Q:

Is it necessary that two bodies must posses unlike charge for their attraction?And Is it necessary that two bodies must posses like charges for their repulsion?

- Varun (age 15)

India

- Varun (age 15)

India

A:

Hello Varun,

This is a very interesting question. The answer depends on what kind of charge you are talking about. In the case of electrical charge the usual answer you have been taught in elementary school is correct; like charges always repel each other and unlike always charges always attract. There are, however, different kinds of forces that act differently. For example you might ask if the gravitational force between anti-matter and ordinary matter is attractive or repulsive?'. In this case the answer is that both are attractive, matter-matter and matter-antimatter. Sensitive experiments at the CERN accelerator have shown that beams of neutrons and anti-neutrons both fall downward in the earth's gravitational field. Similarly, the strong nuclear forces between quark-quark pairs and quark-antiquark pairs are both attractive.

This always puzzled me so one day I asked my theoretical guru why. His answer was quite technical involving quantum field theory, but it came down to the fact that if the force carriers, photons in the case of electrical force, have an odd numbered value of intrinsic spin then there will be a repulsive force between like-charged particles. The If the force carrier's spin is an an even number then both like and un-like particles will be attracted to each other. The spin I am talking about is the intrinsic angular moment of the carrier. It is zero for the strong nuclear case where the force is carried by spin zero gluons, one for the electrical charge, and two for the gravity case where the force is carried by spin two gravitons. The spin units are Plank's constant divided by 2 pi.

LeeH

*(published on 06/06/2013)*