Newton’s laws of motion and universal gravitation, the laws of conservation of energy and momentum, the laws of thermodynamics, and Maxwell’s equations for electricity and magnetism were all more or less nearly complete at the end of the Nineteenth Century. They describe a universe consisting of bodies moving with clockwork predictability on a stage of absolute space and time. They were used to create the machines that launched two waves of industrial revolution — the first one powered by steam and the second one powered by electric current. They can be used to deliver spacecraft to the ends of the solar system with hyper-pinpoint accuracy. They are mathematically consistent in the sense that no one rule would ever violate another. They agree with reality to a high degree of accuracy as tested in experiment after experiment.
At the end of the Nineteenth Century, physics appeared to be at an apex. Several people are reported to have said something like this
There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.
This has been attributed to William Thomson a.k.a. Lord Kelvin (1824-1907) in an address to the British Association for the Advancement of Science in 1900, but I haven’t been able to find the primary source. A similar statement was made twice by the German-American scientist Albert Michelson (1852-1931) as was discussed earlier in this book. It is often reported that Michelson got the idea from Kelvin, but there is little evidence to back this claim up.
At the turn of the century, Kelvin wasn’t saying that physics was finished. In fact, I think he was saying quite the opposite. There were two clouds hanging over Nineteenth Century physics. Here’s the essential quote from a lecture he gave 110 years ago today.
The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds. I. The first came into existence with the undulatory theory of light, and was dealt with by Fresnel and Dr Thomas Young; it involved the question, How could the earth move through an elastic solid, such as essentially is the luminiferous ether? II. The second is the Maxwell-Boltzmann doctrine regarding the partition of energy.
Kelvin is describing two problems with the physics of his time. They are highly technical in nature and not something you could easily describe to your grandmother (unless she had some training in physics). The first one refers to the now discredited theory of the luminiferous ether. The second one describes the inability of electromagnetic theory to adequately predict the characteristics of thermal radiation.
In essence, the first argument went like this: light is a wave, waves require a medium, the medium for light was called the luminiferous ether, it must be extremely rigid (since light travels so quickly), it must be extremely tenuous (since we can’t detect its drag), rigid and tenuous are adjectives that are incompatible (strong yet soft), Nineteenth Century physics cannot handle this, therefore Nineteenth Century physics is in trouble. The ray of sunshine that dispersed this dark cloud was the theory of relativity devised by Albert Einstein. The major revelations of this theory were that there is no ether, there is no absolute space, there is no absolute time, mass is not conserved, energy is not conserved, and nothing travels faster than light. For awhile, this was the most revolutionary theory in all of physics.
The second dark cloud is the solution to the problem Kelvin called "the Maxwell-Boltzmann doctrine" which lead to the most revolutionary theory in all of physics — quantum mechanics. The major revelations of this theory are that all things are both particles and waves at the same time and that nothing can be predicted or known with absolute certainty.
These arrival of these two revolutionary theories divided physics up into two domains. All theories developed before the arrival of relativity and quantum mechanics and any work derived from them are called classical physics. All theories derived from the basic principles of relativity and quantum mechanics are called modern physics. The word modern was chosen since the foundations of these theories were laid in the first three decades of the Twentieth Century. This the era of modern architecture, modern dance, modern jazz, and modern literature. Modern technologies were starting to appear like electric lights, toasters, refrigerators, sewing machines, radios, telephones, movies, phonograph records, airplanes, automobiles, subways, elevators, skyscrapers, synthetic dyes, nylon, celluloid, machine guns, dynamite, aspirin, and psychology. The early Twentieth Century was filled with revolutionary ideas and inventions. Life now seems unimaginable without them. Modern physics was just one important part of the modern era.