The humility of quantum physics

The unusual philosophical approach behind one of the great scientific theories

Quantum physics occupies a somewhat enigmatic place in our scientific consciousness. It is often described as mind-bogglingly weird, wacky, spooky, strange and confusing. There have been various attempts to discredit it, most notably by Albert Einstein. For many scientists, it feels incomplete as the details of how it works physically are stubbornly unexplainable.

And yet, “quantum mechanics is the most accurate theory we have to describe the world”. Theoretic predictions match experimental data incredibly precisely - up to 10 decimal places. Only one or two other scientific theories come remotely close to this accuracy.

So we have a theory that works, but understanding it seems beyond us. To use the term we have defined elsewhere, we are forced into a position of epistemic humility: we don’t actually know how quantum physics works. This position, however, is not so surprising as a strong epistemic humility is an assumption built in to the structure and equations of quantum physics. Thus while many may feel that the unexplainability of quantum physics is some kind of flaw, it is more accurate to see it as an essential feature.

As mentioned in a previous post, there are situations where assuming epistemic humility (that there are things we can't know) can help deliver far greater knowledge down the track. We will show how quantum physics is one such example.

Some history

To see how this works, we first need to understand some of its history. Quantum physics can trace its main ideas to centuries of scientific debate about the nature of light. To simplify, there were two competing theories about how light is transmitted: it had to be either a wave - analogous to what we see on the ocean - or a particle - analogous to a bullet or an arrow. Understood physically, these two methods of transmission are fundamentally different. If light (or any energy) is transmitted as a particle, it means that some thing travels from A to B. If it transmitted as a wave, no thing travels but energy travels by moving whatever it travels through: the ocean goes up and down while the wave travels sideways.

James Maxwell seemed to resolve this question when he published his four famous equations that defined the basis of electromagnetism in 1865. A corollary of these equations was that electromagnetic waves traveled at the speed of light, hence light had to be a wave. This explanation worked brilliantly, but not perfectly. Some puzzles remained, one of which was the photo-electric effect. Albert Einstein provided a neat scientific explanation of this in 1905 that built on work by Max Planck and, contrary to the dominant thinking, assumed that light behaved as discrete particles. This theory was confirmed experimentally in 1916 and Einstein received his Nobel Prize primarily for his work on the photo-electric effect, not on relativity.

This re-opened the puzzle: was light a wave that somehow sometimes behaved as a particle, or vice versa? Or something else?

Duality

The defining conceptual breakthrough that allowed the development of quantum mechanics feels rather like a fudge, or an easy way out. Louis de Broglie, effectively, gave up on resolving the debate and argued in 1924 that both light and matter can each be both a wave and a particle. He backed this up with a mathematical equivalence between wave and particle properties.

Wave-particle duality, as this idea is known, is foundational to quantum physics and has significant experimental confirmation - most clearly and famously through various double slit experiments. Interestingly, both light and elementary particles act like either waves or particles in different situations. But any attempt to detect how the shifts between these two states happens itself changes the situation and prevents us being able to understand how the shift occurs.

If we look at this breakthrough philosophically, de Broglie essentially admitted a profound epistemic humility and accepted that human cognition isn't completely up to the task. The sub-atomic world behaves in ways that our concepts or cognition aren't really able to grasp.

Uncertainty

Wave-particle duality was quickly followed by a second mathematical statement of epistemic humility: Heisenberg's uncertainty principle. Introduced in 1927 by Werner Heisenberg, it states (we’ll use plainer language) that the more precisely we know where some particle is, the less precisely we can predict its speed, and vice versa. So we can either know where it is extremely precisely, or how fast it is going (including direction) very precisely, but not both. While the mathematical limit on this is extremely tiny,¹ it is an explicit limit on what we can know. Versions of this uncertainty principle or relation are central to, or derivable from, all core statements of quantum physics.

Thus, at the centre of quantum physics, are two assumptions of epistemic humility - two assumed limits to what we can know. This has left us with an extremely interesting epistemic situation. We have a mathematical theory that works incredibly well, but there is no agreed conceptual interpretation of what the theory means physically. To give a sense of the state of play, Wikipedia lists 15 different influential interpretations, and none of them are definitive.

Limits to knowledge

Whichever of these interpretations, if any, is correct, the epistemic limits built into quantum physics mean that an experiment to decide between them is currently impossible. There may possibly be some other theory that replaces quantum physics and allows us to transcend the epistemic limits, but currently there are no likely candidates. Thus we are in the situation where we can describe the world at the quantum level with mathematics, but we can't understand it conceptually.

This is clear evidence for a broader principle: human knowledge has limits and there are some things we cannot actually know. In other words, this is evidence for a greater epistemic humility, at least in certain circumstances.

What is particularly interesting about quantum physics is that it assumed principles of epistemic humility in the foundation of the theory and then turned out to be the most accurate scientific theory we have developed. In this case, at least, by starting from a position of humility, it was possible to end up knowing more than if we presumed greater epistemic confidence.

We will explore this idea, that epistemic humility is a productive approach to building greater knowledge, in the future including how it might apply more practically for each of us every day. It does also suggest a way forward for various areas of research that have found themselves in dead ends. Perhaps the key is to identify what we can't know or resolve - and see if a theory based on not trying to resolve those things works.

For all of us who aren't at the cutting edge of any research, the example of quantum mechanics may be an inspiration: sometimes admitting we don't or can't know something is the starting point to discovering a whole lot more than we otherwise would have.

1. It is related to the ‘reduced Planck's constant’, which equals 1.054 x 10^(-34)Js.