Ken Libbrecht turns up the dial on his pressure-cooker-size chamber to 2,000 volts. As I watch, a tiny needle of ice comes shooting out the end of a probe inside what serves as the world’s premier laboratory for studying snow crystals. Most of us might use the term “snowflakes,” but meteorological definitions distinguish a single “snow crystal” from that more generally applicable term, which may also refer to clusters of crystals. Libbrecht, a professor of physics at Caltech, is the reigning authority on the formation of snow crystals, and in his brand-new chamber he is testing out a groundbreaking hypothesis about the mechanisms that underlie these microcosms of ice.
As the current flows through the probe, further needles spike off the main stem, forming a little bundle of ice branches. Libbrecht turns off the voltage and lowers the probe a few inches, sliding it down an internal-temperature gradient to a point he tells me is around minus 15 degrees Celsius (5 degrees Fahrenheit). Immediately, petal-like protrusions begin to sprout from the top of each needle, and minutes later I am looking at an impossibly delicate bouquet of flowers, each crystalline blossom a mere millimeter across.
When he began to think about ice 15 years ago, he was amazed to find how little was known. For all physicists’ understanding of subatomic particles and Big Bang cosmology, we know next to nothing about snow crystals, he says. “I thought, Gosh, there are 6 billion people on this planet. Someone ought to understand this stuff, it falls out of the sky.” Libbrecht decided that that someone should be him, and he embarked on a quixotic journey that has led him finally to a theory which makes concrete predictions about snow-crystal growth rates.
Raised on a farm in North Dakota, where “snow is usually associated with shoveling,” Libbrecht is the first to admit that it is not exactly a research priority. The work is a sideline to his real job as one of a team of hundreds of scientists building a gravity-wave detector known as LIGO (Laser Interferometer Gravitational-Wave Observatory). A collaboration between Caltech and MIT, LIGO is one of the most difficult and complicated experiments ever conducted. Its aim is to detect the gravity waves predicted by Einstein’s general theory of relativity, which should in principle be issuing from such cosmological events as supernova explosions.
Snow crystals are clearly at the other end of the scientific spectrum. Their study attracts neither funding nor fame; nonetheless, snowflakes offer the opportunity for a kind of hands-on engagement with nature that has largely disappeared from big-budget contemporary physics. As a homage to his ephemeral subject, and just in time for the Christmas market, Libbrecht has authored a book, The Snowflake: Winter’s Secret Beauty, a sparkling little gem in its own right that also offers the first substantial collection of new snowflake images since W.A. Bentley’s 1931 classic, Snowflakes in Photographs. For the new book, Wisconsin photographer Patricia Rasmussen was behind the lens, using microscope objectives and mountings custom-built by Libbrecht.
The first person to look at snowflakes from a scientific perspective was the great German astronomer Johannes Kepler. In 1611, Kepler published a treatise called “The Six-Cornered Snowflake,” in which he compared the symmetry found in snow crystals to that observed in flowers. Kepler was apt to see affinities in the most diverse objects, yet in this case he reasoned that the similarities must be a coincidence, for blossoms are alive, whereas snowflakes are not. For Kepler, every plant possessed “a single animating principle of its own,” but “to imagine an individual soul for each and any starlet of snow is utterly absurd.”
In the absence of a soul, Kepler deduced that there must be simple physical principles guiding the formation of these tiny stars. He had noted that cannonballs also display a hexagonal pattern when stacked, and conjectured that the two symmetries were related. Three hundred years later, the invention of X-ray crystallography revealed the atomic lattice underlying the structure of ice, for indeed the six-pronged symmetry of snow crystals results from the basic hexagonal matrix that forms naturally as water freezes.
Yet regular ice, technically known as Ice 1h, is just one of 14 different types. Libbrecht notes that “each represents a unique way in which water molecules can be stacked into a solid” — more than for any other known compound. “In the crystal world,” he says, “ice is an inspirational material.” All variants except regular ice exist only at extremely low temperatures or extremely high pressures, where the atoms can be crushed into dense and unorthodox arrangements.
Even normal ice is mysterious stuff. How is it that as a snow crystal grows, all six branches develop the same ornate shape? Or as Libbrecht puts it: “How do the branches coordinate the intricacies of their growth?” The real puzzle of snowflakes, he says, is “the combination of symmetry and immense complexity.” Snow crystals come in a dazzling variety of forms. Aside from the classic starlike dendrites, there are also six-sided plates, and sectored plates, plus hexagonal prisms, needles and hollow columns. There are even hollow columns capped with plates, like two wheels joined by a miniature axle.
On a voyage through the Arctic in 1820, English explorer William Scoresby made detailed sketches of the snow crystals he encountered, including some rather rare types, but it would take another hundred years before rigorous analysis was brought to bear on the problem of their formation. The giant of snow-crystal science is the Japanese physicist Ukichiro Nakaya, who in the 1930s laid the groundwork on which Libbrecht is currently building. Nakaya had received his training as a nuclear physicist, but like many young graduates was unable to find a job in his field. Eventually he took a position at the University of Hokkaido, where there were no nuclear facilities but a great abundance of snow. Inspired by the myriad forms of natural snowflakes captured by Bentley, Nakaya set about growing his own under laboratory conditions.
Natural snow crystals develop slowly as they float to the ground, the details of their branches encoding a record of the atmospheric conditions at each stage of their descent. In Nakaya’s laboratory, however, the short fall available precluded meaningful results. He thus sought to grow crystals suspended on a thread. Unfortunately, the threads quickly became encrusted with frost, and individual crystals proved elusive. Nakaya tried strings of cotton and silk; he experimented with fine wires, and with spider’s web. All resulted in icy clumps. Eventually, he found the solution in a strand of rabbit’s fur, where, as Libbrecht explains, natural oils on the hair discouraged nucleation of ice and prevented the formation of frost.
From the unlikely platform of rabbit hair, Nakaya embarked on a systematic study of how snow crystals form at differing temperatures and vapor saturations. The research led ultimately to his famous morphology diagram, which describes the kinds of crystals that form under various conditions. At minus 2 degrees Celsius, for example, Nakaya found that platelike crystals predominate. Lowering the temperature to minus 5 produced long, thin needles. At minus 15, thin plates rule again. Yet below minus 25 he found a mixture of thick plates and hollow columns. When Libbrecht began looking into snow crystals, he discovered that no one could explain the physics underlying Nakaya’s diagram.
Crystal growth in general is a well-studied topic, in part because it is critical to the semiconductor industry, but ice is a very unusual material. Its peculiar properties relate to the fact that it is surrounded by a quasi-liquid layer, a semi-melted coating first described by the English physicist Michael Faraday that deeply complicates the process of crystal growth. Much of Libbrecht’s research is focused on trying to work out how ice crystals form in the presence of this layer. He calls the theory he has developed so far “structure-dependent attachment kinetics,” a name even he acknowledges is pretty bamboozling. Then again, nothing about the formation of snow crystals is easy to comprehend. “It’s an unbelievably complicated process,” Libbrecht says. “How do you get one molecule of water to coordinate with 1018 others?”
Physicists have always been jealous of nature’s ability to self-assemble. Without the aid of computers or higher mathematical algorithms, nature effortlessly builds these minuscule marvels of ice. “People like to talk about general relativity and quantum mechanics,” Libbrecht notes, “but we should never forget that there is physics going on all around us.” Snowflakes may not grab newspaper headlines like string theory and gravity waves, yet each crystalline blossom encodes a microscopic universe of exquisitely complex science.
For further information on manmade snow crystals, or to see more images of them, visit www.snowcrystals.com.
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