Nineteenth-century scientists faced a conundrum: How does light work? While they understood that light was a wave–Newton and others had proved that it behaves like a wave in water, refracting and reflecting in the same way that waves do–they began to theorize about what the wave traveled through. The main candidate toward the end of the 19th century was something called the “‘luminiferous aether’, or “‘the aether’ for short.
The origins of the aether are with the Greek philosophers, who saw it as the divine element, different to the four classic elements (earth, air, fire, and water). Ather was the fifth essence (or quintessence), representing the breath of the gods–a divine element that the gods alone could use.
By the 19th century, this theory was still around–but it had evolved through the ideas of many scientists, including Newton. While he was vague on the nature of the aether, he believed it was there, and that it underlay the structure of light and other phenomenon. He wrote in his textbook Opticks:
“Doth not the refraction of light proceed from the different density of this aetherial medium in different places “¦ ?”
By the end of the 19th century, this idea had been refined into the aforementioned “‘luminiferous aether’, an invisible medium through which electromagnetic radiation traveled. It was the water that these electromagnetic waves moved through, and it was the reference frame against which everything could be measured. The aether was everywhere, forming the fundamental and absolute underlying framework of the universe.
Unfortunately, the theory had some complications. Scientists knew that the Sun was moving through our galaxy and that the Earth was orbiting around the Sun. If this was the case, this meant that the Earth was constantly moving through the aether, producing a phenomenon they called the aetheric wind.
Think of it as an airplane flying through still air: The air is not moving, but the airplane still feels pressure against it, like a wind. Similarly, if an electromagnetic wave were to move through the aether against this wind, it would experience drag, slowing it down slightly.
As the Earth is spinning, scientists should have been able to detect this drag. They expected that the light would move slightly slower in one direction when moving into the aetheric wind. If they compared the speed of light in these two directions, they would see a difference, scientists said.
That’s what Albert Abraham Michelson and Edward Morely set out to test in 1881 in a lab at Case University, Chicago. By splitting a beam of light and bouncing the two beams between several mirrors, they created a device that could measure if there was any difference in the speed of light moving in one direction versus light moving at a 90-degree angle. If the aether had the expected effect, one of the light beams would move slightly slower in one direction, due to the drag of the aetheric wind.
This experiment was phenomenally difficult, though, because the difference they expected to find was tiny. It was small enough that even a slight movement of the device from a horse riding down the street threw the experiment off, as it made the device shake, which changed the distance between the mirrors. It took the scientists many years and many prototypes to finally get a measurement they thought was right, but in 1887 they published the results: They found a slight difference, but it was much less than expected. They wrote:
“It appears from all that precedes reasonably certain that if there be any relative motion between the Earth and the luminiferous aether, it must be small “¦ “
In fact, they conceded, the differences noted were so small that they could have been experimental error. Others repeated the test using larger devices and different light sources, but by the early 20th century the luminiferous aether was in trouble. Experimenters couldn’t find any evidence to show that it existed. However closely they compared the speed of light from sources moving at different speeds or in different directions, they could not find any difference. Light, it seemed, moved at the same speed in a vacuum, however the observer was moving.
This conclusion caused a crisis in the scientific community. While Michelson and Morely had set out to prove the existence of the luminiferous aether, their work called the whole concept into question and led to a fundamental change in how scientists regard time and space.
Spurred on by this negative proof (and other similar experiments that showed the same thing with increasing accuracy), scientists decided to rethink the nature of space. A physicist named Hendrik Lorentz came up with a theory to suggest that an observer’s frame of reference changed the observation. Briefly, your speed and location changes how you see something–a phenomenon called a Lorentz Transformation.
Shortly afterwards, Albert Einstein published his theory of special relativity, which suggested that there was no absolute and underlying reference frame for the universe. Instead, any measurement is made within a frame of reference for the observer. In short, if you were to do the same measurement from another frame of reference, you might get a different result. The laws of physics remain the same in any frame of reference, but the observation can be different.
To explain relativity, Einstein used the example of a railway carriage being hit by lightning at both ends, watched by someone inside the train and someone on the platform as the train approaches. For the person sitting in the middle of the carriage, the two lightning strikes seem to happen at the same time. But to the observer on the platform, the lightning appears to hit the front first, then the back because of the longer distance from the back to the eye of the platform observer. Both observations are right, but they are different because they are seen from a different place, or frame of reference. In other words, there is no absolute, independent, and correct frame of reference. It all depends on the observer.
Meanwhile, some physicists tried to rescue the aether, theorizing that the motion through it might compress an object, rather than produce the drag that had been expected. Einstein himself discounted this approach, writing:
“Michelson and Morley performed an experiment involving interference in which this difference should have been clearly detectable. But the experiment gave a negative result–a fact very perplexing to physicists.
Lorentz and FitzGerald rescued [aether] theory from this difficulty by assuming that the motion of the body relative to the aether produces a contraction of the body in the direction of motion, the amount of contraction being just sufficient to compensate for the difference in time mentioned above.
But on the basis of the theory of relativity the method of interpretation is incomparably more satisfactory. According to this theory there is no such thing as a “specially favoured” (unique) coordinate system to occasion the introduction of the aether-idea, and hence there can be no aether-drift, nor any experiment with which to demonstrate it.”
For Michelson and Morley, special relativity (and the theory of general relativity that Einstein published in 1915) meant that however fast the Earth was spinning around the Sun and tumbling through the universe, they would never see a significant difference between the speed of the two beams, because they were measuring them from a point that was moving at the same speed as the device.
The speed of light is constant in every frame of reference, but everything else is measured from a frame of reference that only exists for the observer. In other words, there was no absolute frame of reference that they could measure against to detect the aether. All of their measurements were valid only in their frame of reference. To boil it down even further: Everything is relative.
That left the existence of the aether open, though. The accepted wisdom at the time was that light was a wave, which could still be described as traveling through the aether. Einstein and others challenged this, proposing that light existed as a particle that he called a photon, which had a certain, fixed energy level.
This seemed to contradict the wave theory of light, which suggested that waves could be divided, but a photon could not. Eventually, Louis de Broglie made a startling suggestion in 1924: Light was both a wave and a particle. This led to the development of quantum theory, which held that photons and other particles existed as both waves and particles at the same time.
In the odd world of quantum mechanics, there is no need for the aether, because the wave/particle duality of light can only be measured when you observe them. A divine medium that made things work was no longer a necessity. Because the observer is inextricably linked to the observation, there is no “outside” to look in from. By the late 1920s, the aether was dead, replaced by quantum mechanics.
The aether still has its charms, though. The ancient Greek creation of a divine element beyond our reach sounds to some like the current arguments for dark matter and dark energy. We can’t see it, touch it, or feel it, the proponents of this say, but it must be there for the universe to work. This has caused some to refer to dark energy as quintessence, one of the many names that was applied to the aether over its history. Scientists argue that the two are not the same, though. We may not know exactly what dark matter and dark energy are, but experimental evidence shows they exist, which was never the case with the aether.
Although the various types of aether theory have all been superseded, they still deserve some space in the science hall of fame. The desire of scientists in the 19th century to detect and measure the aether led to lots of great research. Their work contributed directly to many new theories and technologies, including quantum mechanics, radar, nuclear bombs and microwaves, to name but a few.
Michelson and Morley failed to show that the aether existed, but in doing so they spawned a radical rethink of the nature of space and time that we are still struggling to understand. While at the time they may have felt they wasted several years in a dark basement, their work and persistence changed the world in ways they never could have dreamed.
How We Get To Next was a magazine that explored the future of science, technology, and culture from 2014 to 2019. This article is part of our Histories of”¦ section, which looks at stories of innovation from the past. Click the logo to read more.
