Wednesday, October 16, 2013

Light can break Newton's third law – by cheating

Running circles around Newton

Isaac Newton just got cheated. Laser pulses have been made to accelerate themselves around loops of optical fibre, seeming to break the physicist's law that every action must have an equal and opposite reaction. The work exploits a trick with light that only makes it appear to have mass, so it is a bit of a cheat, but it may one day lead to faster electronics and more reliable communications.

According to Newton's third law of motion, when one billiard ball strikes another, the two balls should bounce away from each other. But if one of the billiard balls had a negative mass, then when the two balls collide they will accelerate in the same direction. This effect could be useful in a diametric drive, a speculative "engine" in which negative and positive mass interact to accelerate forever. NASA explored using the effect in the 1990s in a bid to make a diametric drive for better spacecraft propulsion. But there was a very big fly in the ointment: quantum mechanics states that matter cannot have a negative mass. Even antimatter, made of particles with the opposite charge and spin to their normal matter counterparts, has positive mass.

"Writing a negative mass in quantum field theory doesn't make any difference," says Archil Kobakhidze at the University of Sydney, Australia. The equations involve terms that are always squares of mass, so any negative mass will become positive anyway. "It has no observable meaning."

For the rest of the story: http://www.newscientist.com/article/dn24411#.Ul7uF1Pwrs3

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