Cocktail Party Physics

rubber band man

[Caveat for the easily offended: today's blog post is mildly NSFW.] The Time Lord is off gallivanting in Avignon with his fellow cosmologists, leaving Jen-Luc Piquant and me to fend for ourselves in terms of ferreting out interesting science topics. Today's inspiration comes courtesy of The Late Late Show with Craig Ferguson, specifically, his April 13 monologue in which he riffed on the NHL playoffs and various national obsessions (or lack thereof) with hockey; Canada takes their hockey very seriously indeed, while Los Angeles... not so much. He mentioned one game in particular that was delayed when fans from Sweden tossed all manner of rubber sex toys onto the ice -- presumably in celebration, or to cheer on their team, or... well, who the hell knows why they did it? Those Swedes love their sexy fun time. The point is, Ferguson wondered whether tossing a rubber dildo onto the ice, and the ensuing change in temperature, would result in what has been immortalized on Seinfeld as "shrinkage." And this, people, is a physics question. Oh, yes, it is.

Rubber, and rubber bands, are of great interest to physicists, being an example of a "deformable material," and a terrific means for studying elasticity under varying conditions (temperature, yes, but also response to electrical current). This fascination dates back to 1880, when Wilhelm Roentgen -- yes, the guy who discovered X-rays -- attached a rubber band to an apparatus and dangled an unspecified "mass" from the other end, then zapped it with electrical current to see what happened. The result: the rubber band changed in length in response to the current. (Fill in your own off-color quip here.) This was the first electroactive polymer. By 1945, scientists had figured out that collagen wires would expand or contract when dipped in acidic or alkali solutions, respectively.

So electrical current and alkali solutions cause "shrinkage" in rubber bands and collagen fibers. What about cold or heat? I can think of no better expert to comment on the amazing physical properties of rubber bands than the great Richard Feynman:

Feynman Physics Lectures: Rubber Bands by FeynmanPhysicsLectures

As Feynman says, "The world is a dynamic mass of jiggling things if you look at it right." So if heat drives the rubber band, what happens when rubber gets cold? If you're like the students who have asked this question online, you'll have a hard time getting a straight answer unless you're very, very specific about your chosen parameters for the experiment. It's hard to get a straight answer out of a scientist, not because they're deliberately being difficult, but because they know so much about the subject that they can think of innumerable variables that might affect your outcome. That said, in general, when rubber gets cold, it gets harder -- but not in a good way. It loses its elasticity and becomes increasingly brittle, and if it gets cold and brittle enough (we're talking very cold, like liquid nitrogen temperatures), it might even shatter if you drop it.

Probably the best example of the catastrophic failure of rubber at cold temperatures comes courtesy of Feynman again. Shortly before he died of cancer, he testifed before Congress in the wake of the Challenger disaster, memorably demonstrating -- with a simple cup of ice water -- that the rubber O-rings failed on the space shuttle because of the unusually cold conditions on the launch pad that fateful day. Specifically, the rubber lost its elasticity; it didn't go back to its original shape like it was supposed to, a property that was critical to maintaining sealing on the spacecraft.

Feynman Physics Lectures: Challenger Crash O-Ring by FeynmanPhysicsLectures

Yeah, okay, so that's a very bad thing. But there's good news for the sex toys tossed into the hockey rink: they likely didn't get anywhere near cold enough to become brittle and shatter on the ice. Frankly, even if they did get a little cold, it would be very easy to re-heat them using friction -- and friction, to put it delicately, is kind of inevitable during the traditional uses for these items. Zap them with electricity or dip them in alkali, and it might be a different story, depending on the kind of polymers used to make them. And here you thought physics had nothing to offer that could be relevant to your sex life!

Scientists are still interested in rubber bands. Last year, a team of physicists from MIT and Ecole Polytechnique in Paris decided to study how a rubber band "deforms" as it rolls around, of particular interest because it is elastic -- and I'm using that term in the technical sense to describe something that regains it shape after being deformed by an outside force (like one of those squeezable stress balls, for example, or a sponge). The problem: have you ever tried to "roll" a rubber band down an inclined surface, a la Galileo? "It would have taken several meters to get the loops rolling fast enough to deform, and that was larger than the size of our room," team member Cristophe Clanet told Science. Apparently lab space is at a premium these days.

The solution: put the rubber band loops into a big steel drum and spin it, like a makeshift centrifuge, so you can control the conditions of the experiment. Then you'll see, as the researchers did, that the faster the rubber band spins, the more squashed it gets. Frankly, this doesn't strike me as especially surprising, but that's because I don't know very much, and it's not my job to measure and describe as precisely as possible what's going on at a deeply fundamental level. In contrast, the scientists created intricate mathematical models for the rubber band "system" and the various forces that were acting upon it.

"They combined equations for the effects of gravity, bending forces, centrifugal forces, and internal stiffness to explain the loop's movement. Initially, the rubber band's natural stiffness balances with gravity to keep it in shape. As the rolling begins, gravity works to help the loop keep its shape. But at higher speeds of rotation, the other forces overwhelm gravity, and the loop begins to collapse."

Just this past week, rubber bands (or rather, dielectric elastomers of the sort Roentgen discovered) were back in the news, with the announcement that scientists at the Auckland Bioeningeering Institute have used them to create "wearable energy harvesters" that can convert your movement as you walk down the street, for example, into electrical energy to re-charge your cell phone. (Not surprisingly, Sminton Comics had a bit of fun with this concept relating to prophylactics.)

These "dielectric elastomer generators" are more commonly known as "artificial muscles," and are of great interest in the field of robotics, since they make terrific sensors and actuators that together can ably mimic the behavior of real muscles. I'll bet these things would also make terrific materials for dildoes, which have their own kind of underlying physics, apart from the materials from which they are made. Just think, you'd never need to replace the batteries in your vibrator! When it comes to the physics of the device's operation, it's all about oscillations, resonances, and strategic dampening of vibrations, according to the lustily curious folks over at the Physics of Sex blog:

In battery-powered models, vibrations generally come from an electric motor attached to a rotating disk, with its weight placed off-center. The principle is the same thing that causes unbalanced washing machines - which are some, other, people's favorite lovers - > to buck violently when more of the laundry is on one side of the washer drum than the other. The faster the motor turns, the higher the vibrator frequency. ...

Some dildos have a cavity that allows you to insert vibrators into them. The softer and heavier the dildo, the more it will dampen the vibrations and slow the resonance. Many manufacturers of plug-in vibrators offer soft sleeves and attachments to allow you to dampen oscillations in the same way. Damping is the reason that vibrators are more comfortable when used in the anus or vagina than on a hard penis or clitoris. A rigid clitoris or penis has much less damping than softer and more enveloping anal and vaginal tissue. The vibrational motion, and resulting energy, is transmitted at full intensity to sensitive nerves in a small area where the vibrator makes contact with the rigid tissue. In the vagina and anus, the energy is distributed to more tissue and nerves, which feels less intense.

Ahem. I just report the science news, people, I do not make this stuff up. That said, I'm delighted to finally have an excuse to post this hilarious British TV appearance by particle physicist Brian Cox, who shows himself to be quite the broad-minded good sport when host Jonathan Ross presents him with a mini-scale model of the Large Hadron Collider -- built entirely out of sex toys, including a vibrating butt plug. I'd wager Cox knows his oscillations from his resonances.

when fluids get weird

You may have noticed a few changes to the blog this week, most notably the departure of my esteemed co-bloggers who came on board in 2008. Basically, co-blogger Diandra reluctantly informed me this week that she can no longer blog for the cocktail party because she needs to focus more on her personal and professional obligations (among other things, she recently relocated to head up West Virginia University's WVNano, an interdisciplinary research center that targets materials, devices, and biomolecular systems for use in public security, health, energy, and environmental applications). Lee and I had already been chatting about her lack of time to blog, and Allyson and Calla made infrequent (though always welcome!) appearances. It just seemed like a good time to revert back to the individual format. I am sad to lose such awesome contributors, and I know the readers will miss them too -- but I told them they're welcome to pop into the cocktail party any time with a guest post should they feel the urge.

The other changes are to the design, namely, a switch to a three-column format with a few other minor tweaks. We've had the same look for five years now, and over the next few months I'll be revamping the design further with a new banner, color scheme, updated blogroll and so forth. What won't change: the content -- although the goal is to mix up with long monster posts with shorter ones, thereby reining in somewhat my habitual long-windedness. And Jen-Luc Piquant will preside as always over the festivities.

With that out of the way, we now now return to our regularly scheduled science! Via the Improbable Research blog, I came across this nifty video for New Scientist, in which a guy named Chris Chiswell jogs on the non-Newtonian fluid known as "oobleck" (basically cornstarch-mixed-with-water), until he stops jogging and sinks into it.

Oobleck is a favorite substance of mine, because it just can't decide whether it wants to be a solid or a liquid -- it swings both ways! Isaac Newton first delineated the properties of what he deemed an "ideal liquid," of which water is the best example. One of those properties is viscosity, loosely defined as how much friction/resistance there is to flow in a given substance. The friction arises because a flowing liquid is essentially a series of layers sliding past one another. The faster one layer slides over another, the more resistance there is, and the slower one layer slides over another, the less resistance there is. Anyone who's ever stuck their arm out of the window of a moving car can attest that there is more air resistance the faster the car is moving (air is technically a fluid).

That's the basic principle.  But the world is not an ideal place, and not all liquids behave like Newton's ideal liquid. In Newton's ideal fluid, the viscosity is largely dependent on temperature and pressure: water will continue to flow -- i.e., act like water -- regardless of other forces acting upon it, such as being stirred or mixed. In a non-Newtonian fluid, the viscosity changes in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior. Physicists like to call this a "shearing force": stirring a cup of water produces a shearing force, and the water shears to move out of the way. The viscosity remains unchanged. But non-Newtonian fluids like oobleck? Their viscosity changes when a shearing force is applied.

  Ketchup, for instance, is a non-Newtonian fluid, which is one reason smacking the bottom of the bottle doesn't make the ketchup come out any faster; in fact, it slows it down, because the application of force increases the viscosity.  Blood, yogurt, gravy, mud, pudding, and thickened pie fillings are other examples. And so is oobleck. They aren't all exactly alike in terms of their behavior, but none of them adhere to Newton's definition of an ideal liquid.

The substance becomes thicker, or more viscous, in response to agitation, compression or other similar applied forces: punch the oobleck, and it hardens into a solid, softening into a fluid again once the energy dissipates. Compress it into a ball and toss it in the air, and it will quickly lose its shape and flatten before it lands. (Side note: non-drop paint exhibits the opposite effect, brushing on easily but become more viscous once it's on the wall. And under rare circumstances, liquid hydrogen and liquid helium can become superfluids, exhibiting zero viscosity at extremely low temperatures.)

It's very similar to how quicksand behaves. If you want to escape quicksand, it's best not to struggle too frenetically, but slowly and patiently work your way to firmer ground. That's because quicksand is also a non-Newtonian fluid, despite being made up of fine grains of sand or silt; when mixed with clay and salt water, it becomes a colloid hydrogel. So quicksand appears solid when it is undisturbed, but even a tiny (like, 1%) change in the stress on it will cause its viscosity to decrease quite suddenly, and the person walking across it will sink into the sand, after which the sand and water mixture will separate to form something akin to a solid. (Apparently it's the salt that's the blame for that trapping power.)

What could be more fun than a big tub of non-Newtonian fluid? Oobleck: make a batch today!

UPDATE: A Facebook pal pointed me to this awesome appearance by Dr. Who's Matt Smith on British TV explaining about Non-Newtonian fluids (a.k.a. batter). Why don't we see this on The View or Good Morning America?

 

it's a dog's life, part 2

In part 1, I asked for guesses about the genetic identity of my beloved mutt, Darwin, to see if anyone was closer than the genetic test we did to try to uncover his ancestry.  Surprisingly, of the four breeds that the test identified as being present in him at some level, none of them was beagle -- or a hound dog of any type. (As Susan point out in the comments, a dachshund is a hound.  I actually was the one predicting he had some dachsund in him before we even did the test!) The test came back and told us that Darwin has contributions (at some level) of mastiff, dachshund, Saint Bernard and Shih Tzu.   

Let's back up and see whether that could possibly be true.  All the interest in doggie DNA is not just due to a lot of overindulgent pet owners.  Dogs are very good models in which to study the correlation between disease and genomics. 

A DNA Primer

Let's start with some definitions, which you probably already know, but that I had to look up.  DNA is a double helix that looks like someone took a ladder and twisted it.  The backbone, which corresponds to the long uprights of the ladder, is made of phosphate and sugar pieces.  Periodically attached to the backbone and forming one half of each rung is one of four types of amino acids nitrogenous bases (Thank you Shane): A (adenine) , T (thymine), C (cytosine) and G (Guanine).  The amino acids are particular about how they link to each other:  A bonds to T and C bonds to G.   My diagram is adapated from a diagram on wikipedia.

Genes, which are made from DNA, tell cells what proteins to make and how much of each to make.  For example, genes tell your body how much melanin to make, and that helps determine your skin color.  Some traits are attributable to single genes (like cyctic fibrosis), while other traits are due to an interwoven combination of genes. We have a lot of genes: estimate range from 25,000-35,000 genes in the average person.  The collection of genes is called the genome, and the genome is stored on one or more chromosomes.  When two creatures make a third, the child (or puppy) inherits DNA from each of the parents, although not always equitably.  Some genes are recessive, which means that they yield to genes with stronger opinions about how the body should work.

DNA is stored in the nucleus of mammals.  Every time a cell divides, the DNA has to replicate itself so that both cells have DNA.  This doesn't always happen identically - the process can be akin to a game of telephone.  If you were to examine a hundred copies of a gene in your body, you'd find a number of different variations.  Each unique variation is called an allele.    

In addition to physical features like hair color or eye color, many diseases are transmitted genetically.  (Other diseases may have a genetic proclivity, but bad habits (drinking, smoking and eating wrong) can change your risk for the disease.)  So understanding how a disease is correlated to genetics is important from the perspective of making sure a person doesn't contract that disease.

I have to say that I have been a cat person all my life.  Cats have been around for just as long as dogs (more than 40,000 years); however, humans have been much more obsessed about controlling which dogs mate than which cats mated.  (Look, if you can't even herd them, how are you going to control their sex lives?) 

With the exception of some exotic cats, like Persians or Bengals, cats look pretty much alike.  Most dog breeds were created by people in just the last 200-300 years, and our obsession with bloodlines means that you can have a population of dogs where you know exactly who is related to whom.  The diversity of traits between different dog species is pretty amazing.  You just don’t see the diversity in humans that would compare to the difference between the Chihuahua and the Great Dane.  Genetic variation between people is about five to seven percent.  Genetic variation between dog breeds is around 25-28 percent. 

In order to be certified as a purebred dog, the parents have to be certified, which means that the breeding population pool is fairly small and closed.  In fact, some breeds have been in danger of wiping themselves out because they have been so inbred.  There are more than 350 inherited disorders in purebred dog populations, with many of them specific to specific breeds.  Given our closely aligned heritage (same environment, food, etc.), it isn't so surprising that people and dogs share many of the same diseases.

I was kidding a cow researcher about chances of cancer being greater because of the larger number of teats when he informed me that cows don't get breast cancer.  Dogs, however, have many of the same diseases people get, and specific breeds are often predisposed toward specific diseases. Doberman Pinschers and Labrador Retrievers are among the few animals that can have narcolepsy.  (Narcolepsy is due to a single recessive gene.)  Chow Chows are predisposed toward stomach cancer.  Dogs DNA is more similar to human DNA than mouse DNA, making dogs a better predictor of the impact of drugs or other treatments. Dog cancers are much more similar to human cancer than rodent cancers.  Humans often participate in family studies, where a disease is studied among members of a family to try to identify genetic links.  A breed of dog is like a very large family, making dogs ideal models for understanding human illnesses.

The first dog genome to be sequenced was a standard poodle in 2003 that was about 80% complete.  Trivia:  The poodle belonged to Craig Venter. Their study showed that dogs and people have about 18,473 genes in common.  Mice and people share 18,311 genes. In July 2004, a 99% complete boxer genome was released. 

What they're looking for are variations that distinguish the breeds.  Two different types of variations have proven to be most important in correlating traits and genes.   Microsatellite repeats are areas that have different repeats of some sequence. For example, CA is frequently repeated different numbers of times.  The diagram below shows three genes that are identical, except for the numbers of times the "CA" is repeated.

The other kind of variation is single-nucleotide polymorphism, which is called SNP and pronounced "snip".  In this variation, a chain of amino acids is identical, except for a single acid.  These types of differences in ordering are key to finding out which parts of which genes are relatable to some trait. 

Classifications

So now to classify the dogs.  The AKC, which recognizes about 169 different breeds, divides dogs into classes that are primarily determined by morphology (size, size) and behavior:  sporting, hunting, non-sporting, lie-around-the-house-looking-cute, etc. Greyhounds and Italian greyhounds are in the same class, as are basenjis and beagles.

Genetically, the classes fall out a little differently.  A 2004 Science paper by Parker, et al. looked at 85 representative breeds, with five unrelated (no common grandparents) dogs per breed. They were able to demonstrate a significant genomic difference, but not necessarily between breeds per se.  When they analyzed the doggie DNA, they were able to separate the dogs into one of four clusters.  Please look at the diagram from their paper before you keep reading - you may want to refer back to it.

Cluster 1 is 14 dogs that originate from the most ancient animals in Africa and Asia.  This group includes dogs like the Malamute, the Afghan Hound, the Shiba Inu, and Basenji.  When the researchers looked at a set of wolf genes (the wolf being the most closely related relative to the dog) from eight different countries, those animals also fell into this group. 

Cluster 2 is a group of 14 dogs consisting of ‘Mastiff-type’ dogs and include the Boxer, German Shepherd, Lab, Pomeranian and Rottweiler.  These dogs have square heads and boxy bodies, but their size ranges from the toy Pomeranian to the Rottweiler. 

Cluster 3 are 8 breeds that are characterized as "herding dogs", like Sheepdogs, Collies, Greyhounds and Pugs.  (Pugs, I believe, herd gerbils if properly motivated.)

Cluster 4 is the largest cluster and the group most affected by human control over breeding.  These dogs were among the most intentional: people bred them to assist them in very specific tasks:  bloodhounds, border collies, spaniels, retrievers, setters, terriers, beagles and basset hounds.  These are animals that were bred not just to hunt, but to hunt particular animals ranging from little varmints that live underground to lions.  Interestingly, some breeds like the Pharaoh Hound and the Ibizan Hound, which are often cited as being among the ancient breeds, actually identify with this group and not the first cluster.  The authors speculate that this is because these breeds were basically recreated in the modern time from combinations of other breeds. 

Eight-five breeds is really not that many, but you would expect that if you come up with a genome for them, you ought to be able to determine the genome for an unknown dog and then tell what type of dog it is.  That's the basis of the dog testing that's offered.  BioPet (the company we used) uses 63 validated breeds, which they claim cover 92% of all mixed breed dogs.  Each breed that comes out of their analysis is assigned a level, as follows

• Level 1: Over 75% of the DNA found in your dog is from the breed listed.
  
• Level 2: Each breed listed represents between 37-74% of your dog's DNA.
• Level 3: Each breed listed represents between 20-36% of your dog's DNA.
• Level 4: Each breed listed represents between 10-19% of your dog's DNA.
• Level 5: Each breed listed represents less than 10% of your dog's DNA.

Darwin came back as Mastiff, Dachshund, Saint Bernard and Shih Tzu, all level 3.  What that basically says is that he's a mutt through and through. He's probably muttier than most, as he's got contributions from a very diverse gene pool:  One breed for each of the four clusters.  

If you search around on the web, you'll find plenty of testimonials from people who say that their purebred Komondor came back identified as a whippet, which is absurd because the two look nothing alike.  A Komondor has wooly white dreadlocks and a whippet looks almost bald.  But genetically, the two dogs are actually very close, even though their looks are far apart.  A pointer and a bassett hound are also close genetically.  So are the Saint Bernard and the greyhound.  There is an awful lot we still have to figure out about dog genes.

Below, you see Darwin's buddy Kona, who is also a mutt and 15+ years old.  Kona had laid claim on Darwin's sleeping pad, which is why Darwin is squeezed into Kona's bed.  Kona is a cannonical terrier:  he looks like a senior citizen Toto (from the Wizard of Oz). 

The questions over Darwin's origins were so widespread that he got two DNA test kits that Christmas, so we sent one to Kona to see if he came out any clearer.  Here's Kona's "papers":

So he out-mutted Darwin without a bit of terrier there.  The Bichon Frise and Keeshond, incidentally, are right next to the Manchester terrier on the chart. 

Is the measurement wrong?  Probably not.  You would naively think that dogs that look the same ought to have some similar genetic make up; however, dogs can have very similar looks and very different genes (the Italian greyhound and the greyhound are in different categories), or very similar genes and entirely different looks. In short, if you're wondering about your dog's ancestry, keep wondering because you're not likely to get definitive answers from the tests that are commercially available.  That's not to say that they won't improve in the future, but there is still much we need to learn.

Darwin, by the way, was entirely nonplussed by his results.  He laid down on the floor, yawned, and demanded a belly rub.  I'm often overly anthropomorphic, but I'm pretty sure we was thinking "I'm glad you guys were amused, but y'know that money could have been donated to the spay and neuter program to decrease the number of unwanted dogs and cats that end up in shelters."

happy darwin's birthday: it's a dog's life, part 1

About six years ago, we adopted an eight-month-old "beagle mix" from a wonderful organization called Hearts United for Animals out of Auburn, Nebraska.  HUA takes in dogs of all types and origins, especially dogs that have been prisoners of puppy mills. Dogs that aren't adoptable for whatever reason become 'shelter sweethearts" and stay at the multi-acre facility for the rest of their lives.

Being clever scientists and already having two cats named Vector and Chaos, we had to think hard about what we would name our new furry friend.  I actually came up with the name:  Darwin.  When we figured out his age (he was more likely 4-5 months old when he joined us), we back calculated that he was probably born in February.  Since Charles Darwin's birthday is February 12th, that seemed like the natural day to denote as "Darwin's birthday".

And here he is:  If he's a beagle mix, it must be beagle mixed with elephant.  Said pup is about 45 pounds.  I have to admit that I am not a natural dog person.  I have always had cats; however, my best friend once described Darwin as "a dog for people who think they don't like dogs".

Although he's calmed down considerably from his puppy days, he remains a serious mooch for affection, making everyone we meet sure that this poor dog gets absolutely no love at home since he is so starved for affection.  If you start rubbing his tummy, you'd better not stop because he will use his right front paw to direct your hand back to his belly.

Darwin is a mutt without a past:  he was found wandering on the Nebraska-Iowa border and nothing more is known about his parentage or lineage.  From my six years with him, I can tell you that he likes everyone and everything, including cats and especially small children.  He chases rabbits and squirrel, but not nearly with the enthusiasm he had when he was younger.   About the only thing he doesn't like are Dachshunds, as one bit him when he was younger and he's been a little wary of them ever since.  He also rolls his eyes when the toy poodles next door start their chorus of yaps every time he goes out for a walk, but in general, he loves tiny dogs - quiet tiny dogs.

Among Darwin's Christmas presents was one of those genetic tests that promises to tell you what breeds have contributed to your dog.  I don't know how he got the test.  He doesn't fly.  I don't think he's ever seen a SkyMall magazine and his best buddy, Kona, is a stay-at-home dog  whom I can't picture shopping over the Internet.  The rawhide present got Darwin's attention a bit more than the test, but the humans in the family were intrigued at the prospect of finding out what patchwork of chromosomes and DNA make up our much-loved buddy.

The kit you use to do the test is fairly straightforward (and shown below) :  Some cotton swabs and strict instructions not to cross-contaminate your sample.  For example, if your dog and another dog share drinking bowls, they can mix DNA.  We have it easy, as Darwin is the only dog in the family.  You swab the inside of the dog's mouth, seal the swabs in the submission envelope and send them in.

I put the pictures up not just to show off my pup,  but to invite CPP readers to submit their guesses about what the test revealed as to Darwin's ancestry. 

The test gives you four contributory breeds, each with a contribution level.  For example, a purebred poodle would be poodle (1).  A labradoodle (a labrador/poodle) would be labrador(2)/poodle (2).

What do you think?   I'll leave the post open for predictions for a while and then reveal the results. 

  

tripping the light fantastic

Jen-Luc Piquant sez, "Phooey on C.P. Snow and his outmoded 'two cultures' argument!" (She remains a diehard fan of his excellent novel, The Search.) The science-and-art interface is thriving more than ever, evidenced by our recent discovery of Finnish artist Janne Parviainen. (h/t: The Daily What) Among other techniques, Parviainen is a fan of light painting, in which "exposures are made usually at night or in a darkened room by moving a handheld light source or by movin the camera." (Thank you, Wikipedia!)

He recently produced a series of light paintings -- dubbed "Light Skeletons" -- in the snow, with light as his paintbrush and, well, the night as his canvas. The image below, straight from the camera, is not only haunting but also attests to Parviainen's commitment to his art: not even the freezing cold could stop him! See, light painting requires long exposure times and a great deal of patience. So... "When you're standing two hours in a knee deep snow in minus 20 celsius degrees taking photos, playing with fire starts to sound like a great idea, haha!" he writes. "I only burnt my coat just a little while doing this."

Few people know this -- excepting Dr. SkySkull over at Skulls in the Stars, who is a veritable trove of early sci-fi trivia -- but the emergence of photography was eerily prefigured in a mid-18th century science fiction novel by Charles Francois Tiphaigne de la Roche called Giphantie. He envisioned an imaginary world where it was possible to capture images from nature on a canvas coated with a sticky substance, which would preserve the image after it had been dried in the dark. It would take more than a century for chemistry to catch up with de la Roche’s imagination. Scientists already knew that silver chloride and silver nitrate both turned dark when exposed to light, and the first silhouette images were captured by Thomas Wedgwood at the start of the 19th century. But it still hadn’t occurred to anyone that this photochemical effect could be used to make images permanent.

Photography essentially freezes a moment in time by recording the visible light reflected from the objects in the camera lens’s field of view. The reflected light causes a chemical change to the film inside the camera, which is coated with grains of silver-halide crystals. These crystals are naturally sensitive to light. By opening a camera’s shutter for a split second, you expose the crystals to light and transfer energy from the photons to the silver halide crystals. This induces the chemical reaction, forming a latent image of the visible light reflected off the objects in the viewfinder.

If too much light is let in, too many grains will react and the picture will appear washed out. Too little light has the opposite effect: not enough grains react and the picture is too dark, as anyone who has ever taken an indoor photo without a flash can attest. Changing the size of the aperture or lens opening controls the amount of light. In modern cameras, this is the job of the diaphragm, which works the same way as the pupil in the eye. Chemicals are used in the developing process, which react in turn with the light-sensitive grains, darkening those exposed to light to produce a negative, which is then converted into a positive image in the printing process.

The modern digital camera works on the same principle as a conventional camera, but instead of focusing light onto a piece of film, it focuses it onto an image sensor made of tiny light-sensitive diodes that convert light into electrical charges. It turns the fluctuating waves of light (analog data) into bits of digital computer data. Digital cameras have contributed to the rise in light painting, too, since it's easier for artists to see the results immediately.

It was only a matter of time before artists and photographers (and artist photographers) figured out how to "paint" with light. The first to do was Man Ray, back in 1935, when he "signed" his series "Space Writing" with a penlight -- something that wasn't discovered until 74 years later, by photographer Ellen Carey. (Apparently, for Man Ray, his light painting signature was just one big in-joke.) And in 1949, LIFE photographer Gjon Mili was assigned to photograph Pablo Picasso, and produced a series of shots of Picasso making impromptu sketches with a small flashlight. Per LIFE:

Why not have him draw in the dark with a light instead of a pencil?" mused the photographer... as he was on his way to the Riviera to photograph [Picasso]. At Madoura Pottery, Mili accomplished just that; he showed Picasso some of his photographs of light patterns formed by a skater's leaps -- obtained by affixing tiny lights on the points of the skates. Picasso reacted instantly and this photo of Pablo Picasso drawing a centaur in the air was born. "This spectacular 'space drawing' is a momentary happening inscribed in thin air with a flashlight in the dark -- an illumination of Picasso's brilliance set off by the spir of the moment," wrote Mili in Picasso's Third Dimension.

 
The Interwebz tell me that doing your own light painting is remarkably simple: all you need is a camera capable of long exposures (preferably digital); a tripod (because of the long exposure times); a flashlight; and a dark location. You can create a light painting by moving the camera itself, but the easiest way is to mount a camera on a tripod and use a moving light source as your "paintbrush" -- anything from a basic flashlight or candles, to glowsticks and fiber optic light pens. You'll probably also want to use manual focus, which is better for dim light situations, and/or a slow film speed/low ISO setting. For sharper images, use a smaller aperture (f16 or f22), although this means you'll need a longer exposire time. If you want to get all artsy and blur things up a bit, larger apertures (f6) can be used.

So get out there and give it a shot one of these dark and stormy nights -- especially for anyone still snowbound in the wake of last week's blizzard and going stir-crazy being trapped inside for days. Light painting is all the rage, people: it's even made its way into cell phone commercials:

 

the unbearable lightness of LEGO

Is there anything LEGO can't build? Jen-Luc Piquant is positively giddy over a recent find while trawling Twitter. It seems science journalist Adam Rutherford (who once visited us here in Los Angeles and got to be all fabulous, attending film premieres and all) and some pals have put together a behind-the-scenes video of a guy who built a working replica of the famed (in certain circles) Antikythera Mechanism -- entirely out of LEGOs.

Allow me to plagiarize one of my own earlier posts to give you a bit of background. In 1900, a Greek sponge diver named Elias Stadiatis discovered the wreck of an ancient cargo ship off the coast of Antikythera island in Greece. He and other divers recovered all kinds of artifacts from the ship. A year later, an archaeologist was studying what he thought was just a piece of rock recovered from the shipwreck, and noticed there was a gear wheel embedded in it. It turned out to be an ancient mechanical device -- perhaps the earliest example of a geared device -- now known as the Antikythera mechanism and housed in the Bronze Collection of the National Archaeological Museum of Athens. The device was originally housed in a wooden box roughly the size of a shoebox, with dials on the outside and containing a complex assembly of gear wheels within. Its very existence offers strong evidence that such technology existed as early as 150-100 BC but the knowledge was subsequently lost. Similar machines with equivalent complexity didn't appear again until the 18th century.

It took decades just to clean it off, and in 1951, a British science historian named Derek J. de Solla Price began his life's work investigating the theoretical workings of the device. Based on X-ray photographs of the fragments, he published a handful of minor papers before the first major paper appeared in Scientific American in June 1959.

Entitled "An Ancient Greek Computer," the article detailed Price's hypothesis that the mechanism had been used to calculate the motions of stars and planets -- making it the first known analog computer.  A November 30, 2006, article in Nature included a new reconstruction of the device based on the high-resolution X-ray tomography conducted by the study. Based on the new discoveries, the mechanism has been pretty much confirmed to be an astronomical computer used to predict the positions of heavenly bodies in the sky.

And here's where the LEGO comes in, because now that the Antikythera Mechanism has been cleaned off, x-rayed, and reconstructed (at least on paper), those "blueprints" could be used to build a real working model with moving parts and everything. Out of plastic building blocks. The ancient Greeks didn't have LEGOs, otherwise I'm sure they would have done exactly what Andy Carol did. Per the Small Mammals blog:

This is a 2000-year-old analog computing device reconstructed out of Lego. It predicts solar and lunar eclipses, accurate to within two hours — all using plastic gears. Andy Carol, its designer, builds mechanical computers out of Lego as a hobby. He made this device basically because Adam Rutherford, an editor and producer at Nature, dared him to. When Adam heard that Andy had actually built the device, he called me and said, “Well, clearly we have to make some sort of film about this thing now.”

That was almost a year ago.

Read on to find out exactly how they did it. A year they spent on this -- just the film part! -- and the painstaking effort and professionalism definitely shows. This is so many kinds of win, I really have nothing much to say about it, other than to go all Keanu on you: "Whoa. Duuuudes....." Adam et al., you are my heroes.

helium: a weighty question

My research focuses on how magnets behave when you make them very, very small. (The broad answer is: really interestingly and often unpredictably.)  Like many people who study the physical world, it is much easier to get at the basic nature of a material if you cool the material to very low temperatures.  Cooling literally "freezes out" many effects, making the system simpler to understand.

Our practical understanding of temperature is primarily the thermometer.  We experience temperatures from maybe a few tens of degrees below zero Fahrenheit to 107 degrees°F (number picked because we actually hit that here in Texas last month.)  On the Fahrenheit scale, freezing is 32°F and boiling is 212°F), while the Celsius scale sets freezing at  0°C and boiling at 100°C).

On a more fundamental note, however, temperature is actually a measure of molecular motion.  The faster molecules move, the higher their temperature.  This means that there is an absolute lower limit on temperature.  The Kelvin scale, which often is more handy for scientists, is an absolute scale in that 0 K is the lowest possible temperature (corresponding to  -459°F and -273°C).  Zero kelvin corresponds to no motion, not even at the atomic level.  Needless to say, we can't just cool things to 0K.

The last I heard, the world record lowest temperature is somewhere around 100 picokelvin (which is 0.0000000001 or 10^-10 kelvin), but that's really overkill for me.  Most scientists rely either liquid nitrogen (77 K) or liquid helium (4 K).  You can get to slightly lower temperatures by pumping on these liquids.  (PV=nRT), but throwing a little liquid helium in a dewar (a fancy thermos bottle) is by far the easiest way to cool samples and get rid of those annoying degrees of freedom that get in the way of understanding.

Of course, it's not that easy.  Liquid helium (LHe) turns into a gas above 4K and liquid nitrogen does the same above 77K, so you can't just pour these cryogens from one container to another.  You have to transfer LHe using vacuum lines and keep it in a dewar so that it stays cold.  It's a minor pain to do so, but the results are well worth the trouble.

When we encounter Helium in the environment, it is naturally around as a gas. About 7% of the helium used each year is used for balloons, parade floats, etc.  Helium is the second most abundant element in the universe, but it is present on Earth at a concentration of about only 5 parts per million.  Helium is element number two on the periodic table - the second lightest element - and because it is so light, helium is easy to get moving fast, so it rapidly diffuses out of the Earth's atmosphere. 

Helium is produced inside the Earth.  When heavy elements (like uranium) radioactively decay in the Earth's crust, helium atoms are a by-product.  Most of these atoms diffuse to the surface and escape the Earth, but some of the helium gas is trapped in the Earth the same way natural gas is. Helium is usually extracted in the process of natural gas processing, but since helium is a very small fraction of natural gas deposits (it varies from location to location, but often on the order of a percent or less), sometimes it isn't economically advantageous to bother with it.

The annual global use of helium is larger than the amount of helium produced each year.  That seems impossible, but it is made possible by the fact that America controls the biggest store of helium gas in the world.  Stored near Amarillo Texas are nearly a billion cubic feel of helium gas.  The U.S. started the store in 1925 with the idea that we needed a reliable source of helium for blimps.  (Helium, unlike hydrogen, is not flammable.) 

By the mid 1990's, the Federal Helium Reserve was deeply in dept and the U.S. Congress decided that it should sell off the helium reserve and privatize the 'helium economy'.  (The linked article notes that the debt was a paper debt - the Bureau of Land Management wasn't making enough money off selling helium to other parts of the government, but it was a good excuse to let private companies take over the market.)  The reserve is supposed to be sold off completely by 2015.

In the last decade, helium prices have doubled - at least.  I know that the price I pay for helium has risen rapidly.  The graph below is from the National Academy of Science's report "Selling the Nation's Helium Reserves", just out this year.  The pink line is the 'Grade A' price, which is the price of privately owned helium.  The blue line is the Bureau of Land Management crude price.

Robert Richardson, Nobel laureate from Cornell for discovering superfluidity in the isotope helium-3, was the chair of a National Academy of Sciences study.  Richardson believes the U.S. is squandering a precious resource. 

It is always interesting to read different reports of an issue in different media outlets.  Regardless of whether the article is from the American or European press, two quotes stand out in just about every article covering his interview with New Scientist.  The first is that helium supplies will run out in 25 years.  The second is that helium balloons for kids parties ought to cost $100 each.  

Helium won't actually "run out", since the nuclear decay process is ongoing, and there are some potential processes to produce helium from radioactive decay of other elements; however, there isn't much practical difference between "run out' and "so expensive we can't afford it".  The situation is going to be the same as with oil:  as the price rises, it makes more and more economic sense for companies to separate it from the natural gas with which it is usually found; however, the price will likely rise very high.

You may think that's easy to compensate for:  We can all just increase the 'supplies' budget line in our NSF grants; however, scientists are likely to not take the most significant hits.   One-fifth of the world's helium supply is used in MRIs. The typical MRI requires superconducting magnets and, since we haven't figured out room-temperature superconductivity yet, they require liquid nitrogen or liquid helium to keep them in the superconducting state.  Most systems use a closed cycle - helium cools the magnet, warms up in the process, turns to a gas, and is re-liquefied.  A typical MRI magnet, however, requires 1700-2000 liters of liquid helium.  Older models have to be refilled on a timescale from months to years, while newer models advertise that they "never" need to be refilled.  (I'm about to buy a system like that.  We'll see how long 'never' is.)  MRI resolution gets better the larger the magnetic field.  Larger magnetic fields require larger magnets and thus more liquid helium.

The situation is unlikely to get much better, as worldwide demand continues to rise.  The graph at right (from the NAS book) shows a slight leveling off of US demand that has continued throughout 2008-2009 due to the economic recession; however, global demand more than made up for our plateau.  

The Congressional act in 1996 mandated a review by the National Research Council and one was undertaken in 2000.  That panel came to the conclusion that "privatizing the reserves should not adversely affect the production and use of the gas over the next two decades", although they did recommend some actions be taken to ensure that adequate supply would be available.

They also did what most committees that issue reports do:  they recommended that there be another study in 10 years or "whenever there was some change in supply, demand or prices".  That's the genesis of the study that Richardson co-chaired, which was released in August.  It's very interesting to compare the tenor of the two reports. Having served on an NRC committee, I know that an awful lot of the tone of the report is determined by the people on the panel - despite the often-unappreciated work of the NRC staff trying to keep things as objective and even-keeled as possible.

The Earth is 4.7 billion years old and it has taken that long to accumulate our helium reserves, which we will dissipate in about 100 years. One generation does not have the right to determine availability for ever." - Robert C.Richardson

And, of course, I have to comment on the quote about kids' balloons costing $100.  Richardson was trying to make the point that if the price of helium were determined on an open market, the cost would be so high that that's what the amount of helium in a balloon would cost.  One of Richardson's main points is that helium prices have been artificially low, they shouldn't be, and therefore the US is losing an opportunity to make money off this resource.  A lot of outlets used this comment in their headlines or taglines, but the gist was more of as a warning to parents and kids, or it came off as a comment that sort of suggested that helium was too precious to be used in trivial applications like balloons and scientists were killjoys who wanted all the helium for themselves. 

I was never trained to conserve helium - it was cheap and it was fun to watch the giant plumes of white vapor rising from the 100-liter stainless steel dewars.  We weren't purposefully wasteful, but we also didn't go out of our way to minimize how much we used.  In Europe - and increasingly in the U.S. - buildings are being designed to recover as much helium as possible and re-liquefy it.  Recovery and liquefaction is an expensive process, but (as is the case in many contexts) people become interested in conservation only when it becomes expensive to be wasteful.

snowball fight in saturn's f-ring

Some interesting news broke last week from NASA's Cassini spacecraft, which has been orbiting Saturn since, oh, 2004 or thereabouts, checking out the planet's mysterious rings. The infamous "F" ring, discovered in 1979 by Pioneer 11, is one of the most unpredictable in its appearance: it's not a smooth thin band of icy particles, but has lots of kinks in it, and is flanked by two small "shepherding" moons, Prometheus and Pandora (no relation to the Na'vi).

Here's some handy background information (adapted from an earlier blog post from 2008). Scientists have been fascinated by Saturn's rings ever since Galileo Galilei trained a rudimentary telescope on the planet in 1610 and noticed their blurry outlines. The instrument's resolution wasn't quite sufficient for Galileo to make out what those blurry bits actually were, but by 1655, the technology had improved to the point where Christian Huygens  could accurately describe them as a disk surrounding the planet. Ever-better telescopes provided ever-better views of Saturn's rings in the ensuing centuries, but eventually we had to go see for ourselves. Voyagers 1 and 2 sent back loads of images in 1980 and 1981, and as is often the case in science, the images raised as many questions as they answered.

Sure, the rings are pretty and all. But there's some really fascinating physics going on, too. Here's what we do know: The rings are made of lots of small particles that range in size from microns (very tiny) to meters, with the bulk falling in the range of the size of seashells to the size of surfboards, as Burns colorfully described them. Those particles have clumped together as they orbit Saturn, "innumerable small objects orbiting a dominant central mass." They're made of water ice, mostly, with bits of dust and other chemicals creeping in from time to time.

What don't we know? Well, for one thing, scientists aren't sure how old the rings are; Saturn's rings guard that secret as zealously as any aging star in Hollywood. The rings could be a mere 100 million years old, given the relative brightness and purity of the "water ice" that make up the ring particles. Incoming meteoric dust should have darkened the rings substantially by now. They might be ancient, dating back to when the planet first formed. Maybe the particles are the remnants of an old moon broken apart by a collision. Those pieces in turn collided with each other, losing energy in the collisions while still conserving angular momentum, with the end result that the systems flattened out into the thin disks we now see around Saturn.

The most likely circumstances for such an initial "trigger collision" (an old moon with another celestial object) existed during a period known as the Late Heavy Bombardment (would Wikipedia lie to me?) some four billion years ago. But even so, if collisional physics were the sole mechanism for the ring formation, there would be more diffusion, i.e., the edges would be blurry. They're not: they're sharply delineated, although they do show evidence of "sinusoidal scalloping" along the edges: waves that kind of "flow" along the rim. Also, there are areas where the ring's texture becomes ropy in places and strawlike elsewhere, indicating that the simple angular momentum model is insufficient to explain all the rings' features.

There are three main rings, rather unimaginatively dubbed A, B and C, by far the densest of the rings with larger particles. There are also several fainter rings (Dusty Rings) that have been discovered in recent years. The D ring is faintest, and closest to the planet itself. Just outside the A ring, one finds the narrow F ring, and beyond that are two very faint rings known as G and E.

While we tend to think of Saturn's disk as a series of little ringlets, there are very few true "gaps"  in the disk. The best known gap is the Cassini Division that separates rings A and B, and has its own Huygens Gap near its inner edge; the inner C Ring contains the Colombo Gap, while the outer C Ring contains the Maxwell Gap; and within the A Ring we have the Encke and Keeler Gaps. Those gaps aren't always empty, either: both the Encke Gap and the narrower Keeler Gap contain embedded moons, for instance. In 2006, Cassini images revealed evidence of four tiny "moonlets" in the A ring, roughly 100 meters in diameter, too small to be directly observed. But they produce propeller-shaped disturbances that span several kilometers. Scientists now believe the A ring could contain thousands of similar objects; 250 "propeller" moonlets have been detected to date.

Why do things like these gaps occur? Perhaps the spreading outward is interrupted by destabilizing orbital resonances -- essentially gravitational relationships -- that arise when the local orbital period of a particle corresponds in a simple ratio to the orbital period of a nearby satellite (a moon or moonlet). For instance, the G ring is basically a faint, rubble-filled arc that orbits Saturn seven times for every six orbits of Mimas, one of Saturn's moons. So that would be, say, a 7:6 ratio orbital resonance. At these resonant locations, a moon's gravity can stir up a ring's mass distribution, giving rise to spiral density or bending waves, which transfer angular momentum between the moons and the rings and ultimately drive them apart." Such a process (a kind of shepherding mechanism) would serve to "truncate rings and pry open gaps." Should Cassini identify changes in any moon's motion, this hypothesis could be verified.

Scientists are interested in studying all these features of Saturn's rings in greater detail because the processes at work are analogous to other systems, including protoplanetary disks (the nurseries where nascent planets are born), our own asteroid belt, the formation of spiral galaxies (such as the whirlpool galaxy), and even the accretion disks around black holes. It's not the exact same processes at work, but there are striking similarities, so the expectation is that studying Saturn's rings can teach us about the mysterious underlying physics of some of these other celestial objects, as well as the origins of our own solar system. How do hordes of orbiting bodies crowd together? How does material accumulate in such systems to form moons or planets? And how do nearby masses disturb -- and are in turn perturbed by -- adjacent disks?

The latest images from Cassini shed some light on what might be going on specifically inside the F ring, giving rise to its uniquely complex structure. The culprit: Prometheus! Apparently the little potato-shaped moon can't leave well enough alone; it's just got to shake things up now and then. Every time Prometheus passes by the F ring, all the tiny icy particles within it get agitated by the moon's gravitational pull, and start engaging in the particle equivalent of slam-dancing.

These rippling perturbations create "streamer channels," and the particles start to bunch up and clump together into much bigger "snowballs" -- and I do mean big. Some measure as much as 12 miles in diameter, which means they are large enough to have "self-gravity" -- that is, they can attract more icy particles and continue to grow.

Prometheus swings by this particular segment in Saturn's F ring roughly every 68 days, and most of the smaller snowball clumps get ripped apart with the next pass. But that's not enough time for the streamer channels to subside, and with each pass, more channels are created and complicate an already complex kinky pattern. It's like a giant churn in space.

"Every time [the snowballs] survive an encounter [with Prometheus], they can grow and become more and more stable," said Cassini imaging scientist Carl Murray of Queen Mary, University of London. While it's likely that even the largest snowballs have a lifetime of just a few months, the new data could improve our current understanding of how space debris gets transformed into denser objects.

You can see this whole process in action in the animation below. Cassini scientists believe this could be why the F ring's structure is so surprisingly complex. And it just might help explain another mysterious objects first detected by Cassini scientists back in 2004: it's called S/2004 S 6 -- being too new to warrant a more formal moniker -- measures between 3 to 6 miles in diameter, and every now and then it bumps into the F ring and produces a spray of debris. Maybe it's one of those giant surviving snowballs. Maybe it's something else entirely. We'll have to wait to see what else Cassini tells us.

FROM THE ARCHIVES: singing sands

In honor of Comic-Con, here's an early post from 2006 inspired by an episode of Dr. Who, about the phenomenon of singing sands. Sand: a marvelous substance!

here's a classic story line in Dr. Who, in which the time-traveling TARDIS crew crash-lands in the middle of the Gobi Desert and conveniently bumps into famed explorer/trader Marco Polo and his caravan. They are on their way to the palace of Kublai Khan. There's a great deal of convoluted plot, including a young Chinese woman on her way to an arranged marriage with an elderly man, and a devious Mongol warlord; also, Marco Polo becomes enchanted by the TARDIS (he calls it a "magic caravan") and confiscates the key, forcing Dr. Who and his entourage to travel with him to the Kublai Khan's stately pleasure dome. Along the way, they witness a strange desert phenomenon: singing sands.

One doesn't expect a high level of historical accuracy in a campy sci-fi series, but Marco Polo did claim to have journeyed across the Gobi Desert, and said the dunes filled the air "with the sounds of all kinds of musical instruments, and also of drums and the clash of arms." It's a fairly rare phenomenon. Only about 30 dunes worldwide are musically inclined -- the rest, apparently, are tone deaf -- although astronomers and geologists suspect that the sandy hills on Mars might also be alive with the sound of music.

Marco Polo's storied adventures took place in the 13th century, but accounts of singing, or booming (and sometimes squeaking) sand dunes date as far back as 9th century China. According to an ancient manuscript, people used to climb Mount Ming-Sha-Shan (now known as the "Singing Mountain") on a special festival day, and slide down the sand, producing the sound of rolling thunder. Centuries later, Charles Darwin commented on singing sands while traveling in Chile.

Popular folklore has blamed underground rivers or even genies; Marco Polo blamed evil spirits.  Scientists have also puzzled over the phenomenon. There have been numerous hypotheses as to the cause, most centered on the notion that it comes from vibrations of the dune as a whole, not the individual grains of sand acting in concert (pun intended). For example, some surmised the effect was similar to how a violin bow moving across the strings produce a musical tone, with the sandy sounds coming from blocks of sand stick-slipping across the body of a dune. Another instrumental hypothesis likened the effect to how a flute produces a pure tone via resonating air in the hollow tube.

It turns out that neither of these hypotheses are right. A team of scientists -- hailing form the University of Paris, the CNRS Lab in Paris, Harvard University, and the Universite Ibn Zohr in Morocco -- have conducted field studies all around the world, supplemented with controlled lab experiments, and published their findings in Physical Review Letters. They found that the sounds come from the sliding motion of large-ish grains of dry sand acting together -- a very active field of study known as avalanche dynamics. (You can listen to the nifty sounds they recorded from sand dunes in China, Oman, Morocco and Chile here. And if you want to tour the globe and visit those melodic dunes yourself, check out this map.)

Apparently, the team stumbled on their insight a few years ago by accident while studying the formation of crescent-shaped sand dunes in Morocco. As they scrambled up a particularly steep dune, they set off an avalanche, producing a 100-decibel singing sound. And they found they could reproduce the sound by sliding down the dunes with their legs swung out. (Yes! Just like those 9th century Chinese!) They took recordings back to the lab in France and managed to replicate the same sound in a donut-shaped sandbox.

Specifically, they were able to measure vibrations in the sand and air, thereby detecting surface waves emanating from the avalanche. According to head researcher Stephane Douady, the face of the dune seems to serve as a kind of loudspeaker and the surface waves produce the sound in the air. The waves result from collisions between individual grains -- about 100 times per second, according to the lab measurements -- that create a kind of feedback loop of synchronized collisions at a specific frequency, and voila! The dune begins warbling a sandy song. It just so happens that if sand is packed tightly, it can't move without expanding its volume. Douady thinks that a dune avalanche serves to compress and decompress air among individual sand grains, and this causes the singing sound. In fact, the grains themselves are unique: round, with a coating of silicon, iron and manganese.

That doesn't solve all the outstanding questions about sand, which is pretty fascinating stuff, from a physics standpoint. It's a type of substance known as granular material, since it acts both like a liquid and a solid: dry sand collected in a bucket pours like a fluid, yet it can support the weight of a rock placed on top of it, like a solid, even though the rock is technically denser than the sand. So sand defies all those tidy equations describing various phases of matter, and the transition from flowing "liquid" to a rigid "solid" happens quite rapidly. It's as if the grains act as individuals in the fluid form, but are capable of suddenly banding together when solidarity is needed, achieving a weird kind of "strength in numbers" effect. (Jen-Luc finds this metaphor rather inspiring, but she's feeling a bit emotional at present.)

Nor can physicists precisely predict an avalanche. That's partly because of the sheer number of grains of sand in even a small pile, each of which will interact with several of its immediate neighboring grains simultaneously -- and those neighbors shift from one moment to the next. Not even a supercomputer can track the movements of individual grains over time, so the physics of flow in granular media remains a vital area of research. Among other agencies, NASA is funding work in this area.

The frivolous summer pastime of building sand castles might also be considered interesting physics. Everyone knows sand has to be wet to build sand castles -- just enough to cause the dampened grains of sand to stick together via surface tension. (Jen-Luc Piquant has experimented extensively and found that the perfect ratio for an optimum castle is one pail of water for every eight pails of sand.) The water forms "liquid bridges" between the contact points of the grains, and the resulting tension creates an attractive force between them.

A few years ago scientists at MIT and Clark University studied sand in a rotating drum, observing that the sand would build up into a pile and reach a steep slope before collapsing. (I believe physicists call this self-organized criticality, which I've always thought to be a pretty nifty moniker.) They did this with both dry and damp sand, and found that adding even a tiny bit of water made the grains stick to each other more effectively, so that the pile could reach steeper angles and collapse less drastically than when the sand was dry. And just like sand castles collapsing on a beach in clumps, some of the "bridges" in the experiment stayed intact.

Quicksand is another special case, since it's a mixture of fine sand, clay and saltwater. At rest, it's little more than loosely packed grains of sand resting on top of water. But if some unfortunate soul happens to fall into quicksand and start flailing about, the movement transforms that delicate balance, turning it into a dense liquid soup. This means the victim sinks deeper, at which point the the water and sand starts to separate again into water-rich and sand-rich levels. The wet sand sediment becomes densely packed, trapping the victim. Fortunately, the average density of a human body is about 62 pounds per cubic foot, much less than the 125 pounds per cubic foot of most quicksand. So the trick to survival, apparently, is not to panic.  Instead, relax, stay still and stretch out on your back to increase your surface area, and then just wait until your legs pop free.

Studying quicksand could lead to insights into the shifting of wet soil underground during earthquakes that causes buildings to sink into the ground (soil liquefaction). The water-soaked soil "liquefies" because of the vibrations of the quake. The shock waves compress the soil faster than the water can escape, raising the pressure so that the water bears more of the load than the sand. And the buildings start to sink. For safety reasons, obviously scientists would like to understand this process better, perhaps developing better building foundations to prevent liquefaction.

My point is that you think you know a substance, it's so familiar, you see it every day, but somehow it keeps surprising you with unexpected or inexplicable behavior. That's how scientists feel about their ongoing relationship with sand and other forms of granular materials, which is looking like a pretty long-term commitment, thanks to this constant element of surprise. So while you're hanging out at the beach this summer, attempting to escape the sobering news of the day, take a moment to appreciate the intricate complexity that can be found in that collection of millions of grains of sand.

why my christmas shopping isn't done

Most people are scurrying around malls this time of year looking for those last-minute presents.  My colleague Yoda, however, got sidetracked during a visit to an outdoor mall called Watter's Creek at Montgomery Farms in Allen, TX.  He described a tree with "decorations on it that look like lights are falling.  They're plastic tubes with falling lights inside.  Tell me how that works."

He was so enthusiastic about the decorations that I made a trip Tuesday night up to the Mall to see them.  I normally avoid malls like The Plague this time of year; however, I'd just returned from some dental work that left me not feeling like getting much work done, the temperature has been in the sixties and it's a short trek up the freeway.  I was skeptical:  the decorations on our house consist of candles and red bows, with some greenery over the fireplace and on the stair railings.  And the mall in question is very close to Frisco, which was recently documented in a really interesting book about how a couple of Texas families celebrate Christmas.  If you look on the Amazon website for the book Tinsel: A Search for America's Christmas Present, you can see an example of an understated Texas yard at Christmas time.  I'm not a big fan of the 'bust the electric bill' mode of decorating, so I wasn't expecting that the effect would be impressive.  But Yoda was right: it is pretty cool.

I've shown two pictures at different times on the right.  It would be easier to see if they didn't have the blue globes on the tree (I told you I'm a decorating minimalist) , but I guess they wanted something you could see during the daytime as well.  I've highlighted one device with a faint yellow ellipse and pointed to it with a grey arrow.  See how the light is positioned at different places with respect to the static blue globes (So those blue globes are good for something - they are constants in my tree experiment.)

It didn't take long to figure out how they worked.  I was actually hoping they would be a little more science-y, but they work on the simple principle of persistence of vision -- the same thing that allows us to view movies. 

Or so I thought.  In film school, many years before I decided to stick with physics because I wouldn't have to spend my life begging for money and listening to idiots criticize my work, I remember learning the theory of 'persistence of vision'.  You've probably seen those little books that have drawings in the upper right hand corner.  You flip the pages fast enough and it looks like the drawing is in motion.  In film, the minimum rate to make things appear to move is 16 frames per second, although most modern films run at 24 fps.

Your eye works like a movie projector.  The image comes in through the lens and is projected on your retina, which is like the movie screen.  (It's projected upside down and your brain is smart enough to account for that, but that's another blog.)  Persistence of vision says that there is an afterimage on your retina - the image remains on your retina for another 40 milliseconds after the object creating the image is gone.  The entry for afterimage on wikipedia has an example of a negative afterimage.  If you stare at a word written in red on a green background for 20-60 seconds, then look at a blank piece of white paper, you'll see the word in cyan on a magenta background.  (cyan in the complement of red and magenta the complement of green.

The original theory of persistence of vision claimed that the reason people see motion from still pictures is because the eye sees an afterimage, which persists long enough for the next frame to appear and take its place, giving the eye a continuous image where there actually isn't one.  I learned this in the early 1980's, although apparently back in 1912's, people realized that the ability to see motion from still pictures is not due to the eye's mechanical persistence of vision.  It's due instead to complex phenomena in the brain called 'phi phenomenon' and 'beta movement' that psychologists and film critics are still somewhat confused about. 

A website from Purdue compares the beta and phi effects.  If you go to their website and click the 'phi' button, see if you can make the image transition from two separate dots blinking on and off to a single dot moving left and right.  I can see both, but one is easier for me to see if I move back from the computer screen and sort of stare.  (I'm still digesting the detailed difference between beta and phi, but if you're interested, ignore how annoying it is to have to click for each line of text in their presentation and look at the numerous neat demos toward the end of the presentation.)

Even if you look at the photos I took really quickly, you still won't see any motion, so take a look at the video below that shows the same kinds of lights as those I saw at the mall.  (You really only have to look at the first 45 seconds or so to get the idea, but it's a nice video.)

The first thing you notice when you look at the lights close up is that they are LEDs.  The cool color is the giveaway, since LEDs emit a very different spectrum of light than incandescent lights.  Incandescent lights are much warmer in color.  The fancy dropping light fixtures are called Snowflake LED lights or LED Motion lights or similar names and they are expensive, which is why you probably won't see them on houses this year.  LEDs or Light Emitting Diodes are gaining popularity with consumers because they don't get hot and use a lot less energy than incandescent bulbs.  They also respond much faster.  An incandescent light bulb produces light because electrons moving through a high-resistance filament make the filament hot.  It takes time to raise the temperature of the incandescent light's filament to 2500 degrees Celsius (more than 5000 degrees Fahrenheit), which is how hot it needs to be to provide light.

In contrast, LEDs are semiconductor diodes that produce light when electrons combine with holes.  A hole is a place where there could be an electron, but there isn't.  The combination electron/hole pair has less energy than the electron and the hole have by themselves. They excess energy is emitted in the form of light.

The electron-hole recombination process happens much more quickly (and much more efficiently) than heating a filament.  LEDs turn on almost instantaneously, while incandescent bulbs may take 200 milliseconds to start producing light.  That may not sound like a lot of time, but if you're going 70 mph, you travel 20 feet in 200 milliseconds.  That gives you twenty extra feet to avoid the car in front of you when it slams on its brakes.

The fast response time allows the LED snowfall lights to work.  The diagram below shows the sequence of lights with time going to the right.  Groups of lights are lit at different times and your brain interprets that sequence as motion, much in the same way the racing lights around a cinema marquee give the impression of motion.  That's much easier to make happen with LEDs than with incandescent bulbs.  

As I mentioned, the lights are a little pricy.  Five double-sided tubes, each about 39 inches long have 240 LEDs each and are between $700 and $800.  The tree was probably 50 feet tall and there were at least 25 LED fixtures, so we're talking about a whole lot more than the price of a dozen red velvet bows at Michaels.

So that's my excuse for why my Christmas shopping isn't done.  Between the lights and the convection currents in the pond that were made visible by a thick film on the water's surface, I didn't make it to a single store before closing time. 

I guess one might say being perennially behind is the price you pay for being easily distracted. On the other hand, it's sort of nice that the wonder and amazement of Christmas that one had as a child can be recreated (at least for a few moments) by something as mundane as a bunch of electrons and holes pairing up in a brilliant, well-orchestrated process.