How to stop a bullet

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.

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