Читать книгу Quantum Physics is not Weird. On the Contrary. - Paul J. van Leeuwen - Страница 20

Nowadays everybody agrees that light is an electromagnetic wave phenomenon. How did we discover this? From ancient times we were already familiar with electrical and magnetic phenomena. De term 'electricity' is said to be derived from electrum, the Latin name for amber. Rubbing amber with a woolen piece of cloth evokes small sparks. We discovered two types of electrical charges. Similarly charged objects repelled each other and dissimilar ones attracted each other. Magnetic compasses were used in navigation.

It turned out in the 18^th century that applying the same mathematical methods as with Newton's gravity mechanics, one was able to calculate the dynamic behavior of objects with attracting or repelling electric charges or with magnetic properties. The mathematical tools that Newton had laid down for gravity could be applied in the same way to the electrical attraction between two charged objects. The electrical or magnetic force is, just like gravity, inversely proportional to the square of the mutual distance and directly proportional to the product of the two electrical or magnetic charges. However, when one wanted to calculate with these methods the simultaneous behavior of more than two charged objects, mathematical problems arose. Exact mathematical tools are simply not available for multiple body interactions. In those early days one had to make do with approximative methods and calculations with pen and paper. Nowadays we use computers for more accurate approximations.

To simplify the multiple body calculation problem, the field concept was developed. A good approximation of the behavior of more than two attracting or repelling bodies was found by assigning to every location in the empty space around an electric or magnetic charge a field vector property. A vector is a mathematical object representing a both magnitude and direction. An electric field vector ^[8] represents the direction and the magnitude of the force that an electric unit charge would experience in that location of space. The same principle applies for a magnetic field vector ^[9]. Assigning these mathematical vectors to every location in space around a charge, defines thus the electric or magnetic field of that charge. Ask yourself now if you think that such a field is an objective tangible thing.

Figure 3.5: Electric field lines.

This idea of a vector field idea simplified calculations greatly. Figures 3.5 and 3.6 show the graphic presentation of electric and magnetic fields.

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 19^th 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.