A Personal Voyage

by Shane L. Larson

Today would have been Carl Sagan’s 78th birthday.  This December, it will have been sixteen years since he left us, returned to the star stuff from whence we all came. I was first exposed to Carl in the fall of 1980, a few weeks after my eleventh birthday.  Cosmos was broadcast for the first time that fall, and my parents somehow had known to sit me down in front of our boxy television set and let me be enchanted by Carl’s poetic rapture with everything that is, or was, or ever will be.

Cosmos was not full of the dry, pithy rigidity most of us remember from our science classes.  Carl was a master of exposing the magic of science, seemingly unafraid of voicing the inner wonder that the beauty of the natural world inspires, carrying us along on his personal voyage with flair and poetic majesty.  Many of my scientific colleagues find the lyrical narrative of Cosmos annoying and disingenuous.  They feel like it belittles science, taking away some of the deep meaning of our quest to understand the secrets of Nature. But for myself, I find it exhilarating and uplifting; to this day it has the power to enchant me.  Language, like science, is one portal into the human spirit, and it is the only way by which we can communicate the wonder and mystery of the Cosmos from one person to another. It’s not grandiloquence; it’s a passionate attempt to express the deep inspiration and magic of Nature that moves us as scientists.

If there is any essential consequence of my exposure to Cosmos, it is that Carl’s voice is the voice that rings in my head when I read almost anything about science. When my profession has been boxing me soundly about the ears, or I get depressed about the state of science literacy in modern America, or I need some simple soul massage, I always turn to Cosmos.  It lifts my spirits, it reminds me of why I do this — why I do science — in the first place. In many ways, Cosmos is why I write for this blog every month.  I aspire (perhaps foolishly) to provide a mechanism for someone else to discover science, the way Carl helped me discover science.

So, in honor of Carl’s birthday, let me tell you about one small weave from the tapestry science. This is a story that evokes in me a little bit of wonder about the natural world, and astonishes me with the ingenuity of our curious young species. This is a story about how we have come to understand the bejeweled structure of matter.

One of the great wonders of modern science is that we have extended our ability to see far beyond the reach of the senses that Nature provided us through a billion years of biological evolution.  There are many places our curiosity leads us to where our biological sensors are simply not designed to work. This is the case when we peer deep into the vast reaches of the Cosmos, and it is the also the case when we peer inward to see the microstructure of the world.  In both cases, we have invented technology that allows us to see.

Suppose we look inward.  What is the matter around us made from?  How is it put together?  Are the micro properties of matter connected to the things that I can see on the scale of fuzzy bunny slippers and wine glasses and fly swatters?  Most of us at some point early in our education learn that the world is made of atoms, and learn to draw the classic picture of atoms as a dot (the nucleus) ringed by a set of ellipses threading other beady little dots (electrons).  Another axiom that we learn is that the stuff of the world is actually made from connected groups of atoms, called molecules.  How do we know that?

Crystals of table salt showing cubic symmetry (left), and snowflakes showing hexagonal symmetry (right).

That matter has underlying structure seems almost obvious when you look at some objects.  A single crystal of table salt has spectacular cubelike symmetry; snowflakes unerringly show hexagonal symmetry.  Many states of matter form themselves into regular structures that physicists call a lattice.  The exact shape and layout of the lattice depends on the properties of the atoms that make it up.  Salt crystals are made up of an alternating array of sodium and chlorine atoms.  Atoms use electrons as hooks to link themselves together (something our friends in chemistry call “valence bonds”), and where those hooks are in the atom affects the shape of molecules that include the atom.  Sodium and chlorine can link together in straight lines, and so naturally settle into a rectangular form when piled together.

A model of the lattice of table salt (sodium chloride, left) and of a water molecule (right).

By contrast, snowflakes are made from water molecules, which are comprised of a single oxygen atom bound to two hydrogen atoms.  The structure of the hooks on the oxygen atom keeps the hydrogen atoms from lying in a straight line, so the molecule ends up being bent at an angle of 120 degrees. That angle is one of the symmetry angles of a hexagon!  When you lay water molecules down together to form the ice crystal of the snowflake, they naturally take on the famous hexagonal symmetry because that is the easiest way to lay side by side.

It sounds plausible and reasonable, but how do we know this? Atoms and molecules are far too small to be seen by human eyes, but being clever students of the natural world we have learned how to use other physical phenomena as probes that are capable of teasing out mysteries that our eye cannot discern. One such phenomenon that can be exploited for this purpose is light, particularly light that we cannot see with our eyes, like x-ray light.  X-ray light is very energetic, and it can fit into very small areas (like the spaces between atoms!).  If one illuminates a material, like a salt crystal, with x-ray light, the light worms its way into the spaces between the atoms then bounces out of the crystal where it combines with x-ray light that bounces out of other parts of the crystal.  The result is a dizzying array of bright dots whose spacing and geometry are a representation of the structure of the crystal.

A single salt crystal (left) will, when illuminated with xrays, create a geometric pattern representative of the underlying crystal structure.

You can do a similar experiment at home with a CD and a laser pointer. If you shine the laser pointer on the surface of a CD and look at the reflection on the wall, you’ll see an array of dots — the spacing of those dots depends on the structure of the tracks on the CD that encode the music you listen too.

You can probe the structure of a CD using a laser pointer at home. The pattern is called a diffraction pattern.

Perhaps the most famous and inspiring picture taken of matter by this technique is an image known as “Photo 51.” It was taken in 1952 by a young scientist named Rosalind Franklin and her research student, Raymond Gosling. It is the first picture ever taken of DNA, the master molecule of all life on Earth.  Before Franklin’s work, the double helix structure of DNA was unknown.

Rosalind Franklin (left) and the famous Photo 51 (right) that revealed the double helix structure of DNA.

The structure of DNA had been hinted at, but the geometry and shape were confused.  Franklin had been the first person to understand that the nucleotide bases pointed inward from the double helix spine, a fact that she had personally relayed to Francis Crick and James Watson.  Unknown to Franklin, Photo 51 had been shown to Watson, a turning point event that helped inform Watson and Crick’s  ultimate deduction of the double helix structure.  Franklin and Gosling’s paper on the structure of DNA was one of three that were published simultaneously in the journal Nature in 1953.  Franklin went on to continue her work in imaging, notably working on the characterization of the polio virus.  Sadly, she died of ovarian cancer in 1958, at the age of 37.  Watson and Crick, together with their colleague Maurice Wilkinson, were awarded the Nobel Prize in 1962 for the discovery of the structure of DNA.

Franklin’s story, like so many tales of scientific discovery, is now lost to us, taken when she died. We can’t know the wonder she and Gosling must have felt when the shadows of the double helix first emerged from their film, but we can imagine.  It is that imagining that is important now, long after the glow of discovery has faded. Imagining the simple joy of discovery after a long personal voyage; imagining those few moments when you and no one else knows some deep secret wrested from Nature.  Those are altogether human moments, when our scientific minds are flooded with endorphins and overwhelmed by the joy of discovery.  Those are the moments that we live for.

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