Three Scientific Revolutions: How They Transformed Our Conceptions of Reality (23 page)

Since Maxwell's time, Physical Reality has been thought of as represented by continuous fields . . . not capable of any mechanical interpretation. This change in
the conception of Reality
is the most profound and the most fruitful that physics has experienced since the time of Newton.
⁷⁸
(italics added)

It was his dream that all the diverse explanations could finally be reduced to one set of laws in a unified field theory to which he dedicated his life—though it eluded him.

While the unification of the electromagnetic force with the strong and weak nuclear forces in the projected grand unified theory (GUT) has partially vindicated his dream, other unifications would prove elusive or unattainable, such as the reduction of gravity to electromagnetism and the elimination of the uncertainties in quantum mechanics. But his emphasis on simplicity, generality, and elegance in the formulation of theories has been enhanced by the recent discoveries of symmetries.

His correct predictions at the end of his 1918 article on the general theory of relativity of the precession of Mercury, the red shift due to the recession of stellar bodies, and especially photographs of the telescopic confirmation by British astronomer Arthur Eddington of the bending of light during a solar eclipse in 1919, and the more recent prediction of black holes from his theory support his claim to have created, at least partially, a new physical reality to replace Newtonian mechanics.

Chapter VII

CONSTRUCTION OF THE ATOM IN THE TWENTIETH CENTURY

Yet the full account of the revolutionary developments in the twentieth century still has not been related, such as the inquiries leading to a more precise conception of the interior structure of the atom. In England one of the first attempts was the “plum pudding” model of J. J. Thomson that consisted of a positively charged mushy sphere on which the negatively charged electrons were embedded, like plums in a pudding, so that their exterior negative charges balanced the positive charge of the mushy interior producing a neutrally charged atom. Among the obvious faults of this model was his attributing the mass of the atom not to the interior substance, but to the exterior electrons, which would prove to be the reverse of the actual structure.

Rutherford, who began his research under Thomson and later would succeed him as Director of the Cavendish Laboratory, having left Cambridge for Montreal, now accepted a position in Manchester, England, were he conducted his own more sophisticated experiments to investigate the interior of the atom. Working in an excellent laboratory and aided by two talented assistants—Hans Geiger, who would invent the Geiger counter for measuring radiation, and Ernest Marsden, who had emigrated from New Zealand to study with his famous compatriot—Rutherford, decided to use his discovered α particles with their positive charge, large mass, and great velocity to probe the interior of the atom.

Instructing his assistants to radiate α particles at thin gold foil and measure the percentage of deflections striking a scintillating screen set at various angles, they found that most of the particles passed directly through the thin foil with a few deflected at
slight
angles by the presumed existence of the interior atoms. He then suggested that Marsden alter the angle of the screen to see if any of the α particles would be deflected at a greater angle and was astonished when Marsden reported that a few had actually been deflected straight backward to the eyepiece, as if they had been repelled by some massive component within the interior of the gold foil. As an indication of his astonishment, Rutherford described his reaction as “quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
⁷⁹

Undeterred by his astonishment, he began experiments to find a more precise explanation of the cause. Based on the measurements of the percentages of deflections at various angles, he devised a formula for measuring the angles of deflection, the velocity of the particles, and their charges. He also conceived of the nucleus as consisting of particles of a certain mass, along with a positive charge calculated by his formula indicating the atomic number of the element. Accordingly, he defined two of the properties of the nucleus, the atomic mass and the atomic number, that were independently confirmed by J. J. Thomson and H. G. J. Moseley based on X-ray emissions from the atom.

Apparently, from these X-ray emissions it was inferred that the atomic nucleus was surrounded by the negatively charged electrons, which are relatively massless, about 9 × 10
^-28
g, or 1,836 times less than that of the proton, but whose shells occupy most of the volume of the atom confirming an earlier conjecture by Jean-Baptiste Perrin in 1901 that the structure of the atom might resemble the solar system:

Each atom might consist . . . of one or more positive suns . . . and small negative planets. . . . If the atom is quite heavy, the corpuscle farthest from the centre . . . will be poorly held by the electrical attraction . . . . The slightest cause will detach it; the formation of cathode rays [electrons] will become so easy that [such] matter will appear spontaneously “radioactive. . . .”
⁸⁰

Indicative of how much progress was being made in explaining the structure of the atom Segrè states that the

new science of X-ray spectroscopy not only allows the study of deep electron shells and elementary chemical analysis on an unprecedented level of sensitivity and certainty: it also opens the way to the exploration of crystalline lattices and, more generally the architecture of solids and of molecules.
⁸¹

It was decided that electrons in the outer shell cause the visible spectra, while the electrons in the inward shell are the source of the X-ray spectra. Apparently, it was Moseley's and Thomson's induced X-ray spectra that provided the evidence for the electron shells and Moseley who determined that the electron shells are related to the nuclear charge, thus providing independent evidence of the atomic number.

Becoming convinced that the experimental evidence supported his calculations, Rutherford presented his results first to the Manchester Literary and Philosophical Society in March 1911—the same society to which Dalton had submitted his atomic theory—and then sent a more detailed account in May to the
Philosophical Magazine
followed by another article entitled “The Structure of the Atom” in February 1914. Though he was unable to explain the exact causes of either atomic stability or radioactive instability, his conception of the composition of the nucleus and structure of the electron orbits was sufficient to enable physicists to formulate a clearer notational designation of the nuclear components and properties of the atom. For example, depicting the
charge
as plus or minus
e
and the
number
of the charged units as Z, then +Z
e
stood for the total charge of the nucleus with –Z
e
representing the total charge of the number of electrons in a particular atom.

Thus if there is an equal number of +Z
e
s and –Z
e
s, the charge of the atom is neutral, while ionization consists of the loss or gain of electrons and radioactive transmutations as a change in the nuclear number due to the emission of α, β, or γ rays. As Ne'eman and Kirsh state:

While emitting an alpha particle, the nucleus loses electric charge of +2
e
and a mass of about 4 amu. The process, which is also called alpha decay, or disintegration, lowers the atomic number Z by 2, and the mass number A by 4. The equation representing the alpha decay of uranium 238, for example, is:
₉₂
U
²³⁸
→
₉₀
Th
²³⁴
+
₂
H
e
⁴
.
⁸²

Since the number of –Z
e
represents the number of electrons in the atom, which accounts for the chemical properties, it also indicates its place in Mendeleev's Periodic Table. Since in neutral atoms the number of +Z
e
s equals the number of –Z
e
s, isotopes are atoms with identical chemical properties but different atomic weights. Yet while the chemical properties were attributed to the electrons, it still was not known what accounted for the
nuclear
numbers and weights. In 1919 Rutherford had discovered that when an a particle interacts with a hydrogen atom a hydrogen nuclear particle is ejected, but did this mean that atomic nuclei were all hydrogen nuclei?

After a number of experiments probing the nuclei of other atoms produced the ejection of the same entity, physicists decided they had discovered the first nuclear particle, naming it “proton,” after the Greek word
protos
meaning “first.” But they were still puzzled by the fact that the number of the nuclear particles of an atom did not match its mass or atomic weight. However, when James Chadwich, at the Cavendish Laboratory, probed the nuclei of lighter elements, such as beryllium, he discovered that a new particle was ejected that was quite massive but neutral in charge. Determining the mass of the particle to be nearly that of the proton and finding that it had a neutral charge, for obvious reasons he named it a “neutron” and received the Nobel Prize in 1935 for his discovery. Thus the discovery of subatomic particles was resolving many problems at a stroke, as Ne'eman and Kirsh affirm.

The discovery of the neutron is a classical example of the way in which the addition of a new building block clarifies as if by magic many previously inexplicable facts. For example, it became clear that the mass number A is just the total number of protons and neutrons in the nucleus. The fact that the atomic mass is always quite close to an integral number of amu found simple explanation: the masses of both the proton and the neutron are close to 1 amu. Different isotopes of an element are atoms the nuclei of which have the same number of protons but not the same number of neutrons. (p. 18)

Following his success in detecting the proton, deciding to eject α particles into the air Rutherford succeeded in producing nuclear disintegration. In an article titled “Collision of Alpha Particles with Light Atoms,” published in the June 1919 issue of the
Philosophical Magazine
, he describes in the fourth part “An Anomalous Effect in Nitrogen” in which the

nitrogen atom is disintegrated under the intense forces developed in a close collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus. . . . The results . . . suggest that if α particles—or similar projectiles—of still greater energy were available for experiment, we might expect to break down the nucleus structure of many of the lighter atoms.
⁸³

Yet the model was mainly due to the great Danish physicist from Copenhagen, Niels Bohr (1885–1962), whom most physicists consider, along with Einstein and Rutherford, one of the three greatest physicists of the new era. Along with Einstein, he was a dominant influence on the development of theoretical physics in the first half of the twentieth century due to his own contributions and those of his famous Institute of Theoretical Physics in Copenhagen at which all the famous physicists of the era, such as Hendrik “Hans” Kramer, Wolfgang Pauli, Werner Heisenberg, Erwin Schrödinger, and Paul Dirac, etc., at one time attended.

The Cavendish Laboratory under the direction of Thomson being the most outstanding research center in physics, Bohr decided to do his postgraduate study there under the supervision of Thomson. However, having studied Thomson's model of the atom carefully and found some faults in it, during their very first meeting after his arrival in the fall of 1911, he had the temerity to point them out to the famed physicist. Apparently not being accustomed to being corrected by a twenty-six-year-old graduate student who barely spoke English, Thomson did not appreciate the criticisms so the relationship, though not hostile, was not friendly.

Fortunately, however, in November Bohr met Rutherford in Manchester and then heard him lecture in Cambridge on his developing theory of the atom. These encounters led to his amicable departure from the Cavendish to study in Rutherford's laboratory in Manchester where his association with Rutherford flourished to the extend that he considered him a kind of surrogate for his father who had died earlier. They remained close friends and admirers until Rutherford's death in 1937.

Arriving in Manchester in March 1912, Bohr took courses in radioactivity and then, at the suggestion of Rutherford, began his own experiments investigating the nucleus with α particles. Realizing, however, that these nuclear experiments were not conducive to his primary theoretical interest in locating the electrons in the atom he decided to forego them. At the suggestion of two other research associates, Gorge von Hevesy, who was to become famous for his ingenious research in radioactivity, and Charles Darwin, the grandson of the famous evolutionist, who was experimenting on the effects of α particles on electrons, Bohr also began to investigate the impact of α particles on clusters of electrons to determine their placement within the atom.

In the summer of 1912 he prepared a draft article “On the Constitution of Atoms and Molecules,” known as the “Rutherford Memorandum” (which he showed to Rutherford), containing his criticisms of Thomson's conception of the atom. First among the criticisms was the static placement of the electrons in the plum pudding model that Bohr thought untenable, yet, according to classical electrodynamics, if they revolved around the nucleus they would continue to radiate energy and eventually spiral into the nucleus, which also was unacceptable. Second, they could not rotate in the same shell because on classical principles that, too, would prove unstable. Third, on Thomson's model the atom's radius was not determined by the rotating electrons, but by the mushy positive interior, another apparent misconception.