Science

Autonomously Moving Colloidal Objects that Resemble Living Matter

That’s the name of a new paper by Akihisa Shioi, Takahiko Ban, and Youichi Morimune. Abstract:

The design of autonomously moving objects that resemble living matter is an excellent research topic that may develop into various applications of functional motion. Autonomous motion can demonstrate numerous significant characteristics such as transduction of chemical potential into work without heat, chemosensitive motion, chemotactic and phototactic motions, and pulse-like motion with periodicities responding to the chemical environment. Sustainable motion can be realized with an open system that exchanges heat and matter across its interface. Hence the autonomously moving object has a colloidal scale with a large specific area. This article reviews several examples of systems with such characteristics that have been studied, focusing on chemical systems containing amphiphilic molecules.

The journal is called Entropy, which I love. The paper discusses a variety of different systems that can travel, wiggle around, and respond to stimuli in ways that resemble living organisms. Not exactly building life in a test tube, but the boundary grows increasingly blurry.

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Mapping the Dark Matter

Have any friends or colleagues who don’t believe in dark matter? Showing them this should help.

Dark Matter in Abell 1689

That ghostly haze is dark matter — or at least, an impression of the gravitational field created by the dark matter. This is galaxy cluster Abell 1689, in the constellation Virgo. (We feel compelled to add that information, in case you’re going to go looking for it in the night sky tonight or something.) It’s easy to see that the images of many of the galaxies have been noticeably warped by passing through the gravitational field of the cluster, a phenomenon known as strong gravitational lensing. This cluster has been studied for a while using strong lensing. The idea is that the detailed distribution of dark matter affects the specific ways in which different background images are distorted (similar to what was used to analyze the Bullet Cluster). Astronomers use up massive amounts of computer time constructing different models and determining where the dark matter has to be to distort the galaxies in just the right way. Now Dan Coe and collaborators have made an unprecedentedly high-precision map of where the dark matter is (paper here).

This isn’t all about the pretty pictures. We have theoretical predictions about how dark matter should act, and it’s good to compare them to data. Interestingly, the fit to our favorite models is not perfect; this cluster, and a few others like it, are more dense in a central core region than simple theories predict. This is an opportunity to learn something — perhaps clusters started to form earlier in the history of the universe than we thought, or perhaps there’s something new in the physics of dark matter that we have to start taking into account.

But the pretty pictures are certainly a reward in their own right.

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The Pi-on

I am in love with this comment and want to have its babies:

pi appears as a constant in many formula of physics. General relativity says that it isn’t constant. Is it the origin of the pi particle, aka pion?

A curmudgeonly literalist might, when faced with a question such as this, harrumph a simple “No.” A more loquacious sort might explain that general relativity does not say that π is not a contstant. Pi is not a parameter of physics like the fine-structure constant, which could conceivably be different or even variable from place to place. It’s a universal answer to a fixed question, to wit: what is the ratio of the circumference of a circle to its diameter, as measured in Euclidean geometry? The answer is of course 3.141592653589793…, or any number of representations in terms of infinite series.

But the point of the question is that GR says we don’t live in Euclidean space; we move through a curved spacetime manifold. That’s okay. In a curved space, we could imagine defining the “diameter” of a circle as the maximum geodesic distance connecting two of its points, and taking the ratio of the circumference with that diameter, and indeed it would typically not give us 3.14159… But that doesn’t mean π is changing from place to place; it just means that the ratio of circumference to diameter (defined this way) in a curved space doesn’t equal π. If the circumference/diameter ratio is less than π, you are in a positively curved space, such as a sphere; if it is greater than π, you are in a negatively curved space, such as a saddle. Geometry can also be much more complicated than that, with different ratios depending on how the circle is oriented in space, which is why curvature is properly measured by tensors rather than by a simple number.

Taken from Mathematics Illuminated, which says that pi really does depend on the geometry of space, which is crazy.
Taken from Mathematics Illuminated, which says that pi really does depend on the geometry of space, which is crazy.

(Parenthetically, one of the dumbest mathematical arguments ever given was put forward by the world’s smartest person, Marilyn Vos Savant. The columnist wrote an entire book criticizing Andrew Wiles’s proof of Fermat’s Last Theorem. Her argument: Wyles made use of non-Euclidean geometry, but what if geometry is really Euclidean? Touche!)

However … despite the fact that π doesn’t really change from place to place in general relativity, the geometry does change from place to place, and there is a particle associated with those dynamics — the graviton. Although the formulation of the original question isn’t accurate, the spirit is very much in the right place. And I, for one, will henceforth be perpetually sad that the physics community missed a chance by attaching the word pion to the lightest quark-antiquark bound state, rather than to the particle associated with deviations from Euclidean geometry. That would have been awesome.

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Against Space

The Philosophy of Science Association meeting in Montreal was great fun. For one thing it was in Montreal; for another I got to hang out with Doctor Free-Ride; and as a bonus there were some interesting and provocative talks about the nature of time. I chatted with Tim Maudlin, Huw Price, Craig Callender, Nick Huggett, Chris Wuttrich, David Wallace, John Norton, and other people I always learn from when I talk to. Philosophers always force you to think hard about things.

Here are the slides from my own talk, which was supposed to be about time but ended up being more about space. Not much in the way of original research, just some ruminations on what is and is not “fundamental” about spacetime (with the caveat that this might not be a sensible question to ask). I made two basic points, which happily blended into each other: first, that the distinction between “position” (space) and “momentum” is not a fundamental aspect of classical mechanics or quantum mechanics, but instead reflects the particular Hamiltonian of our world; and second that holography implies that space is emergent, but in a very subtle and non-local way. This latter point is one reason why many of us are skeptical of approaches like loop quantum gravity, causal set theory, or dynamical triangulations; these all start by assuming that there are independent degrees of freedom at each spacetime point, and quantum gravity doesn’t seem to work that way.

Sadly the slides aren’t likely to be very comprehensible. There’s a lot of math, and the equations don’t come out completely clearly — my first time using Slideshare, so perhaps they would look better if I uploaded a pdf file rather than PowerPoint. (Hint: the slides are much more clear if you switch to full-screen mode by clicking on the bottom right.) Also I didn’t make any attempt to have the slides stand by themselves without the accompanying words. But at least this will serve as documentation that I really did give a talk at the conference, no just hang out in restaurants in Montreal.

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Food In Space

Editor’s note: Crap. I wrote this post within milliseconds after the first of these awesome images came out, but somehow didn’t publish it. Now they are all over the place, and the message is ancient news in internet-time. But the science is timeless!

This tweet by Alicia Chang says it all: “Comet Hartley 2 looks like a peanut.”

Comet Hartley 2

This is the first close-up image from a fly-by of the comet by NASA’s Deep Impact mission. Expect more coming in. Despite the delicious appearance, however, it wouldn’t be prudent to take a bite; the comet is spewing out cyanide.

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Is Dark Matter Supernatural?

No, it’s not. Don’t be alarmed: nobody is claiming that dark matter is supernatural. That’s just the provocative title of a blog post by Chris Schoen, asking whether science can address “supernatural” phenomena. I think it can, all terms properly defined.

This is an old question, which has come up again in a discussion that includes Russell Blackford, Jerry Coyne, John Pieret, and Massimo Pigliucci. (There is some actual discussion in between the name-calling.) Part of the impetus for the discussion is this new paper by Maarten Boudry, Stefaan Blancke, Johan Braeckman for Foundations of Science.

There are two issues standing in the way of a utopian ideal of universal agreement: what we mean by “supernatural,” and how science works. (Are you surprised?)

There is no one perfect definition of “supernatural,” but it’s at least worth trying to define it before passing judgment. Here’s Chris Schoen, commenting on Boudry et. al:

Nowhere do the authors of the paper define just what supernaturalism is supposed to mean. The word is commonly used to indicate that which is not subject to “natural” law, that which is intrinsically concealed from our view, which is not orderly and regular, or otherwise not amenable to observation and quantification.

Very sympathetic to the first sentence. But the second one makes matters worse rather than better. It’s a list of four things: a) not subject to natural law, b) intrinsically concealed from our view, c) not orderly and regular, and d) not amenable to observation and quantification. These are very different things, and it’s far from clear that the best starting point is to group them together. In particular, b) and d) point to the difficulty in observing the supernatural, while a) and c) point to its lawless character. These properties seem quite independent to me.

Rather that declare once and for all what the best definition of “supernatural” is, we can try to distinguish between at least three possibilities:

  1. The silent: things that have absolutely no effect on anything that happens in the world.
  2. The hidden: things that affect the world only indirectly, without being immediately observable themselves.
  3. The lawless: things that affect the world in ways that are observable (directly or otherwise), but not subject to the regularities of natural law.

There may be some difficulty involved in figuring out which category something fits, but once we’ve done so it shouldn’t be so hard to agree on how to deal with it. If something is in the first category, having absolutely no effect on anything that happens in the world, I would suggest that the right strategy is simply to ignore it. Concepts like that are not scientifically meaningful. But they’re not really meaningful on any other level, either. To say that something has absolutely no effect on how the world works is an extremely strong characterization, one that removes the concept from the realm of interestingness. But there aren’t many such concepts. Say you believe in an omnipotent and perfect God, one whose perfection involves being timeless and not intervening in the world. Do you also think that there could be a universe exactly like ours, except that this God does not exist? If so, I can’t see any way in which the idea is meaningful. But if not, then your idea of God does affect the world — it allows it to exist. In that case, it’s really in the next category.

That would be things that affect the world, but only indirectly. This is where the dark matter comparison comes in, which I don’t think is especially helpful. Here’s Schoen again:

We presume that dark matter –if it exists–is lawful and not in the least bit capricious. In other words, it is–if it exists–a “natural” phenomena. But we can presently make absolutely no statements about it whatsoever, except through the effect it (putatively) has on ordinary matter. Whatever it is made of, and however it interacts with the rest of the material world is purely speculative, an untestable hypothesis (given our present knowledge). Our failure to confirm it with science is not unnerving.

I would have thought that this line of reasoning supports the contention that unobservable things do fall unproblematically within the purview of science, but Chris seems to be concluding the opposite, unless I’m misunderstanding. There’s no question that dark matter is part of science. It’s a hypothetical substance that obeys rules, from which we can make predictions that can be tested, and so on. Something doesn’t have to be directly observable to be part of science — it only has to have definite and testable implications for things that are observable. (Quarks are just the most obvious example.) Dark matter is unambiguously amenable to scientific investigation, and if some purportedly supernatural concept has similar implications for observations we do make, it would be subject to science just as well.

It’s the final category, things that don’t obey natural laws, where we really have to think carefully about how science works. Let’s imagine that there really were some sort of miraculous component to existence, some influence that directly affected the world we observe without being subject to rigid laws of behavior. How would science deal with that? …

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Well, Lads

It is a truth universally acknowledged that, among the world-class theoretical physicists of our time, the one with the most entertaining web page is Gerard ‘t Hooft. (Even though he would be annoyed to see that WordPress refuses to display the apostrophe in his name correctly.) See for example the Constitution for 9491 Thooft, an asteroid that was named in his honor. Sounds like a place I would like to visit, once the hotel situation has advanced a bit.

I’m mentioning it because I was struck by this succinct answer to the question, “Will the Higgs be found?” Nothing especially newsworthy, I just enjoyed the spirit of the reply.

More and more frequently, I receive letters and mails from wise people outside physics, telling me that “they know” that the Higgs will not be found, that our theories are baloney, how dare we spend billions of public funds to build machines such as LHC, “to prove, against better judgment, that our theories still stand a chance of being correct”, and so on.

Well, lads, I am not going to answer all of you in person. What you have in common is a blissful ignorance of the scientific facts concerning the Standard Model. Fact is that the W+, W- and the Z boson each carry three spin degrees of freedom, whereas the Yang-Mills field quanta, which describe their interactions correctly in great detail, each carry only two. Those remaining modes come from the Higgs field. What this means is that three quarters of the field of the Higgs have already been found. The fourth is still missing, and if you calculate its properties, it is also clear why it is missing: it is hiding in the form of a particle that is difficult to detect. LHC will have to work for several years before it stands a chance to see the statistical signals of this Higgs particle. What compounds the matter even more is that there may well be several sets of Higgs fields. If there are two, which is eight quarters of the field, we will get five Higgses rather than one. This would be a quite realistic possibility but it would make the detection of each one of them even harder, because they cause more complex statistical signals that are more difficult to predict.

Recently, Dvali, Giudice, Gomez and Kahagias proposed an extremely clever way to get around the need for an explicit Higgs particle, involving extended non-perturbative states they call “classicalons.” This isn’t the kind of thing ‘t Hooft is objecting to — these are wise people inside physics! My money is still on finding the Higgs, but it’s always good to know what the options are.

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Ghost Moon?

Does anyone know about this phenomenon? My friend Benson Farb, under the charming misimpression that I am some sort of astronomer, sent me the following image, taken by his uncle Henry Farkas, MD.

Ghost Moon

That’s the Moon on the right, somewhat overexposed. On the left is another image of the Moon — substantially dimmer. So what is going on?

Consulting the Google, I was able to find multiple examples of similar phenomena, but no explicit explanation: see here, here, here, here, here, here. My first guess was that we were glimpsing a giant Death Star that had been hiding behind the Moon, but upon further thought I regretfully concluded that it’s unlikely we would have alien invaders clever enough to build a Death Star but sloppy enough to reveal it prematurely like that.

Actually there is only one sensible explanation: some sort of lensing/reflection phenomenon that is giving rise to multiple images. The two obvious culprits would be the camera lens itself, or the atmosphere. But Henry took the picture in the first place because he saw the ghost image with his naked eyes, so the camera lens is out. Atmosphere it is! This is somewhat corroborated by the fact that different exposures show different separations between the images — something that could be explained by changing atmospheric conditions.

Ghost Moon 2
Ghost Moon 3

The atmosphere, whose layers can have very different humidity and temperature, can be a very effective reflector and refractor. Here is an image of a “sun pillar,” to show how dramatic the effects can be.

sunpillar

So I’m pretty convinced that the atmosphere is to blame. On the other hand, it’s a little funny that the images aren’t vertically aligned, which is what I would naively expect. And this wouldn’t be the first time that my lack of real-world knowledge steered me dramatically wrong. Anyone familiar with this phenomenon?

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The Fine Structure Constant is Probably Constant

Cross-posted to Sarah Kavassalis’s blog, The Language of Bad Physics.

A few weeks ago there was a bit of media excitement about a somewhat surprising experimental result. Observations of quasar spectra indicated that the fine structure constant, the parameter in physics that describes the strength of electromagnetism, seems to be slightly different on one side of the universe than on the other. The preprint is here.

Remarkable, if true. The fine structure constant, usually denoted α, is one of the most basic parameters in all of physics, and it’s a big deal if it’s not really constant. But how likely is it to be true? This is the right place to trot out the old “extraordinary claims require extraordinary evidence” chestnut. It’s certainly an extraordinary claim, but the evidence doesn’t really live up to that standard. Maybe further observations will reveal truly extraordinary evidence, but there’s no reason to get excited quite yet.

Chad Orzel does a great job of explaining why an experimentalist should be skeptical of this result. It comes down to the figure below: a map of the observed quasars on the sky, where red indicates that the inferred value of α is slightly lower than expected, and blue indicates that it’s slightly higher. As Chad points out, the big red points are mostly circles, while the big blue points are mostly squares. That’s rather significant, because the two shapes represent different telescopes: circles are Keck data, while squares are from the VLT (“Very Large Telescope”). Slightly suspicious that most of the difference comes from data collected by different instruments.

Variations in the fine-structure constant

But from a completely separate angle, there is also good reason for theorists to be skeptical, which is what I wanted to talk about. Theoretical considerations will always be trumped by rock-solid data, but when the data are less firm, it makes sense to take account of what we already think we know about how physics works.

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John Huchra

jph.2005John Huchra, a leading astronomer at the Harvard-Smithsonian Center for Astrophysics, passed away on Friday. I’m not sure of the cause, but he had been suffering from heart problems; he was 61 years old.

John was most obviously known for his scientific accomplishments, especially as a guiding force behind the CfA Redshift Survey. For you youngsters out there, this project was the pioneering effort at mapping the large-scale structure of the universe. It revealed, to the surprise of many, that there was a lot of structure out there! The iconic image of cosmology in the 1980’s was the “CfA Stickman” reveal in the famous A Slice of the Universe paper by Valerie de Lapparent, Margaret Geller, and Huchra.

CfA Redshift Survey

The stickman was not the universe being playfully anthropomorphic, it was simply the Coma cluster as distorted in redshift space. (You measure positions on the sphere of the sky, but velocities along the line of sight; converting these velocities to distances is inevitably distorted because galaxies in a cluster have peculiar motions inside the cluster.) Before this map was released in 1986, many people assumed that the galaxy distribution would be basically uniform on these scales. They shouldn’t have thought that, in retrospect (you need to go to larger scales before the uniformity becomes apparent), but sometimes it takes real data to get a point across. The survey went on to discover the Great Wall of galaxies, arguably the largest known “object” in the universe.

John had a number of other important contributions, including measurements of the Hubble constant and the discovery of Huchra’s Lens, one of the most dramatic early examples of gravitational lensing. He was also very active in the community, serving as president of the American Astronomical Society and numerous other roles.

But many of us will remember him mostly for his spirit and good humor. When I was a graduate student at CfA, he was one of the most friendly and helpful senior faculty members around, someone you were always happy to bump into in the hallways. There is a guestbook here for people to leave their reminiscences about John; he will be greatly missed.

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