Light and physics

Where the turn in Western philosophy is attributed to Kant with Descartes firing the starting shot, the start of the scientific revolution is associated with Copernicus, who moved from an earth-centric to a sun-centric view. It can be argued that Descartes’ contribution is at least as important — the mind-body split created a mechanistic world that was ‘written in the language of mathematics’ to put it in the words of Galileo. No need for purpose or intent, no stones falling because they seek their natural place. The mind-body split freed science to treat the physical world as pure mechanism — no teleology, no hidden intentions, no souls in stones.

Newton gave this approach its mechanics. Unifying celestial and earthly gravity. Working with particles with definite positions and velocities caused by forces acting between them. Laws that are deterministic, universal. The same everywhere and at every scale. A worldview in absolute space with absolute time. In short, a fully deterministic picture of reality.

There was the contrasting view of Leibniz, who rejected absolute space entirely. In his opinion space is not a container, but the ordering of things that coexist. No space without things. But without a mathematical theory, this relational view remained scientifically inert. It would take another 200 years for the tools to become available.

In the meantime, classical mechanics grew into a rich mathematical framework. Over two centuries Euler, Lagrange, Fourier, Hamilton and many others extended Newton’s toolbox. The approach was spectacularly productive. But it was Laplace that brought the worldview to its logical endpoint — an intellect that knows all positions and forces at one instant could calculate everything, past and future. The clockwork universe.

The crowning achievement of Newton’s classical mechanics and the mathematical toolbox was Maxwell’s electromagnetism. Where Newton had worked with particles and forces, Maxwell worked with fields and waves — unifying electricity, magnetism, and light into a single framework. A triumph of classical physics.

Maxwell’s equations predicted a fixed speed of light, the same for all observers. Waves travelling at constant speed. This inspired Einstein — first came special relativity, where space and time are coupled and localised, then general relativity, which treats gravity as geometry. Absolute spacetime had disappeared, now it was tied to a local observer.

But Maxwell’s theory also exposed a shortcoming. The Rayleigh-Jeans radiation law, derived from classical physics, predicted that energy should increase without limit at high frequencies — the ultraviolet catastrophe. It doesn’t. Planck found that the experimental data only made sense if energy comes in discrete packets — quanta. The smooth, continuous world of classical physics had a grain.

Einstein took this further with the photoelectric effect: light itself comes in quanta. And de Broglie turned it around — if waves behave as particles, particles also behave as waves. This duality made the wave treatment of particles natural, leading to Schrödinger’s wave equation.

But the wave nature brought a consequence nobody expected. Heisenberg showed that position and momentum cannot both be definite at the same time — not a limit of instruments, a feature of nature. A wave with a precise frequency is spread out over all of space; a wave localised in space is a mixture of many frequencies. Once particles are waves, this trade-off is unavoidable. Some properties don’t have definite values until measured.

And what the waves describe became the next question. Schrödinger thought he was describing a real wave; Born read it as a probability amplitude, not certainties. The equation is deterministic; what it describes is probabilistic. Classical mechanics tells you what is there. Quantum mechanics tells you what could be there if you look.

The success of QM was thanks to the impressive results it yielded. But at the same time it created a big interpretative nightmare: a lack of a satisfactory worldview, which persisted for decades.

What is the wave function — a real physical thing, or a tool for calculating probabilities? What happens when you measure — does the wave function collapse, and if so, how? Why do measurements produce definite outcomes when the equations describe superpositions? Bohr and Heisenberg offered complementarity as a response. Einstein rejected the underlying randomness — “God does not play dice.” Bohm proposed hidden variables. Everett proposed that all outcomes happen in branching worlds. No consensus emerged. None has emerged since. “Shut up and calculate.”

At the heart of every interpretation: what is the role of the observer? Is it a privileged role? In classical physics, the observer stands outside the system and reads off its properties - privileged. In quantum mechanics the observer stands inside, is part of the system. What about the privileged role?

The strangeness didn’t stay theoretical. Bell’s theorem showed that quantum correlations violate any classical explanation — and Aspect’s experiments confirmed it. He shared the 2022 Nobel Prize with Clauser and Zeilinger. The questions about what QM means are not just philosophy. They are experimentally real. The whole is not the sum of the parts. Separability fails.

The result is a split that defines modern physics. On one side, spectacular success — atomic spectra, semiconductors, lasers, nuclear physics, three of the four fundamental forces unified within quantum field theory. On the other, an impasse — no agreed picture of reality, no resolution of the observer’s role, and gravity still outside the quantum framework. The most productive theory in history, and no worldview to go with it.

This post is part of the positioning series. See also The Turn in Science.

Photo: Grianghraf / Unsplash