Electric dipole moments are everywhere around us. Most antennas produce electric dipole fields and if you’ve ever held a battery in your hand, you’ve held an object that has an electric dipole moment. Batteries like the D-battery on the left make things like flashlights work because they have an internal separation of charge with an excess of positive charges Q at the top of the battery and a corresponding excess of negative charges -Q at the bottom. When you put a battery in a circuit that has a bulb for example, the excess charges can flow through the circuit and light up the bulb. Whenever you have such a separation of negative and positive charges, you have an electric dipole moment that produces an electric dipole field. Electric dipole moments are proportional to the overall separation d of positive and negative charges as well as proportional to the size of the charge Q that has been separated, in other words they are equal to Q·d. Bigger separation of charges d equals bigger electric dipole moment. Larger excess charges Q equals bigger electric dipole moment.

Even point-like particles like electrons can have an electric dipole moment which is strange since there can be no separation of charges in a particle that has no volume. To create a point-like electric dipole moment from the battery, we have to shrink its size to zero which means setting d to zero. To keep the electric dipole moment from going to zero as you shrink d, you have to simultaneously increase Q in order to keep the product Q·d constant and therefore, keep the electric dipole moment constant as well.

The big deal about subatomic particles like electrons or nuclei having electric dipole moment is that their existence makes the appearance of the universe depend on the direction of time. For example, if particles have electric dipole moments, the energy of these particles depends on the direction of the time arrow. Scientists say that “time-invariance” is violated. It turns out that time-invariance violation is a necessary condition for the appearance of excess matter over anti-matter at the beginning of the universe; that excess of matter is what we and everything we can interact with is made of. This is why scientists are tracking down all possible known sources of time-invariance violation, including subatomic electric dipole moments. Axions are linked to electric dipole moments, and would have been produced in huge quantities at the beginning of the universe making the axion an excellent candidate for cold dark matter.