In the second half of the 19th century, physical views on the nature of the propagation of light, the effect of gravity, and some other phenomena more and more clearly began to encounter difficulties. They were connected with the etheric concept prevailing in science. The idea of conducting an experiment that would resolve the accumulated contradictions, as they say, was in the air.
In the 1880s, a series of experiments was set up that were very complex and delicate at the time — Michelson’s experiments to study the dependence of the speed of light on the direction of motion of an observer. Before dwelling on the description and results of these famous experiments in more detail, it is necessary to recall what the concept of ether was and how the physics of light was understood.
19th Century Views on the Nature of the World
At the beginning of the century, the wave theory of light triumphed, receiving brilliant experimental confirmation in the works of Jung and Fresnel, and later the theoretical substantiation in Maxwell's work. Light absolutely undoubtedly showed wave properties, and the corpuscular theory was buried under a pile of facts that it could not explain (it will be reborn only at the beginning of the 20th century on a completely new basis).
However, the physics of that era could not have imagined wave propagation otherwise than through mechanical vibrations of any medium. If light is a wave, and it is able to propagate in a vacuum, then the scientists had no choice but to assume that the vacuum is filled with a certain substance, due to its vibrations conducting light waves.
Luminiferous ether
The mysterious substance, weightless, invisible, not registered by any devices, was called ether. Michelson’s experiment was precisely designed to confirm the fact of its interaction with other physical objects.
Hypotheses on the existence of etheric matter were expressed by Descartes and Huygens in the 17th century, but it became necessary as air in the 19th century, and at the same time led to unsolvable paradoxes. The fact is that in order to exist, in general, the ether must have mutually exclusive or, in general, physically unrealistic qualities.
The contradictions of the ethereal concept
To match the picture of the observed world, the luminiferous ether must be absolutely motionless - otherwise this picture would be constantly distorted. But his stillness came into irreconcilable conflict with Maxwell's equations and Galileo's principle of relativity. For the sake of their preservation, I had to admit that the ether is carried away by moving bodies.
In addition, etheric matter was thought to be absolutely solid, continuous and at the same time in no way preventing the movement of bodies through it, incompressible and, moreover, possessing transverse elasticity, otherwise it would not conduct electromagnetic waves. In addition, ether was thought of as an all-pervasive substance, which, again, does not fit well with the idea of his passion.
The idea and the first statement of the Michelson experience
American physicist Albert Michelson became interested in the ether problem after reading Maxwell’s letter in Nature magazine, published after the death of the latter in 1879, describing an unsuccessful attempt to detect the Earth’s motion in relation to ether.
In 1881, Michelson’s first experiment was held to determine the speed of light propagating in various directions relative to the ether, moving together with the Earth by an observer.
The Earth, moving in orbit, should be exposed to the so-called ethereal wind - a phenomenon similar to the flow of air flowing onto a moving body. A monochromatic light beam directed parallel to this “wind” will move towards it, losing some speed, and vice versa (reflecting off the mirror) - vice versa. The change in speed is the same in either case, but it is achieved in different times: a slowed-in oncoming beam will be longer in transit. Thus, the light signal emitted parallel to the “ethereal wind” will necessarily be delayed relative to the signal overcoming the same distance, also with reflection from the mirror, but in the perpendicular direction.
To register this delay, an interferometer, invented by Michelson himself, was used, whose operation is based on the phenomenon of superposition of coherent light waves. If one of the waves is delayed, the interference pattern would shift due to the resulting phase difference.
Michelson's first experiment with mirrors and an interferometer did not give an unambiguous result due to the insufficient sensitivity of the device and the underestimation of numerous interference (vibration) and caused criticism. A significant increase in accuracy was required.
Repeated experience
In 1887, the scientist repeated the experiment together with his compatriot Edward Morley. They used an advanced installation and especially took care of eliminating the influence of side factors.
The essence of the experiment has not changed. A light beam collected using a lens was incident on a translucent mirror mounted at an angle of 45 °. Here he shared: one beam penetrated the divider, the second went in the perpendicular direction. Each of the rays was then reflected by an ordinary flat mirror, returned to the beam splitter, and then partially fell onto the interferometer. The experimenters were convinced of the existence of an “ethereal wind” and expected to get a measurable shift of more than a third of the interference band.
It was impossible to neglect the movement of the solar system in space, so the idea of experiment provided the ability to rotate the installation in order to fine-tune the direction of the "ether wind".
To avoid vibrational noise and distortion of the picture when turning the device, the entire structure was placed on a massive stone slab with a wooden toroidal float floating in pure mercury. The foundation under the installation was buried to rock.
The results of the experiments
Scientists conducted careful observations throughout the year, rotating the plate with the device clockwise and counterclockwise. The interference pattern was recorded in 16 directions. And, despite the unprecedented accuracy for its era, the Michelson experiment, conducted in collaboration with Morley, gave a negative result.
Common-mode light waves leaving the beam splitter reached the finish without phase shift. This was repeated every time, at any position of the interferometer, and meant that the speed of light in the Michelson experiment did not change under any circumstances.
Verification of the experimental results was carried out repeatedly, including in the 20th century with the use of laser interferometers and microwave resonators, reaching an accuracy of one ten billionth of the speed of light. The result of the experiment remains unshakable: this value is unchanged.
Experiment value
From the experiments of Michelson and Morley it follows that the "ether wind", and, therefore, this elusive matter itself simply does not exist. If a physical object is not fundamentally detected in any processes, this is equivalent to its absence. Physicists, including the authors of the brilliantly posed experiment themselves, did not immediately realize the collapse of the concept of ether, and with it the absolute reference system.
A consistent and yet revolutionary new explanation of the results of the experiment could be presented only to Albert Einstein in 1905. Having examined these results as is, without trying to attract speculative ether to them, Einstein received two conclusions:
- No optical experiment can detect the rectilinear and uniform motion of the Earth (the right to consider it as such gives the short duration of the act of observation).
- With respect to any inertial reference frame, the speed of light in a vacuum is unchanged.
These conclusions (the first - in combination with the Galilean principle of relativity) served as the basis for Einstein to formulate his famous postulates. So the Michelson-Morley experiment served as a solid empirical basis for the special theory of relativity.