Is it possible to think about quantum physics?

Quantum physics is at the frontier of understanding. One of the great unanswered questions is the search for a unified explanation of things at the subatomic scale and the behaviour of the universe that we perceive with our senses.

 At the same time, the tools that physicists use to investigate and think about this problem are so expensive – think of the Large Hadron Collider – and so esoteric – the most advanced mathematics – that ordinary people like me struggle even to understand the problem. If one can't think about quantum phenomena in terms of the large scale world, how can they ever be joined up? Can one, indeed, even think about this without the advanced mathematics?  That is the challenge.

One needs to start from the beginning with the two slit experiment that led people to believe, for a hundred years, that light was a wave. It showed that when light passed through two narrow slits it didn’t behave like a succession of particles. It showed interference patterns like those where the crests and troughs of two waves cancelled each other out.

It was only early in the twentieth century that a more sophisticated version of the experiment showed that light actually traveled as discrete particles – photons – and that, if one monitored which slit individual photons passed through, then the interference patterns disappeared. This is really strange but there’s no point in my spending a lot of time explaining this because YouTube already has many excellent videos on the subject. Take a look at Jim Khalili (shorter) and Richard Feynman (an hour) doing the honours.

My take on this is:

1. Photons, electrons, atoms etc have some characteristics of particles and some of waves.
2. When they are observed they behave like particles
3. When not observed, two slits are treated as a wave treats them and generate interference.

Which takes one back to de Broglie (cited by J S Bell [1]) who, ‘in 1926 … answered the conundrum wave or particle by wave and particle’. The wavelike aspect, however, is always inferred. All observations end up being of particles. Whenever quantum entities interact with anything, the wave seems to disappear and only the particle remains.

Perhaps the answer is that quantum entities propagate as waves but only interact as particles.

This seems to me an interesting thought, especially in relation to the idea that quanta have fixed sizes because, as soon as an interaction occurred, the whole quantum of energy in the entity would have to be localised so the wave would immediately have to disappear. As soon as the entity interacts, its potential to be somewhere other than the point of interaction expires. So measuring the exit of an entity at a slit would immediately remove any residue that might emerge from the other slit to interfere with it positively or negatively.

This interpretation generates a prediction which may well have been tested already:

• In the case where the entity has been identified coming through one or other slit, it would continue on its way as a wave so it would be possible to pass it through a second pair of slits and show an interference pattern on the other side. 

It also allows us to think about the probabilistic nature of quantum mechanics. One can think of the characteristics of the particle as not being determined purely by the entity itself but as characteristics of the interaction. A spinning coin is neither heads not tails until it lands on the table. Similarly, the spin (for example) of the resulting particle is not in the wave but emerges as it interacts with the external world. So no amount of knowledge about the earlier state of the entity would enable its prediction and no need of an ‘intelligent observer’, just an interaction.

[1] Speakable and unspeakable in quantum mechanics – J S Bell (1987) p. 171