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  The parity of a system, in physics, is its handedness, or symmetry. An object and its mirror image have opposite parity, as do a left and a right hand.

When a system conserves parity, for every object within the system, there can also exist a similar object with opposite parity. This is the same as saying that every object has a mirror image which would prove physically possible to construct.

The world as we know it appears to conserve parity. It is almost inconceivable that there could exist an object whose mirror image would be impossible to create. And yet, subtle experiments have shown us that this is in fact the case. The universe exhibits a slight, but measurable asymmetry; it does not conserve parity.

This asymetry may be measured in various ways. It only occurs in processes involving the weak force. The most obvious example is the neutrino. This tiny, neutral particle only exists in a ‘left-handed’ state; that is, its direction of spin is determined by its direction of motion. If a neutrino is coming towards you, its spin is always clockwise. It can be seen that this does not conserve parity, for if we looked at the neutrino in a mirror, its spin would be reversed but its direction of travel would not. The mirror image, a neutrino with a right-handed spin, is never found in reality.

Another example is atomic parity non-conservation. The weak force at work within the atom produces small, but measurable, effects, which may be seen by passing polarised light through a gas of atoms.

The plane of polarization of the light undergoes a small rotation during its passage through the gas. However, the rotation is always in the same direction, and no system may be set up which looks like the mirror image of the experiment. Thus again, parity is not conserved. JJ

See also weak force/strong force.



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