Gun stops bullets in mid-air
Scientists have figured out how to stop a bullet, albeit a very tiny one, in mid-air.
US researchers at the University of Texas at Austin have built a so-called coilgun that works in reverse, stopping atom- and molecule-sized bullets in flight.
The research, which sounds like something from an X-Men movie, could eventually help measure the mass of one of the most elusive and ubiquitous particles in the universe, the neutrino.
The study appears in the journal Physical Review Letters.
"What motivated this was weapons research," says Mark Raizen, a professor of physics 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 the military's attempts to make such weapons practical have yet to succeed.
Conventional guns use chemical explosives to create hot gas, which propels lead bullets down a barrel.
But 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," says Raizen.
The coilgun consists of 64 hand-made units and is powered by its own capacitor.
The researchers trap particles from the air, store them in a tiny chamber and release them towards the gun.
When a particle reaches the gun and encounters the coils, each coil's magnetic field progressively slows down the particle without touching it.
The coilgun brings atoms and molecules travelling at 500 metres per second 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 says.
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 weigh neutrinos that spit out as the atom decays. Scientists have so far been unable to capture tritium.
"I think this research is fantastic," says Professor 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."