Comets: A key to a parallel universe

Why do comets burn up and vanish far quicker than they should? One theory - that they are made of 'mirror matter' - could help to explain the greatest mystery of our universe. Marcus Chown reports

Comets may be composed of matter hitherto unknown to science, according to a physicist in Australia. If he's right, we may at last have found the identity of the mysterious "dark matter" believed to make up at least 90 per cent of the mass of the universe. What's more, it could mean there is another universe occupying our own, complete with invisible stars, planets and even life.

Comets may be composed of matter hitherto unknown to science, according to a physicist in Australia. If he's right, we may at last have found the identity of the mysterious "dark matter" believed to make up at least 90 per cent of the mass of the universe. What's more, it could mean there is another universe occupying our own, complete with invisible stars, planets and even life.

Why would anyone want to make such an outrageous claim about comets? "Because they are disappearing and nobody knows where," says Robert Foot of the University of Melbourne.

To appreciate the comet mystery, it's necessary to know that comets orbit in their countless billions in the frigid wastes far beyond the outermost planets. This mass of comets is known as the Oort cloud. Occasionally, the gravity of a passing star nudges the cloud so that one falls Sunward. There are then two possibilities: either it swings round the Sun and is catapulted back to the Oort cloud, never to be seen again; or a close encounter with a giant planet such as Jupiter or Saturn steals some of its speed so it becomes trapped in the inner solar system, condemned to swing round the Sun at regular intervals, like Halley's Comet.

The problem is that every time a trapped comet passes close to the Sun, a metre or so of its surface ice boils off into space. Comets are dirty snowballs, typically a few kilometres across, so common sense says they ought to be able to survive scores of orbits around the Sun before being totally melted away. "The puzzle is they don't," says Foot. "Most vanish for ever after passing close to Sun for the first time."

The leading explanation for this disappearing act is that the comets are largely made of rock with a thin coating of surface ice. They therefore "turn off" after one passage by the Sun, having depleted the ices that generate their glowing heads and tails. In effect, they become asteroids.

Recently, however, Harold Levison and his colleagues at the Southwest Research Institute in Boulder, Colorado, have put paid to this theory. They have found that the number of asteroids is far too low to account for the expected number of cometary remnants. "These objects are simply not where we expect them to be," says Levison.

Foot is convinced he can explain the mystery of the disappearing comets. In his new book, Shadowlands, he speculates that many comets might be made of a new type of matter called "mirror matter", which gives out no light. The idea is that they contain only a small amount of ordinary matter, and that this ordinary matter evaporates after passing by the Sun, leaving an invisible core of mirror matter.

Mirror matter is a hypothetical type of matter required to restore nature's flawed left-right symmetry. Nature, for reasons nobody understands, has chosen fundamental laws that exhibit the maximum possible symmetry. For instance, the laws are the same today as they were yesterday, the same in New York as they are in London; and they respect myriad other, more abstract symmetries too. But in one respect nature is not symmetric. The laws are not the same when reflected in a hypothetical mirror. "Electrons and other elementary particles are, in a sense, left-handed," says Foot. "Although most scientists have come to accept that God is left-handed, somehow it has always bothered me."

Foot's unease has led him to embrace a radical idea – that nature's left-right symmetry isn't really flawed, it only looks that way. For every known particle, there is actually a particle that interacts in a right-handed way. So, in addition to electrons and quarks and photons of light, the universe contains mirror electrons and mirror quarks and mirror photons. These combine to make mirror atoms and mirror matter.

Mirror atoms would interact with ordinary atoms very weakly and so produce no light. They are a prime candidate for the dark matter, the "missing mass" that astronomers know must exist (from the gravitational pull it exerts) but cannot see. They believe it makes up at least 90 per cent of the mass of the universe. "If I'm right, there is an invisible mirror universe occupying the same space as our universe, complete with mirror galaxies, mirror stars and perhaps even mirror life," says Foot.

All this would be little more than science fiction if it weren't for baffling experiments at CERN, the European centre for particle physics. Ortho-positronium, an atomic system in which an electron orbits a like-spinning anti-electron, or "positron", disintegrates 0.1 per cent faster than the theory of quantum electrodynamics (QED) says it should. It might not seem much of a discrepancy, but QED is the most successful physical theory ever devised, and it usually predicts experimental results to many, many decimal places. According to Foot, the discrepancy can be perfectly explained if there is an additional decay route, speeding up the loss of ortho-positronium. "I believe that ortho-positronium is 'oscillating' into mirror ortho-positronium, then decaying in the mirror world before it can oscillate back," says Foot.

Efforts are now under way at CERN to repeat the experiment. According to Foot's CERN colleague, Sergei Gninenko, the mirror matter explanation could be confirmed or refuted by next year.

Mirror matter, if it exists, would have been formed in abundance alongside normal matter in the Big Bang in which the universe was born 12 to 14 billion years ago. "It would therefore be surprising if some did not get incorporated into the interstellar dust cloud out of which the Sun, planets and comets condensed 4.6 billion years ago," says Foot.

Foot believes that there is in fact abundant evidence of mirror matter in the solar system. In addition to the puzzle of the disappearing comets, he points out that there have been many sightings of fireballs streaking down the sky and hitting the ground but leaving no debris or crater. The most famous occurred near the Tunguska River in Siberia in 1908, when a 100,000-tonne body devastated a huge area of forest yet left no crater or substantial debris. "A space body made of mirror matter perfectly fits the bill," says Foot.

According to Foot, Tunguska was not unique: "Mysterious impacts that could be due to mirror bodies occur almost yearly." He points to an impact on 18 April last year 50km from Amman, the capital of Jordan. More than 100 people in a funeral procession saw a yellow fireball hit the ground, burning rocks and a tree. "Mysteriously, there was no debris," says Foot. "I think it could have been a small mirror body."

But perhaps the most tantalising evidence for the existence of mirror matter in our neighbourhood is coming from Nasa's Pioneer 10 and 11 probes, launched in 1972 and 1973 and currently streaking out towards the stars. For the past 30 years, both have been experiencing a mysterious force of one ten-billionth of a G, which is pulling them back towards the Sun and slowing them down. "The Nasa astronomers are totally baffled," says Foot. "They've ruled out all mundane effects such as fuel leaks and heat leaks."

John Anderson, an astronomer at Nasa's Jet Propulsion Laboratory in California, has worked on the Pioneer project since 1970. He says: "For the life of me, I can't think of what could be causing the effect."

Foot, of course, is ready with a mirror-matter explanation. "The anomalous force could simply be drag as the probes fly through a thin cloud of mirror matter," he says. "You'd only need a few Earth masses' worth – too little to have any noticeable effect on the orbits of the outer planets, Uranus and Neptune."

Foot could be accused of seeing mirror matter everywhere; an ironic observation, as mirror matter is invisible. However, he says he is well aware of the danger and freely admits that there is a chance he is wrong. Quoting Bertrand Russell, he says: "It's good to have an open mind, but not so open that your brains fall out."

'Shadowlands' by Robert Foot, Universal Publishers, £18.95