Sean Carroll

Author: Sean Carroll

  • Propagating Waves

    A devastating earthquake, 8.9 on the Richter scale, hit Japan today, causing extensive damage and a large tsunami. I can’t imagine what it would be like to look out your window and see something like this headed your way. Our thoughts go out to everyone affected by the disaster.

    A force this big propagates around the world, so beaches here in Southern California were expecting heightened wave activity — nothing very serious, but certainly noticeable. Scientists of course immediately leapt into action to estimate what kind of effects should be expected. The National Weather Service circulated this map of predicted wave heights. Click to embiggen.

    Naturally, the House of Representatives is trying to cut funding for tsunami warning centers.

  • Modal Logic and the Ontological Proof

    The ontological proof for the existence of God (really “proofs” or perhaps “arguments,” as there are various versions) has popped up in the blogs a few times recently: e.g. Ophelia Benson, Josh Rosenau, Jerry Coyne. You’ve probably heard this one; it was most famously formulated by Saint Anselm, and most famously trashed by Immanuel “Existence is not a predicate” Kant. A cartoon version of it would be something like

    1. God is by definition a perfect being.
    2. It is more perfect to exist than to not exist.
    3. Therefore, God exists.

    Now, this is a really cartoonish version of the argument — it’s not meant to be taken seriously. This kind of ontological proof is a favorite whipping-argument for atheists, just because it seems so prima facie silly. Just ask Jesus and Mo.

    This kind of mockery is a little unfair (although only a little). What’s important to realize is that the ontological proof is perfectly logical — that is, the conclusions follow inevitably from the premises. It’s the premises that are a bit loopy.

    It’s instructive and fun to see this in terms of formal logic, especially because the proof requires modal logic — an extension of standard logic that classifies propositions not only as “true” or “false,” but also as “necessarily true/false” and “possibly true/false.” That is, it’s a logic of hypotheticals.

    So here is one formalization of the ontological argument, taken from a very nice exposition by Peter Suber. First we have to define some notation to deal with our modalities. We denote possibility and necessity via:

    Just given these simple ideas, a few axioms, and a fondness for pushing around abstract symbols, we’re ready to go. Remember that “~” means “not,” a “v” means “or,” and the sideways U means “implies.” Take “p” to be the proposition “something perfect exists,” and we’re off: (more…)

  • Bloggy Facelift

    Some minor but nice improvements to the look and feel of the blog today. We have a shiny new banner, so that it’s easy to tell what blog you are actually reading. And if you find to your horror that you’ve been reading Cosmic Variance when you meant to be reading Not Exactly Rocket Science, there’s a helpful widget in the right column that can take you directly to any of the other Discover blogs. Thanks to the crack team at Discover for the upgrades.

    I’ve also added a widget that keeps you updated on our Twitter feeds. Right now that means mine and Daniel’s, as well as the Cosmic Variance feed (mostly, but not exclusively, links to each new blog post), and one I set up for From Eternity to Here. But who knows what the exciting future may bring? Actually I worry a bit that the blog has become less likely since I started with Twitter, since one-liner-type links to interesting things generally go there rather than here. Leaving all the more substantial stuff for the blog, which is great, but there’s only so much time for substantial stuff, so the posting frequency has gone down. Or maybe I just worry too much.

    In older but no less fascinating news, we have a Facebook fan page. And — it’s down a bit, so you may have missed it — the long-lamented “recent comments” widget has reappeared.

    Last summer we passed our fifth year of blogging at Cosmic Variance, and didn’t even really notice. Here’s to many more years!

  • Fractal Black Holes on Strings

    Here’s a fascinating new result about black holes in five dimensions — actually from last October, but I missed it when it came out. I just noticed it this week because of a write-up by Gary Horowitz in Matters of Gravity, the newsletter of the gravity group of the American Physical Society. (I obviously missed David Berenstein’s post as well.)

    You might be thinking that black holes in five dimensions can’t be that interesting, since they are probably pretty similar to black holes in four dimensions, and after all we don’t live in five dimensions. But of course, there could be a fifth dimension of space that is compactified on a tiny circle. (Of course.) So then you have to consider two different regimes: the size of the circle is much larger than the size of the black hole — in which the fact that it’s compact doesn’t really matter, and you just have a regular black hole in five dimensions — or the size of the circle is smaller than the black hole — in which case, what?

    The answer is that you get a black string — a cylindrical configuration that stretches across the extra dimension. This was figured out a long time ago by Ruth Gregory and Raymond LaFlamme. But they were also clever enough to ask — what if you had that kind of cylindrical black hole, but it stretched across a relatively large extra dimension? That sounds like a configuration you can make, but it might be unstable — wiggles in the string could grow, leading it to pinch off into a set of distinct black holes. One way of seeing that something like that is likely is to calculate the entropy of each configuration; for long enough black strings, the entropy is lower than a collection of black holes with the same mass, and entropy tends to grow. Indeed, Gregory and LaFlamme showed that long black strings are unstable. However, it wasn’t clear what exactly would ultimately happen to them. (more…)

  • Bruce Winstein

    Bruce Winstein, an experimental physicist at the University of Chicago, passed away this morning. He had been fighting cancer.

    Bruce was a fantastic physicist and person. He became well-known as a particle experimentalist, forgoing giant collaborations to work in small groups where he could do something unique. He was the leader of the KTeV experiment at Fermilab, which measured the very subtle “direct” CP violation effect. He won the Panofsky Prize from the American Physical Society for this work.

    In an especially impressive move, he then decided that he wanted to switch fields, into cosmology. He took a sabbatical year and went to Princeton, where he basically worked as a grad student in Suzanne Staggs’ lab, learning the trade of cosmic microwave background observations from the ground up. Then he came back to Chicago, where he started and was the founding director of the Center for Cosmological Physics, later the Kavli Institute for Cosmological Physics. Once that was up and running, he moved back into research full-time, becoming a leader of the QUIET collaboration.

    Bruce was a great friend, and a valued mentor while I was at Chicago. He was one of the few faculty members to reach out and invite me into his office when I arrived, and was always ready to talk about physics — or music. He was a true audiophile, and connoisseur of jazz in particular. It was Bruce who introduced me to the music of Von Freeman (who just won the prestigious Rosenberger Medal from the University of Chicago).

    Bruce died far too young. We’ll miss him greatly.

  • World Science Festival TV

    Carl Zimmer is annoyed that other people are getting work done, so he points us to the launch of World Science Festival TV. It seems mostly to be snippets from various WSF events, of which there have been scads. The kind of time-sink I can get behind. Here’s (former blogger) Monica Dunford explaining how we can look for new particles when we’re not sure what it is we’re looking for.

  • Dark Matter: Just Fine, Thanks

    Astrophysical ambulance-chasers everywhere got a bit excited this week, and why wouldn’t they? Here are some of the headlines we read:

    Wow. More evidence against dark matter? I didn’t know about the original evidence.

    Sadly (and I mean that — see below) there is no evidence against dark matter here. These items were sparked by a paper and a press release from Maryland astronomer Stacy McGaugh, with the rather more modest titles “A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies” and “Gas rich galaxies confirm prediction of modified gravity theory,” respectively.

    I’m the first person to defend journalists against unfair attacks, and we all know that headlines are usually not written by the people who write the actual articles. But we can legitimately point fingers at a flawed system at work here: these articles are a tiny but very clear example of what is wrong wrong wrong about our current model for informing the public about science.

    McGaugh’s new paper doesn’t give any evidence at all against dark matter. What it does is to claim that an alternative theory — MOND, which replaces dark matter with a modification of Newtonian dynamics — provides a good fit to a certain class of gas-rich galaxies. That’s an interesting result! Just not the result the headlines would have you believe.

    It’s obvious what happens here. Nobody would read an article entitled “Gas rich galaxies confirm prediction of modified gravity theory” — or at least, most editors doubtless feel, fewer people would be interested in that than in evidence that went directly against dark matter. So let’s just spice up the story a bit by highlighting the most dramatic possible conclusion we can imagine drawing, and burying the caveats until the end. Net result: a few more people read the articles than otherwise would have, while many more people just read the headlines and are left with less understanding of modern cosmology than they started with. Scientists and journalists together have a responsibility to do a better job than this at making things clear, not just making things sound exciting.

    But let me take this opportunity to lay out the problems with MOND. (more…)

  • TEDxCaltech Talks

    Talks from the TEDxCaltech event are gradually coming online. You’ll definitely want to check out Scott Aaronson’s contribution, which was a clear highlight. (The biggest highlight was the closing jam session featuring keyboard wizard Lyle Mays and Tuvan throat-singer Ondar. You heard right.)

    Bloggy narcissism demands that I feature my own talk. It was about the arrow of time, with some Feynman thrown in. As usual, the clip is frozen at at point that shows me to best advantage.

    Cosmology and the arrow of time: Sean Carroll at TEDxCaltech

    But just to show it’s not all about me, you should also see this talk by awesome undergrad Jordan Theriot. I would have been a puddle of nervous jelly in this venue at that age.

    TEDxCaltech – Jordan Theriot – The Pleasure of Finding Things Out

  • Eat the Rich

    In times of economic turmoil, nothing has a calming effect like a few colorful charts. Here are a couple of thought-provoking ones via E.D. Kain at Balloon Juice.

    First, originally by Alex Knapp, we have the distribution of wealth in the U.S.:

    If it looks like a more dramatic amount of inequality than you are used to seeing, it may be because this is plotting total wealth rather than yearly income. Knapp also points out that the tax system doesn’t really redistribute wealth very much; the top one percent pulls in 19% of the pre-tax income, which after taxes is whittle away to … 17%.

    Of course their share is growing with time, courtesy of Mother Jones:

    We can compare that reality to what people think it is, and what it should be:

    What does it imply that most Americans think the distribution of wealth is much more even than it really is, and would like it to be more even still? By itself, nothing at all. These are just data — descriptions of the world — and science doesn’t imply morality. The data are just useful to keep in mind when we do think about how a just society should be ordered, and what strategies (“share the pain!”) might be most appropriate when thinking about how to recover from our recent economic pratfall.

    How many comments do you think we’ll get before someone claims that taxation = slavery? I’m guessing five.

  • Atoms, the Equivalence Principle, and Dueling Laureates

    Good to know that our Secretary of Energy, Steve Chu, is still able to unwind from a long day of bureaucracy by thinking about atom interferometry and the Principle of Equivalence.

    Equivalence Principle and Gravitational Redshift

    Michael A. Hohensee, Steven Chu, Achim Peters, Holger Mueller

    We investigate leading order deviations from general relativity that violate the Einstein equivalence principle (EEP) in the gravitational standard model extension (SME). We show that redshift experiments based on matter waves and clock comparisons are equivalent to one another. Consideration of torsion balance tests, along with matter wave, microwave, optical, and M”ossbauer clock tests yields comprehensive limits on spin-independent EEP-violating SME terms at the $10^{-6}$ level.

    The Principle of Equivalence says that, if you’re in free fall, there’s no way of detecting the gravitational field around you in a local region of spacetime. (You’ve seen Inception, right?) Unlike electromagnetism, with gravity there’s no local “force” that can be detected by comparing what happens to particles of different charges. In other words, all particles feel the same “charge” as far as gravity is concerned; they all fall in the same way.

    So to look for violations of the EP (which are certainly conceivable, even if it sometimes just sounds like technobabble), you do experiments that look for particles doing different things in different kinds of gravitational fields. For example, you can use the EP to predict the gravitational redshift, which can be thought of as “time running more slowly when you are deep in a gravitational potential.” (Not the most precise formulation, but it will do.) And therefore you can test the EP by measuring the different amount of time elapsed by sending clocks on different trajectories. (more…)