MV#2 — The Emergent Multiverse — David Wallace

“When you come to a fork in the road, you should take it”

So goes the jocular advice of Yogi Berra.

But what if this is what the universe actually does? What if we live in a garden of forking paths, of events that can go one way or another and in fact do both?

This is the subject of today’s podcast with David Wallace. David holds the Mellon Chair in Philosophy of Science at the University of Pittsburgh. He is one of the leading advocates of the Everett interpretation of quantum mechanics.

Originally proposed in 1957 by Hugh Everett and also called the Many Worlds Interpretation it offers a way of unpacking the precise mathematical predictions of this theory. 

The issue is this: quantum mechanics permits small things — atoms, electrons, photons — to be in frankly weird states. A single particle can be in many places at once, or traveling at many different speeds. 

These superpositions are key to Quantum Mechanics. Integral to this theory which explains how stars work, has enabled us to build computer chips and, indeed, lies at the very foundations of our understanding of matter — the uninspiringly named “Standard Model” is a quantum mechanical theory. 

Perhaps you have no issue with these tiny particles being in such weird states — you can’t see them. But quantum mechanics does not draw a qualitative distinction between small and big things. Like the rest of physics, it treats big things as agglomerations of small ones, the difference between them is of quantity, not kind. So quantum mechanics appears to tell us that big things like people, or (famously) cats can be in multiple places at once or even in seemingly contradictory states of being dead and alive. This is not what we observe.

The original attempt to explain this away was to say that when a measurement takes place these superpositions break down and crystallise into a single state. You might have come across phrases like “the collapse of the wavefunction” to describe this idea that things go from being spread out, or wavelike, to being localised. But what’s so special about measurement that it should provoke such a change of behaviour, what even is measurement if not just another physical process?

Other attempts propose modifying quantum mechanics — adding a new mechanism that would cause the crystallisation or collapse that doesn’t privilege measurement. However, it is no mean feat to try to modify a theory that has had such predictive success.

But what if we do not try to explain anything away?

What if we take seriously this idea of superpositions at all levels, not just the microscopic but all the way up to human and even universal scale?

Does quantum mechanics tell us we will observe something being in two states at once? No. Hugh Everett, David Wallace, and many others reason that quantum mechanics tells us that the world branches and that as the small superpositions become large those large ones represent worlds and each world looks much like the world we inhabit — where objects are one thing or another but never both at once.

When a photon can follow two different paths, it does, when the detection apparatus can observe it in two different places it will, when I can see that apparatus registering two different things, I will. But there is one me, and another me and neither sees anything extraordinary. The world has followed the fork in the road.

This is a theory with almost incredible consequences. But it has unassuming origins. It does not assume that there is anything special about measurement nor that quantum mechanics is incomplete. It is a radical theory for it goes to the roots of quantum mechanics, from these the branches emerge.

David was my tutor when I studied Physics and Philosophy as an undergraduate at Oxford, my thanks to him for giving his time to my curiosity once again.


The Yogi Berra line is one Harvey Brown used in his lectures on MWI.