In July 1999, I appointed an expert committee to assemble material relating to the safety of RHIC experiments. The committee, composed of Robert Jaffe (chair) and Wit Busza of MIT, Jack Sandweiss of Yale University, and Frank Wilczek of the Institute for Advanced Study at Princeton, has submitted its Report. The Report summarizes technical arguments that conclude there is no danger of a "disaster" at RHIC. Because the language of the Report includes technical terms and concepts that may be unfamiliar to many interested readers, I have summarized the contents below in a less technical form. Details may be found in the full Report that is available on the Brookhaven National Laboratory website: www.bnl.gov. Material in quotations is excerpted from the Report.
RHIC, the Relativistic Heavy Ion Collider, is a pair of circular particle accelerators located at Brookhaven National Laboratory that will accelerate the nuclei of gold atoms in opposite directions to nearly the speed of light. The energetic nuclei will be directed toward each other to form head-on collisions within experimental observation instruments at four stations around the ring. Digital "pictures" of the collisions will be analyzed with extensive computation to infer the behavior of matter at the instant of collision. The experimenters hope to see evidence of a form of matter called the "quark-gluon plasma", a state of matter that is thought to have existed throughout the entire universe a few microseconds after its creation in the Big Bang. Some people have expressed concern that the RHIC collisions could trigger a complicated process that would have disastrous consequences.
Three different kinds of "disaster" scenarios have been discussed in connection with high energy particle collisions:
A. Creation of a black hole that would "eat" ordinary
matter.
B. Initiation of a transition to a new more stable universe.
C. Formation of a "strangelet" that would convert ordinary
matter to a new form.
In connection with these scenarios the authors of the Report ". . . have reviewed earlier scientific literature as well as recent correspondence about these questions, discussed the scientific issues among ourselves and with knowledgeable colleagues, undertaken additional calculations where necessary, and evaluated the risk posed by these processes. Our conclusion is that the candidate mechanisms for catastrophic scenarios at RHIC are firmly excluded by existing empirical evidence, compelling theoretical arguments, or both. Accordingly, we see no reason to delay the commissioning of RHIC on their account."
The authors state that "Issues A and B are generic concerns that have been raised . . . each time a new facility opens up a new high energy frontier. . . . There are simple and convincing arguments that neither poses any significant threat." Issue C is a new concern raised specifically with respect to RHIC collisions, and the Report gives extensive information on this scenario.
A. Black holes A "black hole" is a spherical concentration of matter so great that its gravity causes the escape velocity at the surface of the sphere to equal or exceed the speed of light. Since light cannot escape, the sphere would appear black. Large black holes attract matter to themselves and effectively "consume" the material in their vicinity. Very small black holes evaporate quickly by a quantum mechanical mechanism that essentially overcomes the escape velocity limitation on radiation. Extremely small amounts of matter such as the two gold nuclei in a RHIC collision must be compressed to exceptionally small sizes to form the concentration necessary to create a black hole.
Appendix A of the Report uses a well-known formula for escape velocity to assess the possibility of black hole formation. If all the mass available were compressed to the size of a single proton or neutron (of which there are 197 in each gold nucleus) the escape velocity would be eleven orders of magnitude (powers of ten) less than the speed of light. No mechanism exists to compress the matter even to this size. The report states that "collisions at RHIC are expected to be less effective at raising the density ... than at lower energies where the 'stopping power' is greater." In other accelerators operating at both larger and smaller energies, say the authors, "in no case has any phenomenon suggestive of gravitational clumping, let alone gravitational collapse or the production of a singularity [i.e. a black hole], been observed."
B. Vacuum instability This is an exotic possibility of which the report states that "Physicists have grown quite accustomed to the idea that empty space what we ordinarily call 'vacuum' is in reality a highly structured medium, that can exist in various states or phases, roughly analogous to the liquid or solid phases of water. . . . Although certainly nothing in our existing knowledge of the laws of Nature demands it, several physicists have speculated on the possibility that our contemporary 'vacuum' is only metastable, and that a sufficiently violent disturbance might trigger its decay into something quite different. A transition of this kind would propagate outward from its source throughout the universe at the speed of light, and would be catastrophic."
The vast stretches of interstellar space are penetrated continually by swarms of energetic particles called 'cosmic rays'. Before particle accelerators, scientists studied high energy particle phenomena by examining the tracks of cosmic rays in special detectors. To date, no accelerator, including RHIC, has been able to produce collisions so energetic that they could not be found in cosmic rays.
Stating that "cosmic rays have been colliding throughout the history of the universe, and if [a catastrophic] transition were possible it would have been triggered long ago," the authors cite a relevant 1983 study by physicists P. Hut and M.J. Rees. That study used data on cosmic ray properties that have since been updated, and the RHIC committee re-examined the issue using more modern data. Appendix B of the Report gives technical details on this issue. Their conclusion is that the work of Hut and Rees remains valid, and that "We can rest assured that RHIC will not drive a transition from our vacuum to another."
C. Strangelets Of this disaster scenario, the authors say that "theorists have speculated that a form of quark matter, known as 'strange matter' because it contains many strange quarks, might be more stable than ordinary nuclei. Hypothetical small lumps of strange matter, having atomic masses comparable to ordinary nuclei have been dubbed 'strangelets'. Strange matter may exist in the cores of neutron stars, where it is stabilized by intense pressure. A primer on the properties of strange matter . . . is contained in Appendix C."
Appendix C gives a great deal of technical information that will be of interest to those who wish to know more about the kind of physics RHIC is designed to explore. In this brief summary, I will only give some additional very simple information about the composition of matter, and summarize the argument, again based on cosmic ray data, that reassures us that RHIC collisions will not lead to a disaster through strangelet formation.
The best theory of matter we have today, called the Standard Model, explains essentially all experimental observations on matter to date at scales smaller than atoms. This model regards all other small scale objects as composed of families of particles called 'quarks' and 'leptons' and the forces that bind them together that have their own particles called 'gauge bosons'. There are six quarks with the somewhat whimsical names 'up', 'down', 'strange', 'charm', 'bottom', and 'top'. Only 'up' and 'down' quarks occur in ordinary matter. Protons have two 'up's and one 'down', and neutrons have one 'up' and two 'down's. Atoms have nuclei made of protons and neutrons in which nearly all the mass resides, and a number of electrons which are leptons moving about the nucleus at distances up to hundreds of thousands of times the radius of the nucleus.
All particles ever observed to contain 'strange' quarks have been found to be unstable, but it is conceivable that under some conditions stable strangelets could exist. If such a particle were also negatively charged, it would be captured by an ordinary nucleus as if it were a heavy electron. Being heavier, it would move closer to the nucleus than an electron and eventually fuse with the nucleus, converting some of the 'up' and 'down' quarks in its protons and neutrons, releasing energy, and ending up as a larger strangelet. If the new strangelet were negatively charged, the process could go on forever. This is a simplified picture of the strangelet disaster scenario.
The Report presents arguments that suggest that the conditions for this scenario will not be satisfied in RHIC collisions. In particular, say the authors,
"1. At present, despite vigorous searches, there is no evidence whatsoever for stable strange matter anywhere in the Universe."
"2. On rather general grounds, theory suggests that strange matter becomes unstable in small lumps due to surface effects. Strangelets small enough to be produced in heavy ion collisions are not expected to be stable enough to be dangerous."
"3. Theory suggests that heavy ion collisions . . . are not a good place to produce strangelets. Furthermore, it suggests that the production probability is lower at RHIC than at lower energy heavy ion facilities like the AGS and CERN. . . ."
"4. It is overwhelmingly likely that the most stable configuration
of strange matter has positive electric charge."
"However," the authors state, "one need not assess
the risk based on theoretical considerations alone. We have considered
the implications of natural 'experiments' elsewhere in the Universe,
where cosmic ray induced heavy ion collisions have been occurring
for a long time." Theoretical considerations indicate that
strangelet formation would be even greater for collisions of iron,
of which some cosmic rays are composed, than for gold, which will
be used in RHIC. The total number of collisions that will occur
in RHIC over ten years turns out to be far fewer than the number
of potentially 'dangerous' iron-iron collisions that occur on
the surface of the moon in a single day. For every production
of a dangerous strangelet at RHIC, one expects one hundred thousand
trillion to have been produced on the moon during its lifetime,
any one of which would have converted the moon explosively to
strange matter a phenomenon that is known not to have occurred.
During the preparation of the Report for RHIC, the authors were in contact with a European group of physicists studying similar disaster scenarios (A. Dar, A. De Rujula, and U. Heinz). This group made "worst case" assumptions about what kind of cosmic ray collisions would produce dangerous strangelets in order to achieve the greatest possible assurance that they would not occur in RHIC. The Report says of the European study that "they assume that strangelets are produced only in gold-gold collisions, only at or above RHIC energies, and only at rest in the center of mass. Under these conditions . . . it is necessary to consider ion-ion collisions in interstellar space, where strangelets produced at rest with respect to the galaxy would be swept up into stars. Dangerous strangelets would trigger the conversion of their host stars into strange matter, an event that would resemble a supernova. The present rate of supernovae a few per millennium per galaxy rules out even this worst case scenario." Thus, conclude the authors, "we demonstrate that cosmic ray collisions provide ample reassurance that we are safe from a strangelet initiated catastrophe at RHIC.
Readers wishing to learn more about RHIC physics will find a wealth of detail on the RHIC website at www.rhic.bnl.gov.
- John Marburger