KENSINGTON — In his essay, ''The Value of Science,'' the late physicist Richard Feynman comments on a report citing a two-week half-life for radioactive phosphorus in a rat's brain.
''Now what does this mean?'' he asks ''It means that phosphorus that is in the brain of a rat -- and also in mine, and FTC yours -- is not the same phosphorus as it was two weeks ago. It means the atoms that are in the brain are being replaced: the ones that were there before have gone away.
''So what is this mind of ours: what are these atoms with consciousness: Last week's potatoes? They now can remember what was going on in my mind a year ago . . .''
Somehow the memories that were in last year's meat loaf are now in last week's potatoes. Lifelong memories survive in a flux of atoms; the very same atoms that once stored the memories of Montezuma and Michelangelo, and that made the bodies of every ancestral creature since the beginning of time on earth, are the same atoms that now make our bodies and hold our memories.
Memories and shapes are transient and superficial, but the underlying matter is eternal. The evening breeze carries atoms breathed by dinosaurs and metabolized by ammonites and trilobites, and by Hittites and Israelites. Pieces of Chernobyl, Challenger, Hiroshima and Rome and Dresden are in the air, along with atoms from Arabian and Texas and Venezuelan oil, plus soot from Kuwait and Krakatoa. Caesar's flesh and Moses' breath -- here, now, around us, in us, in each breath and glass of water.
Several falls ago, I was a ''science adviser'' on a movie about the planetary-scale movement of materials. The producer invited me a ''shoot,'' in western Virginia, of a woman who grew ''special tomatoes.'' He wouldn't elaborate. A surprise, he said.
The tomatoes were being grown in sand in an air-tight greenhouse that sealed them from contact with the outside world. The only material the plants had access to arrived by way of the woman's body: her breath and her urine (fortified with phosphate from cokes, and diluted by the sheer volume of intentional excess water) supplied the entirety of the plants' substance. Throughout the growing season she had stayed for several hours each day in the sealed enclosure, exhaling carbon dioxide to be taken up by the plants.
We photographed the woman and the tomatoes made of the water and phosphorus, potassium and carbon and nitrogen that had been part of her. Naturaly, no one in the film crew wanted to sample the tomatoes, to participate in that cannibalism by proxy -- until, that is, the producer's young daughter reached for one.
The child's move was at least partly possible because of her ignorance of the standard view of the frank dirty earthiness of such matters. But that was the intent of the movie, to show how matter moves in the world, how it has been cycled and recycled for billions of years, from plant to animal to air to water. The material of those tomatoes had been the substance of their grower -- as it had previously been the substance of the countless other creatures in the long history of life on earth.
In each breath, we inhale millions of molecules of carbon dioxide exhaled by any living adult person, and about five molecules of carbon dioxide exhaled by any 33-year-old person who lived 2000 years ago. In each breath every person re-inhales more than millions of molecules of carbon dioxide that he or she has previously exhaled, and rebreathes hundreds of millions of air molecules inhaled earlier in his or her life -- or that had been inhaled in the lifetime of any person that ever lived.
All the air in the world has been breathed many times. All the water in the world has passed many times through land animals in the history of life on earth; more than half it has been in the bodies of dinosaurs. Last month's Appalachian rains brought the water in this morning's coffee, water that was in someone else's coffee last week or last year; the cup contains water molecules that were drunk one morning by Ben Franklin, and by Hannibal -- by any person, wherever, whenever.
Nearly all the carbon in the world has been in the bodies of living things. And it keeps moving. The amount of carbon in your body that was in the body of Alexander, or Caesar, or Cleopatra is enough to be visible under a small magnifying glass.