For hundreds of years scholars believed in the absolute perfection of the Heavens. Although the Earth could be changed, and therefore corrupted, the heavenly realm of the planets and stars was seen as unchanging and forever incorruptible. These ideas of perfection inspired Aristotle, the Ancient Greek philosopher, to develop an aesthetically pleasing model of the universe in which he proposed that celestial bodies moved in perfect circles around a stationary Earth.
The natural motion of the planets in the night sky did not easily fit with Aristotle’s ideas, as planets were sometimes observed to move backwards, an apparent contradiction of the idea of perfect circular motion. To explain these, and other, difficulties, ancient astronomers developed increasingly elaborate theories of the movements of the planets around the Earth. Aristotle’s simple ideas gradually evolved into complicated theories, but the ideas of perfection that lay at the heart of the Aristotelian Universe were too seductive for scholars to abandon.
The invention of the telescope finally forced astronomers to confront the imperfection of the universe beyond the Earth. Moons were seen around Jupiter, proving that not everything revolved around the Earth, and spots were observed on the Sun, demonstrating for the first time the changing nature of celestial objects. These shocking discoveries sparked a crisis in the Medieval World, eventually resulting in the embrace of the controversial idea that the Earth moved around the Sun, and leading to the development of new theories of gravity, physics and humanity’s place in the Universe.
The English physicist Isaac Newton was responsible for developing many of the new theories about how the natural world behaved. He is perhaps best known for the Principia, a series of books in which he introduced the theory of gravity, but he also made significant contributions to the study of light in Opticks, one of the first books to seriously analyse the nature of light through the use of lenses and prisms. Newton suggested that light was made of tiny particles, which he named corpuscles, an idea that helped to explain why light seemed to travel in straight lines. But Newton struggled to use this model to explain other phenomena he observed, particularly diffraction, where light appears to bend around the edges of objects.
An alternative theory of light was developed by the Dutch physicist Christiaan Huygens, who proposed that light was instead transmitted as a wave of vibrating particles, in a similar way to sound. Further studies of optics revealed new properties of light, like interference and polarisation, that Newton’s particle theory couldn’t explain, giving further weight to Huygens’ wave theory. Even though, the idea the light moved like a sound wave suffered from one big problem — physicists knew that since such waves could not move through a vacuum, there must be a substance available to vibrate and to allow the wave to spread. Since we can see the Sun, planets and even distant stars, the wave theory of light implied that all of space must be filled with some kind of invisible material.
This was not a new idea to science. The concept of a vacuum, an area of completely empty space, had been discussed since the time of the Greek philosophers, but was dismissed as an abhorrent, illogical idea. Plato and Aristotle concluded that a vacuum, even if it existed, would quickly disappear as surrounding matter rushed into fill it, leading to the influential idea, strongly held even in the 19th Century, that nature abhors a vacuum.
The Ancient Greeks believed that the world was constructed of four fundamental elements — earth, water, air and fire. When Aristotle was developing his models of a perfect universe he added a fifth element, aether, which he thought filled the heavens and held the celestial bodies in place. Aether was a truly perfect material — weightless, resistant to change and capable of moving only in ideal circles around the central Earth.
Thus, when Huygens and Newton were studying the behaviour of light, it seemed much more natural that space would be filled with something, rather than nothingness, and the idea that light might move through space as a wave was not seen as especially problematic. The ancient idea of the aether was adopted and refined to fit the new ideas of physics, in particular to explain how light moved through the Universe. In doing so it became known as the Luminiferous Aether, the light-carrying aether.
With the need for the luminiferous aether seemingly theoretically proven by Huygens’ wave theory, scientists started to speculate about its properties and how the effects of the aether might be detected. Observations soon allowed physicists to determine that the aether must be a fluid filling all of space, but it had to be one that had no mass or friction, since the orbits of the planets seemed to be unaffected by its presence. It must also be extremely strong to be able to withstand the rapid vibrations of light waves. And the aether itself, although it carried light, must be invisible.
A series of experiments during the 19th Century attempted to pin down the exact nature of the elusive aether. One key question surrounded the motion of aether around the Earth as the planet revolved around the Sun. Scientists realised that either the aether must move with the Earth, and so appear to be stationary from the Earth’s surface, or alternatively the aether moved at a constant speed through space, thus creating an “aether wind”, blowing against the Earth as it moved.
A series of experiments and astronomical observations appeared to rule out the first option, and so focus shifted to attempts to detect the aether wind. As the aether carried light, scientists calculated that the speed of light should appear to change slightly as the aether wind varied in strength throughout the year. As the Earth moved and rotated the speed of light should also seem to vary depending on the time of day and the month of the year.
This idea led two American scientists, Albert Michelson and Edward Morley, to construct an instrument that was capable of experimentally measuring these changes in the speed of light, thus, they expected, proving the existence of the aether. Michelson and Morley had no reason to doubt the presence of the aether or the aether wind, and like most physicists at the time they believed their experiment would easily find evidence of the luminiferous aether. The experiment was conducted over several months in 1887, but, in a result that shocked scientists around the world, found no evidence of changes in the speed of light, and therefore no sign of the aether wind or the aether itself.
The failure of Michelson and Morley to detect the existence of the aether seemed to rule out both possibilities for the motion of the aether around the Earth. Like the invention of the telescope and the revelations of imperfection in the heavens, the result raised a deep contradiction at the heart of physics, leading some physicists to nickname the Michelson-Morley experiment as the most famous failed experiment in history.
Other contradictions in the Victorian understanding of light were starting to emerge. James Clerk Maxwell, a Scottish scientist, had developed four key laws covering electricity and magnetism in the 1860s. Maxwell’s laws showed for the first time that waves travelling through a vacuum could be formed from oscillating electric and magnetic fields, and more importantly, that such waves would travel at exactly the speed of light. Maxwell soon realised that light itself existed as one of these electromagnetic waves, although he was influenced by Aristotle’s ideas dismissing the possibility of a vacuum, and he continued to believe that space was filled by the invisible aether.
In response to the Michelson-Morley experiment and Maxwell’s breakthrough in understanding of electromagnetism, scientists started developing new models of the aether that could fit with these puzzling results. One serious attempt was made by Hendrik Lorentz, a Dutch physicist at Leiden University. Lorentz assumed the aether moved at a constant rate, as Michelson and Morley were hoping to prove, but he made a significant step in allowing space and time to contract and expand to explain the observed unchanging speed of light.
Lorentz, and almost every physicist since Aristotle, was wrong. The aether did not exist. Lorentz did at least introduce some concepts that were taken further by one Albert Einstein, who used the ideas of variable space and time to develop his famous theory of relativity. Einstein showed that light always appears to have the same speed, not because it is moving through a stationary substance like the aether, but instead because as objects move closer to the speed of light, space contracts and time dilates, ensuring that Maxwell’s equations always hold.
Einstein’s theory, together with Maxwell’s equations for light, removed the need for the aether, but also revealed light to be something very different from any earlier understandings. The insights that the two theories allowed in the nature of the universe opened up the modern world, allowing a revolution in technology by giving us mastery over electricity and magnetism, and providing the foundation for technologies crucial to the modern world, from radio communications to satellite GPS systems.
The long belief in the non-existent luminiferous aether holds important lessons for modern physicists. Despite experimental evidence showing severe problems with the concept, most scientists regarded the aether as too essential to proven physical theories to be abandoned. As with the geocentric model of the universe, scientists clung strongly to historical ideas of innate perfection in the heavens based on little more than philosophical reasoning.
Like with Copernicus’ Sun-centred model, once Michelson and Morley opened a crack in the world of Newtonian physics the whole structure would come tumbling down. Old ideas, held passionately by physicists for centuries, were cast aside in a few decades. Newton’s world was replaced by a new and bizarre world of relativity and quantum mechanics, leading to an explosion in new ideas and theories, and paving the way for the modern world we live in today.
Relativity was not quite enough to solve the problems with light. While Maxwell and Einstein showed that light existed as an electromagnetic wave, other experiments suggested that light instead existed as a particle, the photon. Other particles seemed to display similar behaviour — in particular, the electron, believed to be a particle, seemed to have a wave-like nature at times. These contradictions were solved by quantum mechanics, which demonstrated that all particles have a wave-like nature, meaning that light occasionally appears to behave as a wave, and at other times as a particle.
In 21st Century physics the nature of light is well understood, but many questions still remain around the nature of gravity. Observations of distant galaxies have shown phenomena that cannot be explained by our current understanding of physics, and in an attempt to resolve these discrepancies scientists have introduced the concept of dark matter. Other mysteries concerning the accelerating expansion of the universe have been resolved by invoking dark energy. Despite many years of experiments probing the exact nature of these two hypothetical concepts, physicists have to yet to find any direct evidence of the existence of either idea.
The last two major revolutions in physics were sparked when experimental observations exposed stark contradictions in existing theories. Once the old theories could no longer be sustained, scientists were eventually able to cast aside old certainties and introduce radical new ideas. Might the questions of dark energy and dark matter be one day solved in a similar fashion?