**Pre-script** (dated 26 June 2020): This post got severely mutilated by the removal of material by the dark force. It may, therefore, be difficult to follow the main story-line.

**Original post**:

I’ve done quite a few posts already on electromagnetism. They were all focused on the math one needs to understand Maxwell’s equations. Maxwell’s equations are a set of (four) differential equations, so they relate some function with its derivatives. To be specific, they relate **E** and **B**, i.e. the electric and magnetic field vector respectively, with their derivatives in space and in time. [Let me be explicit here: **E** and **B** have three components, but depend on both space as well as time, so we have *three *dependent and *four *independent variables for each function: **E** = (E_{x}, E_{y}, E_{z}) = **E**(x, y, z, t) and **B** = (B_{x}, B_{y}, B_{z}) = **B**(x, y, z, t).] That’s simple enough to understand, but the *dynamics *involved are quite complicated, as illustrated below.

I now want to do a series on the more interesting stuff, including an exploration of the concept of *gauge* in field theory, and I also want to show how one can derive the wave equation for electromagnetic radiation from Maxwell’s equations. Before I start, let’s recall the basic concept of a field.

**The reality of fields**

I said a couple of time already that (electromagnetic) fields are real. They’re more than just a mathematical structure. Let me show you *why*. Remember the formula for the electrostatic potential caused by some charge q at the origin:

We know that the (negative) gradient of this function, at any point in space, gives us the electric field vector at that point: **E** = –**∇**Φ. [The *minus *sign is there because of convention: we take the reference point Φ = 0 at infinity.] Now, the electric field vector gives us the force on a unit charge (i.e. the charge of a *proton*) at that point. If q is some positive charge, the force will be repulsive, and the unit charge will accelerate away from our q charge at the origin. Hence, energy will be expended, as force over distance implies work is being done: as the charges separate, *potential *energy is converted into *kinetic *energy. Where does the energy come from? The energy conservation law tells us that it *must* come from *somewhere*.

It does: the energy comes from the field itself. Bringing in more or bigger charges (from infinity, or just from further away) requires more energy. So the new charges *change* the field and, therefore, its energy. *How *exactly? That’s given by Gauss’ Law: the total *flux* out of a closed surface is equal to:

You’ll say: flux and energy are two different things. Well… Yes and no. The energy in the field depends on **E**. Indeed, the formula for the energy *density *in space (i.e. the energy *per unit volume*) is

Getting the energy over a larger space is just another integral, with the energy density as the integral *kernel*:

Feynman’s illustration below is not very sophisticated but, as usual, enlightening. 🙂

Gauss’ Theorem connects both the math as well as the physics of the situation and, as such, underscores the *reality* of fields: the energy is *not* in the electric charges. *The energy is in the fields they produce*. Everything else is just the principle of superposition of fields – i.e. **E** = **E**_{1 }+ **E**_{2 }– coming into play. I’ll explain Gauss’ Theorem in a moment. Let me first make some additional remarks.

First, the formulas are valid for *electrostatics* only (so **E** and **B **only vary in space, not in time), so they’re just a piece of the larger puzzle. 🙂 As for now, however, note that, if a field is real (or, to be precise, if its energy is real), then the flux is equally real.

Second, let me say something about the units. Field strength (*E* or, in this case, its normal component *E*_{n} = ** E**·

*) is measured in newton (N) per coulomb (C), so in N/C. The integral above implies that flux is measured in (N/C)·m*

**n**^{2}. It’s a weird unit because one associates flux with flow and, therefore, one would expect flux is some quantity

*per unit time and per unit area*, so we’d have the m

^{2}unit (and the second) in the denominator, not in the numerator. But so that’s true for heat transfer, for mass transfer, for fluid dynamics (e.g. the amount of water flowing through some cross-section) and many other physical phenomena. But for electric flux, it’s different. You can do a dimensional analysis of the expression above: the sum of the charges is expressed in coulomb (C), and the electric constant (i.e. the vacuum permittivity) is expressed in C

^{2}/(N·m

^{2}), so, yes, it works: C/[C

^{2}/(N·m

^{2})] = (N/C)·m

^{2}. To make sense of the units, you should think of the flux as the

*total flow*, and of the

*field strength*as a

*surface density*, so that’s the flux

*divided*by the total area, so (field strength) = (flux)/(area). Conversely, (flux) = (field strength)×(area). Hence, the unit of flux is [flux] = [field strength]×[area] = (N/C)·m

^{2}.

OK. Now we’re ready for Gauss’ Theorem. 🙂 I’ll also say something about its corollary, Stokes’ Theorem. It’s a bit of a mathematical digression but necessary, I think, for a better understanding of all those operators we’re going to use.

**Gauss’ Theorem**

The concept of flux is related to the *divergence *of a vector field through Gauss’ *Theorem*. Gauss’s Theorem has nothing to do with Gauss’ *Law*, except that both are associated with the same genius. Gauss’ Theorem is:

The **∇**·** C **in the integral on the right-hand side is the

*divergence*of a vector field. It’s the

*volume*density of the outward flux of a vector field from an infinitesimal volume around a given point.

** Huh? **What’s a

*volume*density? Good question. Just substitute

*for*

**C****E**in the surface and volume integral above (the integral on the left is a surface integral, and the one on the right is a volume integral), and think about the meaning of what’s written. To help you, let me also include the concept of

*linear*density, so we have (1) linear, (2) surface and (3) volume density. Look at that representation of a vector field once again: we said the density of lines represented the magnitude of

**E**. But what density? The representation hereunder is flat, so we can think of a

*linear*density indeed, measured along the blue line: so the

*flux*would be six (that’s the number of lines), and the

*linear*density (i.e. the

*field strength*) is six divided by the length of the blue line.

However, we defined field strength as a *surface* density above, so that’s the flux (i.e. the number of field lines) divided by the surface area (i.e. the area of a *cross-section*): think of the square of the blue line, and field lines going through that square. That’s simple enough. But what’s volume density? How do we count the number of lines *inside *of a box? The answer is: mathematicians actually *define *it for an infinitesimally small cube by adding the fluxes out of the six individual faces of an infinitesimally small cube:

So, the truth is: *volume* density is actually defined as a *surface *density, but for an infinitesimally small volume element. That, in turn, gives us the meaning of the *divergence *of a vector field. Indeed, the sum of the derivatives above is just **∇**·* C* (i.e. the divergence of

*), and ΔxΔyΔz is the volume of our infinitesimal cube, so the divergence of some field vector*

**C***at some point P is the flux – i.e. the outgoing ‘flow’ of*

**C***–*

**C***per unit volume*, in the neighborhood of P, as evidenced by writing

Indeed, just bring ΔV to the other side of the equation to check the ‘per unit volume’ aspect of what I wrote above. The whole idea is to determine whether the small volume is like a *sink* or like a *source*, and to what extent. Think of the field near a point charge, as illustrated below. Look at the black lines: they are the field lines (the dashed lines are *equipotential* lines) and note how the positive charge is a *source* of flux, obviously, while the negative charge is a *sink*.

Now, the next step is to acknowledge that *the total flux from a volume is the sum of the fluxes out of each part*. Indeed, the flux through the part of the surfaces common to two parts will cancel each other out. Feynman illustrates that with a rough drawing (below) and I’ll refer you to his *Lecture *on it for more detail.

So… Combining all of the gymnastics above – and integrating the divergence over an entire volume, indeed – we get Gauss’ Theorem:

**Stokes’ Theorem**

There is a similar theorem involving the *circulation *of a vector, rather than its flux. It’s referred to as Stokes’ Theorem. Let me jot it down:

We have a *contour *integral here (left) and a surface integral (right). The reasoning behind is quite similar: a surface bounded by some loop Γ is divided into infinitesimally small squares, and the circulation around Γ is the sum of the circulations around the little loops. We should take care though: the surface integral takes the *normal component* of **∇**×** C**, so that’s (

**∇**×

**)**

*C*_{n }= (

**∇**×

**)·**

*C***. The illustrations below should help you to understand what’s going on.**

*n***The electric versus the magnetic force**

There’s more than just the electric force: we also have the magnetic force. The so-called *Lorentz *force is the combination of both. The formula, for some charge q in an electromagnetic field, is equal to:

Hence, if the velocity vector ** v** is

*not*equal to zero, we need to look at the magnetic field vector

**B**too! The simplest situation is

**, so let’s first have a look at that.**

*magnetostatics*Magnetostatics imply that that the flux of **E** doesn’t change, so Maxwell’s third equation reduces to *c*^{2}** ∇**×

**=**

**B****/ε**

**j**_{0}. So we just have a

*steady*electric current (

**j**): no

*accelerating*charges. Maxwell’s fourth equation,

**∇**•

**B**= 0, remains what is was: there’s no such thing as a

*magnetic*charge. The Lorentz force also remains what it is, of course:

**F**= q(

**E**+

**×**

*v***B**) = q

**E**+q

**×**

*v***B**. Also note that the

*,*

**v****and the lack of a**

*j**magnetic*charge all point to the same: magnetism is just a relativistic effect of electricity.

What about units? Well… While the unit of E, i.e. the electric field strength, is pretty obvious from the F = qE term – hence, E = F/q, and so the unit of E must be [force]/[charge] = N/C – the unit of the magnetic field strength is more complicated. Indeed, the **F** = q** v**×

**B**identity tells us it must be (N·s)/(m·C), because 1 N = 1C·(m/s)·(N·s)/(m·C).

*Phew!*That’s as horrendous as it looks, and that’s why it’s usually expressed using its shorthand, i.e. the

*tesla*: 1 T = 1 (N·s)/(m·C).

*Magnetic*flux is the same concept as electric flux, so it’s (field strength)×(area). However, now we’re talking

*magnetic*field strength, so its unit is T·m

^{2 }= (N·s·m

^{2 })/(m·C) = (N·s·m)/C, which is referred to as the

*weber*(Wb). Remembering that 1

*volt*= 1 N·m/C, it’s easy to see that a

*weber*is also equal to 1 Wb = 1 V·s. In any case, it’s a unit that is

*not*so easy to interpret.

Magnetostatics is a bit of a weird situation. It assumes *steady fields*, so the ∂**E**/∂t and ∂**B**/∂t terms in Maxwell’s equations can be dropped. In fact, *c*^{2}** ∇**×

**=**

**B****/ε**

**j**_{0}implies that

**∇**·(

*c*

^{2}

**×**

**∇****=**

**B**)**∇**·(

**/ε**

**j**_{0}) and, therefore, that

**∇**·

**= 0. Now,**

**j****∇**·

**= –∂ρ/∂t and, therefore, magnetostatics is a situation which assumes ∂ρ/∂t = 0. So we have electric currents but**

**j***no change in charge densities*. To put it simply, we’re not

*looking*at a condenser that is charging or discharging, although that condenser may act like the battery or generator that keeps the charges flowing! But let’s go along with the magnetostatics assumption. What can we say about it? Well… First, we have the equivalent of Gauss’ Law, i.e.

*Ampère’s Law*:

We have a *line* integral here around a closed curve, instead of a *surface* integral over a *closed* surface (Gauss’ Law), but it’s pretty similar: instead of the sum of the charges inside the volume, we have the current through the loop, and then an extra *c*^{2} factor in the denominator, of course. Combined with the **∇**•**B **= 0 equation, this equation allows us to solve practical problems. But I am not interested in practical problems. What’s the

*theory*behind?

**The magnetic vector potential**

The** ∇**•**B **= 0 equation is true,

*always*, unlike the

**∇**×

**E**= 0 expression, which is true for electrostatics only (no moving charges). It says the

*divergence*of

**B**is zero,

*always*, and, hence, it means we can represent

**B**as the curl of another

*vector*field,

*always*. That vector field is referred to as the

**magnetic**, and we write:

*vector*potential**∇**·**B** = **∇**·(**∇**×**A**) = 0 and, hence, **B** = **∇**×**A**

In electrostatics, we had the other theorem: if the *curl* of a vector field is zero (everywhere), then the vector field can be represented as the gradient of some scalar function, so if **∇**×**C **= 0, then there is some Ψ for which **C** = **∇**Ψ. Substituting **C** for **E**, and taking into account our conventions on charge and the direction of flow, we get **E** = –**∇**Φ. Substituting **E** in Maxwell’s first equation (**∇**•**E** = ρ/ε_{0}) then gave us the so-called *Poisson *equation: ∇^{2}Φ = ρ/ε_{0}, which sums up the whole subject of electrostatics really! It’s all in there!

Except magnetostatics, of course. Using the (magnetic) *vector potential* A, all of magnetostatics is reduced to another expression:

∇^{2}** A**= −

**/ε**

**j**_{0}, with

**∇**·

**A**= 0

Note the qualifier: **∇**·**A** = 0. Why should the divergence of **A** be equal to zero? You’re right. It doesn’t have to be that way. We know that **∇**·(**∇**×** C**) = 0, for any vector field

**, and**

*C**always*(it’s a mathematical identity, in fact, so it’s got nothing to do with physics), but choosing A such that

**∇**·

**A**= 0 is just a

*choice*. In fact, as I’ll explain in a moment, it’s referred to as choosing a

*gauge*. The

**∇**·

**A**= 0 choice is a very

*convenient*choice, however, as it simplifies our equations. Indeed,

*c*

^{2}

**×**

**∇****=**

**B****/ε**

**j**_{0}=

*c*

^{2}

**×(**

**∇****∇**×

**A**), and – from our vector calculus classes – we know that

**∇**×(

**∇**×

**C**) =

**∇**(

**∇**·

**C**) – ∇

^{2}

**. Combining that with our**

**C***choice*of

**A**(which is such that

**∇**·

**A**= 0, indeed), we get the ∇

^{2}

**= −**

**A****/ε**

**j**_{0 }expression indeed, which sums up the whole subject of magnetostatics!

The point is: if the time derivatives in Maxwell’s equations, i.e. ∂**E**/∂t and ∂**B**/∂t, are zero, then Maxwell’s four equations can be nicely separated into two pairs: the electric and magnetic field are not interconnected. Hence, as long as charges and currents are static, electricity and magnetism appear as distinct phenomena, and the interdependence of **E** and **B** does not appear. So we re-write Maxwell’s set of four equations as:

**Electrostatics**:**∇**•**E**= ρ/ε_{0}and**∇**×**E**= 0**Magnetostatics**:×**∇**=**B****j***c*^{2}ε_{0}and**∇**•**B**= 0

Note that electrostatics is a neat example of a vector field with *zero curl* and a given divergence (ρ/ε_{0}), while magnetostatics is a neat example of a vector field with *zero divergence* and a given curl (**j**/*c*^{2}ε_{0}).

**Electrodynamics**

But reality is usually *not* so simple. With *time-varying *fields, Maxwell’s equations are what they are, and so there *is *interdependence, as illustrated in the introduction of this post. Note, however, that the magnetic field remains *divergence-free* in dynamics too! That’s because there is no such thing as a magnetic charge: we only have electric charges. So **∇**·**B** = 0 and we can define a **magnetic vector potential**

**A**and re-write

**B**as

**B**=

**∇**×

**A**, indeed.

I am writing *a *vector potential field because, as I mentioned a couple of times already, we can *choose* **A**. Indeed, as long as **∇**·**A** = 0, it’s fine, so we can add *curl-free *components to the magnetic potential: it won’t make a difference. This condition is referred to as ** gauge invariance**. I’ll come back to that, and also show why this is what it is.

While we can easily get** B** from** A** because of the** B** = **∇**×**A**, getting **E** from some potential is a different matter altogether. It turns out we can get **E** using the following expression, which involves both Φ (i.e. the electric or electrostatic potential) as well as **A** (i.e. the magnetic *vector* potential):

**E** = –**∇**Φ – ∂**A**/∂t

Likewise, one can show that Maxwell’s equations can be re-written in terms of Φ and **A, **rather than in terms of** E** and** B**. The expression looks rather formidable, but don’t panic:

Just look at it. We have two ‘variables’ here (Φ and **A**) and two equations, so the system is fully defined. [Of course, the second equation is three equations really: one for each component x, y and z.] What’s the point? Why would we want to re-write Maxwell’s equations? The first equation makes it clear that the *scalar* potential (i.e. the electric potential) is a time-varying quantity, so things are *not*, somehow, simpler. The answer is twofold. First, re-writing Maxwell’s equations in terms of the scalar and vector potential makes sense because we have (fairly) easy expressions for their *value *in time and in space as a function of the charges and currents. For *statics*, these expressions are:

So it is, effectively, easier to first calculate the scalar and vector potential, and then get **E** and **B** from them. For *dynamics*, the expressions are similar:

Indeed, they are *like *the integrals for statics, but with “a small and physically appealing modification”, as Feynman notes: when doing the integrals, we must use the so-called retarded time t′ = t − r_{12}/ct’. The illustration below shows how it works: the influences propagate from point (2) to point (1) at the speed *c*, so we must use the values of ρ and **j** at the time t′ = t − r_{12}/ct’ indeed!

The second aspect of the answer to the question of why we’d be interested in Φ and A has to do with the topic I wanted to write about here: the concept of a *gauge* and a *gauge transformation*.

**Gauges and gauge transformations in electromagnetics**

Let’s see what we’re doing really. We calculate some **A** and then solve for **B** by writing: **B** = **∇**×**A**. Now, I say *some *A because any **A**‘ = **A** + **∇**Ψ, with Ψ *any* scalar field really. Why? Because the *curl *of the gradient of Ψ – i.e. curl(gradΨ) = **∇**×(**∇**Ψ) – is equal to 0. Hence, **∇**×(**A** + **∇**Ψ) = **∇**×**A** + **∇**×**∇**Ψ = **∇**×**A**.

So we have **B**, and now we need **E**. So the next step is to take Faraday’s Law, which is Maxwell’s second equation: **∇**×**E** = –∂**B**/∂t. Why this one? It’s a simple one, as it does not involve currents or charges. So we combine this equation and our **B** = **∇**×**A** expression and write:

**∇**×**E** = –∂(**∇× A**)/∂t

Now, these operators are tricky but you can verify this can be re-written as:

**∇**×(**E** + ∂** A**/∂t) = 0

Looking carefully, we see this expression says that **E** + ∂** A**/∂t is some vector whose curl is equal to zero. Hence, this vector must be the gradient of

*something*. When doing electrostatics, When we worked on electrostatics, we only had

**E**, not the ∂

**/∂t bit, and we said that**

**A****E**

*tout court*was the gradient of something, so we wrote

**E**= −

**∇**Φ. We now do the same thing for

**E**+ ∂

**/∂t, so we write:**

**A****E** + ∂** A**/∂t = −

**∇**Φ

So we use the same symbol Φ but it’s a bit of a different animal, obviously. However, it’s easy to see that, if the ∂** A**/∂t would disappear (as it does in electrostatics, where nothing changes with time), we’d get our ‘old’ −

**∇**Φ. Now,

**E**+ ∂

**/∂t = −**

**A****∇**Φ can be written as:

**E** = −**∇**Φ – ∂** A**/∂t

So, what’s the big deal? We wrote B and E as a function of Φ and **A**. Well, we said we could replace **A** by any **A**‘ = **A** + **∇**Ψ but, obviously, such substitution would not yield the same **E**. To get the same **E**, we need some substitution rule for Φ as well. Now, you can verify we will get the same **E** if we’d substitute Φ for Φ’ = Φ – ∂Ψ/∂t. You should check it by writing it all out:

**E** = −**∇**Φ’–∂** A’**/∂t = −

**∇**(Φ–∂Ψ/∂t)–∂(

**+**

**A****Ψ)/∂t**

**∇**= −**∇**Φ+**∇**(∂Ψ/∂t)–∂** A**/∂t–∂(

**Ψ)/∂t = −**

**∇****∇**Φ – ∂

**/∂t =**

**A****E**

Again, the operators are a bit tricky, but the +**∇**(∂Ψ/∂t) and –∂(**∇**Ψ)/∂t terms do cancel out. Where are we heading to? When everything is said and done, we do need to relate it all to the currents and the charges, because that’s the

*real*stuff out there. So let’s take Maxwell’s

**∇**•

**E**= ρ/ε

_{0}equation, which has the charges in it, and let’s substitute

**E**for

**E**= −

**∇**Φ – ∂

**/∂t. We get:**

**A**That equation can be re-written as:

So we have *one *equation here relating Φ and **A** to the sources. We need another one, and we also need to separate Φ and **A** somehow. How do we do that?

Maxwell’s fourth equation, i.e. *c*^{2}**∇**×** B **=

**/ε**

**j**_{0 }+ ∂

**/∂t can, obviously, be written as**

**E***c*

^{2}

**∇**×

**−**

**B****∂t =**

*∂***/****E****/ε**

**j**_{0}. Substituting both E and B yields the following monstrosity:

We can now apply the general** ∇**×(**∇**×**C**) = **∇**(**∇**·**C**) – ∇^{2}** C** identity to the first term to get:

It’s equally monstrous, obviously, but we can simplify the whole thing by choosing Φ and A in a clever way. For the magnetostatic case, we chose **A** such that **∇**·**A** = 0. We could have chosen something else. Indeed, it’s not because **B** is divergence-free, that **A** has to be divergence-free too! For example, I’ll leave it to you to show that choosing **∇**·**A **such that

also respects the general condition that any **A** and Φ we choose *must* respect the **A**‘ = **A** + **∇**Ψ and Φ’ = Φ – ∂Ψ/∂t equalities. Now, *if *we choose **∇**·**A **such that **∇**·**A **= −*c*^{–2}·∂Φ/∂t indeed, then the two middle terms in our monstrosity cancel out, and we’re left with a much simpler equation for **A**:

In addition, doing the substitution in our other equation relating Φ and **A** to the sources yields an equation for Φ that has the same form:

What’s the big deal here? Well… Let’s write it all out. The equation above becomes:

That’s a wave equation in three dimensions. In case you wonder, just check one of my posts on wave equations. The one-dimensional equivalent for a wave propagating in the x direction at speed *c* (like a sound wave, for example) is ∂^{2}Φ/∂x^{2 }= *c*^{–2}·∂^{2}Φ/∂t^{2}, indeed. The equation for **A** yields above yields similar wave functions for **A**‘s components A_{x}, A_{y}, and A_{z}.

So, yes, it *is *a big deal. We’ve written Maxwell’s equations in terms of the scalar (Φ) and vector (**A**) potential and in a form that makes immediately apparent that we’re talking electromagnetic waves moving out at the speed *c*. Let me copy them again:

You may, of course, say that you’d rather have a wave equation for **E** and **B**, rather than for **A** and Φ. Well… That can be done. Feynman gives us two derivations that do so. The first derivation is relatively simple and assumes the *source *our electromagnetic wave moves in one direction only. The second derivation is much more complicated and gives an equation for **E** that, if you’ve read the first volume of Feynman’s *Lectures*, you’ll surely remember:

The links are there, and so I’ll let you have fun with those *Lectures *yourself. I am finished here, indeed, in terms of what I wanted to do in this post, and that is to say a few words about gauges in field theory. It’s nothing much, really, and so we’ll surely have to discuss the topic again, but at least you now know what a *gauge* actually is in classical electromagnetic theory. Let’s quickly go over the concepts:

- Choosing the
**∇**·**A**is choosing a, or a*gauge**gauge potential*(because we’re talking scalar and vector potential here). The particular choice is also referred to as**gauge fixing**. - Changing
**A**by adding**∇**ψ is called a, and the scalar function Ψ is referred to as a*gauge transformation*. The fact that we can add*gauge function**curl-free*components to the magnetic potential without them making any difference is referred to as.*gauge invariance* - Finally, the
**∇**·**A**= −*c*^{–2}·∂Φ/∂t gauge is referred to as a.*Lorentz gauge*

Just to make sure you understand: why is that *Lorentz* gauge* *so special? Well… Look at the whole argument once more: isn’t it amazing we get such beautiful (wave) equations if we stick it in? Also look at the functional shape of the gauge itself: it looks *like* a wave equation itself! […] Well… No… It doesn’t. I am a bit too enthusiastic here. We do have the same 1/*c*^{2}* *and a time derivative, but it’s not a wave equation. 🙂 In any case, it all confirms, once again, that physics is all about beautiful mathematical structures. But, again, it’s not math only. There’s something *real *out there. In this case, that ‘something’ is a traveling electromagnetic field. 🙂

But why do we call it a *gauge*? That should be equally obvious. It’s really like choosing a gauge in another context, such as measuring the pressure of a tyre, as shown below. 🙂

**Gauges and group theory**

You’ll usually see gauges mentioned with some reference to *group theory*. For example, you will see or hear phrases like: “The existence of arbitrary numbers of gauge functions ψ(**r**, *t*) corresponds to the U(1) gauge freedom of the electromagnetic theory.” The U(1) notation stands for a *unitary group *of degree n = 1. It is also known as the *circle group*. Let me copy the introduction to the unitary group from the Wikipedia article on it:

In mathematics, the **unitary group** of degree *n*, denoted U(*n*), is the group of *n* × *n *unitary matrices, with the group operation that of matrix multiplication. The unitary group is a subgroup of the general linear group GL(*n*, **C**). In the simple case *n* = 1, the group U(1) corresponds to the circle group, consisting of all complex numbers with absolute value 1 under multiplication. All the unitary groups contain copies of this group.

The unitary group U(*n*) is a real Lie group of of dimension *n*^{2}. The Lie algebra of U(*n*) consists of *n* × *n *skew-Hermitian matrices, with the Lie bracket given by the commutator. The **general unitary group** (also called the **group of unitary similitudes**) consists of all matrices A such that A*A is a nonzero multiple of the identity matrix, and is just the product of the unitary group with the group of all positive multiples of the identity matrix.

*Phew! *Does this make you any wiser? If anything, it makes me realize I’ve still got a long way to go. 🙂 The Wikipedia article on gauge fixing notes something that’s more interesting (if only because I more or less understand what it says):

Although classical electromagnetism is now often spoken of as a gauge theory, it was not originally conceived in these terms. The motion of a classical point charge is affected only by the electric and magnetic field strengths at that point, and the potentials can be treated as a mere mathematical device for simplifying some proofs and calculations. Not until the advent of quantum field theory could it be said that the potentials themselves are part of the physical configuration of a system. The earliest consequence to be accurately predicted and experimentally verified was the Aharonov–Bohm effect, which has no classical counterpart.

This confirms, once again, that the fields are real. In fact, what this says is that the *potentials *are real: they have a meaningful physical interpretation. I’ll leave it to you to expore that Aharanov-Bohm effect. In the meanwhile, I’ll study what Feynman writes on potentials and all that as used in quantum physics. It will probably take a while before I’ll get into group theory though.

Indeed, it’s probably best to study physics at a somewhat less abstract level first, before getting into the more sophisticated stuff.

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‘Finally, the ∇·A = −c–2·∂Φ/∂t gauge is referred to as a Lorentz gauge.’

Actually, it is called the Lorenz gauge, named after the fellow who came up with it. That was not Lorentz.

Thanks for pointing that out ! Much appreciated !