Sean Carroll’s Mindscape — Episode: Holiday Message | Hits and Misses

Release Date: December 23, 2024
Host: Sean Carroll

Overview

In this year’s unnumbered, informal holiday episode, Sean Carroll reflects on his scientific journey through the lens of “hits and misses,” inspired by Joni Mitchell’s albums of the same theme. Using his own publication history, Carroll provides not only stories behind key research papers—those that made an impact and those that didn’t—but also broad lessons about the creative, messy, and human process of doing science. The episode offers personal anecdotes, career-vulnerability, philosophical asides, and an inside look at what drives publication attention (or neglect) in theoretical physics.

Key Discussion Points & Insights

Introduction: Joni Mitchell’s “Hits and Misses” and the Science Parallel

[02:10]

• Carroll opens with admiration for Joni Mitchell’s artistic approach and her dual-album idea: “Hits” (popular, successful songs) and “Misses” (her favorites with less public attention).
• Bridges the concept to his own scientific work:

  “The idea here is very, very important to emphasize that it’s not that the Misses were bad songs... she really liked them. But for whatever reason, they didn’t become as popular as the others. I love this idea, and it came to mind when I was thinking about the podcast you’re listening to right now...”
  (Sean Carroll, 02:55)

• Lays out the plan: to discuss papers from across his career, both the “hits” (high impact) and “misses” (personal favorites with modest impact), drawing lessons from both.

The Science of Publishing: How Papers Emerge

[07:20]

• Explains the peculiarities of physics publishing—papers (not books) are the currency.
• Emphasizes making an impact not as egotism, but as a natural goal in scientific communication.
• Outlines the paper “hit” or “miss” metric: primarily citations, but also influence and personal satisfaction.

Early Career: First Paper and Accidental Collaborations

[09:20]

• Tells the story of his first-ever publication:
  • “Limits on a Lorentz and Parity Violating Modification of Electrodynamics,” with George Field and Roman Jackiw.
• Describes the serendipity of advisor-assignment at Harvard and influential collaborations.
• Physics content: Introduced a mathematically novel Lorentz-violating term, inspired by the Chern-Simons approach.
• Experimental tie-in: their modified electrodynamics predicts “cosmic birefringence” (rotation of photon polarization).
• Lesson (quotable moment):

  "We basically pioneered a new way of doing that [testing Lorentz violations] based on terms in a Lagrangian... that opened up a floodgate of possibilities."
  (Sean Carroll, 23:40)

• Outcome: This paper was unexpectedly a “hit” due to its opening up new experimental/phenomenological possibilities.

A Miss: The Time Machine Papers

[24:10]

• Discusses two papers (with Eddie Farhi, Alan Guth, and later Ken Olum) on obstacles to building “time machines” (closed timelike curves).
• Key insight: Technical focus on cosmic strings and how energetic/momentum conditions limit the creation of Gott time machines.
• Candid about the limits (“it wasn’t an airtight proof”) and the experience of academic humility:

  "We wrote a paper... and the abstract was simply four words: 'We prove the title.' ...We put the paper on the archive and people pointed out to us that we had not proven the title." (Sean Carroll, 38:00)

• Culturally, notes “snootiness” about time machine research—perceived as unserious by some in academia.
• Overall: Carroll loves these papers for their clever approach, but they received little recognition.

Pivoting for Impact: The Accelerating Universe (“Quintessence and the Rest of the World”)

[49:30]

• After “career mistakes,” found himself well-positioned during the discovery of the accelerating universe.
• Describes the importance of writing about “quintessence”: a scalar field as dark energy, and the experimental constraints that come with it.
• Contribution: Describes a loophole—a pseudoscalar field’s natural symmetry can evade experimental detection, apart from a subtle “cosmic birefringence” effect.

"Here's how you can have your quintessence without it violating all the usual rules about experimental bounds with other fields... There was one experimental effect: cosmological birefringence." (Sean Carroll, 57:05)

• Result: Paper was a “hit”—the right idea at the right time.

On Citations and Academic “Hits”

[01:01:00]

• Discusses the correlation—imperfect but real—between citations and paper quality.
• Advocates use of Google Scholar for background checks, both as a professional and a podcast host.

University of Chicago Era: Modified Gravity and the “R + 1/R” Model

[01:03:30]

• Motivated by the numerical coincidence between the onset of dark matter in galaxies and the onset of cosmic acceleration.
• Explores modifying Einstein’s equations by adding an “inverse curvature” term—R + 1/R.
• Initially set aside as “not interesting”; re-energized by enthusiastic colleagues.
• Lesson:

  “This is a lesson that I never really learned, but I should learn. I thought it was a fun little model at first; I put it in the drawer... the team ended up pushing it out the door and into Physical Review and it blew up. Full employment for theorists: give people a new playground to play in.”
  (Sean Carroll, 01:14:50)

• This became his highest-cited original research paper (“Is Cosmic Speed-Up Due to New Gravitational Physics?”).

Another Miss that Matters: The Arrow of Time and “Baby Universes”

[01:16:00]

• With Jennifer Chen, tackles the philosophical and cosmological puzzle: why does entropy increase? Why did the Big Bang have low entropy?
• Proposes a speculative model: de Sitter space naturally, via rare quantum events, “buds off” baby universes with low-entropy beginnings—thus explaining the arrow of time without cheating.

“I still think... that is the only attempt at explaining why our early universe has low entropy that doesn’t cheat... it doesn’t fine-tune anywhere. It plays by the rules.” (Sean Carroll, 01:33:30)

• Considered a “miss” in citation terms (~100+), but among Carroll’s proudest contributions.

Caltech Era: Dark Matter and “Dark Photons”

[01:36:20]

• Story behind “Dark Matter and Dark Radiation” with Lottie Ackerman, Matt Buckley, and Mark Kamionkowski.
• Proposes a hidden copy of electromagnetism ("dark photons") interacting only with dark matter.
• Discusses the balancing of parameters: if dark matter is heavy, dark electromagnetism can be surprisingly strong without observable effects.

“But as the particle that is charged gets heavier and heavier, even though dark electromagnetism might exist, it becomes less and less relevant... You could hide a dark photon sector.”
(Sean Carroll, 01:42:10)

• A “hit" but word choices ("dark photon") were later co-opted for a somewhat different class of models.

Compact Dimensions and the Unpopular “Cool” Idea

[01:46:50]

• “Dynamical Compactification from de Sitter Space” with Matt Johnson and Lisa Randall.
• Tackles how higher dimensions might “curl up,” starting from a universe where none are compact, using only the positive cosmological constant and a gauge field.
• Carroll speculates the lack of attention is partly because physicists “don’t really like de Sitter space”—the universe we live in!
• Another “miss” citation-wise, but notable for its minimal assumptions and cosmological ambition.

Late Caltech / Quantum Foundations: Space from Hilbert Space

[01:54:00]

• Carroll transitions research toward quantum foundations—a focus on “emergence” (of space, time, gravity) from entanglement structure in a quantum state.
• Two notable papers:
  • With Charles Tsao and Spiros Michalakis: “Space from Hilbert Space: Recovering Geometry from Bulk Entanglement”
  • With Charles Tsao: “Bulk Entanglement Gravity without a Boundary”
• Key insight:

  “If you didn’t put the metric of space-time in by hand... could you have a quantum state that didn’t have space and time built-in, but recovers a metric structure from entanglement?”
  (Sean Carroll, 01:55:15)

• These papers are still “handwavy” and early-stage, but propose a direction for quantum gravity that is “orthogonal” to the more popular AdS/CFT approaches.

The Firewall Paradox and Many Worlds

[02:06:30]

• Paper: “Branches of the Black Hole Wave Function Need Not Contain Firewalls”
• Context: The information paradox and “firewall” controversy—whether observers falling into black holes see dramatic new physics at the horizon.
• Carroll argues, via the many-worlds ontology, that entanglement structure is relative to branches of the global wave function.

  “Whether or not two subsystems of the universe are entangled with each other can have a different answer depending on whether you mean in the global wave function of the universe, or on a branch…”
  (Sean Carroll, 02:12:10)

• Suggests a logical loophole in the argument for firewalls: perhaps no single observer (branch) ever sees one, even if global information is preserved.
• A creative angle, but not widely adopted by the community—a “miss” so far, but a spur to further research.

Reflections and Lessons for Young Scientists

[02:20:00]

• The process of “hits and misses” is messy, nonlinear, and sometimes humbling.
• Authorship, revision, correction of errors, and rejection are all part of the game.
• Some ideas emerge in a flash (on a plane ride), while others are years in the making.
• Being too far ahead or off-trend can mean creative work gets overlooked, at least temporarily.

“Doing science is just a great thing. And so looking forward to more hits and misses.”
(Sean Carroll, 02:27:00)

Notable Quotes & Memorable Moments

• On the joy and hazard of theorist “playgrounds”:

  “Give people a new playground to play in. That is the lesson for getting songs on your hit record.”
  (01:15:40)

• On being overlooked for dabbling in “taboo” topics:

  “If we had called it ‘classifying the space of violations of global hyperbolicity…’ no one would have objected, even though it was exactly the same content to the paper. But if you call it an obstacle to building a time machine, suddenly they cop an attitude...”
  (Sean Carroll, 41:35)

• On the real reason for “hits”:

  “It's always a story of catching up for me... It does, however, help you quantify whether those papers have been hits or misses.”
  (Sean Carroll, 21:40)

• On the arbitrariness of career progress:

  “I know what you’re thinking. At the end of that period, I was denied tenure and had to leave. Why did that happen if all my papers were getting all these citations? Yeah, good question. I don’t know.”
  (Sean Carroll, 01:06:45)

• On tackling big speculative problems:

  “I don’t think it’s necessarily right… But it doesn’t cheat. It doesn’t fine tune anywhere. It plays by the rules, in other words, so people can disagree with that… But I still think it’s the most promising general framework to go forward.”
  (Sean Carroll, 01:33:30)

Episode Timeline (Timestamps)

| Time | Segment / Topic | |--------------|----------------------------------------------------| | 00:00–03:10 | Sponsor message & introduction | | 03:10–10:00 | Joni Mitchell, “hits and misses” motif | | 10:00–25:00 | Early papers: Lorentz violation (the “hit”) | | 25:00–48:00 | Time machine papers (“misses”) + academic culture | | 49:30–01:02:00| Pivot: Accelerating universe & quintessence (hit) | | 01:02:00–01:16:00 | Chicago era, “R + 1/R” gravity paper (hit) | | 01:16:00–01:33:30 | Arrow of Time, baby universes, entropy (miss, but dear) | | 01:36:20–01:44:00 | Caltech: “Dark photons” & dark matter (hit) | | 01:46:50–01:54:00 | Compactification & extra dimensions (miss) | | 01:54:00–02:06:30 | Quantum foundations: space from Hilbert space (early “hit”) | | 02:06:30–02:20:00 | Firewalls, black holes, many worlds (“miss” with potential) | | 02:20:00–02:27:30 | Reflections, the creative mess, closing |

Final Thoughts

This episode is an in-the-weeds yet engaging meditation on scientific creativity, recognition, and perseverance. Carroll’s candor about failures and successes, the role of luck and zeitgeist, and the ongoing dance between fashion and deep curiosity make the episode valuable for scientists and non-scientists alike.

For would-be theorists or science fans interested in “how the sausage gets made,” the Holiday Message provides a unique blend of philosophical musing, career advice, and scientific storytelling.

Note: All sponsor messages and non-content sections have been skipped in this summary.