35. The Birth of Modern Physics (1900 – 1928)

Summary In the end of the 19th century European natural scientists commonly thought that they had attained a good grasp of how the Universe works. In the beginning of the 20th century, however, long-standing concepts about space, time and matter were shattered by the theory of relativity and by quantum mechanics, which from then on were the core of modern physics which is distinguished from the former classical physics. While the theory of relativity describes the Universe on large astronomical scales, quantum mechanics describes the Universe on small atomic and sub-atomic scales.

Keywords Classical Physics; Quantum Mechanics; Theory of Relativity

Solvay conference 1927

Participants of the Fifth Solvay Conference in Brussels in 1927. Among the participants were Erwin Schrödinger (back 6th from left), Paul Dirac (middle 5th from left), Niels Bohr (middle 1st from right), Max Planck (front 2nd from left) and Albert Einstein (front 5th from left), all of who were pioneers of modern physics in the beginning of the 20th century. (© Benjamin Couprie / Wikimedia Commons / Public Domain)

In the end of the 19th century European natural scientists commonly thought that they had attained a good grasp of how the Universe works. The movements of physical objects on Earth and in outer space which are subject to forces, for example gravity, were described by mechanics – the centerpiece which, as well as of the whole Scientific Revolution of the 17th century, being Isaac Newton’s work Philosophiæ Naturalis Principia Mathematica of 1687. Phenomena associated with the transformation between heat and energy were described by thermodynamics which had been developed during the 19th century. And phenomena associated with electricity, magnetism and light were taken care off by electrodynamics, which was based on the Maxwell equations of the 1860s. These physical theories were developed in Europe and are now known as classical physics, and the knowledge and technologies resulting from them helped the European states to unleash the Industrial Revolution and to dominate the globe politically.

In the early 20th century, however, a group of young physicists challenged the traditional paradigms of classical physics and created the modern physics which we know today. Under the impression of the Michelson-Morley experiment of 1887, which showed the unexpected constancy of the speed of light (at about 300,000 kilometers per second) when it was measured from places with different relative velocities to a light beam, in 1905 Albert Einstein described this experimental finding in the special theory of relativity. This theory stated that space and time are not fixed – as was thought till then – but rather that for an object moving close to the speed of light space contracts and time stretches. The speed of light was now perceived as the speed of causality and as the upper speed limit of the Universe, and its constancy caused that curvature of spacetime. In 1915 Albert Einstein formulated the general theory of relativity, which not only described the perceived curvature of spacetime for fast-moving objects, but also the curvature of spacetime which is caused by masses, such as the Earth, the Sun or a black hole. The general theory of relativity contained Newtonian mechanics as an approximation for physical situations at small velocities and small nearby masses, and it described gravity as a simple geometric property of the curved spacetime rather than as a Newtonian force.

While the theory of relativity had success in describing the Universe on large scales including gravity, a successful description of the Universe on small scales, such as the atomic scale, required different lines of thought. In 1900 Max Planck mathematically described the emitted spectrum of a hot body – the so-called black-body radiation – by introducing the Planck constant, which resulted in the quantization of energy states. This event is today regarded as the birth of quantum physics. Following the discovery of the electron in 1897 and of the atomic nucleus in 1911, the Bohr model of the atom of 1913 described electrons revolving around the atomic nucleus due to the binding electromagnetic force, similar to planets revolving around the Sun due to the gravitational force, with the difference that the energy states of the electrons were quantized. The understanding of the behavior of electrons in atoms – as well as of other small objects – was then improved by the Schrödinger equation of 1926 (later by the Dirac equation of 1928 which also includes special relativity), according to which electrons not physically revolve around the atomic nucleus, but rather reside in a blurry probability cloud around it. This is further illustrated by Heisenberg’s uncertainty principle of 1927, according to which there is a universal limit given by the Planck constant regarding how accurately two complementary variables of a given particle – such as position and momentum, or time and energy – can be known. If one of the two variables is measured very accurately, then the uncertainty in the determination of its complementary variable automatically increases. Also other irritating phenomena in the quantum world were found, such as the wave-particle duality of matter and its behavioral dependence on the fact if it is measured or not.

1933 Nobel Peace Prize awarded to Norman Angell

An example for a Nobel Prize medal. Since 1901, Nobel Prizes were annually awarded for outstanding scholars from different disciplines, one of them being physics. They are regarded as being one of the highest honors for physicists. User:Anubis3 / Wikimedia Commons / CC-BY-SA-3.0)

These new concepts about space, time, the structure of atoms and the fuzziness in the quantum world were sometimes spooky and hard to digest for rational minds. Many of the pioneers of modern physics were awarded with the Nobel Prize in physics, which is regarded as one of the most prestigious awards for physicists till now and was founded by the Swedish engineer and businessman Alfred Nobel. While the advancement of physics continued during the later 20th and the 21st centuries, the unification of the theory of relativity and of quantum mechanics to a combined theory of quantum gravity, or to a theory of everything, was unsuccessful till now.

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