January 3, 2020


The world is not enough: Guessing at the game God is playing (Samuel Graydon, January 3, 2020, TLS)

Photons have the same discrete energy, which depends on the colour of the light. The brighter the light the more photons, but their energy stays the same. "After a hundred years of thinking of light as a wave, physicists had to start thinking of it as a particle." It was for this work, and not for the theory of relativity, that Einstein was awarded the Nobel Prize.

It's no secret, however, that light does act like a wave as well - which is very much where the questions start. And, in fact, wave-particle duality exists not just as a phenomenon of light, but, seemingly, of all matter as well. One of the most famous experiments in all quantum theory is the double slit experiment. It relies on diffraction, which most of us are familiar with from school. Pass a water wave through two slits close to each other and ripples will propagate from them both; as the two sets of ripples encounter one another you will see a pattern of constructive and destructive interference between them, as they either enhance or attenuate the oscillations. The same is true of other at least nominally more corpuscular things. Fire a beam of electrons towards two slits and you will also see peaks and troughs spread out on the wall behind them.

Moreover, if you fire one electron at a time through the slits, so that they cannot interfere with any other particle, you still get peaks and troughs at the other end. An interference pattern emerges over time as you fire electrons one by one through the slits, though individually they arrive on the other side of the slits in a defined place. We are forced to conclude that electrons can interfere with themselves like waves. They pass through both slits at once.

But this isn't the strangest thing about the experiment. Every attempt - every attempt - to measure an electron passing through the slits has destroyed the interference pattern, and produced a definitive answer to which path the electron took. Without a measurement there is concrete evidence for a wave-like nature; but as soon as one is made, however it is made, all evidence of it disappears and we are just left with particles.

This seems baffling - or at least it should. Niels Bohr, one of the fathers, and for a good time high priests, of quantum mechanics, is said to have given a talk on the subject to a group of philosophers, and was disappointed when they simply accepted what he told them about it, instead of protesting vehemently: "If a man does not feel dizzy", he said, "when he first learns about the quantum of action, then he has not understood a word". While it may seem complicated, it is perfectly possible to explain what goes on in the subatomic world, what happens when you do this or that, and in fact, once you have the knack, it can feel quite intuitive - but nobody knows why things behave the way they do. For this reason, in 1964, the same year he was awarded the Nobel Prize in Physics for his work on quantum mechanics, Richard Feynman could tell a lecture hall: "I think I can safely say that nobody understands quantum mechanics".

Once physicists staked out a position of incomprehensibility, intellectuals of all stripes followed.
Posted by at January 3, 2020 7:32 AM