Between them, Bohr and Moseley had restored arithmetic to me, provided the essential, transparent arithmetic of the periodic table which had been intimated, though only in a muddy way, by atomic weights. The character and identity of the elements, much of it, anyhow, could now be inferred from their atomic numbers, which no longer just indicated nuclear charge but stood for the very architecture of each atom. It was all divinely beautiful, logical, simple, economical, God’s abacus at work.
What made metals metallic? Electronic structure explained why the metallic state seemed to be fundamental, so different in character from any other. Some of the mechanical properties of metals, their high densities and melting points, could now be explained in terms of the tightness with which electrons were bound to the nucleus. A very tightly bound atom, with a high ‘binding energy’, seemed to go with unusual hardness and density, and high melting point. Thus it was that my favorite metals – tantalum, tungsten, rhenium, osmium: the filament metals – had the highest binding energies of any of the elements. (So there was, I was pleased to learn, an atomic justification for their exceptional qualities – and for my own preference.) The conductivity of metals was ascribed to a ‘gas’ of free and mobile electrons, easily detached from their parent atoms – this explained why an electric field could draw a current of mobile electrons through a wire. Such an ocean of free electrons, on the surface of a metal, could also explain its special luster, for oscillating violently with the impact of light, these would scatter or reflect any light back on its own path. The electron-gas theory carried the further implication that under extreme conditions of temperature and pressure, all the nonmetallic elements, all matter, could be brought into a metallic state. This had already been achieved with phosphorus in the 1920s, and it was predicted, in the 1930s, that at pressures in excess of a million atmospheres it might be achieved with hydrogen, too – there might be metallic hydrogen, it was speculated, at the heart of gas giants like Jupiter. The idea that everything could be ‘metallized’ I found deeply satisfying.[70]
I had long been puzzled by the peculiar powers of blue or violet light, short-wavelength light, as opposed to red or long-wavelength light. This was clear in the darkroom: one could have quite a bright ruby safelight that would not fog a developing film, whereas the least hint of white light, daylight (which of course contained blue), would fog it straightaway. It was clear, too, in the lab, where chlorine, for example, could be safely mixed with hydrogen in red light, but the mixture would explode in the presence of the least white light. And it was clear with Uncle Dave’s mineral cabinet, where one could induce phosphorescence or fluorescence with blue or violet light, but not with red or orange light. Finally, there were the photoelectric cells that Uncle Abe had in his house; these could be activated by the merest pencil of blue light, but would not respond to even a flood of red light. How could a huge amount of red light be less effective than a tiny amount of blue light? It was only after I had learned something of Bohr and Planck that I realized the answer to these apparent paradoxes must lie in the quantal nature of radiation and light, and the quantal states of the atom. Light or radiation came in minimum units or quanta, the energy of which depended on their frequency. A quantum of short-wavelength light – a blue quantum, so to speak – had more energy than a red one, and a quantum of X-rays or gamma rays had far more energy still. Each type of atom or molecule – whether of a silver salt in a photographic emulsion, or of hydrogen or chlorine in the lab, or of cesium or selenium in Uncle Abe’s photocells, or of calcium sulfide or tungstate in Uncle Dave’s mineral cabinet – required a certain specific level of energy to elicit a response; and this might be achieved by even a single high-energy quantum, where it could not be evoked by a thousand low-energy ones.
As a child I thought that light had form and size, the flower-like shapes of candle flames, like unopened magnolias, the luminous polygons in my uncle’s tungsten bulbs. It was only when Uncle Abe showed me his spinthariscope and I saw the individual sparkles in this that I started to realize that light, all light, came from atoms or molecules which had first been excited and then, returning to their ground state, relinquished their excess energy as visible radiation. With a heated solid, such as a white-hot filament, energies of many wavelengths were emitted; with an incandescent vapor, such as sodium in a sodium flame, only certain very specific wavelengths were emitted. (The blue light in a candle flame which had so fascinated me as a boy, I later learned, was generated by cooling dicarbon molecules as they emitted the energy they had absorbed when heated.)
But the sun, the stars, were like no lights on earth. They were of a brilliance, a whiteness, exceeding the hottest filament lamps (some, like Sirius, were almost blue). One could infer, from the radiation energy of the sun, a surface temperature of about 6,000 degrees. No one in his youth, Uncle Abe reminded me, had any idea what could allow the enormous incandescence and energy of the sun. Incandescence was scarcely the right word, for there was no burning, no combustion, in the ordinary sense – most chemical reactions, indeed, ceased above 1,000 degrees.
Could gravitational energy, the energy generated by a gigantic mass contracting, keep the sun going? This, too, it seemed, would be wholly inadequate to account for the blazing heat and energy of the sun and stars, undimmed for billions of years. Nor was radio-activity a plausible source of energy, because radioactive elements were not present in the stars in anywhere near the needed quantities, and their output of energy was too slow and unhurryable.
It was not until 1929 that another idea was put forth: the notion that, given the prodigious temperatures and pressures of a star’s interior, atoms of light elements might fuse together to form heavier atoms – that atoms of hydrogen, as a start, could fuse to form helium; that the source of cosmic energy, in a word, was thermonuclear. Huge amounts of energy had to be pumped into light nuclei to make them fuse together, but once fusion was achieved, even more energy would be given out. This would in turn heat up and fuse other light nuclei, producing yet more energy, and this would keep the thermonuclear reaction going. The inside of the sun reaches enormous temperatures, something on the order of twenty million degrees. I found it difficult to imagine a temperature like this – a stove at this temperature (George Gamow wrote in The Birth and Death of the Sun) would destroy everything around it for hundreds of miles.
At temperatures and pressures like this, atomic nuclei – naked, stripped of their electrons – would be rushing around at tremendous speed (the average energy of their thermal motion would be similar to that of alpha particles) and continually crashing, uncushioned, into one another, fusing to form the nuclei of heavier elements.
We must imagine the interior of the Sun [Gamow wrote] as some gigantic kind of natural alchemical laboratory where the transformation of various elements into one another takes place almost as easily as do the ordinary chemical reactions in our terrestrial laboratories.
Converting hydrogen to helium produced a vast amount of heat and light, for the mass of the helium atom was slightly less than that of four hydrogen atoms – and this small difference in mass was totally transformed into energy, in accordance with Einstein’s famous e = mc². To produce the energy generated in the sun, hundreds of millions of tons of hydrogen had to be converted to helium each second, but the sun is composed predominantly of hydrogen, and so vast is its mass that only a small fraction of it has been consumed in the earth’s lifetime. If the rate of fusion were to decline, then the sun would contract and heat up, restoring the rate of fusion; if the rate of fusion were to become too great, the sun would expand and cool down, slowing it. Thus, as Gamow put it, the sun represented ‘the most ingenious, and perhaps the only possible, type of ‘nuclear machine’,’ a self-regulating furnace in which the explosive force of nuclear fusion was perfectly balanced by the force of gravitation. The fusion of hydrogen to helium not only provided a vast amount of energy, but also created a new element in the world. And helium atoms, given enough heat, could be fused to make heavier elements, and these elements, in turn, to make heavier elements still.
70
It was also wondered, early in the twentieth century, what might happen to the ‘electron gas’ in metals if they were cooled to temperatures near absolute zero – would this ‘freeze’ all the electrons, turning the metal into a complete insulator? What was found, using mercury, was the complete opposite: the mercury became a perfect conductor, a superconductor, suddenly losing all its resistance at 4.2 degrees above absolute zero. Thus one could have a ring of mercury, cooled by liquid helium, with an electrical current flowing around it with no diminution, for days, forever.