between them, but from that moment on, fields were included in physics, despite their intangible quality. Fields can exist and propagate in and through the utter emptiness of a vacuum, without any physicist seeing a problem with that.
It follows mathematically from Maxwell's equations that moving charges will emit electromagnetic waves that will always propagate through empty space with a constant speed. In fact, when Maxwell himself calculated the speed at which an electromagnetic wave had to propagate, he was surprised to find that it was precisely the speed of light, which is now a physical constant denoted with the letter c. He then made the, now generally accepted, inference that light was an EM wave. So, two new fundamental fields were now introduced into classical mechanics in addition to the gravitational field: the electric and the magnetic field.
The speed of light in vacuum has always to be the same because, as follows from Maxwell's equations, its value only depends on fixed physical constants. Einstein was the one who realized that this constancy of the speed of light meant that it could not depend on the relative movements of different observers. For example, the speed of the light you observe coming to you from an approaching train is not increased by the speed of the train. It will always approach you with 300,000 km/s. It was not until the first decennium of the 20th century that this problem was solved.
Meanwhile, reinforced by Maxwell's success, the field had now become an almost tangible thing. In Maxwell's EM wave, the original physical relationship between the source of the electric and magnetic forces - the electric or magnetic charge - and their fields, had virtually disappeared, except perhaps as their initial cause. Electric and magnetic fields could now propagate without considering their originating charges.
Many physicists of the time regarded the existence of an ether - an ultra-rarefied medium in which light waves would propagate - still very seriously. They had a number of good reasons:
An absolute coordinate system would be established by the ether in relation to which the earth, the sun and its planets and the galaxy moved. This would be a confirmation of Newton's absolute space.
If the ether was a medium that didn't move relative to the absolute space of Newton, the absolute motion of the earth could be determined by comparing the speed of light in different directions.
If the existence of the ether could be confirmed by experiments, the question of what it is that is actually oscillating and propagating in the EM-wave would be answered.
A Nobel prize for a failed experiment
Despite Maxwell's conclusion that the speed of light in a vacuum had to be constant, always and in every situation, scientists kept trying to measure the speed at which the earth traveled through the ether by sophisticated experiments with light. After all, it could still be possible that the ether itself would prove to be the basis of Maxwell's EM fields.
In 1887 Albert Michelson (1852-1931) and Edward Morley (1838-1923) devised and build an extremely accurate and quite advanced interferometric setup [12] to measure the movement of the earth in relation to this purported unmoving ether. They sought to determine the ether wind, an ether movement experienced due to the movement of the earth through this supposed ether, a bit like the wind you experience when driving a motorbike. They applied roughly the same principle with which one could determine the speed, in relation to the stationary air, of a passing police car with its siren on, the so-called doppler effect. Traffic police do something similar using radar. The radar waves reflected from your car are still traveling at the speed of light, however they have changed slightly in frequency due to being reflected from your moving car. In that way, the police can measure your speed. So, beware.
Michelson and Morley's experimental set-up made use of this expected doppler effect of light waves sent out from an object moving through the ether, cleverly combined with the aforementioned interference phenomenon of light waves. See figure 3.8. Using mirrors, they let monochromatic light bounce back and forth in two perpendicular directions and made the reflected light waves interfere with themselves. A rotation of their arrangement would change the direction of the two perpendicular light beams relative to the direction of the assumed movement through the ether. This would affect the speeds of both light beams and their interference pattern had to shift visibly. By measuring that shift, the speed at which their measurement set-up would move through the purported ether could then be calculated.
Compare this with a bike contest between exactly equal fast engines. The start and finish are in city C. City N lies 50 miles north of C, city W lies 50 miles west of C. A strong wind blows exactly from the north with 25 miles/hour. One biker has to reach N before returning to C, the other has to reach W. Which race route would you choose if you were one of the bikers? The answer is that the side wind path is the most favorable if you do the calculations. Biker C-W-C will be the first to finish.
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.
