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As we now approach the 20th century it is easy to be overwhelmed by the amount of scientific innovation and disheartened by the complexity of modern scientific thought. It is therefore important that we keep the main road along which science, civilization and society evolved together clearly in sight.
We saw in Lecture 17 that the foundations for modern European science were laid in the Italian merchant cities of the Renaissance. We followed its growth as capitalism established itself during the Enlightenment or Age of Reason, first in England (Lecture 20), then in France (Lecture 21), and witnessed the scientific revolution. Lecture 22 showed us how the successes of the new science led to a new philosophy and the separation of philosophy from religion.
During the 19th century science discovered electricity and magnetism, phenomena that could not be understood on the basis of classical mechanics. The breakthrough came from philosophy, which directed Hans Christian Ørsted to make his fundamental observations and opened the way for field theory (Lecture 27).
The development of field theory was the first indication that the ordered, deterministic view of the world developed by the Enlightenment could not describe nature in its entirety. The dualism of light, its character as an electromagnetic wave and as a particle stream necessitated the simultaneous use of two mutually exclusive techniques (field theory and particle mechanics). As a result scientists had to accept that science can only describe how the world works, it cannot make definite statements about the true nature of things. This will become more and more obvious as we look at the achievements of 20th century science.
The inability of science to grasp the true form of animate and inanimate nature does not have to be a cause of concern. Humans have used nature for millennia without knowing its true form, and in our everyday lives we do not usually feel uncomfortable not knowing what nature really is like. Does our body really consist of small particles separated by vast amounts of empty space? Do electrons really flow down the telephone wire? Do molecules really crash with great force against the piston of a car engine? These are not questions we ask ourselves when we are on the telephone or drive a car. We do not expect science to answer them either, but we trust that science has the means to describe the processes involved and predict their effect.
For science the realization of its own limits was a painful process. It ended the fascination of the18th century with science and the blind trust of the 19th century into the ability of science to solve all the world's problems. In less than 30 years - between 1890 and 1920 - it changed physics completely. But it was not a revolution that overthrew the accepted science of the day, as the Copernican system had done to the Ptolemaic system. It was evolutionary in character, an extension of Newtonian physics into regions not covered by classical mechanics.
As scientists probed the boundaries of classical knowledge they had to rely more and more on experiments that went beyond the everyday experience of people. Everyone can see the refraction of light rays in a rainbow, although scientists of the 19th century struggled to understand the phenomenon. The structure of atoms or the phenomenon of radioactivity cannot be appreciated with our normal senses of sight, touch and hearing but requires advanced instrumentation. Science thus becomes intricately linked with developments in technology, at a scale that rivals the scale of industrial production: A modern scanning electron microscope is a machine comparable in complexity to an aircraft, and a modern synchrotron requires an investment comparable to a large factory.
During the 19th century Britain, the first country to undergo its industrial revolution, had also become the centre of major scientific development. While French scientists had brought classical mechanics to a satisfactory closure and contributed to research into electricity and mathematics, the new field theory was born in England and Scotland. As the 20th century approached, the industrial revolution had established itself in most of continental Europe as well, and the centre of scientific development returned to the continent.
In France the republic established by the Revolution of 1789 had been replaced by the dictatorship of Napoleon. He shared power with no one, but he was very effective in providing the conditions that allowed the rapid growth of capitalism. The Code Napoléon unified the law, and a nationwide administration created a unified market. During his reign the number of spinning machines in France increased from 89 in 1789 to 3300 in 1805 (Anderle et al., 1966).
After Napoleon's defeat at Waterloo the victorious European powers restored the Bourbon monarchy in France. This brought new tension to the society. Industrialization had created a new working class, which after the first cyclical economic crisis of 1826 was no longer prepared to play the role of assistant to the bourgeoisie in its conflict with the aristocracy. There was thus struggle on two fronts, and revolutionary action was now characterized by popular uprisings against capitalism as well as monarchism.
The July Revolution of 1830 lasted only three days. It brought the downfall of the Bourbon dynasty but ended in the installation of a new king by the bourgeoisie and an acceleration of industrialization. A rapid fall of industrial wages led to increasing unrest, and another deep crisis in 1847 led to the revolution of 1848, which introduced universal (male) suffrage and established a republic.
The republic lasted less than four years. Louis Napoleon III, who had been elected president, established himself as emperor, and France was again under a dictatorship that served the industrialists. In the span of 20 years - from 1850 to 1870 - the use of steam engines increased from 5212 with a total of 67,000 horse power to 27,088 with 336,000 horse power (Anderle et al., 1966).
The time of the Second Empire was a time of great misery and a learning period for the working class. By 1870 Napoleon faced the real threat of a public uprising and tried to deflect it by declaring war against Prussia. The working class answered with the establishment of the Commune de Paris, the first working class government in history. It was brutally crushed, but it brought the end of all monarchism and established the Third Republic, which lasted until the invasion of France by German troops during World War II.
The events of 1830, 1848 and 1871 in France had tremendous repercussions in all European countries. The direct impact depended on the degree of industrialization and the state of political organization in each country. Austria had been a leading European power during the 18th century. At the beginning of the 19th century it became the main defender of the old order. Its foreign minister the Prince of Metternich managed to stave off revolutionary unrest for three decades and made himself the personification of repression and absolutism. When the revolution of 1848 broke out in France he was forced to resign and went into exile. But the bourgeoisie managed to exploit the rivalries between the various nationalities of the Austrian-Hungarian empire and rescued the monarchy. As a result industrialization in Austria was slow, and Austria was overtaken by Prussia as the leading German power.
The Thirty Years' War of 1618 - 1648 had turned Germany into a patchwork of small dukedoms and kingdoms of no European importance. During the 18th century Prussia had emerged as a dominant power under Frederick the Great, but industrialization was still hampered by the multitude of sovereign states: To transport cloth from Silesia to Hamburg involved the payment of customs duties at several borders.
The French Revolution of 1789 sparked a first attempt at the establishment of a republic in 1793. It was quickly suppressed by Prussian troops. Napoleon's occupation of much of Germany instilled a new sense of national unity under the leadership of the bourgeoisie, which demanded Germany's unification. The resulting association of the Rhine states lasted only until Napoleon's defeat, and when the decisions about the European order after Napoleon were made at the Vienna Congress under Metternich's leadership in 1815, Prussia emerged again as the main German winner.
In 1833 Prussia created the Zollverein (Customs Union), which counted most of the German states among its members and established a united market of 23 million people. Freedom of commerce led to unseen misery and only increased the demands for political freedom. When news of the 1848 revolution arrived from France, uprisings in several states led to the convocation of the first National Assembly in the Paulskirche (St. Paul's Church) in Frankfurt.
The year 1848 is generally seen as the year of Germany's bourgeois revolution. The delegates to the Assembly represented a wide spectrum of society, from liberal bourgeois to socialists and communists, with widely differing interests. While the representatives of the working class demanded a republic, the bourgeoisie sided with the existing political order. An uprising of Frankfurt's population increased its fear of "French developments", and the campaign for a German republic was left to the working class. By the end of 1849 Prussian troops had extinguished the last flickers of the German revolution.
The national federal parliament continued to meet in Frankfurt, but in the end the unification of Germany was the result of Prussian hegemony. Prince Bismarck, Prussia's representative in Frankfurt, managed to establish the Northern League by provoking a war with Austria and promising good spoils. The League united the northern German states under Prussian leadership. Bismarck then used the war of 1871 against Napoleon III to get the southern states to join the League. With unification achieved in all practical aspects, the German Empire was founded in 1871, a capitalist state ruled by a monarchy that was supported by the land-owning gentry and run with military discipline.
The year 1848 marked a turning point in Italy's history as well. In the 18th century Italy had not only been fractured into several independent kingdoms but also suffered from foreign occupation by Austria and France and from the collusion of the papal state with the foreign powers. When the revolution of 1848 in France sparked an uprising against the Austrian occupation, the social fabric in Italy already contained enough working class elements that the bourgeoisie was afraid of siding with the general popular movement. The unification of Italy was therefore achieved under the leadership of Guiseppe Garibaldi, the son of a fisherman and a popular hero.
The bourgeoisie was of course also interested in Italian unification but wanted to avoid a republic with universal suffrage. Realizing that Garibaldi's popular support and military successes ruled out any open opposition against him, the king of Piedmont (the major power in Italy) and his prime minister Count Cavour secretly financed Garibaldi but publicly distanced themselves from his actions; on occasions they even had him arrested by the royal troops, only to have him released soon after.
The kingdom of Italy was proclaimed in 1861. Like Germany, Italy had managed to establish a capitalist economy and maintain the monarchy. It achieved this slightly earlier than Germany. But in the fast moving times of the 19th century a decade made all the difference, and by raising the highest taxes in Europe (Anderle et al., 1966) Italy quickly rose to a secondary imperial power with colonial possessions in Africa.
The changed conditions of the new revolutions redefined the role of scientists in the process as well. The French Revolution of 1789 had offered only one choice: You were either for the rising new order and supported the bourgeoisie, or you defended the old order and worked for the reactionary aristocracy. Most scientists supported the new order and served in the revolutionary government.
Two generations later the working class had appeared as a third force, and the fronts were not drawn with the same clarity. Scientists were no longer wealthy private individuals but employees of state-funded universities. They usually did not own factories and therefore had no direct interest in the exploitation of the people; but their generous salaries and their social position depended on good relations with the state authorities, which could easily fracture if they sided too closely with the working class.
The new situation produced a pattern of behaviour that is still operational today and can be observed particularly clearly in developing countries. In their youth, scientists and intellectuals in general are fearless supporters of social progress; wherever people rise up against oppression and exploitation students are at the forefront of the movement. For most students this is a passing phase, which they leave as they enter well paid professions. As established professionals they are closer to the centre of wealth than ordinary people, and only a few continue the social engagement of their student days.
The scientists of the 19th century were the first to follow that pattern. All major uprisings and revolutions in Europe saw the strong participation of university students but had little support from the science establishment. The student movement was particularly strong in Germany, where the first Burschenschaft (Students' Association or Fraternity) was founded in 1818. It demanded a republic and promulgated violence if necessary to achieve this aim. It was banned a year later, but other Burschenschaften emerged in Germany, Switzerland and France and operated as clandestine organizations.
The scientists in closest contact with the activities of the students were the professors at the universities. As a consequence, university academics were more inclined than other scientists to risk their positions and speak out against the politics of power. When in 1837 the king of Hanover suspended the constitution, seven professors of the University of Göttingen protested against the decision and were promptly dismissed. The most prominent of all scientists to suffer such a fate was Dmitry Mendeleyev, who was dismissed for support of student demands in 1890.
The reluctance of scientists to take public positions against governments did not last forever. When the 20th century saw the fruits of science used for the destruction of entire cities and the loss of thousands of innocent lives scientists spoke out again. They were then on their own, and their voices remained unheard. This will be the topic of the next lecture.
Most of the political developments just described formed the background to the birth of field theory discussed in Lecture 27. Germany's unification into a rising imperial empire was already established fact in 1873 when James Clerk Maxwell published his field equations. The scientific developments to be discussed now evolved in a Europe of competing imperial powers, in which Britain was reduced to an empire among several others, France and Italy had obtained their own colonies, and Germany was trying to catch up with its rivals.
The new generation of scientists was born into a time characterized by fierce nationalism. Lecture 29 will see them grow old in two world wars, promote international understanding and speak out against the use of science for purposes of war. The decades before World War I did not show any indication for such engagement, but the transition from classical field theory to relativity set the scene for things to come.
Maxwell's equations had provided the complete description of electromagnetic fields through wave theory. Although it had become more and more likely that the waves did not require a medium to propagate, they clearly shared their basic property - to act as a transmitter of energy - with other, mechanical wave systems: Ocean waves carry energy, as is evident when they end up crashing against a sea wall, but the water particles in a wave move up and down and back and forth and remain at their original place after the wave has gone through. Sound energy is carried from the source to the ear, but the air particles only vibrate back and forth. The question that arose was how to calculate the energy that is carried by an electromagnetic wave.
The answer to this question was of very practical importance. Building a radio transmitter with sufficient output to reach across a country is pure guesswork without accurate assessment of the energy transfer in waves. And this was only one of the many possible applications, because Maxwell's equations held equally true for radio waves as for light, the radiation of heat, and many other electromagnetic phenomena.
The search for the determination of the energy transmitted by Maxwell's waves led to the concept of "black-body" radiation. A black body is defined as a body that absorbs all radiation that falls on it. Any radiation that can be measured as being emitted by a black body must therefore be the product of the body itself. Gustav Robert Kirchhoff showed that all enclosures maintained at constant temperature contain black-body radiation, which can be measured through a small hole in the enclosure. It was soon found that the total energy emitted by a black body was proportional to the fourth power of its absolute temperature. But a black body emits radiation at more than one wave frequency, and the problem was then to find the energy distribution between the various frequencies, or the "energy spectrum."
Several scientists obtained answers to the problem, but all proved to be only partial solutions. They were either valid only for short waves, or only for long waves, or the solution could give the frequency that emitted the most energy but failed to catch the distribution of energy over the remainder of the spectrum. The final solution was presented in 1900 by Max Planck; but rather than bringing universal relief it posed a tremendous challenge for theoretical physics.
Planck had discussed the problem with Ludwig Boltzmann, one of the scientists who worked in the discipline thermodynamics. Heat had been well understood as the result of random movement of molecules, and thermodynamicists were accustomed to the idea that heat energy had to be calculated from the statistics of random particle movement. Boltzmann told Planck that he would never get close to determining the energy of electromagnetic waves unless he abandoned the idea of the continuity of radiant energy.
Following Boltzmann's advice, Planck assumed that radiant energy - whether heat, light or electromagnetic energy - is emitted in bursts. After long and arduous attempts at formulating the idea in mathematical form he found that the quantity of energy contained in each burst is proportional to the frequency of vibration that is emitted (and described by Maxwell as a wave). He called the constant of proportionality the "quantum of action" h. It is now known as Planck's constant.
The idea that energy emission occurs in discrete bursts was close to the concepts of thermodynamics and reasonably easy to accept within the framework of the existing physics. But Planck's theory postulated that energy absorption, too, can only occur in discrete bundles. That the statistical movement of particles results in some kind of vibration that travels through space as a wave was something that could be envisaged in some way. Converting an incident wave back into particles that are absorbed in bursts was beyond all experience, and Planck spent the next years trying to understand what it meant.
The first example of such a process was provided by Albert Einstein in 1905. In 1887 Heinrich Hertz had observed that an electric current can be produced when light falls upon metal. Physicists explained this "photoelectric effect" as the ejection of electrons from the metal but had been unable to explain why this should be happening. Einstein assumed that light consists of particles or light quanta. When the quanta arrive at the metal plate with velocity v they carry the energy hv, which enables them to tear out an electron from the metal. The arriving light wave was thus associated with a stream of particles, now called photons, and the intensity of the light or "wave amplitude" was equivalent to the number of photons that arrived per unit time. While this changed the number of electrons torn out of the metal, the individual interactions between photons and electrons would always be associated with the same energy quantum.
The fact that energy is transmitted both through waves and in quanta was thus formally established. Scientists found it difficult to accept; some were in favour of rejecting wave theory altogether, others like Einstein hoped to find a unifying answer. In 1913 Niels Bohr applied the quantum idea to the model of the atom developed by Ernest Rutherford. In the classical atom model electrons circle the nucleus. Radiation means transfer of energy. If an atom radiates energy this energy has to come from the movement of the electrons. In the classical model this would slow the electron down, and it would spiral into the nucleus, moving faster and faster as it gets closer to the nucleus. If movement around the nucleus is interpreted as the frequency of the associated wave, the spiralling motion would produce an increase in the frequency.
Observations did not show a continuous range of frequencies emitted from atoms but a set of distinct frequencies. Bohr explained this by postulating that
Bohr's theory led to successful explanations of hitherto unexplainable phenomena and was soon widely accepted. It led to the new discipline of "wave mechanics" of Louis Victor de Broglie, in which waves were associated with corpuscles of different type (photons for light, electrons for electromagnetics) and the momentum p of a corpuscle was connected with the wavelength l through l = h/p. It also quickened the pace at which modern science left the realm of everyday experience and studied processes well beyond the reach of the normal human senses.
There can be no doubt that with the 20th century science changed its character profoundly. Science began to study phenomena and processes that occur in nature but are not normally consciously witnessed by humans and require special experiments and apparatus if they are to be observed. At the beginning of the 20th century scientific study was still restricted to naturally occurring phenomena and processes, but this should soon change with the rise of nuclear physics, which led to the generation of elements too unstable to exist on Earth under natural conditions.
The beginning of the 20th century was also the period where the question whether science can tell us anything about the substance of nature came into focus. The new wave mechanics was essentially a method to solve a differential equation, in other words, the scientific description of nature was largely determined by developments in mathematics. Einstein and others may well have been motivated in the search for a unified theory by the belief that science discovers the substance of nature. In practice, their outstanding scientific successes only helped to prove that science provides a description of nature that generates predictions of nature's behaviour. Its true substance remains as inaccessible as ever.
The unified theory remained elusive. Instead, Werner Heisenberg postulated his uncertainty principle that any arrangement for the measurement of the position of a particle precludes the simultaneous measurement of its momentum and vice versa. Bohr expressed the same condition in his complementarity concept that corpuscle and wave are two images of reality and that, once one image is observed, the other is excluded.
The proof of the usefulness of any scientific image of nature is its ability to make deductions that can be verified by experiment. Einstein's theory of relativity, which extends science into regions not accessible before, led to a range of experiments and has shown its predictive strength on several occasions. It sprang from one of the core problems of early wave theory, the search for the ether as the medium for the wave.
Astronomical observations had led to contradictory conclusions about the ether. It was known for a long time that the stars change their positions on the sky over the course of a year as a result of the Earth's movement around the Sun. (This is the "aberration" of the stars; it amounts to 20.49 seconds of arc.) If light travels through ether this suggested that the ether was fixed in space and the Earth does not carry it with it on its path around the Sun. Attempts were made in 1887 to measure the velocity of the Earth through the ether by comparing the velocity of light through space in different directions (the Michelson-Morley experiment). It was the same in all directions, which could only mean that the ether was at rest relative to the Earth, while aberration showed that it was not.
The idea of an ether was eventually abandoned in the 1930s, but before that several scientists began to consider possible modifications of classical Galilean movement. George Francis FitzGerald in 1893 and Hendrik Antoon Lorentz in 1895 suggested that a moving body is shortened in the direction of motion by such an amount that the contraction affects the apparatus to produce identical measurements of the velocity of light. Einstein used these and other works to develop his theory of special relativity. In 1905 he showed in a series of publications that the two fundamental principles of classical mechanics
are unconnected and can therefore be combined with alternative postulates. Einstein extended the first postulate to include electromagnetism and replaced the second by a new principle:
Special relativity reinforced the fact that science provides descriptions of physical phenomena but does not establish reality. It showed that it is impossible to establish the simultaneity of events in different coordinate systems. It shows that distances are shorter and time is slowed down in all systems other than one's own.
The fact that Einstein's publication of relativity theory did not contain a single reference to the work of others has often been taken as an indication for the genius of its author. A fairer assessment has to note that he was put on the right track by the work of Lorentz and FitzGerald. Normal scientific practice and ethics requires that reference to these works should have been made. (The French mathematician Henri Poincaré also published elements of the theory of special relativity in 1906 independent of Einstein; but his work was derived on the assumption of an existing ether, and in any case Einstein was not aware of Poincaré's work.)
In recent years studies of personal letters between Einstein and his first wife Mileva Einstein-Maric have suggested that Einstein developed the core ideas of relativity in close collaboration with her but did not mention her contribution anywhere and possibly actively suppressed her name from his paper on special relativity. Mileva's fate is indeed an important part of the history of science of this period and a vivid demonstration of the discrimination under which women scientists had to suffer.
For people who grew up in an environment dominated by the traditions of European civilization it is difficult to imagine that the concept of simultaneity is lost when events are seen in different coordinate systems. In the 20th century the science of linguistics (the study of the structure of languages and of their implications for thinking patterns) has provided evidence that the concepts of Newtonian mechanics and Euclidian space are intimately linked with the structure of the Indo-European language family. In other words, the structure of our language is tuned to the description of nature in terms of classical physics. The analysis of other language groups (such as the language of the Hopi and other native American societies) indicates that at least some native American societies have a very different way of describing nature, a way that is closer in many respects to relativity theory than Newtonian physics. If this is correct it suggests that the ways in which humans interprete and describe nature were set very early during the development of the species, at the time when the fundamental language groups began to evolve.
A most important result of relativity theory was that unlike the laws of Newtonian mechanics (which govern the interaction between masses), Maxwell's equations (the behaviour of waves) remain unchanged during transformation from one coordinate system to another. To match this with a new transformation for the behaviour of particles, Newtonian mechanics had to be extended to situations where mass is destroyed during the release of energy. This led to the famous equation for the energy E that is released when a mass m is destroyed:
E = m c2
The theory of special relativity extended Newtonian mechanics to situations where velocities can no longer be regarded as small compared to the velocity of light c. It applies to uniformly moving coordinate systems and reduces to Newton's Laws in systems with small velocities. The theory of general relativity extended Newtonian mechanics into the region of accelerated coordinate systems.
General relativity leads into the realm of non-Euclidean geometry (first developed by Gauß and contemporaries) and moved science further away from everyday human experience. It questioned the distinction between "real forces" such as gravity and "virtual forces" such as the Coriolis force and argued that gravity and inertia are expressions of the same force. Just as it is possible to eliminate the Coriolis force by an appropriate choice of coordinate system it should then be possible to eliminate gravity as a force by finding the appropriate coordinate system.
In such a coordinate system matter does not exert a force on other matter but produces a curvature of space in its vicinity, and a body moving through space is not held on its path by the balance of forces but moves along the "straight lines" (so to speak) of the system. Einstein's Law of Gravitation, which he developed from this premise, is again an extension of classical mechanics; for small gravitational fields it reduces to Newton's Law of Gravitational Attraction.
Relativity theory has provided answers to a range of observations that could not be explained by classical physics. For the movement of planets it predicts a slow rotation of their elliptical paths in the orbital plane and in the direction of the orbital motion (an advance of the perihelion of the ellipse). The effect had been observed for Mercury, the planet closest to the Sun. Proposed explanations involved the existence of additional planets between Mercury and the Sun or a departure of the Sun's shape from spherical. Relativity theory gives an advance of the perihelion of 42.9 seconds of arc per century, extremely small but in exact agreement with observations.
Another consequence of relativity theory is that photons are deflected from a straight line when they pass a very large mass, such as for example the Sun. To observe the effect the light has to pass so close to the sun that its source is not visible against the Sun itself except during solar eclipses. In 1919 the Royal Society and the Royal Astronomical Society sent out parties to observe a solar eclipse and measure the deflection of light from a bright star. Both parties measured a deflection of 1.75 seconds of arc; Einstein had predicted 1.69 ± 0.50 seconds of arc.
The development of nuclear physics, to which we turn in the next lecture, produced phenomena on the atomic scale that allowed the study of gravitational forces with extremely small rather than extremely large masses. Relativity theory again provided solutions to questions raised by observations where classical quantum mechanics was lost for answers. While open questions remain, there can be no doubt that relativity theory provided a most significant extension of classical physics and was the most important development in science during the first two decades of the 20th century.
Anderle et al. (1966) Weltgeschichte in Daten. VEB Deutscher Verlag der Wissenschaften, Berlin.
de Broglie, L. (1966) Contemporary atomic and quantum physics. In: R. Taton (ed.) History of Science, Science in the Twentieth Century. Basic Books Inc. New York, 78 - 88. (Translation of La science contemporaine)
Tonnelat, M.-A. (1966) Relativity. In: R. Taton (ed.) History of Science, Science in the 19th Century. Basic Books Inc. New York, 89 - 106. (Translation of La science contemporaine)
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