Figure 3.8 shows the situation in which their interferometer moves in the ether to the right at a speed v. The dotted lines represent the path of the light relative to the assumed ether. The solid lines represent the path of the light in relation to the apparatus.
Figure 3.8: Michelson-Morley experiment.
Source: Stigmata Aurantiaca on Wikimedia Commons.
Half of the light is reflected upwards (grey vertical arrow) by a beamsplitter (a semi-transparent mirror); and then reflected down again by the upper full mirror. On its way back half of it again passes through the beamsplitter (lighter gray downward arrow) reaching finally the interference detector. The horizontal grey arrow represents the other half of the light that passes first through the beamsplitter, reflects back then from the full mirror on the right, returns to the beamsplitter where again half of it is reflected downwards to the detector. At the detector, both light waves (light and dark grey) - after having traveled different paths - meet each other and will interfere there, showing an interference pattern. The more the travel time in vertical direction differs from the travel time in the horizontal one, the more their phases will differ and the more the interference pattern will shift.
Since the direction in which the earth would move through the ether was not known beforehand, Michelson and Morley's interferometer had to be rotatable in a very controlled way. Their final experimental set-up was therefore mounted on a large heavy round granite plate that floated in a mercury bath. This prevented unwanted vibrations disturbing the interference.
For the calculation aficionados: in figure 3.8 you will also find the formulae for the duration Tt of the transversal light wave up and down (gray line) and for Tl that of the longitudinal wave to the right and back (black line). See if you can derive these plus a formula for ΔT = Tt - Tl.
The very small speed differences due to this movement of the earth through the ether would lead to time differences ΔT and therefore phase differences upon arrival at the detector. This would lead to a measurable shift in the interference pattern.
Michelson and Morley's measuring set-up was extremely sensitive. It should be able to detect deviations in the speed of light due to the speed of the earth in its orbit, which is 30 km/s (18.6 mi/s), should that also be the speed of the earth relative to the ether. Their reasoning was that even if the sun did not move with respect to the ether, the earth's orbit would show speed differences in opposite seasonal positions at twice that amount - so 60 km/s (37.2 mi/s). See figure 3.9.
Figure 3.9: The movement of the earth in summer and winter with respect to the ether wind.
If Michelson and Morley's interferometer was now to move through the stationary ether at 30 km/s, their calculations pointed out that the light perpendicular to the direction of movement would lag 10-15 seconds behind the light traveling in the direction of movement of their set-up. The resulting path difference of about 300 nm (nanometer) should have been detected by their very accurate interferometer. This difference is of the same order as the wavelength of visible light, between 400 and 800 nm. A noticeable shift of the interference pattern should be observed.
However, to their astonishment, no shift was detected. So, exit the ether. Or one would have to return to the geocentric universe of Ptolemy, the earth residing immovably at the centre of the universe. Which was of course not a viable option. Nonetheless, Edward Morley maintained his belief in the existence of the ether until his death. Their experiment, by the way, is still being repeated - every time with increased precision. So, in principle, the case is still undecided. But today's consensus is that the ether does not exist unless the opposite can be convincingly demonstrated.
Albert Michelson received the Nobel Prize in physics in 1907 for his performance in optics and his contribution to physics. A considerable prize for a "failed" experiment.
The classical view of the universe at the end of the 19th century
The subject of the previous paragraphs was the rise and great success of classical physics. It has now hopefully become clear why, apart from a few imperfections, classical physics fits so seamlessly into our daily experience of material reality. We can summarize our own everyday experience of the world as follows:
If I want to move something, I have to push it or pull it.
What I perceive exists objectively and independently of me. The world seems to exist with or without me.
Even when I am not looking at the moon, or when it has disappeared behind the horizon, I am sure that it is still 100% materially there.
According to classical physics, my inner experience of reality is an image that coincides with the "real" world around me.
Mathematics is the instrument to make a model of the world that explains exactly how it works.
Causality, cause and effect, reigns absolute.
The whole is just the sum of the parts. After analyzing the parts first, in principle the whole is known.
In short, the world seems solid and permanent. According to most scientists, physics was considered to be almost complete in the second half of the 19th century, with the exception of a few problems. Smart young students like Max Planck were discouraged from pursuing a career in physics, because there would be no longer any honor to be gained in it. However, it would turn out differently.
The electromagnetic spectrum
In 1886, seven years after Maxwell's publication, Heinrich Hertz (1857-1894) confirmed the existence of standing electromagnetic (radio) waves with a wavelength of almost 61 meters (200 ft). It is probably not a coincidence that the depth of his laboratory was 30 meters, so that half that wavelength did fit precisely. Maxwell's theory was confirmed. Hertz saw no practical use for his discovery. Guglielmo Marconi (1874-1937) saw its application potential and realized the first radio communication in 1889. In 1896, shortly after that, the telegram service and the Morse code were invented. Nikola Tesla (1856-1943) forgot to patent his invention concerning sound communication using radio waves and the cunning Marconi received also the credits for that.
In 1895, Wilhelm Röntgen (1845-1923) discovered highly energetic EM-waves he generated by applying a high voltage between the electrodes of a vacuum tube. Röntgen called his discovery X-rays.
Figure 3.10: Albert von Kölliker's hand photographed by Röntgen.
These X-rays have wavelengths between 0.01 and 10 nanometers. The first X-ray photos are those of Röntgen's acquaintances, see figure 3.10 for an X-ray of Albert von Kölliker's hand. Röntgen also took such an X-ray of his wife's hand. Upon seeing the result, she feared her end was near. It is incidentally remarkable that Röntgen made his discovery because he noticed that a screen covered with barium platinocyanide lit up in his darkened laboratory when he switched his vacuum device on. The fact that he had such a screen accidentally standing readily available in his laboratory, the fact that Röntgen noticed the effect of his device and the fact that he draw the right conclusion is an excellent example of serendipity, the accidental and unsolicited discovery [13]. Serendipitous discoveries abound in science.