WHAT would happen if you took the Earth and compressed it down to the size of a tennis ball? No one is saying this is imminent, but the thought experiment leads to an intriguing possibility. If you could squash the Earth down to this size, it would become far denser than any substance found in nature. You鈥檇 think it would be well on its way to becoming a black hole. Yet there is a slight chance that you鈥檇 get something completely different: a preon star.
Preons are hypothetical particles that have been proposed as the building blocks of quarks, which are in turn the building blocks of protons and neutrons. A preon star 鈥� which is not really a star at all 鈥� would be a chunk of matter made of these constituents of quarks and bound together by gravity. According to physicists Johan Hansson and Fredrik Sandin of the Lule氓 University of Technology in Sweden, swarms of preon stars may have formed in the early universe, and might even still be around. If so, they would be strong contenders to be the weirdest stuff in the cosmos.
It鈥檚 a highly speculative idea, but it鈥檚 also a concrete attempt to shake up physics as we know it. The same questions keep cropping up. Will we find a theory of everything? Does dark matter exist, and if so, what is it? These are big gaps in our knowledge. Physicists keep coming up with ideas to try to fill these gaps, and Hansson and Sandin are doing their bit. So far they have published two papers to back up the preon star concept, and have suggested experiments with which to test it.
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Elementary, my dear quark
If preon stars do exist, they would be the first evidence of a class of particles more elementary than quarks, and our current understanding of matter would be overturned. Preon stars might even account for much of the universe鈥檚 dark matter, the invisible stuff that keeps clusters of galaxies from flying apart and that may be responsible for sculpting the structure of the cosmos.
At the root of all this is the standard model of particle physics, part of which is the notion that protons and neutrons are made of quarks. In 1974, Nobel laureate Abdus Salam and Dirac medallist Jogesh Pati proposed that if quarks themselves consist of even smaller particles, which they named preons.
In the 1980s and 90s, however, preons fell out of favour because of the lack of both experimental evidence and theoretical necessity. 鈥淲e do not really need preons to explain the properties of quarks,鈥� says physicist Greg Landsberg of Brown University in Rhode Island. 鈥淪earches for quark substructure are performed every time a new energy frontier opens up, and they have been all unsuccessful so far.鈥�
Hansson was undaunted. He was among a minority of physicists who thought the building blocks of quarks could still be flushed out, and that these might bridge the gap to string theory, a popular approach to a theory of everything. 鈥淚t鈥檚 extremely naive to assume that there is nothing between quarks and the conjectured superstrings,鈥� he says. 鈥淭here probably are many, many layers left before we hit rock bottom, if ever.鈥�
The question was whether you could actually detect any of those layers. For this, Hansson took a cue from Fields medallist and string theorist Ed Witten, who had in the 1980s that some quarks might have condensed out of the hot stew of matter and light forged in the big bang to form quark 鈥渘uggets鈥� before they could combine into protons and neutrons. If such objects exist, they might be observable.
So in 2003, Hansson started wondering whether preons could have condensed out of the same primordial soup. He joined forces with Sandin, who was working on the behaviour of matter in neutron stars. In 2005, they postulated that in regions of very high-density matter, gravity could have caused huge numbers of preons to clump together into preon nuggets, or preon stars, instead of combining into quarks or other, larger particles ().
Preon stars would be many orders of magnitude smaller and denser than a chunk of quark or atomic matter of the same mass, because preons can in theory bunch together more closely than they do when they form quarks. The largest preon star that could exist without collapsing into a black hole would have a mass similar to Earth鈥檚, while one with the mass of a large asteroid would be smaller than a red blood cell. A small preon star would be very difficult to detect unless it happened to strike the Earth, for instance.
What Hansson and Sandin needed was a way to test whether preons or preon stars could ever be observed. In December, they published a paper outlining several possible methods ().
Preons show their hand
One of these relies on the Large Hadron Collider. Like most physicists, Hansson and Sandin will be keeping a close eye on what happens when the LHC is switched on later this year at CERN near Geneva, Switzerland. Reaching energies seven times as high as any previous particle accelerator, the LHC just might have enough oomph to bust quarks apart into their building blocks, if they exist. These would show up as a shower of anomalous particles and radiation, though because they would probably recombine into quarks and larger particles within a tiny fraction of a second, they would be tough to spot.
What about detecting preon stars in space? One way would be to use gravitational lensing, which occurs when an object鈥檚 gravity bends light around it, as predicted by Einstein鈥檚 general theory of relativity. Despite their tiny size, preon stars would have enough mass to deflect the light of distant sources. Shorter wavelengths of light are more likely to bend around small bodies, so gamma-ray bursts 鈥� flashes of short-wavelength radiation originating from stellar explosions, neutron star mergers and the like 鈥� might be ideal.
In fact, Hansson and Sandin point to a gamma-ray burst detected by the Japanese satellite Ginga in 1988 as possible evidence for preon star lensing. The spectrum it recorded revealed two dips of intensity at short wavelengths 鈥� the sort you might expect if there were an intervening preon star 鈥� but most astronomers think the dips were just part of the gamma-ray burst itself.
Failing that, another relativistic phenomenon might do the trick. Gravitational waves 鈥� ripples in the fabric of space-time produced by massive objects moving 鈥� have been predicted but not yet directly observed in nature. Two preon stars whirling around a common centre of mass might, however, stir up enough activity to be detected by next-generation experiments if they lie within a few thousand light years of Earth.
If such a preon star system had a relatively wide orbit, it might emit gravitational waves at several thousand cycles per second. The European Gravitational Wave Observatory, still a decade away from construction, would be sensitive to such frequencies. Its technical upgrades should make it more sensitive than LIGO in the US. The challenge, says Sandin, would be telling the difference between a preon star and ordinary matter.
Other preon stars might whirl around each other in orbits of less than a millimetre. In this case they would radiate gravitational waves at millions of cycles per second. The University of Birmingham in the UK has built a gravitational-wave detector that uses the resonant properties of microwaves to catch such ultra-high-frequency waves. Although it is not sensitive enough to detect waves from preon stars, a next-generation version might be. That would reveal the densities and masses of the wave sources, possibly clinching the case for the existence of preons.
Realistically it鈥檚 a long shot. Even if preon stars exist, it may take decades to build up compelling evidence. 鈥淭he subject is still very young,鈥� Hansson concedes, but he can point to one encouraging sign. 鈥淎 few years ago, if one searched for 鈥榩reon stars鈥� on Google, it got no hits and the inevitable question came back: 鈥楧id you mean porn stars?鈥� That effect has since disappeared as preon stars have become a concept of their own.鈥�
