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Quantum mechanics — our best theory of atoms, molecules, and the subatomic particles that make them up — underpins most of modern physics. But there are varying interpretations of what it means, all of them controversial in their own way.

 Famously, quantum theory predicts that with the right setup, a cat can be made to be alive and dead at the same time. On the face of it, that sounds either meaningless or ridiculous.

 According to today’s guest, David Wallace — professor at the University of Pittsburgh and one of the world's leading philosophers of physics — there are three broad ways experts react to this apparent dilemma:

 1. The theory must be wrong, and we need to change our philosophy to fix it.
 2. The theory must be wrong, and we need to change our physics to fix it.
 3. The theory is OK, and cats really can in some way be alive and dead simultaneously.

 (David and Rob do their best to introduce quantum mechanics in the first 35 minutes of the episode, but it isn't the easiest thing to explain via audio alone. So if you need a refresher before jumping in, we recommend checking out our links to learn more, summary and full transcript.)

In 1955, physicist Hugh Everett bit the bullet on Option 3 and proposed Wallace's preferred solution to the puzzle: each time it's faced with a ‘quantum choice,’ the universe 'splits' into different worlds. Anything that has a probability greater than zero (from the perspective of quantum theory) happens in some branch — though more probable things happen in far more branches.

While not a consensus position, the ‘many-worlds’ approach is one of the top three most popular ways to make sense of what's going on, according to surveys of relevant experts.

Setting aside whether it's correct for a moment, one thing that's not often spelled out is what this approach would concretely imply if it were right.

Is there a world where Rob (the show's host) can roll a die a million times, and it comes up 6 every time?

As David explains in this episode: absolutely, that’s completely possible — and if Rob rolled a die a million times, there would be a world like that.

Is there a world where Rob becomes president of the US?

David thinks probably not. The things stopping Rob from becoming US president don’t seem down to random chance at the quantum level.

Is there a world where Rob deliberately murdered someone this morning?

Only if he’s already predisposed to murder — becoming a different person in that way probably isn’t a matter of random fluctuations in our brains.

Is there a world where a horse-version of Rob hosts the 80,000 Horses Podcast?

Well, due to the chance involved in evolution, it’s plausible that there are worlds where humans didn’t evolve, and intelligent horses have in some sense taken their place. And somewhere, fantastically distantly across the vast multiverse, there might even be a horse named Rob Wiblin who hosts a podcast, and who sounds remarkably like Rob. Though even then — it wouldn’t actually be Rob in the way we normally think of personal identity.

Rob and David also cover:

• If the many-worlds interpretation is right, should that change how we live our lives?
• Are our actions getting more (or less) important as the universe splits into finer and finer threads?
• Could we conceivably influence other branches of the multiverse?
• Alternatives to the many-worlds interpretation
• The practical value of physics today
• And much more

Chapters:

• Rob’s intro (00:00:00)
• The interview begins (00:02:15)
• Introduction to quantum mechanics (00:08:10)
• Why does quantum mechanics need an interpretation? (00:19:42)
• Quantum mechanics in basic language (00:30:37)
• Quantum field theory (00:33:13)
• Different theories of quantum mechanics (00:38:49)
• Many-worlds theory (00:43:14)
• What stuff actually happens (00:52:09)
• Can we count the worlds? (00:59:55)
• Why anyone believes any of these (01:05:01)
• Changing the physics (01:10:41)
• Changing the philosophy (01:14:21)
• Instrumentalism vs. realism (01:21:42)
• Objections to many-worlds (01:35:26)
• Why a consensus hasn’t emerged (01:50:59)
• Practical implications of the many-worlds theory (01:57:11)
• Are our actions getting more or less important? (02:04:21)
• Does utility increase? (02:12:02)
• Could we influence other branches? (02:17:01)
• Should you do unpleasant things first? (02:19:52)
• Progress in physics over the last 50 years (02:30:55)
• Practical value of physics today (02:35:24)
• Physics careers (02:43:56)
• Subjective probabilities (02:48:39)
• The philosophy of time (02:50:14)
• David’s experience at Oxford (02:59:51) 

Producer: Keiran Harris
Audio mastering: Ben Cordell
Transcriptions: Sofia Davis-Fogel and Katy Moore