In the two centuries between Newton and Einstein, arguably the greatest physicist of the 19th century was the Scotsman James Clerk Maxwell. Maxwell made fundamental contributions to thermodynamics, the study of how gases, liquids and solids change when ambient conditions (such as temperature and pressure) change, and how to convert heat to work. One of the results was an understanding of the propagation of sound waves through the air. But Maxwell also applied the new mathematics of differential calculus to create a unified theory of electricity and magnetism. These are the famous “Maxwell’s Equations” that predict the existence of electromagnetic waves, which we see as “light”.
Maxwell saw the relationship between electromagnetic waves and water and sound waves. Being steeped in a mechanical analysis of the world, he was unsatisfied with his abstract mathematical theory, and invested time in building a mechanical model of the “aluminiferous ether” – the medium in which light waves traveled. Having spent years studying his equations and their predictions, I am fascinated by claims of his success. It’s a magical world in which the linear motion of charges creates rotary magnetic effects. My understanding is that the model was not simple, but contained complex systems of interlocking gears.
Now Maxwell’s work was not merely a curiosity – it was the basis for the design of communication networks that broke down distances with the enormous speed of light. More than anything else, this has brought us into each other’s lives and helped to create the sense that we are one human family. (The social and psychological reaction to that reality is complex, and we’re still growing into our responsibilities as neighbors. In The Empathic Civilization, Jeremy Rifkin offers a hopeful analysis of the transition.)
So the world of scientific inquiry hung on Maxwell’s words, and in America, two of them, Michelson and Morley, designed an experiment to detect the presence of the ether. If the ether filled all of space, the Earth must be moving through it. Therefore the speed of light should change depending upon the motion of the observer through it. The analogy was with water waves: an observer moving along with a water wave doesn’t experience its disturbance – while one moving against it feels its disturbance enhanced. This is an example of Newton’s laws concerning the change of reference frames.
Since the Earth rotates around the sun, light emitted from the Earth in a specific direction relative to the sun should have a different speed at different times of the year. To test this hypothesis, Michelson and Morley built a sensitive instrument that compared the speed of light travelling in two perpendicular directions. As the Earth varied its motion through the ether, the pattern of dark and light on a screen was expected to shift slowly. Strangely, the result was negative: the image did not change.
The conclusion was that there was no ether. This was a real crisis, because Maxwell’s Equations don’t behave very well when trying to predict the relationship between observations made by people moving at different speeds. To understand how really terrible this is, consider: in Maxwell’s theory, charges moving through empty space creates a rotary magnetic field. But what if the observer is moving along with the charge? The charge no longer appears to move, so the magnetic field disappears. How can that be possible?
This was the challenge taken up by the Dutch physicist Henrik Lorenz. He analyzed the mechanical properties of rulers and clocks, which are of course held together by electromagnetic forces, and discovered a magical world in which rulers change length and clocks speed up and slow down when the speed of the observer changes.
This was the context in which Einstein introduced his theory of Special Relativity. He did not really add to the results of Lorenz, but he simplified their derivation by proposing two simple principles: First, since the vacuum is empty, we have no way of determining whether we are moving or not. All motion is relative to an observer (thus the title: Special Theory of Relativity), and so no observer should have a preferred view of the universe. The second was that the speed of light is the same to every observer. Einstein’s mathematical elaboration of these principles unified our understanding of space and time, and matter and energy. Eventually, General Relativity extended his ideas to include accelerating observers, who can’t determine whether they are actually accelerating or rather standing on the surface of a planet.
Special and General Relativity were not the only great theories to evolve in the course of the 20th century. Quantum Mechanics (the world of the microscopic) and Particle Physics (describing the fundamental forces and how they affect the simplest forms of matter) were also developed, but ultimately Einstein’s principles permeated those theories as criteria for acceptance.
Then, in 1998, studies of light emitted from distant supernovae seemed to indicate that something is pushing galaxies apart from each other, working against the general tendency of gravity to pull them back together. The explanation for this is Dark Energy, a field that fills all of space. This field has gravitational effects, and its effects in distorting the images of distant galaxies have been observed. However, this field cannot be moving in all possible directions at all possible speeds. Therefore, it establishes a preferred reference frame, invalidating Einstein’s assumptions.
Working physicists resist this conclusion, because they have a means of accommodating these effects in their theories, which is to introduce additional mathematical terms. But science is not about fitting data – it is about explaining it. Einstein used his principles as an explanation to justify the mathematics of his theories. When those principles are disproven, the door opens to completely new methods for describing the universe. We can travel as far back as Maxwell in reconstructing our theories of physics. While for some that would seem to discard a lot of hard work done over the years between (and undermine funding for their research), for others it liberates the imagination (see Generative Orders as an illustration).
So, for example, why didn’t Michelson and Morley detect the ether? Maybe ether is more like air than water. Air is carried along with the Earth, and so the speed of sound doesn’t vary as the Earth moves about the sun. Maybe dark energy, which Maxwell knew as the ether, is also carried along with the Earth. Maybe, in fact, gravitation is caused by distortion in the Dark Energy field when it is bound to massive objects.