When the space shuttle Discovery hurtled off launch pad 39 at the Kennedy Space Center yesterday (June 2), a three-ton detector called the Alpha Magnetic Spectrometer (AMS) was mounted in the rear of its payload bay. Its quarry: any errant bit of antimatter that finds its way through perhaps billions of light years to our neck of the universe.
"If you look at the last 40 years, a tremendous amount of work, at MIT and elsewhere, has been done to measure light rays, gamma rays, X-rays, ultraviolet and infrared radiation in space," said Samuel C.C. Ting, the Thomas Dudley Cabot Professor of Physics. "Charged particles, which may or may not be antimatter nuclei, have never been explored in space with equipment at this level of precision.
"No one knows what dark matter is and whether antimatter is around. But this experiment will for the first time provide precise measurements of the mass of charged particles in space," he said.
The detector, made of a very large and powerful permanent magnet and several other components, took scientists from 37 research institutions three years to build. The international team is led by Professor Ting, who expects this first-of-its-kind experiment to increase our understanding of the composition and origin of the universe. Also involved in the collaboration at MIT are Professor Ulrich J. Becker and Associate Professor Peter H. Fisher of Physics.
This 10-day shuttle mission -- which carries mission specialists Wendy B. Lawrence (SM '88 from the joint MIT/Woods Hole Oceanographic Institute program) and Franklin R. Chang-Diaz (PhD '77 in applied plasma physics) -- is a shakedown cruise for AMS. It is preparing for its real job of spending from 2002-05 hooked up to the International Space Station, trying to detect antiparticles -- oppositely charged counterparts of the protons, neutrons and electrons that make up matter as we know it -- to tell us whether whole antistars and antigalaxies that may have been produced during the Big Bang are still out there, lurking billions of light years away.
It is not new for Professor Ting, who received the Nobel Prize in 1976 for his discovery of a heavy elementary particle he dubbed the J particle, to conduct needle-in-haystack searches for elementary particles.
But this is Professor Ting's first opportunity to search for these tiny building blocks away from the background produced by the Earth's atmosphere and above the North and South Poles, where the influence of the Earth's magnetic field on inbound particles is minimized.
Detecting positrons (the antimatter counterpart of the electron) or antiprotons might also help unravel one of the great mysteries of astrophysics -- the nature of dark matter. Observation of positrons may give some clues about why the mass of a galaxy seems to be greater than the mass of all its visible stars, gas and dust. Scientists hope the energy spectra of the positrons detected by the AMS will indicate or set limits on the nature of the mysterious missing matter.
A MIRROR UNIVERSE
In 1928, the existence of antimatter was predicted by English physicist Paul A.M. Dirac, who said that for every particle of ordinary matter, there is an antiparticle with the same mass but an opposite charge.
Antiparticles such as positrons, antiprotons and antineutrons could create antiatoms, which in turn could form antimatter versions of all the matter we see, such as antistars, antiplanets and even antihumans.
Antimatter doesn't normally stay around very long. When antimatter and matter meet, they destroy each other in a huge burst of gamma rays and other particles. Using a particle accelerator, scientists have managed to very briefly create antihydrogen.
Professor Ting isn't the first to look for antimatter in space. Balloon-borne detectors rising to the upper atmosphere seek to detect positrons resulting from violent collisions of subatomic particles. These detectors have analyzed millions of hits of charged particles, also known as cosmic rays, never finding an antiparticle heavier than an antiproton. But these balloon detectors are only aloft for a matter of hours, while the AMS will be in the vastly improved vantage point of space for three to five years.
Professor Ting hopes to detect heavy antimatter nuclei of anticarbon or antihelium. An antimatter nucleus could signal the presence of a far-off antimatter galaxy because, unlike positrons and antiprotons, these heavy antiparticles are too massive to have resulted from interstellar particle collisions. An antihelium nucleus would prove that some antimatter survived the Big Bang. An anticarbon nucleus could mean that antistars exist, because carbon and heavier elements are created only in stars.
According to the most widely accepted theories, the Big Bang produced almost the same amount of matter and antimatter. At least a local excess of matter became the stars and galaxies we see. But very far away (because scientists agree that everything as far as 20 million light-years from us is all composed of matter), there may be clusters of galaxies of antimatter lurking in space.
Antimatter particles from these galaxies could theoretically move in trajectories around intergalactic magnetic field lines, bouncing from one galaxy to another, slowly making their way toward Earth, needing only the right detector to make their presence known. Scientists are hoping the AMS is that detector.
NOT YOUR EVERYDAY MAGNET
The AMS uses a permanent magnet made of a new alloy called neodymium-ferrous-boron to deflect the cosmic rays, which would curve in one direction or another depending on whether they were positive or negative. AMS will measure the charge and direction of incoming particles and the amount of their deflection.
When particles enter the detector's magnetic field created by a cylinder-shaped shell of magnets, they can be identified by their trajectories. Electrons are bent in one direction while protons and positrons veer off the other way. You can tell a positron from a proton because the heavier proton will travel in a straighter path than a positron with the same velocity.
The AMS's time-of-flight counters are triggered when a charged particle or antiparticle passes through the detector. This starts the readout of the silicon microstrip tracker, made up of 1,921 silicon sensors in six layered horizontal plates.
The anti-coincidence counters do just that: prevent coincidences by flagging the entry of any secondary, or background, particles so these ambiguous signals can be rejected. This is no small task; there may be one heavy antiparticle in a background of 100 million particles.
The aerogel threshold Cerenkov counter enhances the detector's ability to identify particles. As particles pass through a 10-centimeter layer of aerogel, they are viewed by 168 phototubes. This will identify positrons and help distinguish antiprotons from other particles.
When a particle of interest to scientists passes through the detector, the detector signals are digitized by sophisticated electronics, which transmit the data from the "event" to Earth. The electronics also relay commands from computers at Johnson Space Center to the dectector.
The United States' part of the AMS detector construction was funded by the Department of Energy through MIT's Laboratory for Nuclear Science.
Professor Ting pointed out that there has never been a magnetic spectrometer in space to conduct a sensitive measurement of the high-energy particles that bombard the Earth. If the AMS detects a heavy antiparticle, it would be a very significant first. Either way, by the end of the AMS's stint on the space station, he believes we will know for certain whether an anti-universe exists.
A version of this article appeared in MIT Tech Talk on June 3, 1998.