THESIS The fi nal frontier?

Eight years ago, when Brookhaven National comparable collisions at RHIC energies, far Whether or not this sets your mind Laboratory switched on the Relativistic more than the 1011 collisions expected in the at rest, it’s not the fi rst time that physics Heavy Ion Collider (RHIC), it pushed lifetime of RHIC. has confronted such fearful scenarios. experimental physics into unprecedented But most worrying of all, it seems, is During the Manhattan project, of course, territory. It’s not the energy per particle at the possibility that RHIC collisions might physicists worried (briefl y) that the fi rst RHIC that is novel, but the energy density, create so-called strange matter, in which atom-bomb test in New Mexico might suffi ciently high that quarks lose their some up or down quarks get replaced with set the atmosphere on fi re. Similarly, as affi liation with specifi c nucleons, creating heavier strange quarks. Such matter may Joseph Kapusta has recently recounted quark–gluon plasma. Th e collider may, if well exist in the dense interiors of neutron (arXiv:0804.4806) nuclear theorists in the we’re fortunate, reveal signatures of physics stars, where the enormous pressure and 1970s had concerns about the stability of beyond the standard model. density, and the Pauli exclusion principle, nuclear matter, aft er Tsung-Dao Lee and Meanwhile, we all hope that RHIC avoids force some quarks to occupy high-energy Gian-Carlo Wick speculated that a new state stirring up any trouble. Th e hypothetical states. Th e weak transformation of an up of ‘abnormal’ nuclear matter might exist at possibility that a rare physics event might or down quark into a heavier strange quark high density, and just might be more stable. annihilate the Earth isn’t only the demented can therefore be energetically favourable, as “No one really knew”, Kapusta recalls, as nightmare of an anti-science extremist, but a such quarks can then fall into lower states. physicists in the mid-1970s prepared to start possibility considered plausible enough that up the Bevalac facility at Lawrence Berkeley Brookhaven in 1999 appointed a team of Laboratory, “what to expect when nuclear respected physicists to assess the risk. Th eir matter was compressed to three-to-four times decade-old report (R. L. Jaff e et al. arXiv: the density of atomic nuclei”. Th e possibilities hep-ph/9910333) now makes fascinating, if they imagined, somewhat humorously, were somewhat spine-tingling reading. also alarming: Th e team considered three logical “Heavy-ion collisions will compress the possibilities, within currently accepted nuclei to such a degree that abnormal nuclear physics, by which an experiment at RHIC matter will be formed in the core of the might wipe out the planet, or have, as they compressed nuclei. Th is abnormal nuclear delicately put it, “profound implications for matter, being more stable than ordinary health and safety”. First, a heavy-ion collision matter, will accrete stuff around it and grow might induce such a density of mass as to Extreme physics has the to visible size. Being so massive it will drop to create microscopic black holes, which would the fl oor of the experimental hall where one suck in surrounding matter, ultimately taking potential to produce extreme can weigh it and measure its radius, thereby the Earth into a gravitational singularity. Th is consequences. determining its density! possibility, they concluded, is astronomically “Such an object, however, would be unlikely, as even conservative estimates of denser than ordinary nuclear matter… the gravitational forces in RHIC collisions In principle, strange matter might also and hence cannot be supported by steel or put them 1022 times too weak to create a be created in less exotic circumstances, and concrete and would fall to the center of the classical black hole, with quantum eff ects theorists raised the possibility more than Earth! Further, what would prevent it from even far weaker. Moreover, no previous twenty years ago that it could be more stable growing larger and larger until it would experiment had ever even detected signs of than ordinary nuclear matter. Hence, a occupy the entire Earth? Simple estimates any gravitational clumping at all. ‘stranglet’, once created, would naturally tend suggested that this could occur in a matter Other possibilities couldn’t be dismissed to change surrounding matter into the same of seconds…” quite so easily. For instance, there’s nothing state. Yet, here too, Jaff e et al. found that the Physicists decided ultimately that this in principle to rule out the hypothetical available evidence suggests we’ll be safe. threat was exceedingly small, and pressed metastability of the vacuum. Th eoretically, To begin with, there is no credible on. No disasters ever took place. It seems a suffi cient energy density might cause the evidence yet for strange matter existing overwhelmingly likely that we’ll also be safe vacuum to decay into something more anywhere in the Universe. Moreover, the with RHIC, and any other accelerator in the stable and very diff erent, initiating a phase most stable confi guration of strange matter near future. transition, the boundary of which would race would almost certainly have positive electric But at some point, we may fi nd that the outwards at the speed of light: very bad news charge, strongly inhibiting its interaction probability of disaster becomes unacceptably for us all. In this case, Jaff e et al. admitted, with other matter; hence there would be large. Extreme physics really does have the there doesn’t seem to be any knock-down no chain reaction, even if it were created. potential to produce extreme consequences. If theoretical argument against the possibility. Finally, heavy ions have been colliding on the chance of creating an Earth-annihilating Yet there is, they noted, ample evidence the surface of the Moon for billions of years, event were 1 in 10, we would obviously hold that, were this outcome plausible at RHIC, and also in interstellar space. In the case of back. We may one day reach a ‘fi nal theory’, we’d have been victims of it long ago. the Moon, for example, there should have as Steven Weinberg speculated in his popular Estimates of the total number of cosmic-ray been about 1028 collisions between iron book, but hopefully it will be fi nal because collisions that have occurred in our past nuclei with RHIC-like energy densities over it cannot be bettered, not because it was light-cone — hence, the eff ects of which the past 5 billion years, but the Moon has followed by a fi nal experiment. we’d have experienced — fi nd some 1047 not yet turned to strange matter. Mark Buchanan nature physics | VOL 4 | JUNE 2008 | www.nature.com/naturephysics 431 © 2008 Nature Publishing Group