Category: Science

Lectures on Emergence and Complexity

By Sean Carroll | September 17, 2012 6:07 pm

Simon DeDeo is a theoretical physicist and very smart guy, who started out as a cosmologist and has made the transition to complexity theorist at the Santa Fe Institute. (He’s not smart because he made that transition, it just so happens that both statements are true.)

This summer he gave a series of three lectures at SFI’s Complex Systems Summer School, on the general topics of Emergence and Complexity. These are big ideas, and obviously one cannot say everything interesting there is to say about them in three lectures, but Simon manages to cover a lot of extremely important and fascinating topics such as coarse-graining, renormalization, computation, and effective theories. Worth a listen!


New Life for Gravitational Waves in Space?

By Sean Carroll | September 11, 2012 8:42 am

Last year we brought the bad news that NASA had pulled back from the LISA project, an ambitious proposal to build a gravitational wave detector in space. The science reach of LISA would be amazing, teaching us a great deal about black holes, general relativity, and cosmology.

Fortunately, the European Space Agency did not give up on the idea, and has kept it in the queue of possibilities without actually saying they will do it. They began to design a somewhat down-scaled mission, now dubbed NGO for “New Gravitational wave Observatory.” (Hey, nobody said NASA had a monopoly on dopey acronyms.) NGO was put into the hopper along with two other proposals as part of a selection process to decide on the ESA’s next large-scale mission, dubbed L1 (“L” for “large”), as part of the Cosmic Vision program. It lost out to JUICE, a mission to Jupiter’s moons with admittedly a much cooler acronym as well as some very good science behind it.

But if there is an L1, that implies that someday there might be an L2, and NGO is still in the running to be Europe’s next big mission in astrophysics from space. Read More


Looking for Extra Dimensions at the LHC

By Sean Carroll | September 7, 2012 11:13 am

Phd Comics, channeling physicists Daniel Whiteson and Jonathan Feng, explains how the LHC isn’t anywhere near done yet. Now that the Higgs(like) boson has been found, we’ll be looking for all sorts of other things, such as extra dimensions of space.


Guest Post: Doug Finkbeiner on Fermi Bubbles and Microwave Haze

By Sean Carroll | September 4, 2012 8:22 am

When it comes to microwaves from the sky, the primordial cosmic background radiation gets most of the publicity, while everything that originates nearby is lumped into the category of “foregrounds.” But those foregrounds are interesting in their own right; they tell us about important objects in the universe, like our own galaxy. For nearly a decade, astronomers have puzzled over a mysterious hazy glow of microwaves emanating from the central region of the Milky Way. More recently, gamma-ray observations have revealed a related set of structures known as “Fermi Bubbles.” We’re very happy to host this guest post by Douglas Finkbeiner from Harvard, who has played a crucial role in unraveling the mystery.

Planck, Gamma-ray Bubbles, and the Microwave Haze

“Error often is to be preferred to indecision” — Aaron Burr, Jr.

Among the many quotes that greet a visitor to the Frist Campus Center at Princeton University, this one is perhaps the most jarring. These are bold words from the third Vice President of the United States, the man who shot Alexander Hamilton in a duel. Yet they were on my mind as a postdoc in 2003 as I considered whether to publish a controversial claim: that the microwave excess called the “haze” might originate from annihilating dark matter particles. That idea turned out to be wrong, but pursuing it was one of the best decisions of my career.

In 2002, I was studying the microwave emission from tiny, rapidly rotating grains of interstellar dust. This dust spans a range of sizes from microscopic flecks of silicate and graphite, all the way down to hydrocarbon molecules with perhaps 50 atoms. In general these objects are asymmetrical and have an electric dipole, and a rotating dipole emits radiation. Bruce Draine and Alex Lazarian worked through this problem at Princeton in the late 1990s and found that the smallest dust grains can rotate about 20 billion times a second. This means the radiation comes out at about 20 GHz, making them a potential nuisance for observations of the cosmic microwave background. However, by 2003 there was still no convincing detection of this “spinning dust” and many doubted the signal would be strong enough to be observed.

The haze

In February 2003, the Wilkinson Microwave Anisotropy Probe (WMAP) team released their first results. Read More

CATEGORIZED UNDER: Guest Post, Science, Top Posts

Animal Consciousness

By Sean Carroll | August 31, 2012 12:00 pm

At the Francis Crick Memorial Conference in Cambridge last month, a collection of internationally recognized experts on consciousness took an unusual step, as science conferences go: they issued a declaration (pdf). The subject was whether or not non-human animals could be considered “conscious.” (See discussion by Octopus Chronicles, Christof Koch, io9.) The spirit of the declaration was in the direction of saying “pretty much, yeah,” although they tried to stick to what could be scientifically discussed. Here’s the upshot of the declaration:

The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Nonhuman animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.

Even the experts don’t necessarily agree on a definition of “consciousness,” so the declaration doesn’t come right out and say “animals are conscious.” But the authors basically agree that the mental supervenes on the physical, so whatever consciousness may be, it must have some neurological substrate — some parts of the brain that do the work. The point they’re making is, whatever those parts are, some animals have them too.

I don’t have a well-thought-out position on this, at least as far as the big-picture consequences are concerned. Read More


arXiv Find: Reasons Not to Believe in the String Theory Landscape

By Sean Carroll | August 29, 2012 11:32 am

Tom Banks has long been skeptical of the popular picture of the string theory landscape — the idea that there is some extremely large (10500 or more) number of phases of string theory, representing different ways to compactify the extra dimensions, and that all these phases are dynamically connected to each other, possibly by cosmological transitions during eternal inflation. Tom’s reasons aren’t of the curmudgeonly you-kids-get-off-my-lawn sort, but arise from his views about how quantum gravity works. (He thinks different cosmological boundary conditions represent truly different quantum theories, not just different regions of one big spacetime.) Well worth considering, if only because it’s too easy to run off in the direction of conventional wisdom when you’re far away from the realm of experimental testing.

The Top 10500 Reasons Not to Believe in the Landscape

T. Banks

The String Landscape is a fantasy. We actually have a plausible landscape of minimally supersymmetric $AdS_4$ solutions of supergravity modified by an exponential superpotential. None of these solutions is accessible to world sheet perturbation theory. If they exist as models of quantum gravity, they are defined by conformal field theories, and each is an independent quantum system, which makes no transitions to any of the others. This landscape has nothing to do with CDL tunneling or eternal inflation.

A proper understanding of CDL transitions in QFT on a fixed background dS space, shows that the EI picture of this system is not justified within the approximation of low energy effective field theory. The cutoff independent physics, defined by the Euclidean functional integral over the 4-sphere admits only a finite number of instantons. Plausible extensions of these ideas to a quantum theory of gravity obeying the holographic principle explain all of the actual facts about CDL transitions in dS space, and lead to a picture radically different from eternal inflation.

Theories of Eternal Inflation (EI) have to rely too heavily on the anthropic principle to be consistent with experiment. Given the vast array of effective low energy field theories that could be produced by the conventional picture of the string landscape one is forced to conclude that the most numerous anthropically allowed theories will disagree with experiment violently.


Bill Thurston

By Mark Trodden | August 22, 2012 10:41 am

Just a quick note here to say how sad I was to hear (via Terry Tao’s blog) of the death yesterday of Bill Thurston, whose work, particularly on 3-manifolds, endeared him to mathematicians and physicists and resulted in the 1982 Fields medal. I’m certainly no expert on his work, but I encountered it first thirteen years or so ago when I was working on the idea that compact hyperbolic manifolds might provide interesting examples of the extra dimensional spaces used in large extra dimension models.

I found the whole area to be fascinating, and it was particularly interesting for a non-expert because the GeomView program, produced by the Geometry Center (which Thurston had been involved with, and which had just closed) allowed great visualizations of complicated manifolds. They also produced wonderful videos, like this one (that Tao also links to) of Thurston’s method for everting the sphere.

I never met Thurston, but greatly enjoyed the small part of his work that I’ve used, and am very sorry he won’t be around to contribute more.

CATEGORIZED UNDER: Mathematics, Science

A Few Powers of Ten

By Sean Carroll | August 22, 2012 9:10 am

Via the endlessly enjoyable It’s Okay to Be Smart, here’s a gif image that zooms in by about three orders of magnitude. (Not sure of the original source.) We start by looking at an amphipod, a tiny shrimplike critter about a millimeter across. For some reason (vanity?) it’s decorated by an even tinier diatom, a bit of algae that is common in phytoplankton. From there we zoom in on a yet-tinier bacterium, just chilling out near the middle of the diatom.

Human beings have about ten times as many bacterial cells inside them as “human” cells. The bacteria are the passengers, we’re just the bus.


Gravitational Waves in Five Years

By Sean Carroll | August 20, 2012 1:15 pm

LIGO, the gravitational-wave observatory, is currently on ice. After running successfully (although without actually detecting any gravitational waves) through 2007, it got a mini-upgrade and ran as Enhanced LIGO in 2009 and 2010. But in October 2010 it shut off, and the original detectors were disassembled. Not because anything was wrong, but because of a long-anticipated upgrade to Advanced LIGO, a substantially more sensitive observatory.

Those upgrades are still going on, with the new detectors scheduled to come online in 2014. Advanced LIGO should provide more than a tenfold improvement in sensitivity, which allows the search for gravitational waves to pass an important threshold: with LIGO, it would have been possible but quite fortunate to actually detect gravitational waves from predicted astrophysical sources. With Advanced LIGO, it will be a surprise if we don’t detect them.

Clara Moskowitz has nice update on She quotes Kip Thorne as predicting that our first definite direct detection of gravitational waves will come in between 2014 and 2017 — within five years. Start your betting markets! Traditionally, looking at the skies in a new way (radio waves, cosmic rays, X-rays, gamma rays, neutrinos…) has always taught us something new and exciting. I’d be surprised if gravitational waves aren’t equally surprising.


Mars Science Laboratory Touches Down Tonight

By Sean Carroll | August 5, 2012 9:37 am

Nowadays everyone calls it the “Curiosity rover,” but I got to know it as the Mars Science Laboratory, and I’m too old and set in my ways to switch. Launched on November 26, 2011, the mission is scheduled to land on Mars’s Gale Crater tonight/tomorrow morning: 5:31 UTC, which translates to 1:30 a.m. Eastern time or 10:20 p.m. Pacific. See here and here for info about where to watch. Between this and the Higgs boson, the universe is clearly conspiring to keep science enthusiasts on the East Coast from getting a proper night’s sleep.

NASA has done a great job getting people excited about the event, and one of their big successes has been this video, “Seven Minutes of Terror.” Love the ominous soundtrack.

Mars is about fourteen light-minutes away from Earth, so scientists at the Jet Propulsion Laboratory aren’t actually able to fine-tune the spacecraft’s approach, like you used to do playing Lunar Lander in the arcade back in the day. Everything has to be carefully programmed well ahead of time, setting up an elaborately choreographed series of events that guides the lander through the seven-minute journey from the top of the Martian atmosphere to eventual touchdown. I still struggle with parallel parking, which is why I’m a theoretical physicist and not a JPL engineer.

This isn’t NASA’s first rodeo, of course. Read More


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