition.
Strangelets
A “strangelet” is a form of quark matter which contains many strange (s) quarks. Under either high pressure or high temperature, quarks are no longer bound to their individual hadrons. One of the primary goals of the RHIC was to provide evidence of quark-gluon plasma, the state induced by high temperature. Quark-gluon plasma can be “accurately described as a gas of nearly freely moving quarks and gluons”. On the other side of the spectrum, quark matter is the name given to such matter which is under high pressure and low temperature.
Ordinary matter is primarily composed of up (u) and down (d) quarks, the lightest varieties. As quark matter is compressed, the Pauli Exclusion Principle – that no two quarks within the same quantum system can share a state – forces quarks into higher and higher energy state. Eventually, the up and down quarks will become strange quarks in order to reduce energy. By the time equilibrium has been reached, there will be a finite density of strange quarks.
The dangerous strangelets are those that are both negatively charged and stable enough to come to a rest in ordinary matter. Once such a quark has done so, however, the results are catastrophic. It would be captured by some ordinary nucleus within the environment, quickly fall into the lowest Bohr orbit, and react with the nucleus, absorbing several neutrons to form a larger strangelet. The reaction would be exothermic, and afterwards, the strangelet would have positive charge. However, if the energetically preferred charge were negative, it would quickly return to a negative state by absorbing surrounding electrons. This process would continue until the strangelet’s radius approached the electron Compton wavelength 4×10^-11, at which point it begins to behave differently. Its baryon number would be on the order of 10^6, and it would begin to trigger electron positron pair creation. The positrons would surround the strangelet as a Fermi gas. Any atom which approached the strangelet would be stripped of its electrons by electron-positron annihilation, and the bare nucleus would be absorbed by the strangelet core. The panel reviewing the safety of the RHIC remarked “We know of no barrier to the rapid growth of a dangerous strangelet.”
Fortunately, it was concluded that the formation of such a strangelet at the RHIC was extremely unlikely. Strangelets are cold, dense matter, and heavy ion collisions are hot. Thus, the second law of thermodynamics works against the formation of strangelets at the RHIC. Additionally, negatively charged strangelets require many strange quarks. However, it is more difficult to produce a strangelet with many strange quarks. These two reasons not only show that it is unlikely that a dangerous strangelet will be formed in the first place, but also show that a dangerous strangelet would likely be too unstable to reach ordinary matter and begin growing.
Again, rather than simply rely on theory, the RHIC safety panel also brought up experimental evidence from cosmic ray collisions. They computed the number of heavy ion collisions taking place on our (relatively) nearby friend the Moon, and observe that the Moon is, in fact, not made of strange matter (or cheese). They computed that over the 5 billion year lifetime of the moon, roughly 10^11 dangerous strangelets would have been formed from cosmic ray collisions. However, none of these strangelets survived contact with lunar soil. Using extremely conservative estimates, the RHIC safety panel placed a safety factor of nearly 10^22 between the values of constants which would be required to cause alarm and the actual values of said constants. In short, we are not likely to turn into strange matter anytime soon.
Bigger, Badder Colliders
It is quite possible that we will create even larger colliders in the future. Within the span of a decade, we upgraded from the RHIC to the LHC. However, the incredible safety margins placed on current colliders make it extremely unlikely that we will create an Earth-destroying collider anytime soon. Furthermore, these safety concerns are revisited each time we build a new collider. During the creation of the LHC, all the safety concerns about the RHIC were considered, and all were found to have high safety margins, mostly relying on cosmic ray data. So, you don’t have to be concerned about being eaten as a snack by a particle physics experiment gone wrong – at least for now, anyways.
References
F. Carus, “The final countdown?” The Guardian, September 2008. [Online]. Available: The Guardian, http://theguardian.com [Accessed February 20, 2018].
J. Ellis, G. Guidice, M. Mangano, I. Tkachev, and U. Wiedemann, “Review of the safety of LHC collisions,” Journal of Physics G: Nuclear and Particle Physics, vol. 35, no. 11, p. 115004, September 2008.
K. Locock, “A ‘big bang’ machine,” ABC Science, July 1999. [Online]. Available: ABC Science, http://abc.net.au [Accessed February 26, 2018]
M. Tegmark and N. Bostrom, “Is a doomsday catastrophe likely?” Nature, vol. 438, p. 754, December 2005.
R. Jaffe, W. Busza, F. Wilczek, and J. Sandweiss, “Review of speculative “disaster scenarios” at RHIC,” Reviews of Modern Physics, vol 72 iss. 4, October 2000.
R. Matthews, “A black hole ate my planet,” New Scientist, August 1999. [Online]. Available: New Scientist, http://newscientist.com [Accessed February 20, 2018]
S. Giddings and M. Mangano, “Astrophysical implications of hypothetical stable TeV-scale black holes,” Physical Review D, vol 78, August 2008.
T. Leonard, “‘Big Bang’ machine could destroy the planet, says lawsuit,” The Telegraph, April 2008. [ Online] Available: The Telegraph, http://telegraph.co.uk [Accessed February 26, 2018]
W. Wagner and F. Wilczek, “Black Holes at Brookhaven,” Scientific American, p. 8, July 14, 1999.