Making Antimatter: Science's Latest Great Adventure
For decades, scientists have made a virtual industry out of smacking tiny bits of matter into each other to see what secrets lie inside.
Now, physicists at the Fermi National Accelerator Laboratory here are putting some of those Humpty Dumpty-like pieces back together. During the past month, a Fermilab team has begun producing antihydrogen, the antimatter twin to the most abundant element in the universe.
Researchers would like to use hydrogen antiatoms to test theories of the universe, says John Eades, a physicist at the European Laboratory for Particle Physics (CERN), where a small amount of antihydrogen was first produced last fall.
For instance, why is the universe, which in its earliest moments is thought to have had equal amounts of matter and antimatter, now dominated by matter? When antimatter and matter combine, they annihilate each other in a burst of energy. If the two were present in equal parts, they should have annihilated each other. Theories explaining matter's triumph over antimatter have been tested with individual particles, but not with complete atoms.
Some researchers have gone further, suggesting that antimatter isn't just for theorists anymore. Gerald Smith, a physicist at Pennsylvania State University, has proposed that antimatter be harnessed for uses ranging from medical imaging to space propulsion. Such ideas still raise eyebrows among many physicists. But, Dr. Smith says, "the taxpayers ought to get some return for their investment" in antimatter research. He and his colleagues described concepts for antimatter engines in spacecraft at a July meeting of the American Institute for Aeronautics and Astronautics.
Perhaps in response to naysayers, the team has given the idea the upbeat name ICAN II. They envision a 400-metric-ton craft that could take a crew to Mars and back in four months, including a 30-day stay at the planet, or in a smaller version, could put a 100-ton scientific payload in orbit around Pluto in three years.
But far more basic research is still required before scientists know whether antimatter is really suitable for such a purpose.
For now, scientists have enough to do just to measure the basic properties of antimatter. Under the laws of physics as understood today, hydrogen and antihydrogen should obey those laws in precisely the same way, Dr. Eades says. "Even a minute difference ... would make a phenomenal difference over cosmic time scales" in how the universe evolved.
Physicists would like to examine such minute properties of antihydrogen atoms, but first, they have to make them. Then they have to hold onto them long enough to study them.
Fermilab's recipe for antihydrogen was devised about 4-1/2 years ago by Charles Munger, a scientist at the Stanford Linear Accelerator in Palo Alto, Calif., who is part of the antihydrogen team working at Fermilab. He suggested that if you shine a tightly focused beam of antiprotons through a thin layer of hydrogen gas, some of the antiprotons would collide with hydrogen atoms and produce, among other things, positrons.
If some of those positrons happened to be traveling in the same direction and at the same speed as some antiprotons in the beam, and if the two were close enough, they could bond to form an antihydrogen atom.
Fermilab's recipe is simpler than that used by CERN, which tried a similar approach, but using xenon gas rather than hydrogen as a target, to produce nine antihydrogen atoms during a two-week experiment last fall.
Yet antihydrogen atoms produced so far at CERN and Fermilab move much too fast and break up too quickly for scientists to study them in any detail.
So CERN scientists have proposed building a device to make antihydrogen atoms using leftover parts from the center's low-energy antiproton ring (LEAR), which was shut down last month.
Slowing the antimatter particles down is no small feat. When the positrons are created, they are whipping around a particle accelerator at nearly the speed of light. Their energy level must be dropped to about 4 degrees above absolute zero. When the positrons and antiprotons form and combine at that temperature, the atoms' movements would be slow enough to allow researchers to study them with lasers.
If experiments detect even the tiniest deviation compared with regular hydrogen, says Harvard physicist Gerald Gabrielse, who has designed devices to cool and combine the antiparticles, "You'll see a major flurry of activity in physics."
CERN officials have approved the approach, but because the lab is focusing on building the world's most powerful particle accelerator, called the Large Hadron Collider, funding for the new antimatter experiment must come from outside sources.
In a last-minute experiment in the 10 days before LEAR's shutdown, Dr. Galbrielse's team was able to trap and cool positrons and antiprotons and nudge them into the same vicinity. Although no antihydrogen formed, he says, the attempt has given the team additional confidence that the chilly approach to making antihydrogen will work.
In the meantime, LEAR's closing leaves Fermilab as the only production site for antihydrogen in the universe - for now.