Ontologies in physics—2: Electromagnetic fields as understood by Faraday and Maxwell

In this post, we shall begin going through the ontology assumed in—or rather, demanded by—the physical phenomena which are covered by the classical (i.e. Maxwellian) electrodynamics. We call this ontology EM for short.

We will mostly be talking in reference to electro-statics. However, note, this ontology just as well applies also to electro-dynamics.


1. A list of the ontological objects used in the physics theory of electromagnetism (EM):

The EM objects basically are of only two types: (1) massive and electrically charged point-particles (of one of two polarities: positive or negative), and (ii) a background object (sometimes loosely identified with the fields induced in it, by calling it “field”; other times identified with an aether of a certain kind by Maxwell; more, below).

Notably, there are no separate magnetically active objects in this ontology even though magnetism has been known as a force for more than at least one millenium. Magnetism is an effect produced by the electrical charges.


2. Electrically Charged (EC) objects:

The ontology of NM (seen in the last part in this series) is basically that of the uncharged bodies. What the EM ontology now does is to further endow these same objects with an additional attribute of the electric charge. This extra attribute considerably modifies the entire dynamical behaviour of these objects, hereafter called the EC objects for short.

For the elementary charged objects (basically, here, only the electrons and protons), the phenomenon of the induced charge/polarity does not come into picture—the charge of each elementary EC object always remains with it and its quantity too remains completely unaffected by anything or any action in the universe.

The electrical charge is just as inseparable an attribute of an EC object as its mass is. As an EC object moves in space, so does its charge too. An electrical charge cannot exist at any spatial location other than that of the massive EC object which possesses it.

Just as in NM, the EC objects too can be abstractly seen as if all their mass, and now charge too, were to be concentrated at a single point. We call such EC objects the point-charges.


3. Electrostatic forces between point-charges—Coulomb’s law:

Coulomb’s law is an empirically derived quantitative relationship. There is no theoretical basis beyond the fact that such behaviour was actually observed to occur in carefully conducted experiments. The statement of the law, however, is mathematically sufficiently refined that it would be easy to suspect whether it was not derived from some other a priori basis. As a matter of fact, it was not. What does Coulomb’s law describe?

Consider an isolated system of two point-charges fixed in space at some finite distance apart. [Help yourself by drawing a free-body diagram, complete with the structural support symbols for each of the charged bodies too. No, these supports, though they look like the electrical ground, are actually mechanics symbols; they don’t discharge the charges by grounding.] It is experimentally found that both the charged bodies experience forces of certain magnitudes and directions as given by Coulomb’s law.

Let the electrical charges of the two point-charges (EC massive point-particles) be q_1 and q_2, and let their positions be \vec{r}_1 and \vec{r}_2.

Let the separation vector going from the first point-charge to the second be given by \vec{r}_{12} = \vec{r}_2 - \vec{r}_1. (There is no typo in the last equation.) Similarly, let the separation vector going from the second point-charge to the first be given as \vec{r}_{21} = \vec{r}_1 - \vec{r}_2. (Again, no typo.)

[Note, a separation vector measures the difference in the two vector positions of two different bodies at the same time, whereas a displacement vector measures the difference in the two vector positions of the same body at two different times. … There is another related idea: The variation in position is a vector that measures the difference in the two vector positions of the same body, without reference to motion (and hence time), but as imagined in two possible and different configurations within two description of the same system. We will not need it here.]

Let \vec{F}_{12} be the force that q_1 exerts on q_2 at \vec{r}_2 (and nowhere else). Similarly, let \vec{F}_{21} be the force that q_2 exerts on q_1 at \vec{r}_1 (and nowhere else).

Coulomb’s law now states that:

\vec{F}_{12} = \dfrac{1}{4\,\pi\,\epsilon_0} \dfrac{q_1\,q_2}{r^2}\;\hat{r} = - \vec{F}_{21},
where
r = |\vec{r}_{12}| = |\vec{r}_2 - \vec{r}_1| = |\sqrt{\vec{r}_2 \cdot \vec{r}_1}| = |\vec{r}_1 - \vec{r}_2| = |\vec{r}_{21}|, and \hat{r} = \vec{r}_{12}/r = - \vec{r}_{21}/r.

A few points are especially noteworthy:

  1. The two electrical forces so produced are always equal in magnitude, regardless of how big or small q_1 and q_2 may be with respect to each other. Even if q_1 \ll q_2, the first object still ends up exerting just as much of an electrical force on the second object as the electrically much stronger q_2 exerts on the first.
  2. The forces appear on the two EC objects even though they are not in a direct physical contact—exactly as in Newtonian gravity. They also obey an inverse-square law, once again exactly as in Newtonian gravity.
  3. The forces don’t at all depend on the respective masses of the bodies; they are only for the electrostatic interaction.In reality, the two point-charges in the fixed configuration would also experience the force of gravity, not to mention the imaginary forces exerted on them by the mechanical supports. However, we ignore gravity in this entire description, and we exclude the support forces from our system—they are regarded as at all times belonging to the environment. (If a charge moves, so does its support, and along with it, the system–environment boundary. In fact a motion of a boundary is necessary for any system to exchange energy with its environment in the form of the mechanical work. We will need to take into account the mechanical work when it comes to defining the electrical potential energy of the system.)

4. Why the big ontological issue of Action-at-a-Distance remained unresolved before EM came on the scene:

The issue slipped under the carpet the last time (while discussing NM) now once again raises its (ugly or beautiful) head: How come two charges are able to exert forces on each other even if they are separated by nothing but the empty space, i.e., when there is no direct contact of the NM-kind between them?

People had thought a lot on this question right since the time that the issue came up in the context of gravity. In fact even Newton himself had once speculated whether an invisible string might not extend between two gravitating bodies like the earth and the Moon. However, Newton was only too well aware of the limitations of the available experimental evidence. The observational data such as that by Kepler indicated very high speeds for transmission of gravitational forces; the data were not refined enough to capture any effects of a finite but high speed for the forces. So, Newton refrained from adopting any definitive position concerning either a mechanism for the transmission. (Unlike Faraday, the poor fellow could not sprinke asteroids between the earth and the moon, and thereby grow confident enough about ideas like tubes of force or space-filling fields.)

The people from the other side—from the energetics approach to formulating mechanics—were not as deeply interested in the ontological matters anyway. In the market-place of ideas, their main market-differentiator was not a superior or more refined physics but a superior method of calculating solutions when the boundary conditions became complex, e.g., too numerous, as with continuous and curved constraints. (The physics implied by the mathematics of both the approaches—Newton’s momentum-based approach and the energy-based approach—was exactly identical. What differed between the two was mathematics—the methods of calculations.)

So, there arose a feedback circle of sorts: people who didn’t care about ontology and foundations of physics, but wanted to do maths, got attracted to the energetics program; people who already were in the program directed their energy in pursuing their strong point further. So, what they kept on developing was maths. Given this feedback circle, any ontological problem concerning the action-at-a-distance couldn’t have benefitted from them anyway. In actuality, it didn’t.

That’s why even if a term for the gravitational potential energy V had appeared as early as ~1773 in Lagrange’s writing (which was devoid of not just ontology but also of even a single geometrical diagram), and even if luminaries like Laplace (~1799), Gauss (about the same time but published in 1840) and Green (1828) happily developed the potential theory for gravitation, they all were perfectly happy working with just “a mathematical function” of “coordinates” for V—not with an actually existing physical field. It was Green who described V as the “potential function”.

Of course, these physicists couldn’t have used the term “potential energy” for V. The discovery of the first law of thermodynamics and the conceptual clarity on what precisely the concept of energy itself meant, was still only in latency; the explicit identification was at least 20–30 years away.

In any case, the mathematically oriented physicists on the continent didn’t pursue the issue of the physical meaning of potential energy a lot. The quizzical end-result was this: There wasn’t just an equation for the gravitational (and later electrostatic) potential energy of a system, there also was this distinctly further development of an equation for the gravitational potential of a single gravitating body. Thus, the Continental physicists had succeeded in mathematically isolating the interacting system of two charges into components specific to single charges: the potential field due to each, taken in isolation. The potential energy of the system could be found by taking the potential of either charge and multiplying it with the other charge. They were really advanced in maths. They successfully manipulated equations and predicted results. But they didn’t know (or much care) about what ontology their concepts or procedures suggested.

That’s why the ontological issue concerning the action-at-a-distance remained unresolved.


5. Faraday’s lines of force and Maxwell’s fields:

Historically, the crucial step in developing the idea of the field was taken by Faraday. I will not go into the details simply because they are so well known: sprinkling of iron filings near magnets, tubes of force (called “lines” of force by Faraday himself), the broad laws governing their behaviour (attractive force goes with tension, repulsive with sideways pressure), etc.

What is important is to note that Faraday did explicitly advocate the idea that the lines of force physically existed; they weren’t just a device of calculations the way the gravitational potential was to the Continentals. At the same time, he also believed that even the apparently empty space in between two lines of force was also filled with more such lines, that they filled the entire 3D space.

Maxwell got impressed by Faraday’s idea. He even met Faraday, and then, set out to translate the idea into more precise mathematical terms. He arrived at his mathematical description of the electromagnetic phenomena by imagining not just the tubes of force but even many further mechanical mechanisms, invented by him, in order to give a mechanical explanation of the known EM laws (Coulomb, Biot-Savart, Ampere, Faraday, and others.) Eventually, the system of explanations of EM phenomena using mechanical means (essentially, the NM-objects) became too complex. So, in his final synthesis, he simply dropped these complex details, but retained only the differential equations.

However, importantly, he continued to keep the abstraction of the fields in his final synthesis too.

The idea of the field solved the problem of action at a distance. Maxwell put forth the idea that all of space (except for the regions where charged objects are present) is filled with a physically existing field. Charged bodies are in continuous contact with the field, and therefore, are able to induce a condition of force in space—which is the field of force. Maxwell imagined that the force field consists of non-uniform mechanical stresses and strains. (He used the term “displacements” for these mechanical strains; hence the term “displacement current,” which is still in use.) When the same field comes in contact with some other charge, it experiences a net force due to the presence of these mechanical stresses at its boundary. That’s how the forces get transmitted.

Maxwell was smart. What he started out (or invented as he went along) were rather complicated physical mechanisms (all of which were made from NM-objects). But he had a definite sense of which point-quantities to abstract away, using what kind of limiting arguments, and how. Thus showing a refined and mathematically informed judgment, he arrived at an essentialized description of all the electromagnetic phenomena in terms of point-properties of a continuum.


6. The ontological view implied by Maxwell’s ideas:

In Maxwell’s view, the entire universe could be analyzed in purely mechanical terms. This means that all the objects he employed in his synthesis were essentially only NM-objects. These came in two types: (i) point-charges, and (ii) a mechanical continuum for the field.

The two interacted (exchanged forces) using the only mode that NM-objects were allowed: using direct physical contact. The interaction proceeded both ways: from charges to the continuum and from continuum to charges.

The point-charges pressed forces on the continuum of the field at their common boundary, which resulted in there being a mechanical field of stresses and strains inside it. These stress-strain states extended everywhere in the continuum, “up to” infinity.

The continuum, in term, generated forces on the surfaces of the boundary between itself and any charged object embedded in it.

The specific stress-strain field generated by a charge was spherically symmetric around that charge. Hence, it didn’t result into any net force acting on the same charge. However, due to the inverse-square nature of Coulomb’s law, and the fact that charges were point-particles, the field they generated was necessarily non-uniform at all other points. Hence, the continuum did generate a net force on the other charges.

Ontologically, there was little difference between Faraday meant (or strived to indicate), and what Maxwell directly put forth, using mathematical concepts.


7. The basic weakness of what precisely Faraday, and also Maxwell, meant by a field—its ontology:

Maxwell’s proposal of fields had a very great virtue, and a very great conceptual (ontological) weakness.

The virtue was an advocacy of a physically existing condition in what earlier was regarded as completely empty space. This condition was identified with the mathematically defined fields. The idea of fields was not just satisfactory from the viewpoint of broad philosophic principles (we will touch on them in a short while), it also gave a reasonable-sounding solution to the physics problem of action-at-a-distance.

The weakness was that the field, even if defined very carefully (in direct reference to empirically observed electromagnetic laws, and with rigorous mathematical abstraction), still was characterized, explained, and defended as a specifically mechanical kind of a physical existent. To describe the ontology of such fields using our scheme and notation, these EM fields were “pure” NM-objects.[Professional physicists often don’t have very good ontological clarity, but they are referring to the same underlying physical fact when they say that a mechanical aether provides a reference frame that obeys the Galilean-invariance but not the Lorentz-invariance.]

This particular weakness immediately led to conceptual challenges for Maxwell. Eventually, it also led to a lasting confusion for all, a confusion that persists till date (at least in the discussion of EM and aether).

The difficulties posed by the weakness were actually insurmountable. Here is one example. If the field had to be mechanical in nature as Maxwell said, then it would have to possess seemingly impossible combinations of physical properties. It would have to be an infinitely rigid object, and yet allow other massive objects (of the NM-object kind) to pass through them without hindrance. Et cetera. For an interesting history of how creative solutions were sought, and even were supplied see [TBD]. (A candidate explanation: The field acts like a metal ball placed on a block of snow: The ball passes through the block’s thickness even while keeping the block solid everywhere else, but the ball gets reflected when thrown with a sufficiently high speed. Another example I can think of: a jet-plane (say in a tail-pin) that hits the ocean surface. When the speed is great, it first rebounds as if it had hit a solid surface, rather than sinking in the water as it eventually does when it loses speed.) However, such explanations did not bear out—no mechanism would if it produces a mechanical aether in the end.

If Maxwell’s theory were to be less successful, this weakness would be less consequential. However, this was not the case. Maxwell’s was one the most fundamental and most successful theories of his time. (It was what QM and Relativity are to us.) Since the weakness rode on a very strong theory, it irretrivably put people in the mindset that all fields, including the luminiferous aether (which transmits Hertzian waves through it), had to be necessarily mechanical in nature.


8. The consequences of the conceptual weakness of Maxwell’s idea of the field—its ontology:

The weakness had to come out sooner or later, and in the 19th century, it came out rather sooner.

In particular, if the aether was to be space-filling, then the earth would have to move through it during its elliptical motion around the Sun. If this aether was mechanical in nature, then the interaction of the earth with the aether would have to be mechanical in nature. It would be analogous to the motion of a finger through the tub-water. The aether would exert a drag force on the earth’s motion, which could be detected via light interference experiments.

However, experiments such as those by Michaelson-Morely showed that such a drag was not actually present. This momentous experimental finding led to the following situation.

The physicists and mathematicians of the energetics program (mostly on the continental Europe) had labored for almost 70 years to develop the mathematics of fields, but without regarding it as a physical entity. But they had produced excellent mathematics which greatly clarified presentation of physics and simplified calculations. Even Maxwell’s theory had its mathematics developed on the basis of these developments.

The aether-induced drag was only a deductive inference made from Maxwell’s theory. Maxwell’s theory itself was founded on very well established experimental findings. Another deductive inference, namely that light was an EM wave with the speed of c, had been borne out by experiment too. So, all these aspects had to be kept intact.

But the drag implied by Maxwell’s aether was not to be found in the experiments. So, this part of Maxwell’s theory had to be corrected for. The simplest way to do that was to drop the whole idea of the aether from the theory!

After all, in his own development, Maxwell himself had started out with a laundry list of different kinds of physical mechanisms for different aspects of electromagnetics. However, eventually, he himself had come to drop all these mechanical features, because it was hard to get all these mechanisms to work together in a simple manner. So, he had instead decided to abstract out just his mechanical field from them. This field was then identified with the luminiferous aether.

But the aether-wind gave problem. So, why not take just another step of abstraction, and entirely do away with the very idea of the aether itself? Why not regard it as just a mathematical entity? Why can’t space once again be completely empty of any physical being, just the way it had been right since Newton’s times—and even during the entire development of the potential theory and all?

If the situation is to be framed as above, then there can be only one logical way out of it. The physicists came to choose precisely that. Without challenging the specifically mechanical nature of the aether (because no one could think of any other kind of a nature for a physical aether, since none could figure out any good philosophical arguments for having a non-mechanical aether), physicists in the late 19th- and early 20-th century simply decided to remove this whole idea from physics.

In the meanwhile, Einstein was advocating a denial of the absolute space and absolute time anyway. If the space itself was not absolute but depended on the relative motion of the observer, i.e., if space itself depended on motion, then was there any point in filling it with anything?

For the rest, pick up virtually any of the hundreds of the pop-sci books on the relativity theory and/or on Einstein’s (IMO at least in part undue) glorification. … If Maxwell to be less successful as a physicist, his wrong ontological views would be much less consequential. Ditto, for Einstein.


9. A preview of the things to be covered (concerning the EM ontology):

The essential error, to repeat, was to conclude that since Maxwell was successful, and since his field was mechanical, therefore every field has to be taken as being mechanical—i.e., as if it had to follow the NM ontology. Wrong.

Next time, we will cover the correct ontological view to be taken of the Maxwellian fields. We will also look into a few issues about ideas like the electrostatic potential, the electrostatic potential energy, and their fields. We look into the details of these concepts only because they are relevant from an ontological point of view. [And yes, there is a difference between just potential and potential energy—just in case you had forgotten it.] This discussion will also help us prepare for the correct ontological view which is to be adopted when it comes to the quantum mechanical phenomena. Hopefully, the whole portion would be over in two more posts, at most three.

See you next time. Bye for now, and take care…


A song I like:

(Hindi) “o sanam, tere ho gaye hum…”
Singers: Lata Mangeshkar and Mohmmed Rafi
Music: Shankar-Jaikishen
Lyrics: Shailendra

 

 

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