Here’s a thought experiment: Imagine astronomers didn’t really believe that Earth orbits the sun or that our world turns daily on its axis. What if they viewed the heliocentric model of the solar system merely as an abstract mathematical tool to track planets and stars with great precision, not as a literal description of the way things are? What if they claimed we can’t truly know whether the sun orbits Earth or vice versa and, moreover, that such questions were not even worth asking?
It would be preposterous. No respectable scientist would ever entertain such notions — except when it comes to the most powerful theory in the history of physics: quantum mechanics. More than a century after its birth, quantum mechanics, the physics of atoms, photons and other particles, remains as baffling as ever. Experiments have repeatedly confirmed the theory’s weird predictions with phenomenal accuracy — to a dozen or more decimal places in some cases. Technologies derived from it drive the world’s economy: The electronics industry as we know it wouldn’t exist without quantum mechanics. It explains why the sky is blue and how stars generate their light.
And yet, despite the theory’s unquestioned dominance and practical significance, physicists still don’t agree on what it means or what it says about the nature of reality. Some physicists deny that quantum mechanics describes any sort of objective reality.
At least a dozen interpretations of quantum mechanics vie for physicists’ hearts and minds, each with a radically different take on reality. Adán Cabello, a physicist at the University of Seville in Spain, recently summed up the confusing, incompatible gaggle of viewpoints as “a map of madness.”
There’s the Many Worlds model, which posits the existence of innumerable parallel realities. If that seems a tad extravagant, you might prefer QBism (pronounced “cubism”), where the quantum world and the scientists who observe it are inextricably bound together in an unpredictable, interactive universe. The central issue is that physicists don’t know what the most basic equation of quantum theory — a mathematical formulation called the wave function — actually represents. Does it describe a fundamental feature of the physical world? Or is it instead just a handy way to predict experimental results?
