Three Scientific Revolutions: How They Transformed Our Conceptions of Reality (20 page)

As knowledge of Röntgen's remarkable discovery spread throughout the world, other physicists began investigating the phenomena. Learning that Röntgen believed that whatever caused the fluorescent glow on the inner surface at the end of the cathode ray tube also caused the X-rays, a year later Henri Becquerel began experimenting with phosphorescent substances to determine whether they emitted X-rays, but discovering they did not, he decided to use uranium salt. To test his hypothesis he proceeded like Röntgen by covering a photographic plate with black paper and placing the uranium salt on top, assuming that the heat from the light of the sun would activate the uranium and produce an image on the photographic plate. After leaving the arrangement in the sun for several hours and then developing the photographic plate, he did find an image of the uranium salt.

Intending to reproduce the experiment, Becquerel placed a copper cross between the wrapped photographic plate and the uranium salt, but finding the sky overcast he placed the copper cross in a closed cupboard for several days. Expecting the photo to be unexposed because of the lack of sunlight acting on the uranium salt, he was “stupefied” to discover an even sharper image of the cross than of the previous metals. Realizing that his assumption that the uranium salt had to be activated by light to produce the image was false, he concluded that the “rays” were generated spontaneously within the uranium compound itself. But though he was the first to identify radiation, his discovery was generally ignored until the research of Pierre and Marie Curie.

It was a year later in 1897 that Becquerel's discovery began to be accepted when Pierre Curie, director of the laboratory in the school of Industrial Physics and Chemistry in Paris, suggested to a Polish émigré Marya Sklodowska, who had enrolled as a graduate student, that for her doctoral dissertation she might investigate “Becquerel rays.” It also resulted in Pierre and Marya getting married the following year and because of their mutual attraction, common interests, and collaboration they formed the most famous husband and wife team in the history of science, with Marie Curie (her married name) the most scientifically gifted of the two.

Later at her initiative, they began investigating Becquerel rays using Pierre's improved electrometer in place of Becquerel's electroscope to obtain a more precise measurement of the degree of radiation. Marie discovered that the source of the radiation was uranium atoms and introduced the term “radioactive substance” in the title of a paper in 1898 to describe the material emitting the spontaneous radiation. She also developed a technique for isolating radioactive isotopes and discovered two elements, polonium (named after her country of origin) and radium.

Marie and Pierre shared the Nobel Prize in physics with Becquerel in 1903 for their discoveries and she earned another Nobel Prize in chemistry in 1911, the first person to win two Nobel Prizes and one of only two people to have been awarded a Nobel Prize in two fields. Tragically, they both suffered dreadful deaths: Pierre at age forty-six when he was killed by a vehicle while crossing the Rue Dauphine on the left bank in Paris, and Marie at sixty-seven died in a sanatorium as a result of her exposure to radiation from which she suffered for many years. Conceivably their discoveries in radioactivity, which helped convince physicists of the limitations of Newtonian science due to the existence of a deeper level of physical reality, provides some consolation for their tragic deaths.

It is to the first discovery of an element at this deeper level that we now turn, and again it relates to experiments with the cathode ray tube. As was the case with light, there was a division between the English investigators and those on the continent as to the nature of cathode rays, the English adopted a particle interpretation and the Europeans a wave theory. As the Englishman William Crookes stated:

As is well known there are two opposing views on the nature of the cathode rays. The earlier one . . . adopted by the English physicists, considers the rays as negatively-charged particles. According to the second one, more representative of the German physicists, especially Goldstein, Wiedemann, Hertz and Lenard, the cathode rays are processes . . . in the aether. (pp. 81–82)

In 1879 Crooks also claimed that “In studying this fourth state of matter we seem at last to have within our grasp . . . the little indivisible particles which with good warrant are supposed to constitute the physical basis of the Universe” (p. 80). But it was J. J. Thomson who determined that the cathode rays were particles, later called “electrons” after measuring their mass/charge ratio, a property of particles not waves. A year previous to Thomson's announcement of his experimental measurement, Emil Wiechert's University of Königsberg experiments on cathode rays indicated that they had a mass much lighter than hydrogen, a property of particles. He could have been the first to have discovered a subatomic particle, but for ideological reasons he desisted. As Abraham Pais states: “It is the first time ever that a subatomic particle is mentioned in print and sensible bounds for its mass are given” (p. 82).

It was in the following year, 1897, in April that Thomson presented to the Royal Society of London his experimental determination of the ratio of charge to mass (
e
/
m)
of cathode rays that definitely convincing him they were particles:

On the hypothesis that the cathode rays are charged particles moving with high velocities [it follows] that the size of the carriers must be small compared with the dimensions of ordinary atoms or molecules. The assumption of a state of matter more finely subdivided than the atom is a somewhat startling one. . . . (p. 85)

In the following August he submitted an article describing his results to the
Philosophical Magazine
.
⁶³
He asserted first that the particles were negatively charged, having measured it by an electrometer. Then in an experiment with a cathode ray tube in which he had inserted two separated parallel electrode plates of contrasting charge, attached oppositely to the center surface of the tube to generate an electric field perpendicular to the length of the tube, he directed the beam of cathode rays between the electrode plates, noting the rays'
electrical
deflection indicated by the displacement of the luminous glow in the enclosed glass globe at the opposite end. Repeating the experiment but generating a magnetic rather than an electrical field, he detected their
magnetic
deflection, another property of particles rather than waves.

From these experiments he was able to measure the charge-to-mass ratio,
e
/
m
, along with calculating the velocity of the particles and discovering that since they were identical in all the experiments they must be an elemental component of nature. As he concluded:

On this view we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter . . . is of one and the same kind; this matter being the substance from which all chemical elements are built up. (pp. 85–86)

This is a remarkable premonition in that as we now know the electron to be a basic particle of nature that accounts for the chemical properties and interactions of the elements.

Thomson finally confirmed his discovery of electrons in 1899 using the cloud chamber method developed by his student C. T. R. Wilson. In that experiment charged particles forming the nuclei of condensed droplets of supersaturated water vapor enabled him to measure separately the charge and mass of the particles. The charge he found to be
e
≈ 6.8 × 10
^-10
esu (electrostatic units) and the mass of the electron to be 3 × 10
^-26
g, a very accurate measurement for the time (p. 86). Thus electrification could be explained as the result of splitting the electron from the atom. While it was George Stoney in 1894 who introduced the term “electron” to symbolize the unit of electrical charge lost or gained when an atom or molecule acquired a net electrical charge, thus becoming an ion, it was Thomson's measurement of the electron's mass that is the reason he is considered “the discoverer of the electron.” (p. 86)

Although this completes the account of the discovery of the electron it does not explain the curious reason why Wiechert, the person who, a year before Thomson, had experimentally detected that cathode rays appeared to be particles rather than waves because of their mass, was not credited with making the first discovery. The conceded explanation is that in contrast to England, where the existence of atoms and corpuscles had been commonly accepted among scientists since Newton, in Austria and Germany where the “positivist philosophy” of the Viennese physicist Ernst Mach prevailed, it was thought to be “unscientific” to accept the hypothetical existence of entities like atoms or electrons that could not be actually observed, but only inferred from the experimental evidence.

During the eighteenth century the Scottish philosopher David Hume in his critique of science denied the existence of insensible objects, while in Prussia Immanuel Kant had limited all cognition to the phenomenal world of sensory experience declaring that knowledge of imperceptible “things in themselves” was impossible. But while in England Hume's skepticism was offset by Locke's philosophical defense of the existence of the
insensible primary qualities
of microscopic objects and Newton's acceptance of the existence of unobservable atoms and corpuscles, the skepticism of Kant was perpetuated in Germany and Austria by the positivist philosophy of Ernst Mach (1838–1916). Influenced by his positivism, Wiechert did not believe or declare he had made a real discovery and therefore his priority is not acknowledged. Even when I was studying the philosophy of science Mach was still a strongly obstructive influence.

Having acquired a BS degree as an undergraduate and then being attracted to philosophy by the writings of the ancient Greeks and such contemporaries as Alfred North Whitehead, Hilary Putnam, Bertrand Russell, John Dewey, and Thomas Nagel, I had the misfortune of teaching during the second half of the twentieth century when positivism and ordinary language philosophy prevailed, both of which I considered superficial and ill informed, explaining why they no longer have
any
following. Who today would accept Kant's and Mach's positivistic thesis that scientific research is limited to the observable world or that ordinary language philosophy is preferable to the technical language of science as claimed by G. E Moore, Gilbert Ryle, and Ludwig Wittgenstein?
⁶⁴

Returning to the account of the investigations of atomic emissions that greatly influenced the transition from the Newtonian worldview to that of contemporary science, the next contributor was Ernest Rutherford who was born on the South Island of New Zealand where he received his early education. As an indication of his intelligence, on his entrance examination to Nelson College “he scored 580 out of a possible 600 points and was first in English, French, Latin, history, mathematics, physics, and chemistry.”
⁶⁵
After graduating from Nelson College, he received a fellowship to Canterbury College, where he earned an MA degree and then won a scholarship to Cambridge University in 1851, which enabled him to continue his education in England. As related by Emilio Segrè: “It is said that when the announcement of his prize arrived, Rutherford was on the family farm digging potatoes. He read the telegram bringing the news and said, ‘This is the last potato I have dug in my life'” (p. 49). Thus began one of the most brilliant careers ever in experimental physics.

Arriving in London when he was twenty-four years of age in 1895, there could hardly have been a more auspicious time for him to begin his research. Röntgen would discover X-rays in the next month, Becquerel would detect uranium radiation in six months, and the Curies in about a year and a half would identify the existence of the radioactive substances polonium, thorium, and radium. Shortly after his arrival, Rutherford collaborated with J. J. Thomson, who immediately recognized his research talents. Rutherford measured the ionization produced by X-rays and, subsequent to its discovery by the Curies, uranium. From these experiments he learned that uranium radiation was not homogeneous but emitted two kinds of rays identified by their different emission capacities:
alpha
(α) being highly absorbable and
beta
(β) very penetrating. It was the
alpha
rays that were absorbed by the black cardboard wrapper in Becquerel's experiment and the
beta
rays that activated the coated screen.

Despite his close collaboration with Thomson at the Cavendish Laboratory, one of the greatest research centers in the world, when offered a professorship in 1898 at a much higher salary at McGill University in Montreal, Canada, Rutherford accepted. When he arrived at McGill he found newly built physics and chemistry laboratories and such distinguished colleagues as Frederick Soddy and Otto Hahn. As a further reward, when John Cox, the chairman of the physics department, observed Rutherford doing his research, he declared that “I think I'd better take your classes and do the teaching work. You keep on doing what you have to do” (p. 52).

Pursuing his research on α-rays, despite Becquerel's initial claim that they were neither polarized nor refractive, Rutherford proved experimentally that they were indeed deflected by electric and magnetic fields thus inferring that they were particles with a positive charge similar to helium. As early as 1903 he had discovered that, when heated, radioactive substances gave off helium, which convinced him that α-particles were helium ions. Investigating β-rays, the second radioactive material detected by Rutherford, others such as Becquerel, the Curies, and Walter Kaufmann found that they, too, were deflected in electric and magnetic fields, have a negative charge, and an
e/m
ratio similar to cathode rays or electrons. When in 1902 Kaufmann demonstrated that β-rays and cathode rays exhibit very similar properties, from “that time on it was considered settled, the β-rays are electrons.” Paul Villard at the École Normale in Paris had discovered a third penetrating emission of radium, called
gamma
(γ) rays, that resembled X-rays.