To mark time, we decided to go to the formal gardens and to the Ernest Rutherford exhibit, or, as they call it here, “Rutherford’s Den.” Now that we might have a few extra days, I wanted to think about Rutherford, this man from New Zealand; I wanted to think about how he had hollowed out the world, taken the measure of its emptiness; how, like Henry Moore, he had urged us to think of space as much as of substance; how he had turned substance into space. “This is where it all began,” Varner was saying, as he looked into the tiny exhibit room where the tall Rutherford was holding aloft an intricate glass tube. “This is where we really begin to understand.”
There was a day not long ago when matter was thought to be solid and impenetrable, just as it appears. This table to which we have moved in the sunny garden, this table on which I am leaning, supports my coffee cup, my writing pad, my elbow. It supports Varner’s journal, which is lying open and empty before us. Like Sir Arthur Eddington’s famous table,* this one is everything we could hope for: it presses upward against our arms, remains flat and level against the shifts of weight we impose upon it. It is the perfect surface: opaque, rigid, a thing to be counted on. A fine material object.
The English chemist John Dalton would have recognized this table. He would have recognized its properties as mere extensions of the tiny atoms he had been thinking about for years, atoms that were round and massive and hard. There was nothing smaller than Dalton’s atom, and inside there was no space. Like clay spheres in a clay ring, they had no moving parts. As the Greek atmos implied, they were the “uncut,” and, for Dalton, the uncuttable. Solidity, the solidity of this table, was nothing less than one should expect, given the robust atoms of which it is composed.
The world of objects, at its unseen heart, was a collection of microscopic spheres, variously configured bound into aggregates. These spheres had one fundamental and ultimately knowable property, and that was weight. The passion of Daltonian chemistry was the passion for atomic weights.
Atomic weight. We bring that term out from storage, dust it off, recall having heard it somewhere. Chemistry! It is back there with words like mole and chemical formula, words we would rather forget, the intellectual equivalents of spinach and broccoli. I once dreamed of atomic weights: myself on one side of a huge pan balance, metallic cobalt being shoveled from a pickup truck onto the other. All I had to do was count the atoms and I would have the atomic weight of cobalt. I woke up sweating, wondering where I was.
Dalton was far more elegant. While his method could not get you the actual weight of an atom, it could get you its relative weight. It worked like this: Suppose you had some water, say eighteen grams of water, enough to line the bottom of a coffee cup to a few inches’ depth. If you could break that water into its simplest parts—its elements, hydrogen and oxygen—then you could weigh those parts and find out just how much hydrogen combined with how much oxygen to give water. When this was done (and in practice it was done by passing a current through the water, collecting the gaseous hydrogen and oxygen at the electrodes), it always gave the same result: sixteen grams of oxygen at one electrode, two grams of hydrogen at the other. The ratio of the “combining weights” was 16/2 or 8/1.
Did that mean an oxygen atom was eight times heavier than a hydrogen atom? It all depended. It depended on what you thought the formula for water was. If, like Dalton, you thought that water was represented by the formula HO, one atom of hydrogen and one of oxygen, and that you had, therefore, collected equal numbers of oxygen and hydrogen “particles” at your electrodes, it made sense to say, from the combining weights, that an oxygen atom was eight times heavier than a hydrogen atom. But Dalton’s “water” would have made a strange, strange world. Not our world.
Suppose instead water was made. up of two hydrogen atoms for every atom of oxygen, the way that Amadeo Avogadro said it was, in the early nineteenth century. Then what would happen? Now there would be twice as many particles of hydrogen as oxygen, but still the same weights, still sixteen grams of oxygen to two grams of hydrogen. If water is really “H2O,” then oxygen must be sixteen times heavier than hydrogen.
The trick was to know combining weights and formulas so that you could get relative weights. Then you could decide that an oxygen atom weighed sixteen times as much as a hydrogen atom and that carbon weighed twelve times as much. This simple trick, this weighing of atoms, is why we remember John Dalton as the Isaac Newton of chemistry.
Still, those were only relative weights, and it took another fifty-eight years, the congress at Karlsruhe, and the eloquence of Cannizzaro to work out a consistent set of them. It took another fifty years to get absolute weights—how much an atom really weighs in comparison, say, to an autumn leaf. This could only be achieved once Planck and Einstein had determined how many atoms there were in twelve grams of carbon, or one gram of atomic hydrogen, or sixteen grams of atomic oxygen. It turned out to be a lot, and it turned out to be the same in every case: 6.02 X 1023. Avogadro’s number. A mole of particles.
Alas, more broccoli. Yet for the chemist these were the good old days, the days when things were simple, when the atom made sense in a good intuitive way, and when the world it rested on did too. All that changed with Rutherford, and in a way it changed with a single experiment.
By the early part of this century, people knew that Dalton had not quite gotten it right. The atom was a thing of parts, charged parts in fact, fleeting bits and pieces of electricity. J. J. Thomson and Robert Millikan had weighed the electron, defined its charge. Thomson had even proposed a model for how electrons might exist in atoms. What he envisioned was an English pudding, a pudding of positive charge in which were embedded plums of negative electricity—the electrons. The “plum pudding model,” however, mouthwatering as it was to contemplate, was put to the test by Rutherford and found to be wanting.
In 1909, at Cambridge University, Rutherford took a very thin sheet of gold foil and subjected it to a barrage of charged particles. He expected them to rush through the foil, to be bent only slightly from their course. And most of the time that was exactly what happened.
But on occasion, something strange occurred. Some of the particles, moving at half the speed of light, actually rebounded, came back and hit the source, like a tennis ball ricocheting from a wall. Rutherford was stunned. In a letter to a friend he said:
It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.
What was causing this flimsy sheet of “tissue paper” to withstand the mighty rush of an “artillery shell,” and more, to turn it around and send it back from where it had come? Rutherford thought he knew, and his calculations bore him out. The particles could have been repelled with such force, he reasoned, only if all of the positive charge in the gold atom were compressed into a hard, dense, fiercely compacted point of matter: into a “nucleus.” But where were the electrons? Rutherford argued, and his brilliant student Niels Bohr showed mathematically, that the electrons must be orbiting this nuclear sun like so many tiny planets, like vacant moons. Suddenly it was no longer possible to think of atoms as Daltonian billiard balls, or even as plum puddings. With this single experiment, the atom, upon which the hard back of the physical world rested, became mostly emptiness.
Imagine the rounded dome of a great cathedral on a quiet morning. In its center, on an updraft, is supported a single dust mote, a particle of mica. Between this speck and the great dome there is nothing. Place yourself there on that ceiling and look across the broad enclosure, rose-colored at this hour, toward that glittering point. You are at the distance of the first electron shell, looking inward to the heart of matter itself, whirling and caught in the force of that insignificant nub, the nucleus. And you can barely discern it, barely make it out. What you see, the great fact of your experience, is space pure and simple, empty space, awash over matter like a sea.