“Light also has a cultural history”

Interview with Jürgen Renn and Matthias Schemmel from the Max Planck Institute for the History of Science about a phenomenon which is both familiar and mysterious

May 27, 2015

Light is everywhere and not only present in painting, photography or as a tool in science. In daily life as well, the phenomenon plays a decisive role. Its importance for our consciousness, but also for the technical world, can hardly be overestimated. Reason enough for UNESCO to declare 2015 the International Year of Light. In the distant past, light must have appeared to people as a thoroughly mysterious “substance”. And its dual nature as wave and particle still challenges our power of imagination today. An interview with Jürgen Renn and Matthias Schemmel from the Max Planck Institute for the History of Science in Berlin.

Let there be colour: this woodcut from the 19th century shows Isaac Newton dispersing white sunlight with a prism.

In his work Dioptrique which appeared in 1637, René Descartes dealt extensively with the nature of light. But the French scholar was certainly not the first to want to track down this phenomenon. When did light become a topic for science?

Jürgen Renn and Matthias Schemmel: The earliest texts on optics which are still available to us arose in Greek antiquity and are associated with names like Aristotle and Euclid. However, optics was essentially understood there as the science of seeing. The role played by light in this connection had not yet been determined. Therefore, many ancient theories assume that the eye emits a kind of sight beam that “scans” objects.

Ptolemy, for example, assumed that light interacted with the sight beams, thus making the bodies visible. In the 11th century the Arabic scholar Ibn al-Haytham, who in Europe came to be known under the name of Alhazen, synthesized and furthered the optical knowledge handed down to him, developing a theory of vision based on the intrusion of something into the eye, something that he represented by means of rays. But it was not until the decoupling of geometrical optics from questions of the psychology of visual perception - a decoupling which was advanced by the technological developments in early modern times - that optics attained the meaning of a science primarily of light. In Descartes’ time this was a very recent development and occurred in the work of Johannes Kepler and others.

Incidentally, it is remarkable that geometrical optics seems not to have arisen only once - in ancient Greece. From the China of the Warring States, around 300 B.C., a text survives that is attributed to the Mohist school, which discusses shadows, plane, concave and convex mirrors and possibly even the camera obscura. But this text was not granted any reception history, unlike its Greek equivalents. The truncated development of Chinese optics shows that a pioneering approach is not sufficient for the success of traditions on the theory of nature: prolonged favourable social conditions are necessary for their continuation.

What gave researchers the idea that light is propagated at a particular speed?

This question only makes sense for particular ideas about light - such as assuming that light consists of particles which fly through space like projectiles, or that it propagates as a wave, in a medium for example. Such ideas were spread in the early modern era, when attempts were made to explain light in mechanistic ways.

There were also, however, mechanistic ideas according to which it was plausible to assume that light propagates instantaneously, meaning with infinitely high speed. At that time, hardly anyone had any idea how fast light was. And therefore attempts to measure the speed of light, like the observation of remote light sources on the earth, remained unsuccessful.

How was the speed of light first measured?

A breakthrough was achieved by the Danish astronomer Ole Rømer, who had the idea, at the end of the 17th century, of making the propagation of light in the solar system an object of research. However, one needs a kind of cosmic clock for this, in order to compare the time of the emission of a ray of light with its arrival time.

For one of his cosmic clocks, Rømer took the regular time of orbit of one of Jupiter’s satellites discovered by Galileo. He could obtain a rather exact estimate of the speed of light from the deviation of the observed from the calculated time of an event like the occultation of a moon of Jupiter in dependence on the distance from the Earth.

Isaac Newton experimented with prisms and postulated the particle nature of light. Johann Wolfgang von Goethe had his own theory and vehemently contradicted Newton with the words: “Those who compose from coloured light the single and essentially white light, those are the true obscurantists.” Was there really such a thing as a “war of faith” about the nature of light in the 18th and 19th centuries?

Newton and Goethe actually took different phenomena as the object of their research. Whereas Newton’s research took as given the assumption of a separation of light from sight theory, Goethe reintroduced the process of seeing in his thoughts on light. With his physical criticism of Newton he fell far behind the state of knowledge of his time. However, Goethe recognized phenomena from his perspective which played no role in a purely physical theory of light, and gave important impulses for the later development of sensory psychology.

At the end of the 19th century, a consensus was reached among the scientists: light was considered to be an electromagnetic wave. But shortly after that, the researchers began to doubt this explanation. How did that happen?

The wave theory of light was widely accepted long before the recognition that light is an electromagnetic phenomenon. As early as the 17th century, Huygens had developed a wave theory of light. At the beginning of the 18th century, this theory gained credibility from investigations into the refraction and interference phenomena of light. As a result, the Newtonian particle theory was increasingly forgotten, even though it achieved astonishing success.

Interestingly, it resulted in the prediction that there must be something like “dark stars” from which the light particles could not escape due to the strong gravitational forces. However, such predictions, which anticipated in some respects the later idea of black holes, were naturally highly speculative. The rebirth of the particle theory of light took place at the beginning of the 20th century in quite a different way, namely from investigations into the interaction of light and matter.

In November 1922 Albert Einstein received the Nobel Prize for physics – though not for his General Theory of Relativity, which he published 100 years ago, but for the explanation of the photoelectric effect. Could you briefly explain what that is all about?

Since Heinrich Hertz’s experimental investigations in the late 19th century, physicists have been interested in the effects of light on metallic surfaces. Here it became clear that the energy of the electrons ejected from the metal did not depend on the intensity, but on the colour of the light. But this was problematic for the wave theory.

Albert Einstein, already as a student, had been interested in the nature of radiation, and in particular the nature of X-rays and for the explanation of Planck’s law of radiation. In this connection, the idea came to him that many aspects could better be explained by the assumption that light consists of particles.

In 1905 Einstein then published a famous article in which he proposed the hypothesis of light quanta, in which the energy is connected with the colour. His explanation of the photoelectric effect on this basis finally brought him the Nobel Prize.

Light has certainly shed light on a lot, quite literally - from remote galaxies to the internals of the cell. Then, as now, researchers applied light as a tool for their purposes...

Yes! Light also has a cultural history. Since the earliest times, it has stood for illumination and enlightenment. For a long time the access to light was a social privilege and was only democratised by the technical revolutions like gas and electric lighting. At the same time, technical revolutions have repeatedly offered new insights into the information which light transports. These include the invention of the telescope and microscope, then the discovery of spectral analysis in the 19th century and finally the development of super-resolution fluorescence microscopy by Stefan Hell. The latter has shown once again that the last chapter in the history of light has yet to be written.

 Interview: Helmut Hornung

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