Physicists love theories that unify seemingly disparate phenomena, showing that they share the same source. The more powerful a theory, the greater its explanatory power.
Some physicists equate explanatory power with beauty, giving an aesthetical value to a theory. Newton’s account of gravity reigned supreme until the early 20th century, when problems with his worldview started to pile up, as new phenomena were observed that didn’t fit into it.
One such problem was light. Newton thought of light as made of tiny bullets, like the Greek Atomists thousands of years before him. However, during the 18th and 19th centuries, it became hard to explain many of the properties of light this way. Instead, the preferred description was that light was a wave. Not a single wave like those we see at the beach, crashing on the shore, but a train of waves, as when you toss a stone into a lake and you see a bunch of ripples moving out from the point of impact.
Just as there is a typical distance between the crests of these waves (called the wavelength), different kinds of visible light, from red to violet, are just trains of waves with different wavelengths. Following the rainbow from red to violet, the wavelengths get smaller. Waves with wavelengths longer than red and shorter than violet exist, but are invisible to the human eye. They are all around, even if we can’t see them. We call all light waves, visible and invisible, electromagnetic waves or electromagnetic radiation. You’ve probably heard of some of them: radio waves, microwaves, infrared, ultraviolet, X-rays, gamma rays, all invisible types of electromagnetic waves.
Being invisible to the human eye doesn’t make these electromagnetic waves any less real. Indeed, their existence teaches us an important lesson: What we see is a tiny fraction of what’s out there. As the Fox said to the Little Prince in Saint-Exupéry’s fable, “What’s essential is invisible to the eye.” Although the Fox was talking about love, we might as well call the “invisible” the parts of Nature not directly accessible to our senses. Science is our best guiding light to illuminate such invisible natural realms, expanding our vision of reality.
Light waves on what?
Visible or invisible, there was an issue with the light-as-a-wave picture. Quite reasonably, nineteenth century scientists believed that every known wave needed a supporting medium. After all, water waves need water and sound waves need air. (Yes, explosions in outer space make no sound.) Waves are usually disturbances on something. What about light? What was the something it propagated on?
Scientists had no clue. So, like most people, they speculated. Not, of course, with any crazy speculation, but based on what was known. And they knew a few things. They knew that the medium had to be perfectly transparent to allow for light from distant stars to reach us. The medium should also offer no friction. Otherwise, the orbits of the moon and the planets wouldn’t be stable, and they’d have crashed a long time ago. The medium had to be weightless, or it would affect the gravitational balance of the solar system and the Universe. Transparent, frictionless, massless—and, adding to the weirdness, the medium had to be very rigid, to support the propagation of very fast waves. Scientists knew that light traveled at 186,000 miles per second, a ridiculously fast speed: blink your eye and light goes seven and a half times around the Earth.
Clearly, whatever this medium was, it was very strange indeed. To compound the mystery, physicists called it the ether, a name that harked back to Aristotle and his model of the cosmos. In the Aristotelian worldview, the cosmos was filled with ether, and all celestial luminaries were made of it: the ether was immutable, eternal, uncreated. Even if the nineteenth-century ether was different from the old Greek one, the allusion was obvious.
Between Aristotle and the nineteenth century, however, science had fought hard (perhaps too hard) to distinguish itself from philosophy. A scientific hypothesis, contrary to a philosophical argument, should, by definition, be testable. You say light travels on an invisible ether? Prove it! According to the scientific code, reality consisted of tangible things, things that existed and could be measured.
At least, this is how we would like science to work. In reality, however, things are not so clear cut. At the cutting edge of knowledge, where we step into the unknown, scientists must take risks and embrace ideas based on a combination of intuition and faith. This is where the subjective enters the scientific narrative, which then proceeds—until there is (or not) some degree of observational evidence—as a mythic narrative, based on entities that may or not exist.
Testing the ether theory
In 1887, Albert Michelson and Edward Morley set out to prove the existence of the ether. Their idea was that if the ether existed, there should be a kind of ether wind, similar to when you drive in a convertible: even without any wind, you feel the air moving toward you. Instead of a convertible, they used the Earth’s motion around the sun. This motion should generate an ether wind. In their experiment, a light source created a straight beam that they could rotate around, pointing it head-on against the direction of Earth’s orbital motion around the sun or perpendicular to it. They expected, quite reasonably, that the time light traveled between points A and B would be different depending on the direction of the beam: slower if the beam pointed in the direction of the Earth’s motion, head-on against the ether wind.
To their dismay, within the precision of their measurement, the traveling times were identical in every direction the beam pointed. No change at all. For the next 40 years, the experiment was repeated with increasing accuracy. Nothing changed. The conclusion was devastatingly simple: the ether didn’t exist. Now what?
People didn’t give up. The alternative, that light could actually travel in empty space, was outrageous. It sounded very ghostly and otherworldly. All sorts of explanations were proposed to remedy the impasse: perhaps Earth dragged the ether with it, affecting the measurement; perhaps the equipment shrank in the direction of Earth’s motion, compensating for the expected longer travel time.
To everyone involved, sticking with a bizarre medium, with properties that bordered on the magical, was preferable to embracing the possibility that light could travel in empty space.
This story beautifully illustrates the human drama behind research, and how it often embraces mythic elements, even if only temporarily. It explains why we use the “re” in research: we search, and we search again. More often than not, we are stunned by our ignorance, by not having the right answer or even a clue how to get it. Quite naturally, our initial impulse is to try to keep the status quo, avoiding radical changes in our thought process and, even worse, in our worldviews. This was the situation during the last decades of the nineteenth century, as physicists proposed different ideas to explain the negative results of the Michelson-Morley experiment.
In the absence of data, when scientists are blind to certain aspects of reality, they embrace what, in retrospect, may seem to them like magical or absurd, with the same passionate faith as the devoted.
The ether dilemma only started to abate in 1905, when 26-year-old Albert Einstein proposed a radical solution: light always travels at the same speed. This being the case, the Michelson-Morley’s result was perfectly fine: whatever direction you turned their equipment, the result had to be the same. Light, like nothing else known, travels in empty space, on its own. It is, in a sense, its own medium of propagation.
Einstein’s idea was revolutionary. In his worldview, two things had to be true: the laws of Nature had to be the same for everyone, independent of how they moved about; and the speed of light was always the same, even if its source was moving. Was this weird? Of course it was! But it worked.
Einstein’s framework, although called the theory of relativity, is actually about two absolutes, the unchanging laws of Nature and the speed of light. The magic medium was gone, but light remained magical all the same.