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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.