This is one of my favourite episodes of Sean Carroll’s Mindscape podcast. He talks about his recent work in attempting to derive the kind of spacetime geometry we observe from little more than the mere existence of a universal quantum wavefunction.
Shownotes:
I suspect most loyal Mindscape listeners have been exposed to the fact that I’ve written a new book, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime. As I release this episode on Monday 9 September 2019, the book will officially be released tomorrow, in print, e-book, and audio versions. To get in the mood, we’ve had several podcast episodes on quantum mechanics, but the “emergence of spacetime” aspect has been neglected. So today we have a solo podcast in which I explain a bit about the challenges of quantum gravity, how Many-Worlds provides the best framework for thinking about quantum gravity, and how entanglement could be the key to showing how a curved spacetime could emerge from a quantum wave function. All of this stuff is extremely speculative, but I’m excited about the central theme that we shouldn’t be trying to “quantize gravity,” but instead looking for gravity within quantum mechanics. The ideas here go pretty far, but hopefully they should be accessible to everyone.
Personal anecdote: While physics has been a subject of interest to me for a long time, I only recently took a real interest in quantum physics. A big reason for that, I think, is the way it is taught and communicated. It’s presented as something mystical, unknowable, “sort of like [analogy], but like, nothing like that at the same time”. The obvious example is how electron orbits are portrayed “sort of like planetary orbits except they don’t work like planets and they’re not really orbits”. Another example is the “collapse of the wave function”, which is presented as some sort of ‘process’ where the laws of physics appear to suddenly cease evolving according to the Shrödinger equation and begin displaying properties associated with classical mechanics. This is all very unfortunate and damaging to the public perception of fundamental physics.
(Then there is the apparent ad-hoc appearance of the Standard Model, which is not really even trying to explain how anything makes sense - it’s just a collection of mathematical formulas and numbers that always seem to predict experimental outcomes - and that gives the impression that we have absolutely no idea what’s really going on. So that doesn’t help.)
Sean Carroll is one of the very few science communicators I’ve come across that actually takes a more careful approach to these issues. He tries to separate conjecture from observed facts and strip quantum physics down to the essence of what makes something quantum. And it’s fascinating! Reality may be fundamentally different from how we experience it, but it isn’t inexplicable. We can explain quantum systems in ways that are consistent and intuitive once you accept the rules. We just don’t know what kind of theory accurately describes everything we observe.
I’m not sure exactly which paper he was referring to, but here are some relevant ones:
- Mad-Dog Everettianism: Quantum Mechanics at Its Most Minimal (2018)
- Quantum mereology: Factorizing Hilbert space into subsystems with quasiclassical dynamics (2020)
- Decomposition of a Quantum System Into Subsystems in Finite Quantum Mechanics (2021)
Someone named O. Stoica has published a refutation (but I haven’t read it yet): Refutation of Hilbert Space Fundamentalism (2021). They call it “Hilbert-Space Fundamentalism”, but I feel like that has a negative ring to it, given the connotations of the word “fundamentalism”.