Writer and physicist Alan Lightman’s take on what might be termed love at first sight.
by Alan Lightman
It is a Saturday in March. The man wakes up slowly, reaches over and feels the windowpane, and decides it is warm enough to skip his thermal underwear. He yawns and dresses and goes out for his morning jog. When he comes back, he showers, cooks himself a scrambled egg, and settles down on the sofa with The Essays of E. B. White. Around noon, he rides his bike to the bookstore. He spends a couple of hours there, just poking around the books. Then he pedals back through the little town, past his house, and to the lake.
When the woman woke up this morning, she got out of bed and went immediately to her easel, where she picked up her pastels and set to work on her painting. After an hour, she is satisfied with the light effect and quits to have breakfast. She dresses quickly and walks to a nearby store to buy shutters for her bathroom. At the store, she meets friends and has lunch with them. Afterward, she wants to be alone and drives to the lake.
Now, the man and the woman stand on the wooden dock, gazing at the lake and the waves on the water. They haven’t noticed each other.
The man turns. And so begins the sequence of events informing him of her. Light reflected from her body instantly enters the pupils of his eyes, at the rate of ten trillion particles of light per second. Once through the pupil of each eye, the light travels through an oval-shaped lens, then through a transparent, jellylike substance filling up the eyeball, and lands on the retina. Here it is gathered by one hundred million rod and cone cells.
Cells in the path of the reflected highlights receive a great deal of light; cells falling in the shadows of the reflected scene receive very little. The woman’s lips, for example, are just now glistening in the sunlight, reflecting light of high intensity onto a tiny patch of cells slightly northeast of back center of the man’s retina. The edges around her mouth, on the other hand, are rather dark, so that cells neighboring the northeast path receive much less light.
Each particle of light ends its journey in the eye upon meeting a retinene molecule, consisting of 20 carbon atoms, 28 hydrogen atoms, and 1 oxygen atom. In its dormant condition, each retinene molecule is attached to a protein molecule and has a twist between the eleventh and fifteenth carbon atoms. But when light strikes it, as is now happening in about 30,000 trillion retinene molecules every second, the molecule straightens out and separates from its protein. After several intermediate steps, it wraps into a twist again, awaiting arrival of a new particle of light. Far less than a thousandth of a second has elapsed since the man saw the woman.
Triggered by the dance of the retinene molecules, the nerve cells, or neurons, respond. First in the eye and then in the brain. One neuron, for instance, has just gone into action. Protein molecules on its surface suddenly change their shape, blocking the flow of positively charged sodium atoms from the surrounding body fluid. This change in flow of electrically charged atoms produces a change in voltage that shudders through the cell. After a distance of a fraction of an inch, the electrical signal reaches the end of the neuron, altering the release of specific molecules, which migrate a distance of a hundred-thousandth of an inch until they reach the next neuron, passing along the news.
The woman, in fact, holds her hands by her sides and tilts her head at an angle of five and a half degrees. Her hair falls just to her shoulders. This information and much, much more is exactingly encoded by the electrical pulses in the various neurons of the man’s eyes.
In another few thousandths of a second, the electrical signals reach the ganglion neurons, which bunch together in the optic nerve at the back of the eye and carry their data to the brain. Here, the impulses race to the primary visual cortex, a highly folded layer of tissue about a tenth of an inch thick and two square inches in area, containing one hundred million neurons in half a dozen layers. The fourth layer receives the input first, does a preliminary analysis, and transfers the information to neurons in other layers. At every stage, each neuron may receive signals from a thousand other neurons, combine the signals – some of which cancel each other out – and dispatch the computed result to a thousand-odd other neurons.
After about thirty seconds – after several hundred trillion particles of reflected light have entered the man’s eyes and been processed – the woman says hello. Immediately, molecules of air are pushed together, then apart, then together, beginning in her vocal cords and traveling in a spring like motion to the man’s ears. The sound makes the trip from her to him (twenty feet) in a fiftieth of a second.
Within each of his ears, the vibrating air quickly covers the distance to the eardrum. The eardrum, an oval membrane about .3 inch in diameter and tilted fifty-five degrees from the floor of the auditory canal, itself begins trembling and transmits its motion to three tiny bones. From there, the vibrations shake the fluid in the cochlea, which spirals snail-like two and a half turns around.
Inside the cochlea the tones are deciphered. Here, a very thin membrane undulates in step with the sloshing fluid, and through this basilar membrane run tiny filaments of varying thickness, like strings on a harp. The woman’s voice, from afar, is playing this harp. Her hello begins in a low registers and rises in pitch toward the end. In precise response, the thick filaments in the basilar membrane vibrate first, followed by the thinner ones. Finally, tens of thousands of rod-shaped bodies perched on the basilar membrane convey their particular quiverings to the auditory nerve.
News of the woman’s hello, in electrical form, races along the neurons of the auditory nerve and enters the man’s brain, through the thalamus, to a specialized region of the cerebral cortex for further processing. Eventually, a large fraction of the trillion neurons in the man’s brain become involved with computing the visual and auditory data just acquired. Sodium and potassium gates open and close. Electrical currents speed along neuron fibers. Molecules flow from one nerve ending to the next.
All of this is known. What is not known is why, after about a minute, the man walks over to the woman and smiles.
Happy Valentine’s Day!