The Antimatter Mission

A few curious physicists think it's time to find some antimatter stars in antimatter galaxies.

By Gary Taubes|Monday, April 01, 1996
RELATED TAGS: DARK MATTER
In the summer of 1984, Michael Salamon and Steve Ahlen went looking for antimatter in the cosmos the old-fashioned, low-budget way. Salamon, who was then at the University of California at Berkeley, and Ahlen, who was at Indiana University, bootlegged some spare cash from a previous experiment and used it to wire up an antimatter detector--a superconducting magnet and some elaborate electronics. They rigged the contraption to sit on a shelflike structure hanging from a high-altitude balloon. The balloon was capable of rising up to 130,000 feet, where it would be above the bulk of the atmosphere and could capture unimpeded the cosmic rays that rain continuously from the heavens. After a few years building the detector and then a month of 18-hour days readying the equipment for its mission, Salamon and Ahlen launched the balloon from Prince Albert, Saskatchewan, on August 13, 1987. It rose to its heavenly altitude above the stratosphere, floated serenely for 12 hours, and then, like a dying duck, plummeted to Earth in the Canadian wilderness.

When we finally found it, says Salamon, it took a hell of a time to get it out. We had to bulldoze some of the forest so a helicopter could get in and fly the payload out. But then the helicopter couldn’t pick it up, because the payload had impaled itself on the trunks of trees it had knocked down. We had to disassemble the support structure. It then took Salamon and Ahlen three months to analyze the data, which came up empty in any case. They had detected no sign of cosmic antimatter. That didn’t mean it wasn’t there, only that their experiment wasn’t sensitive enough to find it.

While Salamon and Ahlen thrashed around upper Saskatchewan, Samuel C. C. Ting was in Switzerland, establishing the largest experimental physics collaboration in the world--a $350 million, 600-physicist experiment known as L-3, which studies the collisions of elementary particles--at the European Organization for Nuclear Research, a laboratory known by its French acronym, CERN. Ting, who shared a Nobel Prize in 1976 and who has been described as the George Patton of high-energy physics, is a consummate experimentalist. But the experiment he was putting together in 1987 represented the twilight of his career, which was slowly being done in by the complex politics of the world’s most expensive pursuit of pure knowledge. By 1993, Ting would be looking for a way to do one more interesting experiment, as he put it, before he retired.

So it was that one of the most renowned practitioners of big science teamed up with a couple of inveterate low-budget astrophysicists to go searching for antimatter. Now Ting, Ahlen, Salamon, and a few dozen collaborators have convinced NASA to conduct the most thorough antimatter search ever. The $20 million experiment will fly first on a space shuttle in April 1998 and then for three years on the International Space Station Alpha, beginning in 2001. It should establish once and for all whether half the universe is made of antimatter, or whether matter, the stuff we’re made of, is all there is.

The search for antimatter has nothing to do with the paradox of dark matter, which is the roughly 80 percent of the universe that’s obviously there but isn’t emitting visible light and so cannot be seen. Ting and his collaborators are concerned instead with the 50 percent of the universe that isn’t there at all--the half that should be made of antimatter. Quantum mechanics dictates that matter can arise from pure energy, that elementary particles can pop into existence from a vacuum, provided that they do so in such a way that various fundamental quantities, such as electric charge and momentum, are conserved. This happened on a large scale during the Big Bang, when the universe began, and it’s been happening on a much smaller scale ever since.

Any time a particle is created from nothing, the universe will simultaneously spew up its antiparticle, which will have the same mass and other characteristics but will have the opposite charge and will head off at an equal speed in the exact opposite direction. Thus protons are made with antiprotons; electrons with antielectrons, known as positrons. The point, says Ahlen, is that you can’t make a proton without making an antiproton. That law has never been seen to be violated. So if there was a Big Bang, how can we have a universe without equal parts of antimatter as matter?

If antimatter was made along with matter, however, there is no sign of it. Assuming antimatter exists, one would expect chance collisions to bring it at least occasionally into contact with matter, resulting in the annihilation of both and a burst of gamma rays--very energetic photons, the packets of energy that make up light. This is another fundamental law. If matter and antimatter can be made from pure energy, then bringing the two back together again will result in pure energy. So if antimatter exists anywhere in the universe, a sure sign of its presence would be the gamma rays produced by its annihilation.

There is certainly no antimatter in the Earth, says Ahlen, and none in the solar system. There’s none in the solar wind, because if that were different from Earth, we’d be baked in the gamma rays from annihilation. If Mars or Jupiter were antimatter, we’d see the gamma rays from that. Using similar logic, astrophysicists can establish beyond a shadow of a doubt that our galaxy, our galactic cluster, and everything up to and including at least our supercluster of galaxies--some 100 million light-years across--are made of matter and nothing but matter.

But then it gets vague. The universe is a big place, composed of some 2 million galactic superclusters, and the only one we’re sure is made of matter is our own. As for the rest, that’s still an open question. Because the light emitted by antimatter is no different from that emitted by matter--or rather, the antiparticle of a photon is still a photon--the only way to tell an antimatter galaxy from one of matter would be to physically examine samples of matter from the galaxy in question. This is not as crazy as it sounds. The universe is full of cosmic rays--atoms and parts of atoms that have been ejected from stars, or remnants of supernovas and other bits and pieces of stellar detritus. These cosmic rays can escape from their own galaxies and may even wander out of their superclusters and find their way to Earth. Of all the cosmic rays bombarding Earth day in and day out, astrophysicists assume a small percentage may have traveled more than 100 million light-years to get here. And those might even be made of antimatter, proving that the galaxies they came from were made of antimatter as well.

Over the years some physicists have looked for such anticosmic rays. Notable among them was the Nobel laureate Luis Alvarez, who lofted antimatter detectors in high-altitude balloons in the 1960s and 1970s. These searches found a handful of antiprotons, but nothing more. The antiprotons say little about the missing antimatter, though. True, they could be the nuclei of antihydrogen, but antiprotons can also be made when regular cosmic rays collide with dust and debris in the interstellar medium. Heavier nuclei, such as those of anticarbon or antioxygen, could only be made by antistars and thus must be evidence of antigalaxies. But there was no sign of such antinuclei in the 40,000 cosmic rays Alvarez and his collaborators collected.

Since Alvarez last looked for antimatter, however, some new knowledge has been gathered that leaves the question of antimatter wide open. More recent calculations suggest that of all the cosmic rays crashing into Earth’s atmosphere, only one in every 100,000, or even one in a million, might be from beyond the galaxy. In that case, all of Alvarez’s 40,000 cosmic rays probably originated right here in our own galaxy. And if Alvarez never saw a cosmic ray from outside the galaxy, then there is no evidence that the universe doesn’t have its fair share of extragalactic antimatter. There might still be antisuperclusters out there, full of antigalaxies, lit by the light of antistars.

Then again, there might not. Theorists, says Ahlen, will tell you antimatter is the most ridiculous thing in the world to look for because everyone knows there is none. The theorists will say that considering no antimatter has been seen to date, the idea that it’s out there is either impossible or almost impossible. They can’t imagine a scenario that would explain how antimatter could be separated from matter on a large scale (into superclusters and antisuperclusters) but not on a smaller scale (into clusters and anticlusters, say, or galaxies and antigalaxies).

According to scenarios theorists can imagine, the universe originated in a cosmic fluctuation, in which pure energy condensed into matter. Sometime around 10-34 second after time zero, a soup of elementary particles and antiparticles condensed out of this energized void, like water droplets condensing out of humid air as the temperature falls. These particles and antiparticles then began annihilating, so the theory goes, until only one in a billion was left, and that happened to be matter rather than antimatter.

Perhaps the universe began with this one-part-in-a-billion excess of matter over antimatter, and what’s left over from the furious annihilation is our universe; in that case the world violated a few quantum laws when it was young. Perhaps, as the great Russian physicist Andrey Sakharov proposed in 1967, the laws of physics are out of balance, slightly favoring the existence of matter over antimatter for some reason. That lack of symmetry, known as CP violation, says that particles and antiparticles can decay differently. But CP violation, which has been measured in the laboratory, doesn’t seem sufficiently unbalanced to account for the antimatter paradox. David Schramm, a cosmologist at the University of Chicago, describes the situation this way: The intuition of most theorists is that no antimatter is there. This means if you find it, it’s a great discovery and proves all these theorists wrong. But most likely it means you won’t find it.

There are two ways to deal with such a situation, as Sam Ting likes to point out. Ting divides experimental physicists into two kinds: those who let theorists tell them what to do, and those who follow their own demons. I am of the second kind, he has said. I am happy to eat Chinese dinners with theorists, but to spend your life doing what they tell you is a waste of time.

Ting was born in 1936 in Ann Arbor, Michigan, where his parents, both professors, had come for a short visit. A few months later they took him home to China, where he stayed for the next 20 years. He then went to the University of Michigan to earn degrees in mathematics and physics.

Ting won his Nobel in 1976 for the discovery of a particle that he named the J particle (other physicists delight in pointing out that J resembles the Chinese character for Ting, and that Ting thus rather immodestly named the particle after himself). He’d found the particle in an experiment at Brookhaven National Laboratory on Long Island, after two other labs had rejected the experiment--one lab because theorists said Ting would never find anything.

At first Ting couldn’t believe he had, so he refused to announce his discovery for over a month, while he had his collaborators reanalyze the data and collect more. He went public only because physicists at the Stanford Linear Accelerator Center (SLAC) also found evidence of the same particle, which they called the psi particle; the two groups announced the discovery simultaneously on November 11, 1974. Because of his lack of faith in his own data--usually a desirable characteristic in a scientist--Ting ended up sharing the Nobel Prize with Burton Richter, the director of SLAC, and the particle is now called the J/psi, except by Ting and his collaborators, who still call it the J, and by Richter and his collaborators, who call it the psi/J.

In the years since, Ting’s power in physics has grown with the size of the instruments and the cost. His $350 million L-3 experiment, for instance, makes use of CERN’s Large Electron Positron collider, a machine that is 16 miles in circumference and cost in the neighborhood of a billion dollars. When U.S. physicists persuaded Washington to spend $5 billion (which later grew to $10 billion) for the 54-mile-around Superconducting Supercollider (SSC), Ting proposed an experiment that would have cost $800 million and kept a staff of 1,000 physicists busy. It was a prominent and controversial candidate for the SSC but was eventually rejected in favor of an American collaboration, and physicists who discuss the decision tend to mention the politics of their world.

By the time Congress killed the SSC in the fall of 1993, the number of experiments a man of Ting’s stature could lead was down to two, both of them at a proposed accelerator known as the Large Hadron Collider at CERN. Ting was not involved in either of them. He says he realized that by the time that collider is finished, he will probably be long retired. He was ready to do something completely different. And he had also become disillusioned with the process. It’s become a very political process, he explains. Because the experiments are very large and take thousands of people, you have to be political if you’re involved. But you don’t have to be involved.

One month after the SSC project was buried by Congress, Ting called Steve Ahlen, who had moved on to Boston University. Ahlen had worked with Ting on his SSC proposal, and now Ting said he wanted to know everything Ahlen knew about astrophysics. The next two months constituted an intensive session of brainstorming with Ting, Ahlen, Salamon--who had moved on to the University of Utah--and four other physicists: Hans Hofer, of the Swiss Federal Institute of Technology in Zurich, and Yuri Galaktionov and Ulrich Becker of CERN, all three longtime Ting collaborators, and Alvaro De Rujula, a theorist with joint appointments at CERN and Boston University. As Ting tells it, they quickly narrowed their possible experiments to two: either build a huge mirror high in the Tibetan Himalayas to study cosmic gamma rays, or put a device in orbit to look at billions of cosmic rays--not just tens of thousands--and search for the handful that might have come from distant superclusters and be made out of antimatter.

Ting decided the antimatter search from space was the better choice, if only because it had never been done. The great thing about Ting, says Ahlen, is that he is guided by general principles: Is there a compelling argument why there is no antimatter? No. Has anyone looked for antimatter at the sensitivity you need to? No. Then you look for antimatter. It’s as simple as that. You don’t have to have a lot of complicated computation to justify looking for it. Ting’s standard line is ‘We’re exploring the unknown.’ Or as Ting puts it, I just thought that to be an experimentalist, you really should measure this.

Such an experiment would require doing what Alvarez, who died in 1988, had advocated for the last 20 years of his life: putting a powerful magnet in space. Any electrically charged particles passing through the magnet would curve under the influence of its magnetic field. Cosmic rays made of matter would curve one way, and cosmic rays of antimatter would curve the other. Because the particles are moving fast, and they’ve got to curve noticeably in the course of a few feet, the job requires a very strong magnetic field. Alvarez and his colleagues had proposed numerous experiments that would put a superconducting magnet in space. At the time, superconducting technology seemed the only way to get a magnet that was both powerful enough and small enough.

But superconducting magnets come with all kinds of technological complications, which translate into exorbitant costs. For instance, the magnets have to be cooled to near absolute zero and kept there at a constant temperature. This requires elaborate refrigeration, which is especially problematic in space because as the equipment orbits around Earth, it will alternately bake in sunshine and freeze in shadow. Estimates of what it might cost to put a superconducting magnet in space came in at between $150 million and $300 million. While that was comparable to the cost of a high-energy physics experiment, it was a lot to ask for an experiment that most people bet would come up empty.

In March 1994, however, Ting was visiting China when he stopped in, he says, by chance, at the Institute of Electrical Engineering of the Chinese Academy of Sciences in Beijing. I found out, he says, that they have been building very precise permanent magnets with a new type of material, neodymium boron iron, and I learned that 90 percent of the neodymium in the world is produced in China and that they can produce very high quality, very high field magnets. The more I talked to them, the more convinced I became that this would be a good way to build a cheap magnet and do things in space.

As a result, says Ting, the Chinese physicists and engineers agreed to build his magnet for around $600,000--a very small fraction of the market cost. Moreover, flying a permanent magnet in space is a proposition equivalent in difficulty to flying a kitchen magnet in space-- it just needs to be anchored to the surface of the shuttle or space station. You don’t have to worry about a power supply, says Ting, or refrigeration, and you don’t have to worry about how to fix it, because it can’t break.

The magnet is only part of the experiment, however. The physicists would have to put some kind of elementary particle detector inside it that could look at a few billion cosmic rays and notice if a handful were of antimatter, without making mistakes. In other words, says Salamon, you need a large enough instrument to detect a billion cosmic ray nuclei, but even that won’t be very helpful if your instrument screws up and misidentifies every thousandth nucleus, or even every millionth or every hundred millionth, as an antinucleus.

Fortunately, Ting has plenty of experience in the particle identification business. Ting’s experiment at CERN, for instance, looks at billions of collisions between electrons and positrons and then has to sift carefully through the remains and precisely identify each particle created. To do so it makes use of silicon sensors that were designed for L-3 by Roberto Battiston and his collaborators from the University of Perugia and the Italian National Nuclear Physics Institute. Ting and his collaborators have decided to put similar technology to work inside their permanent magnet.

Picture the magnet as a vertical cylinder about a yard in diameter and a little less than a yard in height, explains Maurice Bourquin of the University of Geneva, whose group designed the detector for the antimatter search. At top and bottom are two pairs of planes made of a plastic material called scintillator, which will measure the charge of any particles entering the magnet’s hollow core, though they won’t tell whether that charge is positive or negative. The scintillator planes will also measure each particle’s speed and energy.

Crossing the magnet’s core horizontally are six planes of silicon sensors. When a charged particle passes through the silicon, it releases electrons, and their position is recorded electronically. With six planes, every cosmic ray that passes through will leave behind six hits, tracing out a trajectory through the core. The magnetic field, which cuts straight across the hollow core of the magnet, parallel to the planes of the silicon sensors, will pull particles one way or the other, so that the trajectory will be curved. From the amount of curvature, the physicists can tell how heavy the particle was. From the direction of the curvature, they can tell whether it had a positive or negative charge, which means matter or antimatter. And then, says Salamon, if we were to detect a cosmic ray with a nuclear charge of -6, for example, which is anticarbon, this would conclusively demonstrate the existence of antistars, since carbon--or anticarbon--is produced only by stars, in a process called nucleosynthesis.

The catch is that the cosmic rays are likely to curve no more than 100 microns--one-tenth of a millimeter--as they pass from one end of the magnet to the other. So Ting, Battiston, and company have to make sure they can detect the position of each hit with an accuracy of a few microns, or they might mistake a cosmic ray curving in one direction for one curving in the other. Battiston says that they can build the silicon sensors so they can identify the point at which a cosmic ray passed through to within 10 microns. They’ve also come up with a way to build the sensors so that they will keep that accuracy even through the trauma of a shuttle launch. Among other things, the silicon sensors will be bracketed by thin sheets of carbon fiber, which will keep them rigid during launch. The fibers won’t blow the sensors’ accuracy by expanding or contracting, no matter how the temperature may vary--from 72 degrees in Geneva, for instance, to 3 degrees above absolute zero in space. Once the detector is in space, the researchers figure, they can double-check the accuracy of the sensors by using extremely high energy cosmic rays that will pass virtually straight, even through the high magnetic fields of their experiment.

Having an experiment that will work in space, however, is not the same as getting it there. Although the AntiMatter Spectrometer (AMS), as Ting and company were calling their experiment, could fly as a satellite, both NASA and the European Space Agency had such lengthy procedures for approving satellite programs, and such backlogs in launching satellites, that they wouldn’t be able to put it up before 2017, says Bourquin.

International Space Station Alpha looked like a better bet. The project had begun life as the $80 billion U.S. Space Station Freedom, but in 1993 President Clinton internationalized it. For years the scientific community had fought against the space station, calling it an extraordinarily expensive public relations scheme. If Freedom had a constituency, scientists argued, it may have been the aerospace industry, but it certainly wasn’t scientists. By 1992 some two dozen scientific societies, from astronomers to cell biologists, officially opposed the project.

I can see NASA, says Bourquin, with this big remodeled Freedom project, really needing some science. Now, what kind of good science could be put up there? Maybe a lot of little biology experiments--mice or whatever running around--but what about a big science experiment?

Ting’s project looked like the answer. He wasn’t just offering NASA big-time science for the space station, he was offering big-time science at a bargain rate. With his huge international collaboration, drawing on the engineering skills of a dozen nations, Ting could promise to build the experiment for perhaps $20 million. The Department of Energy would peer-review the experiment, making sure it was scientifically worthy, and then ante up $3 million, and Ting’s foreign collaborators would contribute the rest. All NASA had to do was fly the AMS first on the space shuttle--Sam wanted the chance to fly it once, says Dan Goldin, the head of NASA. He would check it out, bring it back to the ground, and retune it--and then put it back up on the space station. It costs us all of $13 million to take it up and down twice and attach it to the space station, says Goldin. For a very small amount of money we have the potential for blockbuster science.

In return, NASA and Goldin required that Ting and his colleagues abide by their safety regulations and requested that Ting change the name of the experiment to the Alpha Magnetic Spectrometer. The new name avoided the implication that the only science AMS could do would be a search for antimatter. After all, says Ahlen, if you talk to people in hallways, they’ll tell you everyone knows there is no antimatter. On the other hand, those same theorists will say that certain particles that are hypothetical candidates for the dark matter problem--that’s the 80 percent of the universe that can’t be seen--are likely to decay into antiprotons. Ting’s experiment could collect thousands of antiprotons and sift through them for evidence that they originated with dark matter. For example, if a sufficiently large percentage of those antiprotons had the same energy, that would suggest they all came from the same sort of decaying dark matter particles. It’s even possible that Ting’s huge magnet will capture anti- dark matter particles. There was no saying. Dave Schramm, for instance, who reviewed Ting’s proposal for the Department of Energy, says he recommended the experiment not because he thought it would find antimatter but because it was an excellent bet to solve part of the dark matter problem.

Ting doesn’t seem too concerned one way or the other. In his lectures on the AMS, he likes to date the first use of precise light measurements in astronomy to 1054, the earliest exact record of a supernova, which happens to be a Chinese record. In all that time, he says, nobody has ever done a precision measurement of charged particles in space. We’re going to do that. We’re going to study very clearly the composition of cosmic rays, and we will learn a lot from it. Since it’s never been done before, we don’t know what we’re going to get. But we’re going to get something.
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