In the sixties, the physicist Richard Feynman wrote, “I think I can safely say that nobody understands quantum mechanics.” Today, the situation hasn’t changed. Sure, physicists *use* quantum mechanics. They used the theory to anticipate the existence of new particles like the Higgs boson, and now they’re harnessing its rules to build technologies like quantum computers. But if you asked physicists what the equations actually say about reality? They wouldn’t be able to definitively answer.

“Physicists tend to treat quantum mechanics like a mindless robot they rely on to perform certain tasks, not as a beloved family member they care about on a personal level,” writes Sean Carroll in the prologue of his new book, *Something Deeply Hidden*, which publishes today. The Caltech physicist thinks that his colleagues have put off thinking about the true meaning of quantum mechanics for too long.

In particular, Carroll objects to the mainstream approach to quantum mechanics that’s known formally as the Copenhagen interpretation, and informally as “shut up and calculate.” Instead, he favors a five-decade-old idea known as Many Worlds, first proposed by physicist Hugh Everett. It describes the universe as a changing set of numbers, known as the wave function, that evolves according to a single equation. According to Many Worlds, the universe continually splits into new branches, to produce multiple versions of ourselves. Carroll thinks that, so far, Many Worlds is the simplest possible explanation of quantum mechanics. WIRED asked him a series of stoner questions about the nature of reality, and he obliged. The interview has been condensed and lightly edited for clarity.

**What is reality?**

**Carroll**: The best answer we can give is that reality is a vector in Hilbert space. This is the technical way of saying that reality is described by a single quantum mechanical wave function.

**OK, that’s abstract. Please conceptualize this?**

We see tables and chairs and people and planets moving through spacetime. Quantum mechanics says that there are no such things as tables and chairs—there’s just something we call a wave function.

Our classical description of the world is a higher level, approximate way of talking about the wave function. The job of physicists and philosophers is to show how, if we live in a world that is just a wave function, then why does it look like there are people and planets and tables and chairs? We don’t have a definite consensus.

**So, let’s talk about Many Worlds. What is it?**

Quantum mechanics says that an electron can be in a superposition of all possible locations. There’s no such thing as the position of an electron. But when you observe the electron, you see it in one location. This is the fundamental mystery of quantum mechanics. Its description when no one is looking is different from what you see.

Many Worlds says, why don’t we just treat you, the observer, as your own quantum mechanical system? You’re made of quantum mechanical particles also. So what happens when you, the observer, looks for the electron? The electron starts in a superposition of many possible locations. When you look, you evolve into a combined system of you and the electron in a superposition. The superposition consists of the electron being here and you seeing it here, plus the electron being there and you seeing it there, and so on. Hugh Everett’s brilliant move was to say that the different parts of the superposition really exist. It’s just that they’re in separate, non-interacting worlds.

**Say you flip a coin. Heads, you get a million dollars—tails, you die. Many Worlds says that once you flip the coin, both worlds are in existence?**