The American physicist Richard Feynman thought that “nobody understands quantum mechanics”. That is no longer true. Smartphones, nuclear plants, medical scans and laser-operated doors have been built with insights from the physics that governs the subatomic level. What perplexes many is that the quantum world is governed by rules that run counter to classical notions of physical laws.
In quantum mechanics, nature is not deterministic. Subatomic particles do not travel a path that can be plotted. It is possible only to calculate the probability of finding these specks at a particular point. Where such calculations leave physics, that hardest of the hard sciences, has troubled its greatest minds. Albert Einstein thought the idea that an element of chance lay deep in science was absurd. “God does not play dice,” he famously declared.
Physics is full of predictions that could be confirmed or denied once the technology to examine them had caught up. Einstein was proved wrong. In his new book, Helgoland, the Italian theoretical physicist Carlo Rovelli narrates how a scientific revolution was started by a young German physicist, Werner Heisenberg. He first devised quantum theory during a summer holiday in 1925 spent on the barren North Sea island of the book’s name.
The world, thought Heisenberg, could not be stated exactly, merely known through models of uncertainty and probability. He won a Nobel prize in 1932, though his achievements were tarnished by tacit support of Nazi Germany. The theory was that the world people experience is decided upon when many possibilities of the quantum world collapse to become the certainty of the classical one. This led to Erwin Schrödinger’s cat-in-a-box thought experiment. Quantum theory suggested that only by opening the container could it be determined if the feline was dead or alive. If the box remains closed the unfortunate cat is in limbo – in a state between life and death, a superposition of possibilities.
Prof Rovelli dismantles attempts to explain away the indeterminacy of quantum mechanics. First, he takes on the “many worlds” thesis, which claims that every possible alternative exists and we just see one of them. In short, Schrödinger’s cat is alive in one universe and dead in another. Some claim that Heisenberg’s work would collapse for some as yet undiscovered macroscopic entity. In this explanation, the cat is too big to be subject to quantum physics. More recently, it has been argued that quantum systems do have definite properties; we just do not know enough about those systems to precisely predict their behaviour. But in Helgoland, this is dismissed as an attempt to return to a pre-1920s view.
Quantum theory, Prof Rovelli says, views the “physical world as a net of relations. Objects are its nodes.” In his “relational” interpretation, Schrödinger’s cat has properties only when it interacts with something else. When it is not interacting, it has no properties. Prof Rovelli reaches for Buddhist thought to explain his ideas. He claims that if nothing exists in itself, surely everything exists solely through dependence. “Facts are relative,” he writes, opening up a debate that is likely to last longer than the century of argument that it seeks to close.