From Maxwell to Einstein
FEST log
Entry #009
June 03, 2024
The State of Physics in 1865
This entry is a sequel to entry #008, "A Picture Book of Physics Theories." There we followed the history of science starting from its prescientific roots till 1865, when Maxwell published his equations of electromagnetism. This unification of the theories of electricity and magnetism led to an explanation of the nature of light, which in turn enabled wireless communication, from early radio and tv to the daily use of our cell phones.
Our last two diagrams were Fig. 5 and its more compact version, Fig. 6, reproduced here below. As a reminder: AM was superseded by CM as the description of motion in space and time under the influence of UG; after a while extra force fields were measured and described, E and M; and in 1865 Maxwell unified the theories of E amd M into one unified theory of EM. The totality of these theories forms a skeleton summary of the state of the art of Physics in 1865.
Physics in 1900: the problem of the aether
By the turn of the century, in 1900, that same picture still held, but there was only one problem: efforts to determine the presence of an aether were unsuccessful. Whatever the characteristic of that medium might have been, the changing speed and direction of the motion of the Earth with respect to the aether, at different times of year, should have been measurable. However, increasingly accurate measurements all gave the same null result: no difference was detected.
Maxwell's theory had convincingly explained that light is formed by electromagnetic waves. A decade later Hertz had figured out how to generate and detect radio waves. There was no doubt anymore that the electromagnetic field could exhibit waves, in a real and practical sense. But how could there be waves without there being a medium, a "carrier" to "carry" the waves? At first, this seemed like an annoying blemish question mark, as depicted by the question mark in Fig. 7, which otherwise is identical to Fig. 6, a snapshot taken 35 years earlier.
1905: The aether resolved; now the problem of gravity
Before long, in 1905 Einstein came up with an idea that seemed even more preposterous than waves without something to make waves in. He proposed that space and time itself were not absolute, as Newton had assumed, but relative to the state of motion of individual observers. His theory of relativity changed not only the dynamical play of motion, performed by physical objects on the stage of space and time; it changed the stage itself!
Changes that dramatic had occurred only twice in the last 2200 years: first by Aristotle who described a stage with two layers, the Earthly and the Heavenly realm, below and above the orbit of the Moon; and then by Newton, whose unified synthesis introduced a single unified stage.
The third stage that Einstein introduced was of a completely new type: a 4-dimensional spacetime as a continuum that would allow different 3-dimensional ways of providing space and time axes for cutting up the 4-dimensional cake, different depending on the state of motion for each observer. This is indicated in Fig. 8.
However, the "???" mark in Fig. 8 indicates that Fig. 6 no longer holds. Maxwell's beautiful unification of electricity and magnetism into electromagnetism, EM, was no longer compatible with the other long-range field, that of universal gravity, UG, which was based on Newtonian absolute space and time. Even though EM was invented as a theory within classical mechanics, CM, it forced a new theory of space and time, SR, as a new home for EM to live in, superseding CM.
Fig. 9 shows this clash between CM and SR, indicated by the vertically placed question marks in "->?" and "?<-".
The search for an answer
A few years after discovering what later would be called the theory of *special* relativity, Einstein began to search for a more *general* theory, indicated in Fig. 10 with "??", in order to solve the problem of "???". If only Newtonian gravity could be replaced by a theory of gravity that would be compatible with special relativity in the limit of weak gravitational fields (weak compared to that of black holes, as we would expressed it now), all would be well again.
Fig. 11 shows how such a new theory could be seen as a unification of UG and SR, somewhat similar to the unification of E and M into EM.
The answer, in the way it leads us back to 1865
In 1915, Einstein found the answer. As I already discussed in entry #007, general relativity added curvature and elasticity to the 4-dimensional fabric of special relativity. Gravity no longer was a force acting between players on an otherwise passive stage. It was the dynamics of the stage that completely explained the effects of the force of gravity, with no need for anything else: gravity without gravity.
General Relativity, GR in Fig. 12, restored physics as a consistent, and seemingly complete, theory for all of the long-range forces across space and time. By replacing "??" by GR, suddenly physics became as complete as it had been in Fig. 5.
To compare the state of completion of physics in 1865 and 50 years later, in 1915, we can plot the two in compact form in Fig. 13. Both situations looked the same in diagram form. In fact, the success of general relativity shows it was even more complete, since there was no longer the nagging question of what the aether was and how to detect it.
The answer, in the way it leads us back to 1700
An alternative way of depicting the revolutionary aspects of GR is given in Fig. 14. So far, the central horizontal lines in our diagrams have depicted the changing role of the stage of space and time. In contrast, the force fields acting across space and time were shown above and below that line, as players on that stage. However, now that GR had become both a new field explaining the *force* of gravity as well as a new and more powerful description of the nature of *spacetime*, we can equally well put GR on the horizontal center line, as an increasingly more accurate way to represent space and time. So let us adapt Fig. 12, to highlight the fundamental spacetime role of GR, to produce Fig. 14 instead.
At this point we can go even further back in time, to 1700, the time of Newton. We have seen in entry #008, in Fig. 3, how the state of the art of physics can be summarized with one stage, classical mechanics, and one long-range force, universal gravity. Fig. 3 is reproduced in the top half of Fig. 15, while Fig. 14 is reproduced in compact form in the bottom half of Fig. 15.
This single figure shows why and how Einstein was considered the "new Newton" in the nineteen twenties. When his general relativity theory was observationally confirmed through the measurements of the bending of starlight near the sun during an eclipse in 2019, physics regained a state of wholeness and simplicity, both, that had not been seen since Newton.
A glimpse of the future, from 1925 onwards
Alas, this happy state of affairs would only last a mere ten years . . . the only period, so far, that humanity has possessed a single consistent theory of space, time, gravity and electromagnetism. In the next entry, #010, we will see what happened in 1925, when quantum mechanics appeared on the stage. Or, to stay with the previous metaphors, demolished the stage.
From family portraits to genealogy
But before going there, let me add the historical perspective of a genealogy of theories. In Figs. 12 and 14 we saw two family portraits taken in the period 1915 to 1925. They were snapshots. If we want to trace the genealogy of ideas, from Newton to Einstein, we get a different picture, that of Fig. 16. To give GR a balanced place in this figure, starting from CM, I have given GR a place halfway the middle line of space and time and the upper line where gravity would be placed as a force. It really belongs to both.
As for SR, note the difference between its place in Figs. 12 and 14, where it directly follows Newton's GR, at the left, and in Fig. 16, where it is placed as the offspring of EM. Even though SR is more fundamental, in retrospect, than EM, it was EM that historically led to SR.
And as a farewell to the classical period of physics, from 1687, when Newton's Principia was published, all the way to 1915, let me take stock of the developments during that period in Fig. 17, in the form of boxes superimposed on Fig. 16. Seven discoveries and unifications stand out.
There was Newton's unification of Aristotle's separate laws of motion in heavens and on Earth, in the first box. The second box shows the discovery of theories for gravity, electricity and magnetism. The third box shows three more successful unifications, of electricity and magnetism into electromagnetism, of space and time into spacetime, and of spacetime and gravity into a dynamic form of spacetime.
Note that of the seven highlights, more than half were made by Newton and Einstein, CM & UG and SR & GR, respectively, one by Maxwell, EM, and the remaining two, theories of E and M, electricity and magnetism, as well as aspects of their interactions, were the result of a number of different individuals. Of course, all of the seven milestones could only have been reached by building on the foundations laid by many others, whose contributions were crucial in clearing paths towards new insights.
The end of physics, act 1: the world as a mechanism
We have now reached the end of what is called "classical physics", the first Act in the Play of physics as the Science of matter. In the next entry we will move on to physics, act 2, which is "quantum mechanics", an act that started in 1925. Quantum mechanics, QM in our abbreviated diagram notation, is really a misnomer. Yes, what is called quantum mechanics lies at the basis of quantum physics, but in no way does it resemble a mechanism. It was only (Newtonian?) inertia that kept the term "mechanics" in use, since the quantum world is anything but a mechanism, as we will see.
The first mechanical model appeared with what is called Aristotelian mechanics (AM), around 300 BC, prescientific in being partly untestable speculation. Science as we know it, using working hypotheses, got started in the 17th century with Newton's classical mechanics (CM). Maxwell's electromagnetism (EM) was a further extension, still structured as a mechanism, but based on an almost immaterial medium, called the aether, an idea that was dropped when no longer needed in Einstein's special relativity (SR) theory. It was only with Einstein's general relativity (GR) theory that a new fully consistent picture of the physical world had been developed.
General relativity could still be viewed as a kind of mechanical theory, but mechanical in a very different way that the term had been used so far. The reference "mechanical" did not point to the way or working of three-dimensional machines, existing in space and doing their work in time. Rather, the term "mechanics" applied to fully four-dimensional entities, existing in spacetime. However, the theory remained deterministic and in that sense it was still "mechanical".
Beyond mechanistic foundations of physics
The succession of updated physics theories of the physical world, illustrated by the four arrows in the sequence "AM -> CM -> EM -> GR -> QM", consisted of changes happening after ever shorter intervals in time of {2000, 200, 50, 10} years, respectively. The last number indicates the decade from the introduction of general relativity in 1915 to the first formulation of quantum mechanics in 1925.
No, the next shocking new discovery did not take place 2 or 3 years after 1925, as the above series might have suggested. It still has not taken place, 100 years later. And what is more, unlike was the case after earlier shocks, how to interpret quantum physics is still an open debate. It remains a question about which hundreds of books have been written, and several conferences and workshops are organized each year. That hasn't happened with any of the previous updates in a scientific theory of the physical world.
The main disagreement centers around the role of the observer in any experiment involving quantum physics. The end of mechanistic thinking was also the end of the unquestioned acceptance of an objective reality. Seen in that way, the need for a science of mind is a logical outcome of progress in physics, made by physicists and made by their own lights. We will now look at the state of physics in 1925, in our next entry.
– Piet Hut