How a failed experiment led to Einstein's first big revolution

Imagine living in the late nineteenth century, thinking about one of the most important physical phenomena in the universe: light. There are a number of things that we take for granted today, that were already known about light. We knew, for example, that light travels at about 300,000 km/s, exhibits wave-like behaviors such as interference and diffraction, and that it is an electromagnetic wave, that is, two vibrating electric and magnetic fields. Like all known waves, it needs a physical medium to propagate through, just as water waves need water, seismic waves need earth, and sound waves need a medium to propagate. Light was assumed to have a medium as well, known as luminous ether. But the properties of the ether were unknown and difficult to detect; it is not an ordinary medium, since light spreads through the void, like the void that separates the Earth from the sun.

In the eighties of the nineteenth century, Albert Abraham Michaelson devised a method for detecting and measuring the effects of ether. The failure of the experiment known as the Michaelson-Morley experiment led to the greatest revolution in the history of science.

Law of Synthesis of Speeds:

Since the time of Galilee, we know how velocities are combined in the case of conventional low-velocity ordinary objects. When a fish swims in a river, for example, it will take a different amount of time if it travels a certain distance against the current and returns with the current to its starting point, compared to traveling the same distance perpendicular to the current and back to its starting point, due to the effects of the river current on it. Whereas if the fish were swimming in still water, these two times would be equal. That is, the direction in which the fish swims does not matter at all.

Michaelson interfering device:

Michaelson decided to use a similar method to test the behavior of light as it travels through the so-called ether. We don't have a "river" that acts as an intermediary through which light travels, because light simply travels through space. But we have something like a raft over that river: planet Earth, which orbits the sun at an average speed of about 30 km / s.

Michaelson's idea was to send two rays of light in perpendicular directions, as if they were fish swimming in a river: one beam goes against the "current" and then returns to its starting point after being reflected from a mirror, and the other crosses the "river" accidentally and then returns to its starting point after being reflected from a mirror. The current here arises or is assumed to arise from the movement of the Earth through the ether. When these two separate rays of light return to their starting point, we recombine them to see if there is even a small difference in light path between them, which would produce an interference pattern.

As Michael imagined, if the instrument were completely stationary relative to the ether, the light emitted by the two rays would travel the distance traveled in the same time. When these rays merge again, we will not notice any change in the interference pattern. However, if the device is in motion relative to the ether due to the Earth's movement around the sun, similar to the way a floating raft moves over a river due to the current beneath it, the light will take longer to cut the direction that moves with the ether than the direction it moves perpendicular to it and this is the crucial test.

Negative result:

Michaelson created, in two stages, an interferometer in collaboration with Edward Morley. The two prepared and implemented several improvements to this experiment. By 1887 and 1888, they had made an interferometer that could be rotated on a platform at any angle they wanted at any time of the day, at any time of the year. They did not get any difference in path difference and therefore a difference in the speed of light between the two directions whatever the time of day and whatever the time of year. With this negative result they could not show the presence of ether.

But the Michaelson-Morley experiment did not immediately lead to the rejection of ether. Some have claimed that the ether was "dragging" with the earth, meaning that the earth must be in common motion with the ether. But this claim soon turned out to be wrong, as the observed stellar eccentricity phenomenon and Fizo's previously conducted experiment ruled out the ether drag hypothesis. The next attempt to interpret Michaelson-Morley's observations was by George Fitzgerald and Hendrik Lorentz, who suggested that when objects move at a speed close to the speed of light, they experience lengths shrinking in the direction of motion. By reducing the length of light moving with Earth's orbit, a negative outcome can be predicted again, thus saving the ether hypothesis if one insists on it.

Special Relativity:

In fact, the ether hypothesis survived until 1905, when a young physicist named Albert Einstein finally solved the mystery. Instead of relying on ether, Einstein took an entirely new approach, by considering the possibility that the true constant quantity in nature was actually the speed of light. In other words, the question of whether the ether is stationary relative to the Sun, dragged with the Earth, or dragged in a more complex way as Lorentz suggested, can all be eliminated if we assume instead that there is no necessary medium through which light propagates; The only problem was that neither space, nor time, nor motion would be absolute, but all observers must agree that the speed of light is the only constant magnitude.

Thus arose the theory of special relativity.

The end:

Despite its negative result, the Michaelson-Morley experiment remains, to this day, one of the most scientifically influential experiments in the history of science. This was reflected in 1907, when Michaelson was awarded the Nobel Prize in Physics for his "precision optical instruments and spectral studies carried out with their aid." Einstein's special relativity did not win the Nobel Prize.