Scientists have made light appear to break Newton’s third law

: GrrlScientist on Flickr http://www.flickr.com/photos/30540563@N08/8558315041/ (creative commons licence)

Laser pulses have been made to accelerate themselves around loops of optical fibre-  which seems to go against Newton’s 3rd law. This states that for every action there is an equal and opposite reaction. This new research exploits a loophole with light that makes it appear to have mass.

Under Newton’s third law of motion, if we imagine one billiard ball striking another upon a pool table, the two balls will bounce away from each other. If one of the billiard balls had a negative mass, then the collision of the two balls would result in them accelerating in the same direction. This effect could be used in a diametric drive, where negative and positive mass interact for a continuously propulsive effect. Such a drive also relies on the assumption that negative mass has negative inertia.

Quantum mechanics however states that matter cannot have a negative mass. Negative mass is not the same as antimatter, as even antimatter has positive mass. Negative mass is a hypothetical concept of matter where mass is of opposite sign to the mass of normal matter. Negative mass is used in speculative theories, such as the construction of wormholes. Should such matter exist, it would violate one or more energy conditions and show strange properties. No material object has ever been found that can be shown by experiment to have a negative mass.

Experimental physicist Ulf Peschel and his colleagues at the University of Erlangen-Nuremberg in Germany have now made a diametric drive using effective mass.. Photons travelling at the speed of light have no rest mass. Shining pulses of light into layered materials like crystals means some of the photons can be reflected backwards by one layer and forwards by another. This delays part of the pulse and interferes with the rest of the pulse as it passes more slowly through the material.

When a material such as layered crystals slows the speed of the light pulse in proportion to its energy, it is behaving as if it has mass. This is called effective mass, which is the mass that a particle appears to have when responding to forces. Light pulses can have a negative effective mass depending on the shape of their light waves and the structure of the crystal material that the light waves are passing through. To get a pulse to interact with material with a positive effective mass means finding a crystal that is so long that it can absorb the light before different pulses show a diametric drive effect.

Peschel therefore created a series of laser pulses in two loops of fibre-optic cable to get around these requirements. The pulses were split between the loops at a contact point and the light kept moving around each light in the same direction. The key to the experiment was having one loop slightly longer than the other. This meant light going around the longer loop is relatively delayed, as shown by the diagram.

When the light completes a circuit and splits at the contact point, some of its photons are shared with pulses within the other loop. After a few circuits, the pulses develop an interference pattern that gives them effective mass.

The team were thus able to create pulses with both positive and negative effective mass. When the opposing pulses interacted in the loops, they accelerated in the same direction and moved past the detectors a little bit earlier after each trip. The loops are essentially the equivalent of having extremely long crystals.

As electrons in semiconductors can also have effective mass, loops could be used to speed them up and boost computers’ processing power. The loops could also be used to control a fibre’s colour output. It is hoped this will also lead to faster electronics as well as more reliable communications.

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