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Could a particle accelerator destroy the world? Fears that the Relativistic Heavy Ion Collider at Brookhaven in the United States could trigger an unforeseen disaster have been allayed by two studies that show the risk of worldwide catastrophe to be truly negligible.

E. Witten (personal communication) has raised the intriguing possibility that nuclear matter consisting of aggregates of up, down and strange quarks in roughly equal proportions may be less massive than ordinary nuclear matter of the same quark number consisting of protons and neutrons (triplets of non-strange quarks). These nuggets of strange quark matter may be stable for almost any baryon number (A), including values intermediate between those of ordinary nuclei (A≲263) and neutron stars (A ∼ 1057). We use the term ‘nuclearite’ to describe such strange quark nuggets in collision with Earth and suggest experiments to detect these encounters.

Radioactive decays inside the Earth produce antineutrinos that may be detectable at the surface. Their flux and spectrum contain important geophysical information. New detectors need to be developed, discriminating between sources of antineutrinos, including the cosmic-background. The latter can be related to the frequency of supernovas.

The possible existence of a second species of photon which is uncoupled to known forms of matter is considered here. Explicit mass terms in the lagrangian can give rise to photon masses and to oscillations of photon identity, without sacrificing the ability of the gauge theory to be renormalized. Current upper limits on the photon mass are ∼6 × 10−16eV c−2 (refs 1, 2). Photon oscillations corresponding to much smaller masses can significantly alter the spectral shape of the cosmic background radiation (CBR). Indeed, we show that the apparent discrepancy3 between theoretical and observed CBR spectra can be resolved in terms of photon oscillations, and a mass parameter of 5 × 10−18 eV c−2.

An observed deficit of high-energy (8B) solar neutrinos1 and the galactic dark-mass problem2 have both been explained in terms of hypothetical particles (ref. 3 and L. M. Krauss, unpublished work): ‘cosmions’ must have masses mc in the range 5–50 AMU, a mean elastic scattering cross-section per hydrogen nucleus in the Sun of s∼4xl0−36cm2, local mass density ρ∼ 1 AMU cm−3, and cosmological velocity ν∼ 10−3c. Accumulated in the Sun, they enhance its thermal conductivity so as to yield the measured solar luminosity with a lower central temperature than in standard models. Here we show that such particles, should they exist, could be detected and studied in the laboratory by bolometric procedures proposed originally by Cabrera et al.4 for the purpose of measuring solar neutrinos. The event rates are so large that a definitive test of this cosmion hypothesis can be made, even with a relatively small bolometric detector.

Cabrera1 has reported the possible detection of a magnetic monopole in flight with magnetic charge g given by the Dirac condition2 2eg = hc. Here, we accept the Cabrera candidate as a t'Hooft–Polyakov3,4 monopole of mass M ∼ 1016 GeV as expected5 in SU(5) or other grand unified theories. The monopole flux on Earth, on the basis of the single candidate, is f∼(0.1) cm−2 yr−1 (2πsr)−1, presumably consisting of roughly equal numbers of north and south monopoles. Galactic or intergalactic monopoles will have typical velocities of ∼300 km s−1. The Cabrera flux would correspond to a mass density f M/v ∼1 GeV cm−3. Because the mean mass density of the Universe cannot exceed 10−5 GeV cm−3, the mean flux of monopoles in the Universe must be at least five orders of magnitude smaller than f (refs 6,7). This would not be a problem if the monopoles were concentrated in the Galaxy. However, Parker has shown8 that the existence of galactic magnetic fields is inconsistent with a mean galactic monopole flux of 10−7 cm−2 yr−1. It follows that f must be a local flux resulting from the special nature of the observation site. Three possibilities come to mind: the local monopole flux may be associated with the Earth, the Sun, or with the Solar System as a whole.

Quantum electrodynamics, despite its many successes, has left many questions unanswered. Why is electrodynamics so different from other interactions? Why do all particles have commensurate electric charges? Why is the value of the fine-structure constant what it is? Today, we have an apparently correct and essentially complete description of all of elementary particle physics in terms of a gauge theory1. If the underlying gauge group is simple2,3, all three elementary particle interactions—strong, weak, and electromagnetic—are parts of one unified theory. (The earlier attempt to synthesise all three interactions obtained both charge quantisation and baryon number violation. It was not based on a simple gauge group, nor did it incorporate conventional quantum chromodynamics.) Electromagnetism is no longer so unique. Apparent differences among the interactions reflect the pattern of spontaneous symmetry breakdown. Charge quantisation is forced on us: all fields have integer mutiples of one-third the electron's charge2,3; all observable particles have integer multiples. (We assume that colour is a truly hidden variable, that quarks are fractionally charged and never liberated. For another point of view, see ref. 4.) The fine-structure constant cannot be chosen arbitrarily. For unified theories in which the proton is unstable, we find an upper limit to its strength, α