This article focuses on the fact that a single atom of antimatter—in particular, antihydrogen—may unlock fundamental mysteries of our universe and could lead to revolutionary advances in medicine and space travel. Physicists, through experiments due to begin soon in Geneva, Switzerland, hope to produce a relatively large amount of antihydrogen on a regular basis to compare matter and antimatter. Athena and Atrap share the goal of producing antihydrogen atoms at low energies, in a magnetic trap, and comparing the energy levels and behavior of antihydrogen with hydrogen. The Athena collaboration developed out of an attempt to measure the gravitational acceleration of antiprotons toward Earth. Its experiments, which are to cover a range of considerations, will include studies of gravitational acceleration of antimatter. A tiny asymmetry in the way particles of matter and antimatter decay could help substantiate the belief that, at a somewhat later time after the Big Bang, collisions between the matter and antimatter destroyed all the antimatter but left an excess of matter, from which our universe evolved.
A single atom of antimatter-in particular, antihydrogen may unlock fundamental mysteries of our universe and could lead to revolutionary advances in medicine and space travel.
Physicists, through experiments due to begin soon in Geneva, Switzerland, hope to produce a relatively large amount of antihydrogen on a regular basis to compare matter and antimatter. What they learn could provide a better understanding of the universe and perhaps new applications of antimatter in engineering. In addition, through their investigation, they are generating new analytic instruments and technological innovations.
An atom of antihydrogen, or anti-H, consists of a negatively charged antiproton and a positively charged antielectron (a.k.a. positron). A hydrogen atom, on the other hand, is made up of a positively charged proton and a negatively charged electron.
Experiments To Extend The Test
Most physicists assume there is an intrinsic symmetry between Hand anti-H-that is, that the particles are images of each other in a kind of mirror that reflects time as well as space. The symmetry has been demonstrated up to a certain level; experiments that will begin this summer intend to take the test farther.
If the behavior of antihydrogen were shown to differ in even the minutest detail from that of hydrogen, physicists would have to rethink many of their concepts of symmetry between matter and antimatter.
Later this year, physicists will launch the first experiments at the new Antiproton Decelerator, a facility dedicated to synthesizing and studying low-kinetic-energy antimatter, at CERN, the European Organization for Nuclear Research in Geneva.
The first anti-H atoms were produced and detected in 1995 at CERN, but those atoms were high-energy and too volatile (moving almost at the speed of light) and were in the lab for too short a time (about 40 nanoseconds) to be studied. From 1987 to 1996, antiproton production for physics experiments at CERN involved four interlinked devices: the Antiproton Collector, the Antiproton Accumulator, the Proton Synchrotron, and the Low-Energy Antiproton Ring, or LEAR.
The synchrotron produced antiprotons by smashing a beam of protons against a heavy copper target. In the collector, the antiproton beam underwent stochastic cooling, which reduces the momentum spread of antiprotons by applying an electric jolt to kick diverging particles back into the correct direction, thus cooling them and sending them back into the proper orbit. The beam then went to the accumulator, which served as a kind of antiproton refrigerator. This procedure was repeated over and over again, until a stack of antiprotons (about 1012 per day) was accumulated.
The stacked antiprotons went from the accumulator back through the synchrotron, where they were decelerated, to the LEAR, at intervals of 30 minutes to several hours. The process reduced the momentum of the antiprotons to 0.6 GeV/c from 3.57 GeV/c. (At a momentum of 3.57 GeV Ic-that is, gigaelectronvolts divided by the speed of light-the particles are moving at almost 97 percent of the speed of light; at 0.6 Ge V/c they are traveling at more than 50 percent of the speed of light.)
Once a portion of the antiproton group had arrived at the LEAR, it could be further decelerated and cooled for low-energy experiments, or accelerated for higher-energy experiments.
In 1996, after CERN began phasing out production of antiprotons, the urgings of physicists from around the world led to feasibility and design studies for the Antiproton Decelerator. Construction of the AD began in February 1997, and the unit will be operational by late summer. It will be operated for up to six months each year.
The Antiproton Decelerator stores antiprotons as an allin-one machine that can capture them and then decelerate and cool them to low energies. The decelerator incorporates the former collector, now modified to decelerate the antiproton beam, includes one main component from the LEAR, and adds improved vacuum and control systems.
The use of two devices reduces the number of operators and operating costs.
The first antiprotons are expected in September. The initial experiments, which are likely to run five years or longer, first will try to obtain results with moderate precision, and then subsequently refine the equipment to obtain higher and higher precision every year. For instance, ultranarrow and stable laser systems could be added in 2000 or 2001.
Two competing experimental collaborations, Athena (derived from the letters in the phrase "a ntihydrogen apparatus"), led by Rolf Landua of CERN, and Atrap (short for "a ntihydrogen Penning trap"), under Gerald Gabrielse of Har vard University, have been formed to produce and study anti-H.
Athena and Atrap share the goal of producing antihydrogen atoms at low energies, in a magnetic trap, and comparing the energy levels and behavior of antihydrogen with hydrogen. The Athena collaboration developed out of an attempt to measure the gravitational acceleration of antiprotons toward Earth. Its experiments, which are to cover a range of considerations, will include studies of gravitational acceleration of antimatter.
Attracted to Gravity
"Gravity is the one force in nature which is most familiar to us in everyday life, but the understanding of gravity in the context of modern physical theories is still very rudimentary," said Michael Holzscheiter, an Athena participant from Los Alamos National Laboratory. "Very few experimental studies of the force of gravity exist for atomic and elementary particles and none at all for particles of antimatter."
Each experiment involves the production and capture of more than 1 ,000 antihydrogen atoms per hour.
"We're going to test antimatter using the most precise method available, namely laser light," said Michael Doser, research physicist on the Athena team. "The sensitivity. we want to achieve at the beginning of the experiment is equivalent to weighing a jumbo jet and being able to determine whether an ant was on board or not. The level of precision will increase as the experiments evolve. Having said that, the technology that needs to be developed to perform such experiments can have very concrete consequences on mundane technology."
Physicists will compare H and anti-H, to see if the same frequency of laser light excites them in the same fashion, producing the same color and energy for both.
The first lasers produce a very narrow band of color-about a trillionth of a wavelength wide—with the intention of including one that will excite the atom from its normal, unexcited state to its next, excited state. Later, in order to study the precise wavelength of the transition, the laser's color will be fin etuned.
Athena incorporates a two-meter-long superconducting solenoidal magnet, which is cooled by liquid helium to 4K (about-269°C), then energized by an external current supply for its initial magnetic field. When both ends of the coil are connected to each other via a superconducting switch, the external electrical current can be disconnected. The current inside the magnet keeps on flowing. This allows the magnet to generate no heat and remain magnetized forever without further electrical input, so long as it remain s at liquid helium temperature.
According to Landua, Athena may build a magnet with a superposed quadrupolar field, in which force increases in every direction from the center. The design would form a magnetic well in which slow, cold anti hydrogen atoms, once produced at low energy, can be held. It may also add a dilution refrigerator that could cool the central region to about 200 mK, or almost absolute zero.
The collision of an antihydrogen atom with hydrogen produces charged and neutral mesons from. the antiproton annihilating on a proton and two photons at a fixed energy of 511 keV from the positron annihilating on an electron. An important part of all antihydrogen experiments is a set of detectors for these annihilation products.
Silicon strip detectors react to charged particles resulting from antihydrogen annihilation and provide information on the exact position of the event. Surrounding them, Athena physicists planto place photodiodes against cesium iodide crystals. An annihilation photon strikes a crystal, which releases its own photons, which hit a photodiode. The diode releases an electrical signal to a preamplifier; the signal, amplified by a factor of 10, is sent to an electrical system that amplifies the signal a thousand times.
Photodiodes could be developed to replace the photomultipliers that are used in positron emission tomography scanning.
In PET scanning, a radioactive atom bound within a glucose molecule is injected into the body and drawn to a high-metabolic area (for example, the rapid cell growth of a braintumor). The atom decays, emitting a positron, which rapidly annihilates with an electron and produces two photons. These photons are detected by chemicalcompound crystals, implanted in a ring around the targeted body part and connected to photomultipliers containing photocathodes. The crystals react by emitting their own light, which is detected by the cathodes and amplified by the multipliers. By determining which two crys tals were hit, one can mapa line that will pass through the active cells and calculate its distribution.
Photomultipliers are vulnerable to external magnetic fields, but a PET scanner working with photodiodes, like the ones developed in the Athena exp er iment, could be used in combination with magnetic resonance imaging. Putting the technologies together would allow the imaging of soft tissues and the measuring of their activity with high spatial and temporal resolution.
Atrap, which includes members of an earlier collaboration whose experiments at CERN began in 1986, has essentially the same goals as Athena, but with a different approach toward the way antiprotons and positrons are captured and brought together to form antihydrogen.
Atrap physicists will first attempt to reduce the momentum of antiprotons and positrons, then hold the cold ingredients in the same container so they can combine to create anti-H. This neutral anti-H will be unaffected by the trap's electric field the moment the atom forms and will escape to its annihilation. The destruction will release decay products, such as pion particles from the anti pro tons and gamma rays from the positrons. These antiproton and positron annihilation signatures, once detected, will tell the Atrap physicists that the anti-H atom had existed.
A later phase of the Atrap experiments will use a superconducting solenoidal magnet with several metal coils spiraling the magnet in an array of complicated patterns, and a clear plastic sheet with photomultipliers surrounding it. When anti-H forms in the trap and falls freely, the atom will strike the plastic and release a pion. The pion will make contact with the plastic's atoms, which in turn will give offlight, detected by the photomultipliers.
The Atrap physicists aim to build nondestructive tests, such as the use of lasers to find and measure anti-H's properties, and the further confinement of the anti-Hatom. At some future point, once an atom can be located without destruction, laser spectroscopy will be per-formed through windows built into the traps, dissecting an atom into its particle or simply exciting it and measuring the light energy it releases.
A third proposed experiment is a Japanese-Euro pean project called Asacusa, for Atomic Specttoscopy And Collisions Using Slow Antiprotons (and also for Asakusa, a district in Tokyo famous for its shrines and temples). Asacusa will focus on the recent discovery that antimatter is sometimes able to survive longer than was thought possible in our everyday world of matter. The experiment will study antiprotonic helium atoms (helium in which one electron is replaced by an antiproton); these atomcules live for several microseconds, an extremely long time for such atoms. In a second phase, it will study protonium, a hydrogen-like atom made of a proton and an antiproton. Later, Asacusa may also participate in the study of antihydrogen.
Athena and Atrap experiments will study the symmetry of charge (C), parity (P), and time (T), concepts that have been under constant scrutiny over the last 50 years.
Symmetry of electrical charge has an analogy in a photographic negative of a black rubber ball. The negative image of the ball is whit e. A print made from the negative shows the same image, with the same shape, size, and texture of the ball, in the opposite shades. A particle, in a symmetry of charge, would be identical to its opposite particle except for its charge.
Parity symmetry is like the ball bouncing before a mirror. A viewer who could only see the mirror would not be able to distinguish between the bouncing ball itself or its reflection, since the image has its parts arranged in a perfect reversal of the real-world bouncing ball. A particle in a symmetry of parity, in the same way, would be the mirror image of its opposite.
In a symmetry of time reversal, a viewer shown a film of the bouncing ball would not be able to discern whether the film was running backward or forward, because the essential behavior of the ball would be the same. The same notion applies to a particle, either moving forward or backward in time, and reacting in a symmetrical fashion.
There appear to be four basic forces at work in the universe. In ord er of strength, they are the strong force, which holds quarks and oth er subatomic par ticles together; electromagnetism, which deals with magnetism caused by an electric charge in motion and relates to the structu re of atoms; the weak force, responsible for radioactive decay; and gravity, the weakest of the four, which binds solar systems and galaxies together.
The first and second forces are apparently inviolate in terms of symmetry. The weak force apparently violates several symmetries. No one has been able to prove gravity a synm1etrical part of the universal system of forces.
"Regarding the expected outcomes of the CPT and gravity experiments, a straw poll among physicists would overwhelmingly favor 'no surprise,' " said Doser of the Athena group. "In other words, gravity treats antimatter just like matter, and CPT holds to whatever precision we will reach."
However, the observation of any asymmetry in this respect would have profound consequences for the fundamental understanding of matter and antimatter.
"Although there is indirect experimental evidence that, combined with standard understanding, suggests gravity on antimatter is the same as on matter, many new theoretical ideas suggest that it could be different," says Michael Nieto, a research physicist at Los Alamos National Lab and an Athena team member. " What is truly tantalizing, though, is that gravity has never at all been directly measured on antimatter."
The sensitivity desired is equivalent to weighing a jumbo jet and determining whether an ant was on board.
A tiny asymmetry in the way particles of matter and antimatter decay could help substantiate the belief that, at a somewhat later time after the Big Bang, collisions between the matter and antimatter destroyed all the antimatter but left an excess of matter, from which our universe evolved.
"The outcome of the experiments also will have some bearing on the question why our universe seems to be 'matter-oriented,' while the common belief is that the Big Bang started with a complete symmetry between matter and antimatter," said Landua. "The complete absence of any signs of antimatter in the observable universe is still mysterious."
Indeed, since antimatter was discovered, scientists have been seeking it out in the universe, theorizing that a galaxy of antimatter may, in fact, collide with a galaxy of matter, and the annihilation (and the light energy released by it) could be visible. So far, however, no evidence of celestial clashes of this type has been found.
"As far as we know, there is no antimatter to be found naturally anywhere-not in the earth or out to the farthest clusters of galaxies," said John Eades, a research physicist with Asacusa. "It's not like coal or oil, lying in the earth, or like solar energy. It has to be produced, and it requires an enormous amount of energy to produce it, so it is an energy sink rather than an energy source, as far a we are concerned. Our interest in antihydrogen is therefore that, if the invariance between antihydrogen and hydrogen turns out to be not quite exact, we may have an explanation as to why we see no antimatter at all out there. Of course, it may still be there, but lurking in some nether corner of our universe."
If, as the AD experiments plan on showing, antimatter can be manufactured in large quantity and stored, it may prove valuable for diverse applications, such as a fuel source for deep space flight.
Gerald A. Smith, professor of physics and director of the Laboratory for Elementary Particle Science at Pennsylvania State University (which is sponsored by the NASA Jet Propulsion Lab and Marshall Space Flight Center), and his colleagues have designed spacecraft that use low-energy antimatter in the propulsion system. One of the craft, dubbed AIMStar, is an unmanned exploration vehicle that, according to Smith, could travel well beyond the solar system- reaching a distance of 10,000 astronautical units in 50 years. An as tronautical unit, or AU, is roughly equal to the mean distance between the center of Earth and the center of the sun. Pluto is about 39 AU from the sun.
The concept uses antimatter as the catalyst for a fissionfusion reaction (as in those created with hydrogen bombs) within the spacecraft. As Smith sees it, anti-H, stored in a neutral or mechanical trap, would be injected into a reaction chamber and introduced to hydrogen and helium. Antihydrogen, annihilating with the hydrogen and producing gamma rays and pions, would heat up the chamber and spark a nuclear fusion reaction between the hydrogen and helium atoms, generating the enormous amount of energy needed by the spacecraft.
"We see this technology coming out of the present-day, low-yield antimatter genera ting and trapping experiments," said Smith. "We're in the research and development phase for the next five years, and should be able to apply these techniques to spacecraft in 15 to 20 years." Other physicists, however, question any practical applications of antimatter emerging from the experiments.
"The amount of antimatter expressed in grams is extremely small and, therefore, also the energy obtainable from antimatter-matter annihilations is very small," said Landua. "All of CERN's prior antimatter production created about one-billionth of a gram during 10 years, and a billionth of a gram of antimatter and matter would produce the energy equivalent of a 60-watt light bulb burning for 50 minutes."
Doser said, "Technology is very unlikely to be directly influenced by any theoretical outcome of this experiment, in that technology deals with the world surrounding us. We have perfectly good theories, such as gravity, quantum mechanics, and electrodynamics, to deal with the world between the atomic and the macroscopic levels.
"Of course," he added, " if we strike gold, and do discover a difference between antimatter and matter, things could get interesting."
As Holzscheiter put it: "We must probe deeper into our understanding of the universe and look for deviations from the predictions of the so-called 'standard model,' which may lead us to new horizons."