Noticing the obvious

Electrical Neutrality

An artistic visualisation of the concept of equality of positive and negative electrical charges.

I tidied my office last week, and flicking through my pile of ‘papers to read later’ I came upon:

The electrical neutral neutrality of atoms and of bulk matter
C. S. Unnikrishnan and C.T. Gillies
Metrologia 41 (2004) S125-S135

The basic question asked in the paper was:

  • Is the electrical charge on the electron equal in magnitude to the electrical charge on the proton?

Or alternatively:

  • Is bulk matter electrically neutral?

What struck me about the paper was ‘Why had I never asked either of these questions myself?“. I think it is because I had somehow thought the answers were ‘obvious’ or ‘self-evident’, but in fact they are neither. They are -IMHO – bloody good questions!

Ultimately the questions are experimental, and a number of clever techniques have been used to provide the answers. So far we know that the charge on the electron and the charge on the proton are equal in magnitude to within 1 part in 1021. Wow! I don’t know of any other two experimental numbers that are known to be equal with that degree of precision.

Electrons have a completely different internal structure to protons and neutrons. Electrons are part of class of particles called leptons (‘light’* ones) which as far as we know have no internal structure. Protons and neutrons are part of class of particles called baryons (‘heavy’** ones)  and we know they are in some sense composed of quarks. Each proton or neutron is composed of three quarks.

Given the completely different structure of each type of particle, the equality of the magnitude of the electric charge on each goes from being ‘remarkable’  to ‘astounding’! And even though we  have no fundamental explanation of what electric charge is, it is shocking to find that two types of particle with completely different structures should have exactly the same amount of it – whatever ‘it‘ is. In fact, I refuse to believe it. It seems inevitable that we we will eventually discover some tiny difference between the magnitude of the electric charge on the proton and the electron.

What would the consequences of such a discovery be? Unnikrishnan and Gillies covers these in some detail but two points caught my attention. The first was a comment by Einstein noticing that if normal matter were not exactly neutral, then when it flowed, it could generate a magnetic field.

The Earth and Sun have magnetic fields, the orientation of which stand in approximate relationship with the axes of rotation… But is hard to imagine that electrical conduction or convection currents of sufficient magnitudes are really present… It rather looks as if the cyclic motions of neutral matter are producing magnetic fields.

And the second was an empirical law proposed by Schuster who noticed that the strength of the magnetic field around a range of planets and galaxies was linked to their respective angular momenta. Could that be evidence that bulk matter is not perfectly neutral?

Now these suggestions are highly speculative, and I am sure that people cleverer than I have worked out all kinds of reasons why they do, or do not, make sense. But whether or not they turn out to be true, I feel like I have had my sense of what is ‘normal’ re-adjusted. I am left with the sense that something I experience everyday – the neutrality of bulk matter – is not in any sense obvious.


* Light in the sense of low mass rather than in the sense of electromagnetic radiation

Heavy in the sense of high mass rather than in the sense of really serious


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