Sean Carroll

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Hi. I'm Sean Carroll. I'm a physicist at Caltech who occasionally wears other hats. This blog is part of my larger website. With my physicist hat on, I'm a theorist who thinks about the fundamental laws of nature and these days about philosophy and the foundations of physics. Some of my work has been on violations of fundamental symmetries, the physics of dark energy, modifications..

Sean Carroll

2M ago

George Field, brilliant theoretical astrophysicist and truly great human being, passed away on the morning of July 31. He was my Ph.D. thesis advisor and one of my favorite people in the world. I often tell my own students that the two most important people in your life who you will (consensually) choose are your ..read more

Sean Carroll

11M ago

This year we give thanks for a feature of nature that is frequently misunderstood: quanta. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, the Jarzynski equality, the moons of Jupiter, space, black hole entropy, electromagnetism, and Arrow’s Impossibility Theorem.)
Of course quantum me ..read more

Sean Carroll

1y ago

Just a few years ago, Australian Catholic University (ACU) established a new Dianoia Institute of Philosophy. They recruited a number of researchers and made something of a splash, leading to a noticeable leap in ACU’s rankings in philosophy — all the way to second among Catholic universities in the English-speaking world, behind only Notre Dame.
Now, without warning, ACU has announced plans to completely disestablish the institute, along with eliminating 35 other academic positions in other fields. This leaves the faculty, some of which left permanent jobs elsewhere to join the new institute ..read more

Sean Carroll

2y ago

This year we give thanks for Arrow’s Impossibility Theorem. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, the Jarzynski equality, the moons of Jupiter, space, black hole entropy, and electromagnetism.)
Arrow’s Theorem is not a result in physics or mathematics, or even in physical science, but rather in social choice theory. To fans of social-choice theory and voting m ..read more

Sean Carroll

2y ago

Just in case there are any blog readers out there who haven’t heard from other channels: I have a new book out! The Biggest Ideas in the Universe: Space, Time, and Motion is Volume One of a planned three-volume series. It grew out of the videos that I did in 2020, trying to offer short and informal introductions to big ideas in physics. Predictably, they grew into long and detailed videos. But they never lost their informal charm, especially since I didn’t do that much in the way of research or preparation.
For the book, by contrast, I actually did research and preparation! So the topics are ..read more

Sean Carroll

2y ago

As far as I remember, the first time I stepped onto a university campus was in junior high school, when I visited Johns Hopkins for an awards ceremony for the Study of Mathematically Precocious Youth. (I grew up in an environment that didn’t involve spending a lot of time on college campuses, generally speaking.) The SMPY is a longitudinal study that looks for kids who do well on standardized math tests, encourages them to take the SATs at a very young age, and follows the progress of those who do really well. I scored as “pretty precocious” but “not precocious enough to be worth following up ..read more

Sean Carroll

3y ago

This year we give thanks for something we’ve all heard of, but maybe don’t appreciate as much as we should: electromagnetism. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, the Jarzynski equality, the moons of Jupiter, space, and black hole entropy.)
Physicists like to say there are four forces of nature: gravitation, electromagnetism, the strong nuclear force, and the ..read more

Sean Carroll

3y ago

The nature of consciousness remains a contentious subject out there. I’m a physicalist myself — as I explain in The Big Picture and elsewhere, I think consciousness is best understood as weakly-emergent from the ordinary physical behavior of matter, without requiring any special ontological status at a fundamental level. In poetic-naturalist terms, consciousness is part of a successful way of talking about what happens at the level of humans and other organisms. “Being conscious” and “having conscious experiences” are categories that help us understand how human beings live and behave, while c ..read more

Sean Carroll

3y ago

Little things can come together to make big things. And those big things can often be successfully described by an approximate theory that can be qualitatively different from the theory of the little things. We say that a macroscopic approximate theory has “emerged” from the microscopic one. But the concept of emergence is a bit more general than that, covering any case where some behavior of one theory is captured by another one even in the absence of complete information. An important and subtle example is (of course) how the classical world emerges from the quantum one.
And here is the Q ..read more

Sean Carroll

3y ago

You knew this one was coming, right? Why the past is different from the future, and why we seem to flow through time. Also a bit about how different groups of scientists use the idea of “information” in very different ways.
And here is the associated Q&A video:
Related Posts:
The Biggest Ideas in the Universe | 5. Time
The Biggest Ideas in the Universe | 16. Gravity
The Biggest Ideas in the Universe | 12. Scale ..read more