Max Planck und Albert Einstein

Physics in upheaval

At the end of the 19th century, the world of physics seemed to be on a secure footing. It was mainly marked by mechanics, with electrodynamics and thermodynamics gaining in importance. There were problems emerging at the interface between these areas, such as that of heat radiation, for which it had not been possible to find conclusive answers.

Challenges to theoretical physics, the emerging discipline in which Max Planck was working, were arising from the experimental study of these problems. 

Was it still possible to base an explanation of the new heat and radiation experiments on mechanics - for example, using the atomic hypothesis or that of an ubiquitous ether? The seemingly well-ordered world of physics was rapidly coming apart.


In the 19th century, numerous physicists tried to explain the phenomena of electricity and magnetism on the basis of classical mechanics. In 1870, James C. Maxwell formulated his theory of electrodynamics, which is based on the idea of fields and waves, to deal with these phenomena. In 1886, Heinrich Hertz experimentally demonstrated the existence of the electromagnetic waves predicted by electrodynamics. Initially, there were attempts to derive electrodynamics from mechanics. As this failed, electrodynamics itself was considered by some physicists as the foundation for all physical events.

What is light?

Are there light particles or does light, like sound, consist of waves? Isaac Newton (1643 – 1727) had assumed that light consists of particles that are subject to the mechanical laws. In the 19th century, the view that light is a wave phenomenon became prevalent, as it made it easier to explain phenomena such as diffraction. The explanation of light as an electromagnetic wave confirmed this view.

However, it was difficult to bring experiments such as the photoelectric effect or the Michelson-Morley experiment into line with this description. At the beginning of the 20th century, Albert Einstein took the photoelectric effect as point of depart for a radical reinterpretation of Planck’s quantum hypothesis. Einstein called for a quantum theory of light, embracing both its particle and wave nature.

Does light need ether?

Sound waves spread out in air, and water waves in water. But what do light waves move through? For a long time, the answer was the ether. The light waves were supposedly carried by a medium like air or water. Initially, ether was understood to be a mechanical substance; however, it soon became clear that it was fundamentally different from such substances. A crucial problem was the question of whether the ether moves with the Earth or whether it remains still in space with the Earth moving through it. Physical considerations made the idea of ether moving with the Earth problematic. In 1887, Michelson and Morley tried to take direct measurements of the “ether wind” that would have to be created by the movement of the Earth through a static ether. They were unable to prove its existence, which further weakened the ether theory.

Constantly velocity

Albert Einstein based his 1905 Special Theory of Relativity on an explicit postulate that the laws of electrodynamics are the same for all systems that move at constant velocity relative to each other (the principle of relativity). This entails that the speed of light is the same in all directions, even if a system (such as the Earth) is moving through space. That is the explanation for Michelson and Morley’s negative result. Planck was one of the first to recognize the fundamental significance of the Special Theory of Relativity and became an important supporter of the young Einstein.


In the natural sciences, “atomistics” was understood to be the assumption that matter is made up of basic building blocks. The atomist world view originated from classical natural philosophy. In the middle of the 19th century, attempts to understand how chemical reactions work and to explain the thermal behaviour of gases by motion of small particles gave rise to the question of whether the existence of atoms can be proven empirically.

Max Planck initially favoured to the view that atoms could not exist, whereas Ludwig Boltzmann and James C. Maxwell were convinced atomists.

It was not until 1900 that experiments on rarefied gases, which made the discovery of cathode rays, X-rays and electrons possible. Together with studies of Brownian motion they led to the proof of the existence of atoms.

The discovery of radioactivity

At the January meeting of the Paris Academy in 1896, Henri Poincaré (1854 – 1912) spoke about the discovery of X-rays. Antoine Henri Becquerel subsequently looked for a connection to fluorescence phenomena in crystals. He discovered that certain uranium salts independently emit radiation that is different in a variety of ways from X-rays - Becquerel radiation. It is known today that it results from of atomic decay.

In spring of 1898 Marie Curie in Paris found another element that emits Becquerel rays – thorium. This resulted in widespread interest in research into Becquerel rays. Marie Curie and her husband Pierre Curie subsequently discovered the elements polonium and radium, and coined the term “radioactivity.”


Thermodynamics, an area on which Max Planck worked throughout his life, deals with heat as a form of energy, and with the conditions under which it is possible to convert heat into other forms of energy, such as mechanical work.

Thermodynamics became established during the 19th century. Important discoveries made during attempts to construct a more efficient steam engine contributed to its emergence. The importance of this discipline was also in its practical benefit to the growing industrial sector. Hermann von Helmholtz, Gustav Kirchhoff, Rudolf Clausius and Ludwig Boltzmann shaped these insights into a theoretical discipline for which Planck showed enthusiasm early on.

What is entropy?

Entropy specifies the proportion of no longer usable energy in a system and was later interpreted as a measure for disorder. The Second Law of Thermodynamics states that entropy can never decrease in a closed system. Thus, for example, temperature differences even out over time and this process is irreversible barring outside influence.

In 1879, Planck expanded the concept of entropy in his doctoral thesis. The interpretation of the Second Law was disputed at the end of the 19th century. Boltzmann saw it as an expression of the statistical behaviour of a large number of particles. The Second Law could, according to Boltzmann, also be violated in rare cases, whereas Planck was convinced that, like the laws of mechanics, it was valid in an absolute sense.

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