WONDERS
by Philip Morrison


1997: Subatomic Centenary


















By the mid-1880s chemists
had mapped many molecules
without any form of imaging.

Around 1875 James Clerk Maxwell, on whose superb work our physics still rests, described atoms as "foundation stones of the material universe...unbroken and unworn. They continue to this day as they were created--perfect in number and measure and weight."

In 1899, 20 years after Maxwell's untimely death, Joseph John Thomson, third director of Maxwell's own laboratory at the University of Cambridge, told quite a different story. For Thomson had found the electron. Any electrical phenomenon, Thomson explained, "essentially involves splitting up of the atom, a part...getting free and becoming detached from the original." To "split the atom" by transferring electrons was a commonplace of electrical and chemical processes, just as requisite for salting your soup.


Image: Dusan Petricic
Back in the 1830s Michael Faraday had passed direct current between two copper plates through a blue-green solution of copper sulfate and noted that the negative plate, the cathode, grew heavier as the positive one dwindled. Positive charge had traveled along with the atoms of metallic copper. Faraday called these traveling charges "ions." The ions always transferred a charge strictly proportional to the weight of the atoms transferred, corrected for different valences by a factor of two or three. (Wires, however, are not at all like fluid electrolytes. Heavy current can flow indefinitely in a copper wire, yet no detectable atoms of copper come along. Whatever travels inside a wire, it is no atom of the metal.)

Maxwell's equations showed the necessity for a fieldlike contribution, without local charges, to complete one major case of conserved current flow. That fed his shadowy view of charge, which cast doubt on whether currents are simply the flow of charged matter. In 1873 Maxwell wrote of one "molecule of electricity," but at once he termed the phrase "gross...and out of harmony," good mainly as a mnemonic device!

By the mid-1880s chemists had mapped many molecules without any form of imaging. Their purely "chemical logic" firmly disclosed atoms linked in space. In 1881 Herman von Helmholtz, lecturing in Britain, emphasized just what Maxwell had evaded: "If we accept...atoms, we cannot avoid concluding that electricity itself is divided into...atoms of electricity." In 1891 George Stoney even dubbed Faraday's atom of electrical charge, before it had ever been isolated, as the electr[i]on.

The explosive sensation that opened 1896 was Wilhelm Roentgen's demonstration of x-rays--penetrating electromagnetic radiation secondary to high-voltage cathode rays. The cathode rays themselves had been identified by Philipp Lenard the year before as swift negative particles. At the Cavendish Lab, J. J. Thomson had long since developed the best cathode-ray beams, the best evacuated tubes and a deep understanding of electromagnetic forces. Yes, the cathode rays did bend like charges in magnetic fields--everyone saw that--and they felt electrical fields as well.

Then Thomson directed strong fields, electrical and magnetic, across an evacuated glass tube, down whose long axis flowed the cathode-ray beam. What the magnet pushed aside, the electrical field could restore. At the right settings the beam's path, well revealed by the visible fluorescent glow where the rays struck artfully positioned patches of phosphor, passed undeflected. At balance, the ratio of the two fields fixed the ratio of particle charge e to its mass m.

To learn either charge or mass by itself, you needed to know the particle speed. Thomson found that by indisputable means: he measured the heating of a little metal cup that caught the stream of rays. That gave the kinetic energy brought in; the electrons could be counted by the charge they carried. From these data their mass and their charge both followed.

Thomson reported his work widely in lectures that spring of 1897. The charge e was close to Faraday's value, equal though opposite to that of the positive hydrogen ion. The mass m was low, less by 1,000-fold than that of hydrogen, lightest of atoms. Two years later Thomson closed the final logical gap. These electrons were no strangers but parts of ordinary atoms. He had made his beams anew by accelerating electrons released at low voltage. Their charge and mass values matched the cathode rays at high voltage. Any atom could now be parted into electrons and a positive residue. The uncuttable had been cut.

I was disappointed when I realized that Thomson's long direct path had not interested students as eager and able as Albert Einstein or the slightly younger Max Born. Their letters and papers over the next few years admire electrons, but they slight Thomson, and his atom-splitting prowess goes unmentioned. Why?

These budding theorists seem to have regarded Thomson's subatomic electron as merely a confirming result, long expected and indeed anticipated by a year or more. Einstein at age 67 recollected that Maxwell's theory was "the most fascinating subject" of his student years. Like most careful readers he found in it a certain strangeness: matter, not space, was both the source and seat of fields. Instead it was the work of the great Leiden physicist H. A. Lorentz that the young ones had mastered. He had celebrated and validated Maxwell's equations but over the years shifted their physical emphasis, adding one explicit equation, the "Lorentz force" felt by point charges. For Lorentz, fields existed only in the vacuum. Matter was atomic and bore the essential charges, plus and minus, that by placement and motion created all field patterns.

Experiments that made those charges real were done by novice Pieter Zeeman and at once interpreted by senior theorist Lorentz. Their lab in Leiden had, like others, just received from Henry Rowland of Johns Hopkins University an exquisitely ruled new diffraction grating that improved by an order of magnitude the power of spectroscopy. Zeeman read Maxwell's admiring essay on Faraday and was moved to repeat 30-odd years later, with the grating, the same spectroscopic experiment that had shown no effect through Faraday's prisms. A Bunsen flame held between the poles of an electromagnet heated a piece of asbestos soaked in salty water; the yellow glow of hot sodium vapor was dominated by one narrow double spectral line.

As never before, the lines could be seen to widen as soon as the strong magnetic field came on. Lorentz knew that electrons moving in any lasting orbit would slowly rotate faster (or slower, depending on the field direction) once an outside magnetic field was imposed. He could calculate the charge-to-mass ratio of the moving atomic electron from the small wavelength shifts Zeeman measured. Polarized light was predicted and found; it fixed the sign of the radiating electron, negative like the cathode rays.

Clearly, Lorentz's theory could not be final. He had no map of the atom. Yet before 1896 had ended, the Leiden researchers had found the first key: electrons of known charge and mass radiate spectral light as they move within ordinary atoms. When Max Planck introduced the idea of quantized energy in 1901, the dizzying journey of quantum physics began.