Nothing’s wrong with antimatter, new experiment confirms
In science fiction, antiparticles provide the power for warp drives. Some physicists have speculated that antiparticles are being repelled by gravity or even traveling backward in time.
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• DENNIS OVERBYE
NEW YORK: Antimatter just lost a little more pizazz. Physicists know that for every fundamental particle in nature there is an antiparticle — an evil twin of identical mass but endowed with equal and opposite characteristics like charge and spin. When these twins meet, they obliterate each other, releasing a flash of energy on contact.
In science fiction, antiparticles provide the power for warp drives. Some physicists have speculated that antiparticles are being repelled by gravity or even traveling backward in time.
A new experiment at CERN, the European Center for Nuclear Research, brings some of that speculation back down to Earth. In a gravitational field, it turns out, antiparticles fall just like the rest of us. “The bottom line is that there’s no free lunch, and we’re not going to be able to levitate using antimatter,” said Joel Fajans of the University of California, Berkeley.
Dr. Fajans was part of an international team known as ALPHA, the Antihydrogen Laser Physics Apparatus collaboration, which is based at CERN and led by Jeffrey Hangst, a particle physicist at Aarhus University in Denmark. Dr. Fajans and his colleagues assembled about 100 anti-atoms of hydrogen and suspended them in a magnetic field. When the field was slowly ramped down, the anti-hydrogen atoms drifted down like maple leaves in October and at the same rate of downward acceleration, or g force, as regular atoms: about 32 feet per second per second. They published their result on Wednesday in the journal Nature.
Few physicists were surprised by the result. According to Einstein’s theory of general relativity, all forms of matter and energy respond equally to gravity. “If you walk down the halls of this department and ask the physicists, they would all say that this result is not the least bit surprising,” Jonathan Wurtele, a physicist at the University of California, Berkeley, said in an announcement issued by the university. It was he who first suggested the experiment to Dr. Fajans a decade ago. “That’s the reality,” Dr. Wurtele said.
“But most of them will also say that the experiment had to be done because you never can be sure,” he added. “The opposite result would have had big implications.” In 1928, in one of the most astonishing examples of nature following math, the physicist Paul Dirac found that a quantum mechanical equation describing the electron had two solutions. In one, the electron was negatively charged; this particle is the workhorse of chemistry and electricity. In the other solution, the particle was positively charged.
What was that particle? Dirac thought it was the proton, but J. Robert Oppenheimer, later famous for the atomic bomb, suggested it was a brand-new particle: a positron, identical to an electron but with a positive charge and spin. Two years later Carl Anderson, of the California Institute of Technology, detected positrons in cosmic ray showers, a discovery that earned him a Nobel Prize in Physics.
And so the lure of antimatter was born. In principle, there could be entire antiworlds inhabited by antibeings. The joke goes that if you met your antiself, that person would stick out a left hand to shake, but you had better not take it or you would both blow up.
