ANTIMATTER sounds like the stuff of science fiction. When it meets normal matter, both it and the ordinary stuff promptly disappear in a puff of radiation 鈥� the purest demonstration possible of Einstein鈥檚 idea that mass and energy are equivalent. In 1989, the American writer Joel Davis speculated in New 杏吧原创 that this energy might one day provide the fuel for interstellar voyages (鈥淲ith antimatter to the stars鈥�, 24 June 1989). Some crystal-ball gazers have speculated that antihydrogen could provide fuel for package tours to Pluto, or power probes to other stars and solar systems.
So it comes as a shock to learn that physicists have been dealing with antimatter for more than 60 years. They have manufactured antiparticles in high-energy accelerators. These are all subatomic particles, however. No one has ever observed or succeeded in making even the simplest of all possible antiatoms 鈥� antihydrogen. This is especially surprising since its real matter counterpart, hydrogen, is the best known of all the elements. It makes up about 75 per cent of the known Universe, and here on Earth there are more than 100 million tonnes of it in every cubic kilometre of seawater. Because hydrogen is so simple 鈥� just one electron orbiting a single proton 鈥� it has long been a stamping ground for atomic physicists intent on probing the manifestations of quantum theory.
Close encounter
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Now the time has come for physicists to take the step up from antiparticles to antiatoms. They are looking at ways of bringing the antimatter counterpart of electrons, called positrons, and antiprotons close enough for long enough to form stable antihydrogen atoms. This autumn physicists at CERN, the European particles physics laboratory near Geneva, intend to do just this. Then, if they can persuade CERN鈥檚 managers to keep the necessary equipment running, they will tackle the even more difficult task of trapping antihydrogen, and using it to test the fundamental laws of physics.
First, the CERN physicists will need to make the constituent particles. Positrons are emitted naturally in the radioactive decay of some unstable isotopes, including carbon-11 and oxygen-15. They are also produced by the interactions of cosmic rays with the atmosphere. An alternative way of producing positrons is to bombard materials with electrons. If the energy of the collision is high enough, the opposite of annihilation takes place: some of the energy of the collision is converted into pairs of newly created electrons and positrons. Because positrons are positively charged, they can be separated from their negatively charged electron companions using electric and magnetic fields, and steered towards their target.
In a similar way, colliding protons with other protons or neutrons generates proton-antiproton pairs. This was the way in which physicists at CERN in the 1980s, under the Nobel prizewinning leadership of Carlo Rubbia and Simon van der Meer, produced antiprotons for the proton-antiproton collider that first detected the W and Z bosons. These two fundamental particles are crucial to the standard model of particle physics, the underlying theory of forces and matter that physicists now generally accept.
The proton-antiproton collider has since been shut down, but a spin-off is still running and is crucial to the current plans for making antihydrogen. This is the Low Energy Antiproton Ring, which stored the supply of antiprotons for the main accelerator. LEAR has been quietly providing antiprotons that physicists have used to probe exotic states of atoms and subatomic particles. It is here that an international team of physicists, led by Walter Oelert of the Nuclear Research Centre at J眉lich in Germany, hopes to produce antihydrogen later this year. A similar experiment is also planned at the Tevatron accelerator in Fermilab, near Chicago.
These experiments will take the most direct route to making antihydrogen: they will combine the constituents within the accelerator ring. The downside of this setup is that the antihydrogen will have to be destroyed simply to confirm that it is there. 鈥淚t鈥檒l be a nice observation, but there鈥檚 not much we can do with it,鈥� says Michael Charlton, who studies antiparticles at University College London. 鈥淭he antihydrogen moves too swiftly to do much physics.鈥�
These experiments will have to take into account one important principle of antiatom production. However fast the antiparticles are moving through the accelerator, their speed relative to each other has to be small, otherwise the antiatom will not form. Quantum theory tells us that an electron orbiting a nucleus, or a positron orbiting an antinucleus, must occupy one of a series of possible energy levels, the highest-energy orbits being those that extend furthest away from the central nucleus. If the positron鈥檚 speed is too high, it cannot remain in orbit close to the nucleus.
Oelert鈥檚 team plans to direct a jet of protons across the path of the antiprotons as they circulate within LEAR. The collisions will be at high enough energy to create electron-positron pairs. A small fraction of positrons and antiprotons should combine to form antihydrogen atoms. Unlike their constituent particles, the antiatoms will be electrically neutral and will not be influenced by the electric and magnetic fields that keep the charged particles circulating round the ring. So while the uncombined particles are directed in a curve, the antihydrogen atoms will race straight ahead towards a piece of metal foil in the wall. As the antiatoms pass through the foil, the positrons will be stripped away from the antiprotons. The positrons will then be annihilated in one detector while the antiprotons are tracked by others. The physicists will be looking for distinctive signals from these detectors that will reveal that an antiatom has indeed formed and then broken up again.
To show that antihydrogen can be formed in this way will be a significant feat in itself. But physicists are already working on a less destructive strategy that will allow them to capture and store antihydrogen for more systematic experiments. To this end, physicists from a variety of centres in the US, Europe and Japan are planning ways in which the antiprotons and positrons can be slowed down before they are combined. One approach, pioneered by Gerald Gabrielse at Harvard University, is to send the antiparticles through metal foils known as moderators. Some of the antiparticles will be annihilated in the foils, but others will pass through the metal atoms and be slowed by them. In an alternative approach, physicists at the Paul Scherrer Institute, which is part of the Swiss Federal Institute of Technology in Zurich, have established that antiprotons from LEAR can be decelerated by passing them through a low pressure gas.
Following deceleration, positrons and antiprotons could be confined in a container by a combination of electric and magnetic fields. 鈥淒ealing with the stored antiprotons is the major challenge ahead,鈥� says Charlton. The fields would be shaped to keep the particles away from the walls of the box, where they would otherwise be annihilated, and the vacuum in the container would be very high to minimise collisions with gas molecules (see Technology, 17 September 1994) that would also destroy the antiparticles. 鈥淲e need to increase their lifetimes and improve the manipulation of the trapped antiparticles,鈥� says Charlton.
If they are to combine, positrons and antiprotons confined in such a container need to lose energy when they are close together. To encourage this, a nearby third particle such as an extra positron can be used to take up some of the energy, catalysing the formation of the antiatom. Because the antiatoms have a magnetic moment they can be confined in their container by suitable magnetic fields. 鈥淟EAR experiments could yield up to a million antihydrogen atoms a day using this approach,鈥� says John Eades, a physicist at CERN.
The hope is that this copious supply of antiparticles could open the way to new tests of the fundamental assumptions of physics. The many species of antiparticles that flit into and out of existence in laboratories around the world have played a crucial role in developing the patchwork that makes up the standard model. This encompasses quantum chromodynamics, which describes strong nuclear forces and the makeup of protons, neutrons and their antiparticles. It also takes in the electroweak theory, which covers quantum electrodynamics and the theory of weak nuclear forces. Underlying these models of nature are even more basic assumptions, such as the idea that energy and mass taken together are conserved.
The tiniest flaw in such fundamental principles would be enough to start the whole tapestry unravelling. So scientists are always on the lookout for ways of testing basic assumptions to ever greater precision. Antiatoms provide a means to carry out tests that are not possible with antiparticles. 鈥淎 key advantage of working with antiatoms is that they are intrinsically stable 鈥� they don鈥檛 decay,鈥� says Richard Hughes, a theorist at Los Alamos National Laboratory in New Mexico.
Like ordinary atoms, antiatoms should have characteristic spectra 鈥� light of certain wavelengths emitted when positrons jump from one energy level to another. And it should be possible to observe their spectra with the high-precision equipment already well developed by atomic and molecular physicists such as Ted Haensch of the Max Planck Institute for Quantum Optics at Garching, near Munich. 鈥淎ll the spectroscopic techniques of atomic physics can be brought to bear, giving amazingly high precision,鈥� says Hughes.
There are two assumptions in particular that antimatter is well placed to test. One is the hallowed idea of 鈥淐PT symmetry鈥�, which says that any behaviour by a fundamental particle will be identical to that of its antiparticle (which has reversed charge, C) viewed in a mirror (which means reversed in space or 鈥減arity鈥�, P) with a reversed arrow of time (T). If CPT symmetry holds, the spectrum of an antiatom should be absolutely identical to that of its ordinary matter equivalent. If it turns out that their spectra are different, the idea of CPT symmetry would collapse, and quantum electrodynamics and the standard model would be in trouble.
Free falling particles
The CPT symmetry condition tells us that antiparticles falling in a gravitational field created by antimatter 鈥� say an antiplanet 鈥� should do so in exactly the same way as ordinary particles falling in a gravitational field created by ordinary matter. Antiphysicists occupying an antiplanet could test this easily enough, but as the only antiphysicists in our neighbourhood tend to be astrologers or politicians who withhold funding, there is not much hope of ever doing this experiment.
But what about antiparticles falling in the gravitational field of our very ordinary Earth? This is where the second assumption that physicists want to test comes in. The assumption is the 鈥渆quivalence principle鈥�, introduced by Einstein as part of his theory of general relativity. It states that a particle in free fall behaves in exactly the same way as it would if it was not under any gravitational influence at all; in other words, in free fall there is no way of telling whether a gravitational field is present or not. A more restricted statement, known as the 鈥渨eak equivalence principle鈥�, is that all matter, independent of its constituents, will experience the same acceleration in a gravitational field. If antimatter and matter were found to behave in different ways in a gravitational field, the equivalence principle would be shown to have failed.
At LEAR, physicists such as Michael Holzscheiter from Los Alamos are already trying to compare the behaviour of particles and antiparticles in the Earth鈥檚 gravitational field. But even if they see no effect, this would not necessarily prove that the weak equivalence principle is true. Another possibility is that the experimental setup is not sensitive enough to pick up the difference between matter and antimatter. According to theorists such as Hughes, these experiments would be unable to detect any difference of less than 1 per cent in the acceleration of matter compared with that of antimatter.
But if antiatoms become available they will provide a less direct means of comparison that turns out be much more sensitive. If one particle is orbiting another, then there is an energy associated with the fact that they are bound together. That energy, according to general relativity, has weight in any gravitational field, and that weight leads to a tiny change in the atomic spectrum 鈥� a shift toward the red end 鈥� whose size depends on the strength of the field. If antimatter violates the equivalence principle, the gravity-induced shift in the antimatter spectrum will be different from that in the matter spectrum. Modern spectroscopy is precise, and so should be able to pick out extraordinarily small differences between matter and antimatter in the already tiny red shift. If such a difference is observed it will tell us that either CPT symmetry or the weak equivalence principle 鈥� or both 鈥� is being violated.
Under threat
To find out which of these principles has failed it would then be necessary to subject hydrogen and antihydrogen to matching variations of gravitational field, and compare the changes in particular spectral lines. If the matter-antimatter difference changes as the gravitational field changes, then it is the equivalence principle that is violated.
Surprisingly, to change the gravitational field it would be enough simply to trap and analyse antihydrogen and hydrogen atoms over a year or more. This is because the Earth鈥檚 orbit around the Sun is conveniently elliptical, so the distance to the Sun varies through the year. The strength of an object鈥檚 gravitational field varies with distance, so the Sun鈥檚 gravity as measured at the Earth waxes and wanes by a few ten-billionths as the seasons pass. The Sun鈥檚 gravity is so strong that even this minuscule proportional change corresponds to an absolute difference in the gravitational field 300 times as great as the difference in gravity between sea level and the top of Everest. Atomic spectroscopists such as Haensch, working at the limits of sensitivity, could reveal a difference in the frequency of a spectral line as small as one part in a billion-billion. Given the seasonal changes in the Sun鈥檚 gravitational field, this is sensitive enough to pick up a difference of only one part in ten million between the gravitational acceleration of ordinary matter and antimatter.
鈥淔or tests of gravity, nothing else comes close to antihydrogen spectroscopy in terms of sensitivity,鈥� says Hughes. 鈥淵ou can infer the answer from other experiments but have to make lots of additional assumptions. And that鈥檚 true of CPT tests as well 鈥� there are other particles, such as K mesons and their antiparticles, that people use, but antihydrogen tests are very direct.鈥�
Barring obstacles, physicists could be using antiatoms to test the equivalence principle and CPT symmetry within four years. But there is, as it happens, a big obstacle, though it has nothing to do with physics. The council of CERN has decided to build the world鈥檚 next mega-accelerator, the Large Hadron Collider. But CERN鈥檚 member states, especially Britain and Germany, have imposed tight budget constraints, so to fund the LHC CERN is planning drastic cuts in other activities. One of the victims is LEAR, which is currently scheduled for closure at the end of next year.
Can LEAR be saved so that the antimatter experiments can go ahead? At the end of this month CERN鈥檚 SPS-LEAR committee will retreat to a mountain top at Cogne in the French Alps, and deliberate on the experimental programme of LEAR and its neighbouring accelerator, the Super Proton Synchrotron (SPS). The committee will consider ways to extend LEAR鈥檚 life. LEAR鈥檚 annual running costs stand at 17.6 million Swiss francs (拢9.5 million), or just under 2 per cent of CERN鈥檚 budget. But money is so tight at CERN that LEAR users are scrambling to try and complete at least some antihydrogen storage and spectroscopy experiments before the planned shutdown.
If those lobbying for LEAR fail to win even a temporary reprieve, there remains a more distant possibility. LEAR鈥檚 users include physicists from Japan, and some of them have suggested that a reproduction of LEAR might be built at Japan鈥檚 national accelerator centre, KEK, in Tsukuba. But even if this plan was accepted, the accelerator would still take years to build, and the lower energies available at KEK would mean that the production of antihydrogen could be a hundred times slower than at LEAR, according to one LEAR user.
The best hope is that LEAR will stay open long enough for antihydrogen manufacture to become routine. Physicists will then be able to fulfil their dream of testing some of their most basic assumptions, perhaps even to destruction.