Question: Is light a particle or a wave and what is matter made of?

John Hadden answered on 29 Jun 2020: last edited 29 Jun 2020 1:06 pm

That is a great question.

The answer to the first part is light is something that sometimes acts a bit like a particle (it seems to be like a localised ‘blob’ of energy, and carries momentum), and sometimes acts a bit like a wave (it reflects, diffracts and disperses like waves on the sea).

The tricky thing is that generally when we try to describe things we normally use analogies with human sized things which we understand, and normally we make a big distinction between things which act like a particle (billiard balls, etc) and things which act like waves (waves on the sea). We don’t know of something in our everyday experience which both acts like a particle, and a wave at the same time. So with light we have to think of it as ‘something which sometimes acts like a particle and sometimes like a wave’ – which is a bit of a mouth-full, so instead people talk about ‘wave-particle duality’, and sometimes particle waves. The light particle here is called a photon.

In terms of what matter is made of, it depends a little how small you look. The matter we are made of is small particles called atoms. It turns out these atoms are made up of electrons, neutrons and protons. Each different type of atom has a different number of protons and neutrons – which make the atoms act differently. If you look even closer it turns out that the proton and neutrons are made up of even smaller particles which we call quarks. Quarks, – and photons, and electrons, and a few other particles such as neutrinos which I won’t go into here are what we call fundamental particles – which means we think they are not made up of anything else.

The interesting thing though (returning to the first part of your question), is that it turns out that things that we thought were just particles (such as electrons, and protons and neutrons in atoms) actually also sometimes act like waves. It turns out that actually everything can potentially have both particle like and wavelike behaviour, but that the wavelike behaviour is only observable for REALLY small things – like electrons, protons, neutrons etc.

I guess the one thing we have learned about small things is that they act very differently to what we are used to in our everyday experience. As physicists/scientist, the struggle is to find out the new small scale and to use them to be able to understand how things fit together into the large scale world we know around us.