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Sean Carroll

@seanmcarroll.bsky.social

39K followers 580 following 7.9K posts

Physics, philosophy, complexity. @jhuartssciences.bsky.social & @sfiscience.bsky.social. Host, #MindscapePodcast. Married to @jenlucpiquant.bsky.social. Latest books: The Biggest Ideas in the Universe. https://preposterousuniverse.com/

preposterousuniverse.com

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Sean Carroll @seanmcarroll.bsky.social · Sep 5

Postdoc job! I expect to have an opening at Johns Hopkins for a postdoctoral researcher working somewhere in the broad realms of physics, philosophy, and complexity. Apply at Academic Jobs Online:

academicjobsonline.org/ajo/jobs/30496

Johns Hopkins University, Physics and Astronomy

Job #AJO30496, Postdoctoral Fellow in Foundations of Physics, Complexity, and Emergence, Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, US

academicjobsonline.org

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Sean Carroll @seanmcarroll.bsky.social · 1h

Reposted by Sean Carroll

Fiona Tribe @fionatribe.bsky.social · 2h

Happy Birthday Friedrich Nietzsche, Michel Foucault and Italo Calvino! 🎈

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Sean Carroll @seanmcarroll.bsky.social · 4h

Fall break is coming up, so how we might deal with this problem is left for next week. But as philosophers, rather than scientists, the option of "denial" isn't one we will casually accept.

arxiv.org/abs/1702.00850

Why Boltzmann Brains Are Bad

Some modern cosmological models predict the appearance of Boltzmann Brains: observers who randomly fluctuate out of a thermal bath rather than naturally evolving from a low-entropy Big Bang. A theory ...

arxiv.org

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Sean Carroll @seanmcarroll.bsky.social · 5h

So we just reject an eternally-fluctuating cosmology, yes? The problem is that the currently favored cosmological model - ΛCDM, with cosmological constant - evolves into de Sitter space with a nonzero temperature, and therefore (arguably!) thermal fluctuations. A problem for the real world.

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Sean Carroll @seanmcarroll.bsky.social · 5h

This formulation helps emphasize that the "brain" part of BBs is not what matters. Whatever you want - brain, whole observer, whole planet, entire galaxy - will most likely fluctuate into existence from thermal equilibrium, not arise as a thermodynamically sensible outcome of ordinary evolution.

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Sean Carroll @seanmcarroll.bsky.social · 5h

Today in #PhilosophyOfCosmology we learned that an eternally-fluctuating universe doesn't really work because, given any anthropic criterion for what is an "observer," the theory predicts most such observers will be surrounded by thermal equilibrium. I.e. they will be Boltzmann Brains.

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Reposted by Sean Carroll

Chris Lintott @chrislintott.bsky.social · 14h

Cosmologists! We have a faculty job going here in Oxford - happy to answer questions. Brief version is that this remains a great & collaborative place to do science and the students are superb. 🔭 www.jobs.ac.uk/job/DOP628/a...

Associate Professor of Cosmology at University of Oxford

Recruiting now: Associate Professor of Cosmology on jobs.ac.uk. Click for details and explore more academic job opportunities on the top job board

www.jobs.ac.uk

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Sean Carroll @seanmcarroll.bsky.social · 1d

Oops yes I certainly did!

Sean Carroll @seanmcarroll.bsky.social · 1d

Our coping strategy will start by paying close attention to the domain of the operators, which is going to be a subspace of L^2(R). We said that much today, will put it to work next class.

(Almost no intro QM courses cover this stuff, but it's good for the soul.)

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Sean Carroll @seanmcarroll.bsky.social · 1d

Then we admitted that some of our favorite operators, like position and momentum, are actually "unbounded" operators when acting on L^2(R) (square-integrable functions). They take perfectly good functions and pop them completely out of the space (e.g. if they're discontinuous or decay slowly).

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Sean Carroll @seanmcarroll.bsky.social · 1d

But we're here to learn physics, not math. So we turned to Hilbert space, and finally completed the definition. Hilbert space is not only a complex vector space with an inner product, it must be "complete" - every Cauchy sequence converges to a limit in the space. (Unlike rationals in the reals!)

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Sean Carroll @seanmcarroll.bsky.social · 1d

Big shift of gears today in #QM1: we started taking infinite-dimensional Hilbert spaces seriously. First I did a quick intro to "infinity" and what that means - cardinalities, Q and Z are countable but R is not, Q is dense in Z, all that stuff. The class was not familiar with these ideas!

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Sean Carroll @seanmcarroll.bsky.social · 1d

Thank you!

Reposted by Sean Carroll

Josh Dobbin @joshdobbin.bsky.social · 1d

My question about Time that has bugged me ALL MY LIFE, that I sent in got answered at the 25:50 mark! And I was scared it was too dumb or basic to address, but @seanmcarroll.bsky.social set me straight & was kind enough to address it in such a way that I finally *get* it. Yay!

Sean Carroll @seanmcarroll.bsky.social · 1d

Mindscape Ask Me Anything | October 2025. Hopefully this one answers once and for all why the universe isn't a black hole if it was so densely packed at early times. #MindscapePodcast

www.preposterousuniverse.com/podcast/2025...

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Sean Carroll @seanmcarroll.bsky.social · 1d

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Sean Carroll @seanmcarroll.bsky.social · 2d

Len Bias?

Sean Carroll @seanmcarroll.bsky.social · 2d

I wonder whether at some point people who are currently Christian nationalists are going to figure out that Christianity isn’t actually a good fit for their core values - too much empathy, helping the downtrodden, etc - and just become pagan nationalists or something.

Rich Raho @richraho.bsky.social · 3d

Pope to Chicago Labor Leaders: “Please know of my appreciation for your welcome of immigrants and refugees, especially your support of food pantries and shelters….I encourage you to continue to advocate for society to respect the human dignity of the most vulnerable.” www.vatican.va/content/leo-...

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Reposted by Sean Carroll

Jacob T. Levy @jacobtlevy.bsky.social · 2d

The deportation order derives from the crime of which he has now been exonerated.

www.miamiherald.com/news/local/i...

He was wrongfully imprisoned for 43 years. Moments after being released, ICE took him

Subramanyam “Subu” Vedam now faces deportation.

www.miamiherald.com

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Reposted by Sean Carroll

Adrienne Mayor @amayor.bsky.social · 3d

Let me know in a message if you would like a free Spotify code for my audiobook Mythopedia, limited supply

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Sean Carroll @seanmcarroll.bsky.social · 4d

This has made my day.

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Sean Carroll @seanmcarroll.bsky.social · 4d

Tragedy is when I am criticized on social media. Comedy is when you are dismembered with a bone saw and die.

Wajahat Ali @wajali.bsky.social · 4d

With receipts!

www.theguardian.com/stage/2025/o...

‘Amoral, evil’: vitriolic backlash builds against comics who played Riyadh festival

Murder by bone-saw, lashings for rape victims, punishment amputation, jail for satirists … these are the Saudi human rights abuses fuelling the fury being directed at the likes of Louis CK, Dave Chapp...

www.theguardian.com

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Reposted by Sean Carroll

Senator Chris Van Hollen @vanhollen.senate.gov · 4d

This is what courage in the face of authoritarianism looks like. No university should take Trump's bribe & surrender their integrity — bending the knee to a bully only feeds the beast & puts ALL our rights at risk.

Others should follows MIT’s example ASAP.

MIT rejects Trump administration deal for priority federal funding

MIT is one of the nine schools that were asked to agree to adopt conservative priorities and policies in exchange for funding perks.

www.washingtonpost.com

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Reposted by Sean Carroll

Filipe Campante @filipecampante.bsky.social · 4d

It might be hard to convey to people outside economics just how seismic this is. The Trump effect has most certainly arrived to US academia.

Florian Scheuer @florianscheuer.bsky.social · 4d

I am delighted to share that Nobel laureates Esther Duflo and Abhijit Banerjee will join our Department of Economics @econ.uzh.ch at the University of Zurich on July 1, 2026, as Lemann Foundation Professors of Economics.

🧵 1/7

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Sean Carroll @seanmcarroll.bsky.social · 4d

Still can’t get over that my job involves talking and thinking about this stuff.

Sean Carroll @seanmcarroll.bsky.social · 4d

The fun, of course, is thinking about the implications, from the Einstein-Podolsky-Rosen (EPR) paper from 1935. If we move spins A and B far apart, it remains true that the outcome of Alice measuring her spin affects the potentialities for Bob's measurement outcomes. Spooky action at a distance!

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Sean Carroll @seanmcarroll.bsky.social · 4d

How can spin B's options be affected by A's measurement, apparently faster than light? And how does Nature conspire to prohibit us from using this fact to actually send information?

I was honest that people still don't agree on how to talk about this. A modern intellectual tragedy.

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