Ideas: A History from Fire to Freud

The new physics came into view one step at a time, and emerged from an old problem and a new instrument. The old problem was electricity – what, exactly, was it?²¹ Benjamin Franklin had been close to the mark when he had likened it to a ‘subtile fluid’ but it was hard to go further because the main naturally-occurring form of electricity, lightning, was not exactly easy to bring into the laboratory. An advance was made when it was noticed that flashes of ‘light’ sometimes occurred in the partial vacuums that existed in barometers. This brought about the invention of a new – and as it turned out all-important – instrument: glass vessels with metal electrodes at either end. Air was pumped out of these vessels, creating a vacuum, before gases were introduced, and an electrical current passed through the electrodes (a bit like lightning) to see what happened, how the gases might be affected. In the course of these experiments, it was noticed that if an electric current were passed through a vacuum, a strange glow could be observed. The exact nature of this glow was not understood at first, but because the rays emanated from the cathode end of the electrical circuit, and were absorbed into the anode, Eugen Goldstein called them Cathodenstrahlen, or cathode rays. It was not until the 1890s that three experiments stemming from cathode-ray tubes finally made everything clear and set modern physics on its triumphant course.

In the first place, in November 1895, Wilhelm Röntgen, at Würzburg, observed that when the cathode rays hit the glass wall of a cathode-ray tube, highly penetrating rays were emitted, which he called X-rays (because x, for a mathematician, signified the unknown). The X-rays caused various metals to fluoresce and, most amazingly, were found to pass through the soft tissue of his hand, to reveal the bones within. A year later, Henri Becquerel, intrigued by the fluorescing that Röntgen had observed, decided to see whether naturally-fluorescing elements had the same effect. In a famous but accidental experiment, he put some uranium salt on a number of photo-electric plates, and left them in a closed (light-tight) drawer. Four days later, he found images on the plates, given off by what we now know was a radio-active source. Becquerel had discovered that ‘fluorescing’ was naturally occurring radio-activity.⁹

But it was Thomson’s 1897 discovery which capped everything, produced the first of the Cavendish’s great successes and gave modern physics its lift-off, into arguably the most exciting and important intellectual adventure of the modern world. In a series of experiments J. J. pumped different gases into the glass tubes, passed an electric current, and then surrounded them either with electrical fields or with magnets. As a result of this systematic manipulation of conditions, Thomson convincingly demonstrated that cathode ‘rays’ were in fact infinitesimally minute particles erupting from the cathode and drawn to the anode. Thomson further found that the particles’ trajectory could be altered by an electric field and that a magnetic field shaped them into a curve.¹⁰ More important still, he found that the particles were lighter than hydrogen atoms, the smallest known unit of matter, and exactly the same whatever the gas through which the discharge passed. Thomson had clearly identified something fundamental – this was in fact the first experimental establishment of the particulate theory of matter.

The ‘corpuscles’, as Thomson called these particles at first, are today known as electrons. It was the discovery of the electron, and Thomson’s systematic examination of its properties, that led directly to Ernest Rutherford’s further breakthrough, a decade later, in conceiving the configuration of the atom as a miniature ‘solar system’, with the tiny electrons orbiting the massive nucleus like stars around the sun. In doing this, Rutherford demonstrated experimentally what Einstein discovered inside his head and revealed in his famous calculation, E = mc² (1905), that matter and energy are essentially the same.¹¹ The consequences of these insights and experimental results – which included thermonuclear weapons, and the ensuing political stand-off known as the Cold War – fall outside the time-frame of this book.²² But Thomson’s work is important for another reason that does concern us here.

He achieved the advances that he did by systematic experimentation. At the beginning of this book, in the Introduction, it was asserted that the three most influential ideas in history have been the soul, the idea of Europe, and the experiment. It is now time to support this claim. It is most convincingly done by taking these ideas in reverse order.