[This musing first appeared in The Boston Globe, May 2, 1988.—Tom.]
On the floor of New England’s oak woodlands, the Canada Mayflower (Wild Lily-of-the-valley) is making its play for the sun. Like two greedy hands, the paired green leaves of that ubiquitous little plant are reaching for sunlight, softening the winter woods and teasing us toward summer.
Forget the skunk cabbage and the robin as signs of spring. I’ve seen skunk cabbages frozen in ice. I’ve seen robins making tracks in snow. But when you see the leaves of the Canada Mayflower pushing up through last season’s leaf litter, you know its time to take down the storm windows and put away the down comforter.
As I walked in the woods this morning I thought of a well-known line from a poem of Dylan Thomas: “The force that through the green fuse drives the flower.” My thoughts followed the green fuse of the mayflower upward, out of the warming earth, through the still-bare branches of the oaks, through twenty miles or so of blue air and 93 million miles of space to the sun. There, deep at the heart of our planet’s star, is the source of the force that drives the flower.
It’s hot at the center of the sun. About 15 million centigrade degrees hot. What causes the high temperature? Basically, the core of the sun, like the rest of its huge bulk, is gas. Hydrogen, mostly. And a lot of helium. The hydrogen and helium at the core of the sun are under enormous pressure. Half-a-million miles of overlying gas is pressing down. Squeeze a gas and its temperature goes up. The hydrogen at the center of the sun is in the big squeeze.
At 15 million degrees something remarkable happens. Hydrogen nuclei, which carry positive charges, are able to overcome their mutual repulsion and fuse together to form helium. Fuse. That word again! The green fuse. Dylan Thomas was more right than he knew. Fusion is the force that drives the flower. It is fusion at the sun’s core that makes the sun shine.
Every second the sun converts roughly 700 million tons of hydrogen into helium. And as if by some kind of cosmic magic the helium weighs less than the original hydrogen. Five million tons less. Matter has disappeared. Matter has been turned into pure energy. The old Einstein equation—energy equals mass times the speed of light squared. Every second the sun turns five million tons of its own substance into radiant energy.
Five million tons a second. Sounds like a lot. But the sun never misses so tiny a fraction of its bulk. The sun has been shining steadily for more than four billion years, every second turning five million tons of matter into energy, and in all of that time it has used up less than a thousandth of its substance.
The fuse is lit. Now the force that drives the flower begins its journey up and out of the sun. It percolates through the sun’s seething interior, absorbed and reradiated again and again. As the energy approaches the surface, it is carried along by the sun’s churning bulk, in huge convecting loops of hot gas. At last, at the furiously roiling surface, the energy is hurled into space as heat and light.
Several million years are required for the energy to make its way from the center of the sun to the surface, but once disgorged, it travels at the speed of light, outward in every direction. Eight minutes later one two-billionth of the sun’s radiant energy is intercepted by the Earth. That’s five pounds worth of the sun’s vanished mass every second, five pounds worth of sun-stuff turned into pure energy.
In summer, about a millionth of an ounce of the sun’s depleted mass falls each second onto the college campus where I teach. In winter less than half as much. A fraction of a millionth of an ounce of matter is all it takes to tip the balance of the season from winter toward summer. A fraction of a millionth of an ounce of fused hydrogen is all it takes to ignite the roots of trees and rocket the Canada Mayflower out of the ground.
“The force that through the green fuse drives the flower/Drives my green age; that blasts the roots of trees/Is my destroyer.” Dylan Thomas was a poet. Einstein was the scientist who unraveled the mystery of fusion in the sun. Thomas’ poetry and Einstein’s science have the same roots. The two men were contemporaries. They died within a few years of each other in the mid-fifties. They both perceived the essential unity of matter and energy. They both recognized in nature a physical force that drives all things, a force that is both creative and destructive, holy and terrible.
Dylan Thomas often identified with the destructive side of the force, as in the “green fuse” poem; Einstein’s optimism usually embraced the creative side. The writings of both men chronicle the painful progress of all of nature from darkness toward light.