QUANTUM THEORY

The story of quantum theory begins with the work of Joseph Priestly (1733-1804), an English theologian and scientist who collaborated with Benjamin Franklin when Franklin was in England spying on the British and organizing for the American Revolution. Franklin encouraged Priestley to write his first major scientific work, The History and Present State of Electricity, which was published in 1767. Priestley made a number of discoveries while experimenting with electricity, including the fact that graphite is a conductor. His scientific work, however, went far beyond the study of electricity.

Priestley lived next to a brewery, and he experimented with “fixed air” (carbon dioxide) that collects over beer as it is brewing. He discovered for, example, that fixed air would extinguish a flame. He also discovered soda pop when he noticed that carbon dioxide would dissolve in water, and give it a pleasant tangy taste. Among Priestley’s early inventions was the pneumatic trough (Figure 1), an indispensable instrument for experimentation with gases.

FIGURE 1

In 1774 Priestley conducted a famous experiment with red calx (mercuric oxide), which is a substance that forms when mercury is heated to near its boiling point. He placed a piece of red calx in a pneumatic trough, and heated it with a magnifying glass. He discovered that the gas released into the trough from the burning of red calx would cause a flame to burn much more vigorously than normal, and it would support a mouse for much longer than normal air. “I have discovered an air five or six time as good as common air,” he claimed. This discovery, together with his discovery that plants absorb carbon dioxide and emit oxygen, made him famous, and led to a meeting in Paris with the great scientist and statesman, Antoine Lavoisier, who repeated Priestley’s experiments and named the newly discovered gas “oxygen.”

Lavoisier (1743-1794) conducted extensive chemical experimentation, and did more than anyone else to develop the modern notion of an element as “a simple substance which cannot be further decomposed.” He conducted a number of revolutionary experiments in combustion which allowed him to isolate and weigh various elements, leading to the later discoveries of Mendeleev. He discovered, for example, that charcoal, graphite, and diamonds are all made primarily of carbon.

Priestley and Lavoisier were both avid supporters of the American Revolution and the French Revolution in its early phases, before it was destroyed by the British Jacobin insurgency which began with the infamous Bastille Day riot. Priestley’s home in Birmingham, England was burned by a Jacobin mob when he refused to participate in the second anniversary celebration of Bastille Day. Lavoisier was one of the many scientists and intellectuals that were guillotined during the Reign of Terror in France, which led to the installation of Napoleon Bonaparte as the first modern fascist dictator.

Priestley, who had escaped England and moved to France, narrowly escaped the Reign of Terror, and moved to Northumberland, Pennsylvania to continue his work at the suggestion of Benjamin Franklin. He built one of the most sophisticated laboratories of the time, and collected a library of approximately 1,600 volumes. He continued to make a number of important discoveries until his death in 1804.

In separate scientific developments, which are key to later developments in quantum theory, Andre Marie Ampere (1775-1836) built the first solenoid in 1820. (Figure 2) His solenoid was simply a conducting wire wrapped around a cylinder. Ampere noticed that this arrangement produces polar magnetism like that of a bar magnet. He hypothesized that the circular motion of electricity mimicked microphysical orbits which he conceived to be present in the atoms of a magnet.

FIGURE 2

Ampere’s work was continued by Wilhelm Weber (1804-1891), who was one of Carl Friedrich Gauss’s key collaborators. By 1870, Weber developed the theory that the atom consists of a positively charged atomic nucleus, and negatively charged orbiting electrons. This was decades before any empirical evidence was available. Today, this work is written out of textbooks. The method of discovery developed by Ampere, Gauss, and Weber is almost completely ignored.

By 1869, 63 elements had been identified and weighed using various methods pioneered by Priestley, Lavoisier, and others. The same year, the great Russian chemist, Dmitri Mendeleyev (1843-1907), made one of the most important discoveries in the history of science when he arranged the elements in order according to their weights and noticed a periodicity in their occurrences. Mendeleev noted gaps in these periods, where he predicted the occurrence of various elements, and their properties. For example, he predicted the existence of an element with an atomic weight of approximately 68, and that it would be much like aluminum. His predictions proved to be accurate with the later discoveries of gallium, germanium, and scandium. Mendeleev’s original periodic table (Figure 3) has been revised and updated many times.

FIGURE 3

In other relevant developments, Heinrich Geissler (1814-1879), a German glass-blower and inventor, developed in 1855 a technique to blow a glass tube and pump the air out of it. When he put electrodes in the tube and ran a charge through it, he discovered that a beam of radiation is formed beginning at the negative electrode, or cathode. Hence, these rays came to be known as cathode rays. Three years later, Julius Plucker (1801-1869), who is well-known for his work in projective geometry, and was studied closely by Bernhard Riemann, discovered that cathode rays are deflected toward the positive pole of a magnetic field, indicating that cathode rays have a negative charge. (Figure 4)

FIGURE 4

Finally, In 1897, J.J Thomson (1856-1940) designed a famous experiment in which he measured very precisely the deflection of cathode rays in a magnetic field. He argued that cathode rays are made of particles. According to his results, these particles weigh about 2000 time less than a hydrogen atom. Since it had already been shown that the same cathode rays are produced regardless of the composition of the cathode, it was thought that these particles, which came to be known as electrons, are a basic component of all matter.

Based on these results, Thomson formulated what was called the plum-pudding model of the atom, which was the idea that all matter is composed of atoms, and that all atoms are composed of electrons that are distributed like plums in a pudding of positively charged matter. (Figure 5) Since electrons have relatively little mass, the positively charged matter was thought to contain most of the weight.

FIGURE 5

In still other relevant developments, Henri Bacquerel (1852-1908) discovered radiation in 1896 when he was experimenting with uranium. He happened to place a bit of uranium compound on a photographic plate and put it in a drawer. He found that the uranium could produce its image on the photographic plate in the absence of sunlight.

Bacquerel’s discovery caught the attention of Marie Curie (1867-1934), who designed an experiment to measure radioactivity with a device her husband built called an electrometer. Together they discovered that radiation acts as an electrical conductor. Their electrometer measured the precise conductivity of radiation. The Curie’s proceeded to conduct similar experiments with every known element, and found that thorium is also radioactive. When they began to experiment with pitchblende, they discovered that it contains elements with a much higher radioactivity than uranium or thorium. This led to their discovery of polonium and radium in 1898. Marie Curie also developed methods to separate radioactive material from compounds through a process called fractional crystallization.

In 1899, when Ernest Rutherford (1871-1937) conducted an experiment using a Curie electrometer containing a thin plate of aluminum over a piece of uranium, he discovered that aluminum absorbs radiation at two different levels, indicating that there were two different types of radiation present. He called these two types of radiation alpha rays, and beta rays. It was later discovered that alpha rays are actually helium atoms stripped of their electrons.

Rutherford’s most important work began in 1911, when he conducted a famous experiment in which he directed a narrow beam of alpha rays at a thin sheet of gold. The apparatus was encircled by a photographic film which could detect minute radioactive disturbances. If Thomson’s plum-pudding model were accurate, the alpha rays, which were thought to consist of particles, would be deflected only a small amount by their impact with the atoms of gold. Rutherford made the astonishing discovery that, while most of the alpha particles traveled straight through the gold, some of them were radically deflected, and a few of them came almost straight back toward the source. (Figure 6) Rutherford hypothesized that atoms must, therefore, contain a relatively small, yet massive, positively charged nucleus, surrounded by orbiting electrons reminiscent of Weber’s theory of the atom, and much like a miniature solar system.

FIGURE 6

In 1900, Eleven years prior to Rutherford’s famous experiment, Max Planck (1858-1947) discovered the quantum following experiments with blackbody radiation, which is heat radiation emitted by an object that is non-reflective. An ideal black-body is the interior of a sealed container which is absolutely light-tight. Black soot approximates a black-body because it reflects very little incident radiation. Rather, it absorbs it. When soot is sufficiently heated, however, it will begin to glow. As its temperature is increased, its color will change, indicating that it is emitting variable frequencies of heat radiation dependent upon its temperature.

Planck analyzed experiments that involved heating a light-tight chamber to various temperatures, and measuring the radiation emitted from within the chamber. Planck’s calculations revealed that black-body radiation is emitted in discrete frequencies only. He therefore concluded that the spectrum of radiation is not continuous—that it exists only in discrete quanta which he calculated to be divisible by a very small magnitude which came to be known as Planck’s constant.

Albert Einstein (1879-1955) took great interest in Planck’s discovery, and continued the development of quantum theory with his explanation of a phenomena first observed by Heinrich Hertz in 1887 known as the photo-electric effect. The photo-electric effect is the emission of electrons by substances, especially metals, when light falls on their surfaces. While Einstein is best known for his theory of relativity, he won a Nobel Prize in 1905 for developing the idea that light, which was known to consist of waves since at least the time of Leonardo da Vinci’s works in optics, also consists of “quanta of energy localized in points of space.” According to Einstein, these quanta of energy, which came to be known as photons, were responsible for impacting the electrons of matter and knocking them loose. Though Einstein’s photons cohered with Planck’s quanta, the fact that electrons are scattered in every direction by photons remained to be explained.

An explanation came in 1913 with the work of Neils Bohr, who had studied spectral analysis, and thought it held the missing key to the structure of atoms. The simplest spectral analysis begins with the refraction of sunlight through a glass prism, which separates sunlight into its various frequencies in the same way a rainbow is formed by the refraction of sunlight in the atmosphere. (Figure 7).

FIGURE 7

It had long been known that light leaves unique spectral arrays depending upon the element through which the light is discharged. (Figure 8)

FIGURE 8

Bohr hypothesized that each element has a unique array of electron orbits that correspond to their individual spectra. Moreover, electrons orbit at a unique “levels” depending upon their “energy state.” When a photon is absorbed by an atom, an electron orbit is removed to a higher energy state. It is “excited.” At some point, the electron orbit releases a photon, whereupon it jumps back to a lower level. Repetitive absorptions and emissions of photons account for the observed spectra, given the uniqueness of each element. (Figure 9) Finally, when an electron orbit reaches a threshold of excitation, an electron is released in a random direction, accounting for the photo-electric effect.

FIGURE 9

Bohr’s model proved to be unsatisfactory in many ways, especially since it provided an accurate account only for the hydrogen spectrum. Analysis of more complex elements, with more complex spectra, led to a number of post hoc suppositions including elliptical electron orbits, and electron “spin.” Even then, complex spectra could not be fully explained. Both Max Planck and Albert Einstein, for example, thought Bohr’s model was incorrect. In spite of their objections, the model remains a standard, textbook theory of atomic structure.

One of the most serious objections to the Bohr model was put forward by Louis de Broglie (1892-1987) in 1924. De Broglie noted that Bohr seemingly ignored the well-known, wave-like properties of light. De Broglie proposed a radically different conception of atomic structure, based on his theory that photons, electrons, and in fact all matter, is essentially wave-like. Phenomena such as the photo-electric effect could be accounted for based upon the actions of singularities within the waves. De Broglie’s theory also provided a theoretical account for the exact position and velocity of these singularities within their wave-like structures, as well as the supposed electron orbits as a system of stable waves around the nucleus, in which the circumference of each wave corresponds to the orbit. Bohr replied to de Broglie’s challenge by formulating the relatively absurd “principle of complementarity,” which asserts that light is either a wave or a particle, depending upon the way in which it is observed.

Among Bohr’s collaborators was Werner Heisenberg (1901-1976), a young mathematician who attempted to discredit de Broglie with his so-called uncertainty principle, which held that it is impossible to calculate both the position and the velocity of a particle with certainty. Rather, mere probability reigns. Heisenberg’s uncertainly principle was based on the radically empiricist notion that the act of observation itself changes the position of a microphysical object, simply because the act of observation necessarily involves the exchange of photons between the object and the observer. The impact of photons, in turn, changes the position of the microphysical object.¹ Heisenberg was also a strict formalist. He demanded that quantum theory must conform to a strict set of formal mathematical rules.

The stage was set for a showdown in 1927, when the world’s leading physicists gathered at Solvay to discuss their theories. On one side stood de Broglie, Planck, and Einstein; on the other, Bohr and Heisenberg. At Solvay, Einstein put forward an objection to the uncertainty principle with a simple thought-experiment which stated that once a singularity is observed at a particular position, the probability of observing it at any other position is nil. Hence, its motion is determinate. When Bohr and Heisenberg held to their arguments, Einstein made the famous observation that “God does not play dice.”

Much of the conflict revolved around two opposing axiomatic approaches. Where Einstein and de Broglie perceived singularities in relativistic space and time, Bohr and Heisenberg perceived mere corpuscles, much like Newtonian “hard-balls”, floating and colliding in infinitely extendable, and featureless, three-dimensional space. Moreover, these were the days of extreme cultural pessimism which led to the rise of fascism. The prevailing mood had a detrimental impact on science. Heisenberg, for example, ended up heading the Nazi’s unsuccessful program to produce the world’s first nuclear weapons. Fortunately, his strict formalism and epistemological shortcomings proved to be the basis of his ultimate failure.

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