IT SOUNDS like every physicist’s nightmare. If you want to find out the secrets of the Universe, you’ll have to forget your pristine, billion-dollar particle smasher, and try murky seawater instead. You’ll have to do your experiments at the bottom of the sea where your precious equipment will get covered in slime and mud. Passing shrimps might mess up your results, but heck, you’ll just have to deal with it.
That’s what around 150 scientists and engineers have happily agreed to do. They are building a telescope called Antares on the Mediterranean sea floor off the coast of France. The team hopes to spot high-energy neutrinos, particles that could tell us something about conditions in the bustling centre of the Milky Way and about the mystifying dark matter that dominates the Universe.
John Carr of the Centre for Particle Physics in Marseilles, one of the Antares team, calls it a new chapter in astronomy. “It could be revolutionary,” he says. “A whole new kind of astronomy is possible with neutrinos. We might explain the mysterious sources of cosmic rays, and maybe we will even see exotic new astronomical objects that have been completely hidden from us until now.”
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Neutrinos are unique in nature’s zoo of matter particles. Produced in nuclear reactions inside stars like our Sun, they whiz around at a whisker off the speed of light. And because they don’t feel either the strong nuclear force or the electromagnetic force, the vast majority of them just zip clean through everything in their path, including planets and stars.
This means they can reveal the conditions inside dense and dusty astronomical regions, such as the centres of galaxies. Light can’t escape, but neutrinos can stream out pretty much unhindered. The arrival directions of the neutrinos might point to dramatic explosions inside shrouds of dust, while their energies could reveal what detonates them. Neutrinos can also tell us something about cataclysmic events in the most distant reaches of the Universe. Although these regions emit penetrating gamma rays, the rays are intercepted before they can reach Earth by the cosmic microwave background, the radiation left over from the big bang.
But before neutrinos can turn astronomy upside down, you first have to catch them. No easy task, since they fly through matter so easily. A special detector is called for, something like Antares. It will detect “muon neutrinos” that have come up through the Earth and interacted with Mediterranean waters or with the rock underneath to produce muons. These charged particles will have enormous energy, so much in fact that they will be moving faster than light does in water.
Because the particles are moving faster than light, they emit blue flashes called Cherenkov radiation, the visible equivalent of a sonic boom. These flashes are quite faint, but if physicists amplify them using photomultiplier tubes, they can record reliable evidence that neutrinos have passed through.
Physicists have used these techniques in previous experiments to capture neutrinos from the Sun and cosmic rays. But the high-energy neutrinos bearing cosmic secrets are even more of a challenge. For a start they are much rarer, so there is only one way to detect a large number of them: make your detector huge. That’s why the Antares team plans to thread 10 million tonnes of water with 900 photomultipliers to create a telescope 150 metres wide and more than 300 metres high, taller than the Eiffel tower.
That’s way too big for the luxury of a custom-made tank and the specially purified water that is routine in some other neutrino detectors. So Antares will plump for a natural alternative: the deep blue sea.
Designing it has thrown up some issues that particle physicists are, to put it mildly, a little unfamiliar with. They have had to think seriously about everything from deep-sea slime to glowing jellyfish.
The telescope will sit in the Mediterranean, 37 kilometres off the Côte d’Azur, not far from Toulon. The Antares team chose the site because the sea is surprisingly clear and because it is deep, 2.4 kilometres down. That means there is plenty of water above to screen out the muons produced by cosmic rays that constantly bombard the Earth’s atmosphere. But it is also fairly close to the coast, making the telescope easier to install and maintain.
Having decided on the site, the team had to convince themselves that the technology would really work in such uncertain conditions. Since 1999 they have been trying out their photomultipliers inside glass spheres at the site, which is connected to the shore by a 37-kilometre telephone cable.
It was always possible that microbes would colonise the surfaces of the glass balls and that sand or mud could settle on them, blinding the photomultipliers to the faint Cherenkov flashes. Sure enough bacteria did grow but they only formed a thin patchy layer that light could easily penetrate. Sand and mud were no problem, providing the detectors faced downwards towards the seabed. After an entire year in the sea, the light transmission of the glass balls fell by only about 2 per cent.
Another possible problem was background light. Although the telescope is 2.4 kilometres down, there is still enough light around to make the Cherenkov light difficult to see. A main concern is bioluminescence, which is why the team sought advice from marine biologists. Practically all the deep-sea creatures in the region, from fish to crustaceans, generate light signals to attract mates, to deceive predators or to catch prey. Would the telescope confuse the light from a ghostly jellyfish or a fang-toothed viperfish for some exotic astronomical object?
It might if they are not careful. Flashing fishes can look similar to Cherenkov radiation because their light is blue and can be bright. But there is a crucial difference: unlike the random fish signals, Cherenkov light is “coherent” like laser light: the wave motion has a predictable phase as the muon travels through the water. By measuring the phase of a signal at different points in the detector, the scientists can distinguish between an astronomical object thousands of light years away and an amorous Mediterranean fish.
Light produced by the radioactive decay of potassium-40, a common isotope in sea salt, is another worry. Fortunately, this background light is constant and can be subtracted from measurements. But sea creatures and salt aren’t the only concern – the Antares team had to find out what the French military is up to (see “Top-secret neutrinos”).
Confident that the technology will work, the Antares team last year laid the cable that will relay electrical power and data between the real detector and the shore. A prototype detector that has one string of 15 photomultipliers will start observing this month. “Even by the end of this year or the beginning of next, we want to see evidence that the technology is going to work,” says Carr.
They already have reason to believe that it will. Another daring project called AMANDA-II is busy detecting high-energy neutrinos at the South Pole. The AMANDA-II detector has 680 photomultipliers buried 1.5 to 1.9 kilometres deep in the Antarctic ice. Braving the frigid conditions has a big advantage – deep in the ice there is no background light to contend with.
The AMANDA-II team began operating in February 2000 and they have proved that the technology behind this strange new kind of telescope really can work. The scientists are still trawling a mountain of data in search of sources of high-energy neutrinos, but according to Steve Barwick, a physicist who works on the project from the University of California, Irvine, its observations of background muons from neutrinos are promising. “For the first time in about 400 years, since Galileo, astronomers have a new messenger to provide information on the heavens,” he says.
Carr thinks Antares will be even more fruitful when it is completed in 2004. He says its ability to pinpoint details is about 10 times as good as AMANDA-II’s. And its relatively low latitude means that unlike AMANDA-II, Antares can peer at sources of neutrinos in the heart of our own Galaxy. “There are things in our part of the sky that are brighter and our detector will be better, so I’m quite optimistic that we’ll see something,” says Carr.
What might that be? One candidate is microquasars – small black holes a few times as heavy as the Sun that have companion stars rotating around them. Around 20 microquasars have been spotted in our own Galaxy in the past decade. As the black hole gobbles up material from its companion, it emits jets of energetic particles that could be bright sources of neutrinos. “If these things are shooting big lumps of matter towards us, then they’ll be shooting neutrinos our way as well,” says Carr. And if armies of neutrinos are seen coming from known microquasars, they will help explain how these powerhouses work.
Ironically, Antares could also have much to say about the sources of cosmic rays, the very ones the telescope has been designed to ignore. Despite being discovered nearly a century ago, scientists are still unsure where cosmic rays come from and why their energies can be so high. “The features we see in cosmic-ray data are bizarre,” says Carr.
Scientists believe that many probably come from supernova remnants, the debris of exploded stars, where shock waves could crank up their energies to enormous values. Yet the light from these remnants tallies with the idea that the shock waves only accelerate electrons, whereas 97 per cent of cosmic rays are actually protons. “It’s obvious that somewhere in the Galaxy there are intense sources of protons but we don’t know where they are,” says Carr.
Hints that at least one supernova remnant is accelerating protons appeared this April in Nature (vol 416, p 797). The trouble is that the observations are controversial, but Antares could settle the matter for good. If supernova remnants are genuinely accelerating protons, they will also act as bright sources of high-energy neutrinos. “These would be very easy sources for Antares to see,” says Carr.
The answer is important because cosmic rays ferry vast amounts of energy round the Galaxy, as much as starlight does. And they probably have a profound influence on the chemical make-up of interstellar gas, the raw material for new stars and planets.
Antares might also spill the beans on dark matter. Around 90 per cent of the matter in the Universe is in some dark, mysterious form that reveals itself only by its gravitational pull on stars and galaxies. A promising candidate is the neutralino, a hypothetical particle that should exist if a theory called supersymmetry is right.
If neutralinos really do exist, they should build up at the centre of stars like the Sun, where they would decay into other particles such as quarks, at the same time creating very high-energy neutrinos that Antares could detect. “This would be an exceedingly dramatic discovery,” says Carr.
John Bahcall, a neutrino expert at the Institute for Advanced Study in Princeton thinks another possibility is even more exciting: Antares might reveal something we have never dreamed of, astronomical oddities entirely new to science. He compares the telescope to special glasses that could suddenly cure someone of being colour blind. “It could be that there is a world for us to see that we have not discovered until now because we did not have the right ‘glasses’,” he says.
What is certain is that Antares and similar projects will blaze the trail for far bigger and better neutrino telescopes. Providing AMANDA-II and Antares fare well, there are plans to extend both types of telescope to scales of about a cubic kilometre. These enormous neutrino telescopes should be big enough to hear the cries of remote quasars and feel the shock waves of distant explosions such as gamma-ray bursts.
Not all cosmic secrets are written in the sky. Think big enough, look hard enough and soon we will see them beneath our feet, chilling in polar ice or swimming with fishes in the dark blue sea.
Top-secret neutrinos
The Mediterranean waters off the coast of France are a regular haunt of military submarines from the French naval base at Toulon. Their movements are a well-guarded secret, but luckily for Antares, they probably don’t dive deep enough to get tangled in the neutrino detector. What’s more Antares focuses only on high-energy neutrinos, so nuclear-powered submarines, whose reactors all emit low-energy neutrinos, won’t trouble it.
Ironically, submarines could cause problems for land-based neutrino detectors in the future. A few hundred nuclear-powered submarines are thought to be sneaking around the oceans. Many aircraft carriers are nuclear powered, as are a Russian fleet of ice-breakers that guide cargo ships along the northern coast of Siberia and take tourists to the North Pole.
Neutrinos from some of these vessels could trigger the detectors, according to Giorgio Gratta and his team at Stanford University in California. They calculate that if a nuclear-powered aircraft carrier sails into Yokosuka military harbour in Japan, or if a large Russian submarine tiptoes along Italy’s Adriatic coast, certain neutrino detectors in these countries will see a rise in their neutrino count.
In some extreme cases, the neutrino count could rise by nearly 50 per cent (Physical Review Letters, vol 89, p 191802). It is unlikely to be a problem at the moment, but the best sites for future high-precision experiments to study neutrinos from land-based reactors will be in the middle of large continents, as far from oceans and stealthy subs as possible.
Even though Antares shouldn’t be affected, the French navy is aware of the project and its sub commanders will steer clear.