March 12, 2008 -- It may sound like something out of an X-Men movie, but scientists have figured out how to stop a bullet, albeit a very tiny one, in mid-air.
Researchers at the University of Texas-Austin have built a so-called coilgun that works in reverse, stopping atom- and molecule-sized bullets in flight. The research could eventually help measure the mass of one of the most elusive and ubiquitous particles in the universe, the neutrino.
"What motivated this was weapons research," said Mark Raizen, a professor of physics at the University of Texas and one of the study's authors. "It's not often that weapons technology leads to basic research."
Coilguns are a standard feature of science fiction, but attempts by the military to make such a weapon practical have yet to succeed.
Conventional guns use chemical explosives to create hot gas, which propels lead bullets down a barrel.
A coilgun accelerates a magnetic projectile, usually iron-based, with a series of coiled wires that create a strong magnetic field. There is no physical contact between the projectile and the coils.
Only a few materials the size of real bullets are magnetic, but "when you look at individual atoms, just about everything is magnetic," said Raizen.
The coilgun consists of 64 hand-made units (about $25 each) and is powered by its own capacitor.
Here's how it works: The researchers trap particles from the air, store them in a tiny chamber, and release them in the direction of the gun. When a particle reaches the gun and encounters the coils, each coil's magnetic field progressively slows the particle down without touching it.
The coilgun brought atoms and molecules traveling at 500 meters per second (1,118 mph) to a compete stop.
The work builds on a previous 18-stage coilgun Raizen and his group built in 2007.
"Ultimately this is a very simple solution on how to stop almost any atom," he said.
The project's eventual goal is to weigh neutrinos, which are very tiny and devilishly slippery.
By capturing an atom of tritium, the heaviest form of hydrogen, the scientists hope to weighh neutrinos that spit out as the atom decays. Scientists have so far been unable to capture tritium.
"I think this research is fantastic," said Marlan Scully, a physicist at Texas A & M University who was not involved in the work. "It has given us a new way to cool atoms and put us into the game of cooling hydrogen."
"This could be a great tool to help us understand the basic science of atoms," he added.
The study appeared last week in the journal Physical Review Letters.
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