Figure 3.6: Magnetic field lines.
S=Southpole N=Northpole
Source: Wikimedia Commons.
The tangent to the field line in every point gives the direction of the field force at that point. The field line density represents the field force. The denser the field lines, the greater the field force. I will not elaborate on the differences between magnetism and electricity, but I hope you will understand that the field concept assigned real properties to something that we consider to be empty space. In this way the abstract mathematical field became gradually a real object.
This is, in my opinion, a striking example of reification, which is the conversion of an abstract idea into an objectively existing object. It will turn out that this reification of the field, starting in the 19th century, will present an obstacle to the better understanding of electric and magnetic effects.
However, field mathematics, whether applied to gravity, electricity or magnetism, resulted in a significant progress in the results of physics research. In 1831 Michael Faraday (1791 - 1867) discovered how an electrical current generated a magnetic field and how, vice versa, a changing magnetic field generated an electrical current. The first dynamos were built. However, a complete mathematical description of electricity and magnetism was still missing.
This task was taken on by James Clerk Maxwell (1831 - 1879), a Scottish mathematician and physicist. In order to build his equations, he extended the field concept in such a way that field lines became almost tangible objects traversing empty space. In 1861 he published a set of twenty equations which described the dynamic behavior of electromagnetic fields completely. His equations said that alternating electrical fields generated alternating magnetic fields that generated electric fields again and so on. Heaviside (1850-1925) and Gibbs (1839-1903) merged and simplified these twenty equations into just four, which are now known as the Maxwell equations.
These four equations (see Wikipedia: Maxwell's equations [10]) describe mathematically how a changing electric field creates a similarly changing magnetic field, and vice versa. The interaction of these fields generates a self-propagating electromagnetic phenomenon behaving like a wave in empty space. Propagation of such a wave takes place in a direction perpendicular to those alternating electromagnetic fields. This wave phenomenon is called an electromagnetic wave [11] or EM wave. See figure 3.7.
Figure 3.7: Vertically polarized electromagnetic wave.
E: Electric field, B: Magnetic field. λ: wavelength.
Source: Wikimedia Commons.
The entire classical electromagnetic theory has its origin in these four basic equations. Note here that classical physics was actually concerned with matter, energy and the forces 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