Sean Carroll's Mindscape: Ask Me Anything – October 2025

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This episode is a special "Ask Me Anything" (AMA) edition of the Mindscape Podcast, where renowned physicist and philosopher Sean Carroll fields wide-ranging questions from his Patreon supporters. The discussion covers quantum mechanics and possible worlds, probability, cosmology, consciousness, politics, science communication, and the philosophy of science, all analyzed in Carroll's signature style: thoughtful, witty, and deeply informed.

1. Quantum Mechanics: Modal Realism vs. Everettian Many Worlds

Question (Tim Falzone, 07:10):
How do David Lewis's modal realism and the Everettian many worlds interpretation of quantum mechanics compare? Are there connections, and how should we think about counterfactuals in a branching universe?

• David Lewis's modal realism: Philosopher Lewis argued all possible worlds are real—there's nothing special about ours—all possible worlds have equal ontological status. Possible worlds aren’t restricted to obey our physical laws.
• Everettian many worlds: In quantum mechanics, if you take the wave function as real and take the Schrödinger equation literally, the universe branches upon measurement. But not all conceivable outcomes happen, only those not forbidden by quantum rules.
• Similarities/differences:
  • Both frameworks involve branching "worlds," but Lewis's are defined by modal logic (possibility & necessity), whereas Everett's arise from quantum dynamics.
  • Self-locating uncertainty: In both, probability is reframed as uncertainty about which world we occupy.
• Quote:
  “There are absolutely close connections and analogies between David Lewis’s modal realism and Everett’s many worlds. They’re not exactly the same thing by any stretch of the imagination, but we reason with them in similar ways and they have sort of similar implications.” (12:58)

2. Theoretical vs. Experimental Physicists

Question (Sean Bentley, 19:34):
What do theoretical physicists actually do, and how is it different from experimentalists or engineers?

• Theoretical physicists:
  • Ask big “what-if” questions about the universe; generate models, propose new ideas, refine existing theories.
  • “Writing papers is what physicists do… The point of research is to get a publishable result.” (22:47)
  • Inspirations come from others' work; few work on “theories of everything.”
• Experimentalists:
  • Build and operate equipment (labs, telescopes, colliders); analyze and store data; test predictions.
• Collaboration: Best progress occurs when theorists and experimentalists work together closely.
• Practical joke:
  • “One way you can tell a crackpot: if they only write about theories of everything.” (27:00)

3. Can Science Prove a Negative?

Question (James Allen, 32:12):
Can we ever prove a negative in logic or science? Is "proof" the right word for scientific knowledge?

• On logic:
  • “You can always prove negatives. There’s literally no difference in classical logic between proving a positive and proving a negative.” (33:00)
• On science:
  • “Science does not prove things... Science makes hypotheses and then tests the hypotheses and uses something like Bayesian updating.”
  • Absolute certainty isn’t science’s domain: “Science has to be open to changing its mind tomorrow when new data comes in.”

4. Everettian Worlds and "Where" They Are

Question (Frank Rustler, 39:41):
After quantum measurement, where are the resulting Everettian universes, and how can a measurement create an entire universe?

• “There’s no such thing as where the universes are. In Everettian quantum mechanics… the worlds don’t have locations. The kinds of things that have locations are objects in space. And Everetting worlds are not those.” (40:06)
• New worlds arise by branching, not “copying.”
• The “thickness” of a branch corresponds to probability/amplitude squared.

5. Politics: Federal Troops in Cities

Question (Brandon Wheeler, 46:53):
How is Chicago handling federal intervention by the President? Is resistance justified?

• Carroll expresses concern over unprecedented federal involvement, but notes local authorities can’t simply "wall off" cities from federal agents.
• Highlights need for “adults in the room”—measured, competent leadership and preservation of norms: “The goal is that we need to work toward sometimes requires a delicate hand rather than just giving in to your instincts of the moment.” (50:30)

6. Cosmology: Measuring the Age of the Universe

Question (Josh Dobbins, 53:24):
If the universe is expanding, how can we say it’s “14 billion years old”? Doesn’t relativity muddy the definition of time across the universe?

• Time is well defined cosmologically:
  “Time always goes at one second per second.” (54:15)
• Relative velocities in the real universe are tiny compared to the speed of light: relativistic effects don’t matter for clocks defining the universe’s age.
• The reference frame is set by the cosmic microwave background: clocks “at rest” with respect to cosmic matter.

7. Political Violence and Ethics

Question (Ben Lloyd, 59:23):
Is political violence ever justified? What are the lines?

• “Political violence is very, very rarely justified. One’s first move, one’s default, should not be to be violent.” (1:00:22)
• Assassinations like that of Charlie Kirk (contextual reference) are not justified; hypothetical cases (e.g., Hitler) must involve clear, consequential benefits.
• Consequentialism over emotional response: weigh effectiveness and actual improvement of the world.

8. Quantum Discreteness & the Size of the Universe

Question (Lee Vermeulen, 1:09:16):
If the universe is finite and quantum mechanics is discrete, could we estimate the minimal size required for all possible quantum interactions?

• Carroll references his own paper “Finite Discretized Quantum Mechanics.”
• The universe's entropy sets the dimension of Hilbert space; current best guess: “something like 10^{10^{122}}”. (1:14:47)
• Recurrence and “Boltzmann brains” become likely if the universe is completely finite.

9. The Ship of Theseus: Identity and Continuity

Question (Joey, 1:17:20):
What’s Carroll’s take on the Ship of Theseus thought experiment?

• “There’s sort of an obvious correct answer here, which is, there’s no such essence of ‘shipness’ or Theseusness.”
• Identity is a “casual and approximate” convenience, not an essence.
• Applies same reasoning to people—identity and continuity are emergent, not absolute.

10. Gravity and Dimensionful Constants

Question (Ilya Levov, 1:22:56):
Why does only gravity have a fundamental dimensionful constant (G), unlike the other forces?

• Difference arises because gravity’s dynamical fields are about spacetime itself, not “internal symmetries.”
• In general relativity, the leading term requires a dimensionful coupling, unlike the dimensionless coupling constants of other gauge theories.

11. The Density Operator in Quantum Mechanics

Question (David Kudavarian, 1:27:32):
What is a density operator, and why does it matter?

• Analogy: Like a probability distribution in classical mechanics, but for quantum states.
• Density operator captures the statistical mixture of measurement outcomes—not just mixtures of wavefunctions.
• Especially relevant because of entanglement: a system plus environment may be in a pure state, but each subsystem is describable only by a density operator.

12. Science Communication and "Myths"

Question (Marie Roscue & Jeff B., 1:37:22):
How to persuade people out of scientific myths or skepticism, especially in the classroom?

• Many “myths” are oversimplified but not straightforwardly wrong.
• “I’m not a believer in human beings’ ability to use the force of reason to convince other people to believe things they don’t want to believe... If anything, what you should do is try to convince them that it’s cooler and better to be on the side believing the true things.” (1:41:45)
• Logic rarely changes minds not arrived at through logic.

13. Large Language Models (LLMs) and Linguistics

Question (David Harper, 1:45:14):
Can LLMs/AI inform debates about universal grammar or how the human mind works?

• Carroll disagrees with a strong Chomskyan critique:
  • LLMs mimic human language and can provide indirect insights.
  • LLM “insides” are pokable, but with billions of parameters, it’s hard to interpret.

14. Eternal Return: Physics or Philosophy?

Question (Christopher Smith, 1:51:20):
Is the “eternal return” idea (universe cycling or repeating) plausible in physics?

• Some physical models could, in principle, allow for eternal recurrence, but “I don’t really think any of them work for a combination of reasons involving Boltzmann brains and the arrow of time.” (1:53:21)
• Most physics suggests a unidirectional arrow of time; recurrence faces major problems.

15. Fundamental vs. Applied Science

Question (David Maxwell, 1:57:24):
Is fundamental science losing out to applied science for funding?

• It’s always a challenge: “Society has every right to say, what’s it worth to me?” (1:58:00)
• Scientists must make their case for curiosity-driven research.

16. Early Universe & Black Holes

Questions (Marcin Chady & Andrew J., 2:00:52):
Why didn’t the dense early universe collapse into one big black hole?

• The early universe is like a “white hole,” not a black hole; singularity in the past, not the future.
• High density doesn't require black hole formation; general relativity permits a singularity in only the past.

17. Important Skills for Young Scientists

Question (John Kunjumin, 2:09:45):
What skill do you wish you’d developed more as a young scientist?

• Coding/programming as an undergrad and quantum field theory: "I was a really good coder… I wish I’d kept it up." (2:10:32)
• Wish he knew quantum field theory more deeply.

18. Consciousness and Physicalism

Question (Make3333, 2:12:40):
Are you 100% sure consciousness can be explained within a physicalist framework?

• “No, I’m not 100% sure of anything… Once I become sufficiently high credence in something, I’m going to take that as true and move on until I come across some evidence that importantly changes my mind…” (2:12:57)

19. AI and Epistemic Convergence

Question (Zach McKinney, 2:13:45):
Will AI increase societal consensus or reinforce polarization?

• “Evidence such as we have it certainly favors more epistemic divergence than convergence.”

20. Fine Tuning and Physical Constants

Question (Orbital Magpie, 2:26:15):
Does it even make sense to talk about "fine tuning" when all parameters might be arbitrary?

• “We’re doing our best… If within the laws as we know it, you can point to certain numerical parameters that seem to be fine-tuned... then it’s perfectly legitimate to focus on why is that particular parameter like it is.”

21. Translation from Equations to Concepts

Question (Cindy Flake, 2:30:48):
How do physicists know what the solutions to equations mean in real-world terms?

• "You have the equations, you try to solve them, and then you try to interpret what are the equations trying to tell me. And that process can take a long time, even for the smartest people we have." (2:32:03)
• Example: Schwarzschild solution → Event horizon understood as black hole only decades later.

22. Something vs. Nothing & Time

Question (Ellen Lubel, 2:36:32):
Does the assertion that the universe "has always existed" imply we’ll live again?

• “My best guess as to why there’s something rather than nothing is that it’s a brute fact that there is something rather than nothing. That’s all I’d be willing to say.” (2:37:10)
• The universe can be eternal without repeating; eternity doesn’t require repetition.

23. Democracy and Anti-Democratic Movements

Question (David Sotologo, 2:40:00):
Should societies, as Germany does, bar parties and candidates that are anti-democratic?

• No universal rules; practical wisdom needed.
• The U.S. tradition is more permissive than Germany’s, partly because of historical context.

24. Quantum Fluctuations and Decoherence in Cosmology

Question (2:46:10):
How do quantum fluctuations create the inhomogeneities we see in the cosmic microwave background via decoherence?

• In Copenhagen, an "observer" collapses the wavefunction; in Many Worlds, the environment induces decoherence.
• In the early universe, the "environment" can be small-scale fluctuations or regions beyond our horizon; practical approaches approximate well.

25. Aumann’s Agreement Theorem & Evidence Sharing

Question (Kevin o'Toole, 2:51:35):
Can rational agents reach agreement without sharing evidence?

• If two agents are 100% rational and believe each other are rational, they can update on each other's posteriors. If not, sharing data is allowed and used. “That’s the dance that you’re talking about.” (2:53:24)

26. Dark Stars & JWST

Question (Joe Bender, 2:56:00):
What are “dark stars,” and what’s the latest?

• Dark stars: Early stars powered partly by dark matter annihilation.
• Carroll is skeptical but open; more data from JWST will clarify.

27. Antifragility in Physics vs. Biology

Question (Mike VR, 2:59:17):
Is "antifragility" (improving via stress) unique to living systems?

• Possibly. Living things can gather and use information to adapt—a sophistication physics systems lack.

28. Cosmology’s Future: Dark Energy, Big Crunch, Boltzmann Brains

Question (Nick B., 3:02:35):
Are you emotionally invested in the universe’s ultimate fate?

• “No, I absolutely do not matter. I’m just very curious as to what the universe does. I would like to know one way or the other.” (3:05:10)

29. Cloning and Continuity

Question (Julio Contijo, 3:07:23):
Is genetically cloning oneself to continue one’s work plausible?

• “No, it’s a very silly idea... cloning just makes the equivalent of an identical twin… who likely will not follow your work.”

30. Emergence All the Way Down?

Question (Anthony Rubo, 3:16:20):
What blocks “emergence all the way down” in physics?

• Quantum mechanics imposes fundamental limits: e.g., Compton wavelength sets minimum size for particles. “Once you get down to very small scales, things start changing their behavior.” (3:18:00)
• Not impossible, just physically unpromising.

31. Is Time Fundamental or Emergent?

Question (Mikhail Sirotenko, 3:22:30):
Is it reasonable to assume that time is emergent?

• “We should admit we have no idea... One can make arguments both ways. So until one of those sets of arguments wins, we can just say, it’s okay, we’re exploring both possibilities.”

32. The Vastness of Chemical Space

Question (Ed Saidstuff, 3:24:22):
Why does only a vanishing fraction of possible molecules exist? Is there a physical “selection principle”?

• Given the vastness of combinatorial possibilities, using only a tiny fraction is unsurprising.
• Evolution, randomness, and selection help explore a tiny sample of options but can never exhaust all molecular configurations.

33. Scientific Publishing

Question (Schleyer, 3:28:40):
How do you decide where to publish?

• “It doesn’t matter much… The existing system says there are journals that are more prestigious than others. In my world that just doesn’t really exist.”
• Physics arXiv revolutionized sharing; publication venue less important than paper quality.

34. Abiogenesis vs. Technogenesis

Question (Thomas Henry, 3:34:21):
Is technological evolution (like Perseverance Rover) comparable to biological?

• No—machines are designed, don’t reproduce or face pressures as in evolution.

35. Increasing "Urgency" in Science

Question (ALX, 3:38:40):
Would more competition/urgency like Olympiad hackathons make science better?

• “If you want to think deeply and come up with creative ideas, that kind of urgency and competition is the worst possible thing.”

36. Warp Drives & Star Travel

Question (Brian Mendoza, 3:42:00):
Does the impossibility of warp drive mean we'll never travel to the stars?

• Faster-than-light travel is likely impossible, but star travel is still possible—if slow and requiring patience and/or life extension.

37. Entanglement Entropy

Question (Red Antonov, 3:44:01):
What exactly is entanglement entropy?

• Quantum version of entropy for a subsystem entangled with others; calculated with “von Neumann entropy: minus trace rho log rho.”
• In quantum mechanics, a composite system can be in a pure state while its components are mixed.

38. What Would Make You Abandon Many-Worlds Quantum Mechanics?

Question (Peter Bamber, 3:49:55):
What could falsify the many-worlds interpretation for you?

• Experimental evidence of Schrödinger equation breakdown in a closed quantum system.
• A better, more compelling theory.

39. Limits of Knowledge

Question (Sergei, 3:52:02):
Could there be empirical limits so some truths are always out of reach?

• Plausible in principle, but the remarkable thing is that our world “is how accessible and intelligible the rules of the game actually are.”

40. Forces in Modern Physics

Question (Tim Giannitsos, 3:58:41):
Are we using “force” too loosely? How do "forces" like the Higgs or exclusion principle fit in?

• “Force” is not fundamental in quantum mechanics; “forces” are classical language, replaced by quantum fields.

41. “All Questions Answered” in Physics

Question (Michael, 4:03:10):
If we find the final laws of physics, what’s left?

• Almost everything! Most physics is about complex systems, not just fundamental laws.

42. Hope for Democracy

Question (Henry Jacobs, 4:04:55):
Is there any hope for coordinated resistance in the face of threats to democracy?

• “Coordination is hard, but it’s not impossible. And we can still do it. We do absolutely have hope.” (4:05:22)

43. Effective Field Theory Analogy in Sound

Question (Theo Lind, 4:06:30):
Is banning frequencies above 80 Hz like integrating out high modes in EFT?

• Analogy holds in some ways; but quantum virtual particles affect low-energy observables, unlike sound.

44. AI and Consciousness

Question (Ken Wolf, 4:09:55):
If AI reaches human-level output, is that enough evidence for consciousness?

• “No. You have to go beyond input-output and actually look at what is happening inside.”
• Functional equivalence is not enough if the underlying process is unlike biological consciousness.

45. Baby Universe Creation

Question (PJ Wenzel, 4:13:36):
How plausible is “child universe” creation as a cosmological process?

• Spontaneous universe creation is not understood; “anything dealing with both quantum mechanics and gravity… we have to be a little uncertain about that.”

46. Compatibilist Free Will and Determinism

Question (Linear, 4:20:00):
If my decisions are determined at a deep level, should that bother me?

• Not at all. Even if there were genuine randomness, “you just don’t know it. To me, that’s exactly the same as a fact about the future that I don’t know.”

47. Writing Trade Books

Question (Alex Debrow, 4:21:50):
How do you approach writing popular science books?

• Schedules dictated by contracts, but actual writing is irregular.
• Minimal revising—prefers to get it right the first time.

48. The Marvel of Lawfulness

Question (Tucker Hyatt, 4:24:50):
Will we always see the universe’s lawfulness as a brute fact?

• Maybe. “My stance on brute fact is that we have to be open to them.”

49. Strange Attractors & Societal Behavior

Question (Unnamed, 4:29:10):
Can we ever quantify “strange attractors” in human societies?

• Statistical mechanics methods show promise; social systems can exhibit collective, emergent behaviors analogous to physics models.

50. The Schelling Model and Cross-Disciplinary Science

Question (Philip Ricius, 4:33:30):
Are there other cases where sciences independently developed similar tools (like Schelling and Ising models)?

• Happens all the time; science is so diverse that similar techniques get independently invented and rediscovered.

51. Getting Started in Science Writing

Question (Eric, 4:35:45):
How did you break into popular writing? Tips for others?

• “You certainly don’t cold call publishers… but agents are open to pitches from people they’ve never heard before.”

52. Remembering Podcast Guests/Details

Question (Pete Faulkner, 4:39:00):
Any tricks for remembering 300+ podcast guests?

• “I think I’m very bad at remembering previous podcast episodes…” Details are often surprise rediscoveries.

53. Retirement Plans

Question (Michael Honey, 4:40:23):
How do you envision winding down your career?

• No set plan—could continue similar work after official retirement or “completely change gears.” “I’ve been working hard. Maybe I deserve a little kicking back.”

54. Extra Spatial Dimensions & “Size”

Question (Ian Carey, 4:41:20):
Are small extra dimensions really different?

• No—3D space could be finite too; we just don’t notice because it’s so large. “In the smaller dimensions, we have to [talk about size] because there’s a reason we don’t see them, and it’s probably because they’re small.”

55. Teaching & Pedagogy

Question (Darren Vigliotti, 4:43:07 / Peter K. Kane, 4:52:28):
How's teaching going, and what’s the ideal lecture style?

• “I am barely holding up… It's a lot of work.” Enjoys dynamic lectures and prefers writing by hand/online notes over PowerPoint for clarity and engagement.

56. Legacy

Question (Karagayu, 4:46:45):
Which scientific contribution do you most want to be remembered for?

• Evasive, but honest: “I hope it’s one I haven’t made yet… I truly don’t care how I want to be remembered once I’m gone.” (4:47:45)

Notable Quotes

• On “proof” in science:
  “Science does not prove things. That’s number one.” (33:37)

• On identity:
  “The only way to be perfect is to understand that at every moment, there’s a different ship, there’s a different thing.” (1:20:35)

• On the ultimate fate of science:
  “The quest to find the underlying laws is important, but it’s a tiny fraction of everything that science does. So most of physics won’t even notice if we were able to do that.” (4:03:34)

• On teaching:
  “I am absolutely one who still writes at the blackboard during all my lectures.” (4:52:54)

For Further Listening or Reading

• Carroll references several of his own papers and books, and highlights archive.org, Schumacher & Westmoreland’s book on Quantum Information, and classic works in philosophy and physics for further context.

Timestamps provided correspond to the moments in the episode where these topics are discussed. Ad breaks, intros, and outros have been omitted for clarity.