Author: Sharika Dhakappa
Introduction
The Michelson-Morley experiment is the most well-known physics experiment that has been regarded as a failure. It aimed to measure how the speed of light changes with the Earth’s motion due to the effect of the ether (the ether wind) and thus detect the presence of the ether. The idea was that if we could calculate the speed of light in different directions, then, we could measure the speed of the ether relative to the Earth. To calculate the difference in the speeds of light in two directions, Albert Michelson designed what is now known as the Michelson interferometer. However, the experiment famously returned a “null result”.
The Luminiferous Ether
When we think of waves in everyday life, we often imagine water waves or sound waves. These waves have a common property. They propagate through a medium. Depending on what the medium’s density is, the speed of the wave differs. In 1864, Maxwell showed that light is an electromagnetic wave. But what is the medium through which light propagates? Nobody knew. Most physicists came to believe that light travelled through a mysterious medium called the “luminiferous ether” (luminiferous means light bearing).
This ether must then be everywhere since light from the Sun also reaches us after travelling through this mysterious medium. Following this, physicists theorized that as the Earth revolved around the Sun, the ether would move across its surface producing an “ether wind”. Depending on the strength of the ether wind and its direction, the speed of a light beam on the Earth would vary in different directions. Michelson’s interferometer would detect this change.
Michelson’s Interferometer
Michelson’s interferometer is an ingenious instrument working solely on the concept of interference. Michelson designed many such instruments and in 1907, he was awarded the Nobel Prize in physics for his optical precision instruments. Today, Michelson interferometers are used to detect gravitational waves - an important prediction of general relativity.
As we discussed, the Michelson interferometer exploits a property of waves called interference. Imagine throwing two pebbles in a pond. As the pebbles hit the surface of the pond, waves spread out from each source and eventually overlap. This is the basic concept of interference. If two crests of a wave overlap, we say the interference is “constructive”. If two troughs overlap, interference is said to be “destructive”. The same applies to light. Constructive interference produces a bright fringe whereas destructive interference produces a dark one. On placing a screen, we obtain an interference pattern consisting of these bright and dark fringes.
Michelson’s interferometer uses a single beam of light and splits it into two parts. The beam is incident on a half-silvered (or half-transparent) mirror which reflects half of the beam to one side and the rest of it passes through the mirror as shown in the image below. Both parts of the beam travel different paths - each part along one of the perpendicular arms of the interferometer. At the end of their paths, they strike a mirror.
The mirror at the end of one path is movable and the one at the other end is fixed. After reflection from the mirror at each end, the two parts of the beam recombine to form an interference pattern on a screen. As discussed earlier, the resulting interference pattern on the screen is composed of bright and dark fringes corresponding to constructive and destructive interference respectively. However, if one of the mirrors is moved (since it was designed to be movable), the interference fringes on the screen will shift. Even a minute change in the position of the mirror can be detected this way, making the interferometer a very accurate device.
This apparatus was then allowed to float on a trough of mercury, free to rotate slowly. As it rotated, Michelson and Morley expected a change in the speed of light because of the ether wind and hence, a change in the interference pattern.
The Michelson interferometer [Image credits: www.britannica.com]
Experimental Results
The experiment was expected to result in a change in the interference pattern on rotation of the apparatus. However, the results of the experiment came as a shock for most physicists. The Michelson-Morley experiment gave a null result! There was no change in the interference pattern.
Out of disbelief, Michelson repeated the experiment countless times with increased accuracy, in different places, at different heights, at different times of the day, at different parts of the year, only to obtain the same result. The experiment was not at fault. The speed of the Earth relative to the hypothesized ether was zero.
Explanation Of The Results
A possible reason why the Michelson-Morley experiment gave this puzzling result could be that the ether moves with the Earth. If this was the case, the relative motion between the Earth and the ether would be zero - successfully explaining the results of the experiment. This is called the ether-drag hypothesis. According to it, matter drags ether with it producing no relative motion. However, this hypothesis is now considered invalid. This is due to the occurrence of “stellar aberration”.
The position of a star when viewed through a telescope appears to vary every six months by approximately 20.5 arcseconds around a central position. This occurrence is due to the change in the position of the Earth itself - due to its revolution around the Sun. This is called stellar aberration. If the ether-drag hypothesis was true, this would not have occurred as the ether itself (carrying the light from the star) would move with the Earth and the position of the star would coincide with the centre.
In 1905, almost twenty years after the Michelson-Morley experiment, Albert Einstein’s paper on the special theory of relativity was finally able to explain the experiment’s puzzling results. He simply stated that there was no ether - light did not require a medium to propagate. Light was a very special type of wave. He also stated that the speed of light is constant for all observers.
It is now an established fact that all electromagnetic waves can propagate without the requirement of a medium. Today, special relativity is a very accurate model and its consequences have been experimentally verified.
The Michelson-Morley experiment is famous for its shocking results that then led to the development of the theory of special relativity, debunking the establishments of that time. It is regarded as a failure because the expected and observed results did not match and neither Michelson nor Morley stopped believing in the ether. However, as it often happens in science, an unexpected result is not a failure as long as an explanation arises.
The special theory of relativity was able to successfully explain the experiment’s results. Apart from this, the theory has changed the way we view space and time. It had a major impact on the field of physics and has deepened our understanding of Nature itself. Regarding the failure of the experiment, Einstein wrote: "If the Michelson–Morley experiment had not brought us into serious embarrassment, no one would have regarded the relativity theory as a (halfway) redemption."