Thread by @NVNECULT

In the 1950s Hugh Everett blew up quantum mechanics with his Many-Worlds theory and physics is only just catching up.

[Thread]

One of the most radical ideas in the history of physics came from an unknown grad student who wrote only 1 paper, got into arguments with physicists across the Atlantic as well as his own advisor, and left academia after graduating without even applying for a job as a professor.

Everett’s research came out of Princeton in the 1950s, under the mentorship of John Archibald Wheeler, who in turn had been mentored by Niels Bohr (the godfather of quantum mechanics).

en.wikipedia.org/wiki/Niels_Bohr

Bohr and his compatriots established what came to be called the ‘Copenhagen Interpretation’ of quantum theory.

In this interpretation, we distinguish between microscopic quantum systems and macroscopic observers.

Quantum systems exist in superpositions of different possible measurement outcomes, called wave functions.

en.wikipedia.org/wiki/Wave_func…

An observer, by contrast, obeys all the rules of familiar classical physics.

The moment an observer measures a quantum system, that system’s wave function suddenly and unpredictably collapses, revealing some definite spin or whatever has been measured.

There are apparently two completely different ways in which quantum systems evolve.

When we’re not looking at them, wave functions change smoothly according to the Schrödinger equation.

hyperphysics.phy-astr.gsu.edu/hbase/quantum/…

But when we do look at them, wave functions act in a totally different way, collapsing onto some particular outcome.

Now, back to Everett and his Many-Worlds theory...

The seeds of his visionary idea, now known as “the Many-Worlds formulation of quantum mechanics”, can be traced to a late-night discussion in 1954 with fellow young physicists Charles Misner (also a student of Wheeler’s) and Aage Peterson (an assistant of Bohr’s)

All parties agree that copious amounts of sherry were consumed on the occasion.

Under Wheeler’s advice, Everett began thinking about quantum cosmology: the study of the entire Universe as a quantum system.

en.wikipedia.org/wiki/Quantum_c…

If every part of the Universe will have to be treated according to the rules of quantum mechanics, then that includes the observers within it.

In addition, there will be only a single quantum state, described by what Everett called the ‘universal wave function’.

Imagine that we have a spinning electron in some superposition of up and down.

We also have a measuring apparatus, which according to Everett is a quantum system in its own right.

Imagine that it can be in superpositions of three different possibilities:

It can have measured the spin to be up,

It can have measured the spin to be down,

Or it might not yet have measured the spin at all (which we call the ‘ready’ state).

The world has ‘branched’ into a superposition of these two possibilities.

The fact that the measurement apparatus does its job tells us how the quantum state of the combined spin + apparatus system evolves according to the Schrödinger equation.

Namely, if we start with the apparatus in its ready state and the electron in a purely spin-up state, we are guaranteed that the apparatus evolves to a pure measured-up state, like so:

Likewise, the ability to successfully measure a pure-down spin implies that the apparatus must evolve from ‘ready’ to ‘measured down’:

What we want, of course, is to understand what happens when the initial spin is not in a pure up or down state, but in some superposition of both.

Good news is that we already know everything we need.

The rules of quantum mechanics are clear:

If you know how the system evolves starting from two different states, the evolution of a superposition of both those states will just be a superposition of the two evolutions.

Starting from a spin in some superposition and the measurement device in its ready state, we have:

The final state now is an entangled superposition: the spin is up and it was measured to be up, plus the spin is down and it was measured to be down.

This is the clear, unambiguous, definitive final wave function for the combined spin + apparatus system, if all we do is evolve it according to the Schrödinger equation.

Again, the world has ‘branched’ into a superposition of these two possibilities.

Everett’s insight was as simple as it was brilliant: accept the Schrödinger equation.

Both of those parts of the final superposition are actually there. But they can’t interact with each other; what happens in one branch has no effect on what happens in the other.

They should be thought of as separate, equally real worlds.

This is the secret to Everettian quantum mechanics. We didn’t put the worlds in; they were always there, and the Schrödinger equation inevitably brings them to life.

The problem is that we never seem to come across superpositions involving big macroscopic objects in our experience of the world.

The traditional remedy has been to monkey with the fundamental rules of quantum mechanics in one way or another.

As Everett would put it: “The Copenhagen Interpretation is hopelessly incomplete because of its a priori reliance on classical physics … as well as a philosophic monstrosity with a “reality” concept for the macroscopic world and denial of the same for the microcosm.”

We don’t need special rules about making an observation: all that happens is that the wave function keeps chugging along in accordance with the Schrödinger equation.

There’s also nothing special about what constitutes ‘a measurement’ or ‘an observer’ – a measurement is any interaction that causes a quantum system to become entangled with the environment, creating a branching into separate worlds.

And an observer is any system that brings about such an interaction.

Consciousness, in particular, has nothing to do with it.

The price we pay for such a powerful and simple unification of quantum dynamics is a large number of separate worlds.

In an early draft of his thesis, Everett used an analogy of an amoeba dividing to illustrate the branching of the wave function:

“One can imagine an intelligent amoeba with a good memory. As time progresses, the amoeba is constantly splitting, each time the resulting amoebas having the same memories as the parent. Our amoeba hence does not have a life line, but a life tree.”

Ultimately, Everett decided not to continue a career in academia. Before finishing his PhD, he accepted a job at the Weapons Systems Evaluation Group for the US Department of Defense, where he studied the effects of nuclear weapons.

He would go on to do research on strategy, game theory and optimisation, and played a role in starting several new companies.

Everett died in 1982, aged 51, of a sudden heart attack.

He had not lived a healthy lifestyle, over-indulging in eating, smoking and drinking.

His son Mark Oliver Everett has said:

“I realise that there is a certain value in my father’s way of life. He ate, smoked + drank as he pleased, and one day he just suddenly and quickly died...

Given some of the other choices I’d witnessed, it turns out that enjoying yourself and then dying quickly is not such a hard way to go.”

/End