First steps along the road to space's final frontiers

August 21, 1998

Antimatter, the ideal fuel for space travel, is not the invention of Star Trek scriptwriters but does exist. But it will be a long time before antimatter-fuelled cruisers roam the universe. Sonya Dowsett reports

I have a confession to make. When I sat in physics lessons at school I used to fantasise about throwing a lump of antimatter at the teacher's head. A satisfying fizzing noise and there would be nothing left, because, as everyone knows, antimatter and real matter annihilate each other on contact.

But does antimatter actually exist outside the realms of fantasy? The immensely pleasing answer, for schoolchildren everywhere, is a resounding yes. Scientists at the Cern research laboratory in Geneva actually made some for the first time last year. It only lasted for a millionth of a millionth of a second because it collided with real matter, but energy given off from the fatal collision and detected by the scientists showed that it definitely existed during that fraction of time.

Now there is the possibility that antimatter might be used as a form of fuel for astrocruisers, powering journeys around the universe. A little way off perhaps, but not an idea confined to science fiction.

As it has never been created for long enough to analyse properly, there is still much debate about what antimatter is. "The best theories suggest that matter has an exact symmetry equal in all respects except charge," says Frank Close, a scientist at Cern. If this was the case, antimatter would have a mass and a volume and would fall to the ground if you dropped it, because gravity would act on it just as it does for real matter.

Hydrogen is the simplest atom, consisting only of a single proton and a single electron. The antimatter created by scientists at Cern last year was anti-hydrogen. They created it by combining an anti-proton and an anti-electron. "To find out if anti-hydrogen has the same properties as hydrogen we would have to look at its spectrum (the fingerprint of an atom) and compare it with that of hydrogen. If there are any differences it would drastically change our perception of what antimatter is," says Professor Close. "However this is putting the cart before the horse as we are still trying to design the conditions in which we can isolate and study, and even store, antimatter for any length of time.

"One method that has been suggested is to take a specially designed magnetic bottle and evacuate all atoms of matter from inside. Then we could create some atoms of anti-hydrogen and store them in the bottle being careful not to let it come into contact with the sides of the container," he says.

Although finding the technology for isolating atoms of antimatter seems far off, anti-electrons, or positrons, are present in abundance and even have applications in everyday life. Positrons, which are a product of radioactive decay, collide with electrons and annihilate each other. In doing so, they release high energy gamma rays.

Medics use this fact to carry out PET (positron emission topography) scans on living brains. The patient's blood is labelled with something radioactive that emits positrons. The brain is then scanned to find out the areas that emit the most gamma energy. These are the areas of highest blood flow, which in turn are assumed to be the areas of highest brain activity. PET scans can be used to detect tumours, but have also played a key role in studies of the brain to find out why people stutter, why sleep is necessary, and whether people who learn a second language build up new brain areas to handle the knowledge.

Positrons are also used in the aircraft industry to test aircraft turbines. A tiny fault in these turbines could be disastrous. Faults are located by exposing the metal to a stream of positrons. Variations in the intensity of gamma rays released show where there are inconsistencies in the metal. "In fact there are more particle accelerators in hospitals and in industry than in all the physics laboratories worldwide," says Close.

So, in what weird and wonderful way could antimatter be used? A favourite idea of Close is as a form of fuel for astrocruisers. "When antimatter annihilates matter a huge amount of energy is given off. A nuclear explosion only releases about one thousandth of the potential energy in the atom. Antimatter would release all of the energy and then 200 per cent of that as there would be two particles involved. Some people like to think that if you put an antimatter engine in a spacecraft this would make a very efficient way of travelling around the universe.

"In theory this is true, however, there are a few problems. The first is that the total amount of antiparticles created in the whole of history amount to less than one millionth of gram. To make several kilograms would be impossible. And even if you did have a few kilograms, how would you manage to prevent it bumping into matter? Even if you had your cleverly- designed magnetic bottle with the antimatter inside you would have to ensure that it did not come into contact with anything during launching or in space, or the whole back of the spacecraft would be destroyed."

For those depressed at the unlikelihood of being able to go on an antimatter-fuelled astrocruise in their lifetime, Close has some words of comfort. "At least we can put our minds at rest that there aren't any antimatter weapons," he says. Physics teachers everywhere will doubtless breathe a sigh of relief.

ONE HUNDRED YEARS SINCE THE DISCOVERY OF THE ELECTRON

1897 Discovery of electron by J.J. Thomson in Cambridge

1918 Proton discovered by Rutherford in Manchester

1920s Dirac proposes idea of antimatter

1956 First antiproton discovered in Berkeley, California

1983 High intense beams made by Cern to discover the W and Z particles.

Wins Nobel Prize for Carlo Rubbia and Simon van der Meer

1997 First anti-hydrogen particles produced at Cern

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