What is the ultimate nature of reality? 

Are quantum effects constantly carving us into innumerable copies, each copy inhabiting a different version of the universe? Or do all those other worlds pop out of existence as mere might-have-beens? Do our particles surf on quantum waves? Or are we ultimately made of the quantum waves alone? Or do the waves merely represent how much information we could possess about the state of the world? And if the waves are just a kind of information, information about what? Or is the information all that there is—and all that we are? 

Those are the kind of questions in play when a physicist tackles the dry-sounding issue of, “what is the correct interpretation of quantum mechanics?” About 80 years after the original flowering of quantum theory, physicists still don’t agree on an answer. 

And although quantum mechanics is primarily the physics of the very small—of atoms, electrons, photons and other such particles—the world is made up of those particles. If their individual reality is radically different from what we imagine then surely so too is the reality of the pebbles, people and planets that they make up. 

As recounted by our December article, The Many Worlds of Hugh Everett by journalist Peter Byrne, 50 years ago the iconoclastic physics student Hugh Everett introduced the idea that quantum physics is incessantly splitting the universe into alternate branches. Byrne’s article talks about Everett’s life (did you know his son is the lead singer of the rock band Eels?) as well as about his theory and the “Copenhagen Interpretation” he aimed to supplant. But many other interpretations of quantum mechanics exist, and today Copenhagenists have more subtle variants to choose from than the one that Everett once called “a philosophic monstrosity.” Here is an all-too-short run-down on some of them. 

The basic scenario an interpretation must address is when a quantum system is prepared in a combination of states known as a superposition. For example, a particle can be at both location A and B, or in the infamous thought experiment, Schrödinger’s quantum cat can be alive and dead at the same time. The problem is that when we observe or measure a superposition, we get but one result: our detector reports either “A” or “B,” not both; the cat would appear either very alive   or very dead. 

Copenhagen Interpretation 
This interpretation (or variants of it) has long been the party line for quantum physicists. The Schrödinger equation describes how a wave function evolves smoothly and continuously over time, up until the point when our big, clunky measuring apparatus intervenes. The wave function enables us to predict, say, there’s a 60% probability we’ll detect the particle at location A. After we detect it at A or B, we have to represent the particle with a new wave function that conforms with the measurement result. 

What bothers some people about this interpretation is the random, abrupt change in the wave function, which violates the Schrödinger equation, the very heart of quantum mechanics. Everett argued that this approach was philosophically a mess: it used two contradictory conceptual schemes to describe reality, the quantum one of wave functions and the classical one of us and our apparatus. 

Many Worlds Interpretation 
Everett’s theory. Also known as the relative state formulation. 

The superposition of the particle spreads to the apparatus, and to us looking at the apparatus, and ultimately to the entire universe. The components of the resulting superposition are like parallel universes: in one we see outcome A, in another we see outcome B. All the branches coexist simultaneously, but because they are completely non-interacting the “A” copy of us is completely unaware of the “B” copy and vice versa. Mathematically, this universal superposition is what the Schrödinger equation predicts if you describe the whole universe with a wave function.