# In electromagnetism, how do we know that either $F^{\mu\nu}$ or $A^\mu$ is a tensor?

In special relativity the partial derivative $$\partial_\mu$$ is a tensor. Now if some function $$A^\mu$$ was a tensor, then also the quantitiy $$F^{\mu\nu}=\partial^\mu A^\nu - \partial^\nu A^\mu$$ would be a tensor simply by composition.

Maxwell's equations can be broken down to a scalar potential $$\phi$$ and a vector potential $$\vec{A}$$. Now of course I can randomly construct a four potential $$A^\mu=\left(\phi,\vec{A}\right)$$ from these coincidentally 4 quantities which fits nicely in the 4-dimensional space-time formulation. But how do I know that $$A^\mu$$ is actually a tensor?! Is it just a phenomenological premise which cannot be proven? Given $$A^\mu$$ is a tensor and the relation of $$F^{\mu\nu}$$ to the electric and magnetic fields $$\vec{E}$$ and $$\vec{B}$$, I can obtain these fields in any inertial system by a simple Lorentz transformation of the entire thing $$F^{\mu\nu}$$.

But I need to start somewhere and know a priori that either $$F^{\mu\nu}$$ or $$A^\mu$$ is a tensor.

I hope that what I am asking is clear. So can it be proven, or is it just given by experiment?

First, note that if $$A^\mu$$ transforms like the components of a vector, then so does $$A'^\mu = A^\mu + \partial^\mu \chi$$ for any scalar field $$\chi$$ - therefore, the tensorial nature of the 4-potential is gauge-invariant.

Second, note that in the Lorenz gauge, we have $$\partial_\alpha \partial^\alpha A^\beta\equiv \square A^\beta = \mu_0 J^\beta$$ Since $$\square \equiv \partial_\alpha\partial^\alpha$$ transforms like a scalar operator under Lorentz transformations, it follows that $$A^\beta$$ shares the same transformation properties as $$J^\beta$$.

Lastly, we know $$J^\beta$$ transforms like a four-vector because the continuity equation $$\partial_\beta J^\beta = 0$$ holds in all reference frames. Letting primes denote transformed quantities,

$$\partial_\beta J^\beta- \partial'_\alpha J'^\alpha = \partial_\beta J^\beta - \left(\Lambda^{-1}\right)_\alpha^{\ \ \beta} \partial_\beta J'^\alpha = \partial_\beta \left(J^\beta - \left(\Lambda^{-1}\right)_\alpha^{\ \ \beta} J'^\alpha\right) = 0$$

which implies that

$$J'^\alpha = \Lambda^\alpha_{\ \ \beta} J^\beta + C^\alpha$$

for some divergence-free field $$C^\alpha$$. On physical grounds we can demand that if the 4-current vanishes in one frame then in vanishes in every frame (a Lorentz boost can't create charge or current density out of nowhere), implying that $$C^\alpha = 0$$ and that $$J^\beta$$ transforms like a 4-vector.

Therefore, $$A^\beta$$ transforms like a vector in the Lorenz gauge, and by extension, in every gauge.

• I guess this is a good argument. Thanks Commented Oct 25, 2019 at 15:05

One way to prove it could be finding how the fields transform. Another way to prove that $$A^{\mu}$$ is a tensor is by trying to construct the theory of a massless spin-1 particle by using the Lagrangian formalism, since the Lagrangian should be lorentz invariant and gauge invariant you would eventually find that you need the same form of the Maxwell tensor $$F_{\mu\nu}$$ that you gave before and that it has to transform as a Lorentz tensor.

A pretty good derivation of the above is given in M.Schwartz Quantum field theory and the standard model, at the end electromagnetism is also a field theory (A classical one).

• How do you know it has to be a massless spin-1 particle?
– user137661
Commented Oct 25, 2019 at 0:20
• Since Maxwell equations describe waves moving at the speed of light the particle we want to describe needs to follow a massless dispertion relation in order to move at the speed of light and we need it to be spin-1 because it is the minimal spin that allows the theory to have two polarization states. (Three in the case of a massive spin-1 particle) Commented Oct 25, 2019 at 0:28
• Sorry for my ignorance but, why do we need the theory to have two polarisation states?
– user137661
Commented Oct 25, 2019 at 0:49
• Because Maxwell equations (no sources, free space) predict waves with two polarization states. :) Commented Oct 25, 2019 at 0:54

Well... $$A^\mu$$ is not really a tensor. The tensor form of that 4-potential is

$$A = A^\mu \partial_\mu,\ A^\mu \in \mathbb{R}$$

So since you know that $$\partial_\mu$$ transform as a tensor, then $$A$$ is a tensor and you say that its components transform as a tensor. Let me explain this a little bit more:

Let's say I want to change coordinates $$x \rightarrow y$$, then

$$\partial_\mu = \frac{\partial}{\partial x^\mu} \rightarrow \partial_\mu^{\ '} = \frac{\partial y^\nu}{\partial x^\mu}\frac{\partial}{\partial y^\nu}$$

Therefore, since $$A^\mu$$ is just a function, under this change

$$A(x) = A^\mu(x)\partial_\mu \rightarrow A^{'}(y) = A^{'\ \mu}(y)\frac{\partial y^\nu}{\partial x^\mu}\frac{\partial}{\partial y^\nu}, \quad A^{' \mu}(y) = A^\mu(x(y))$$

And now you can make an abuse of language and say that $$A^\mu$$ is a tensor because it transforms as

$$A^\mu(x) \rightarrow A^{'\ \mu}(y)\frac{\partial y^\nu}{\partial x^\mu}$$

Now, how do you know that $$A^\mu = (\phi , \vec{A})$$ gives you the correct theory? As far as I know, that is due to a special relativity extension of classical lagrangian mechanincs. Classically you have that the interaction is given as

$$S_{int} = \int dt\ L_{int} = -\int dt\ \phi$$

Since $$S$$, the action, must be invariant under Lorentz tranformation you suggest that $$\phi$$ is the zero-th component of some 4-vector $$A^\mu = (\phi, \vec{0})$$ so now you can write

$$S_{int} = -\int dt\ A^0 = -\int dx_0\ A^0$$

But this is not a Lorentz scalar, so you extend $$A^\mu = (\phi, \vec{A})$$ but without assuming that $$\vec{A}$$ is the usual magnetic vector potential. Therefore,

$$S_{int} = -\int dt\ A^0 \xrightarrow{extension} -\int dx_\mu\ A^\mu = -\int dt\ \frac{dx_\mu}{dt}A^\mu \tag1$$

This is now a Lorent scalar (so invariant) since the index $$\mu$$ is dummy now. Let's see what $$\vec{A}$$ is by developing Eq. (1) (recall $$x_0 = dt$$):

$$S_{int} = -\int dt\ \phi(dt/dt) - \vec{A}(d\vec{x}/dt)$$

The part on $$\vec{A}$$ is the usual interaction of the form current times magnetic potential if consider $$\vec{A}$$ like that and goes to lagrangian density formulation. By construction $$A^\mu$$ must be a 4-vector (tensor of rank 1) because if it wasn't that, $$A^\mu dx_\mu$$ wouldn't be a Lorentz scalar

• I think that doesn't really address the issue. I can take any function $A^\mu$ and transform as you present to another inertial system. But how do I know that given $A^\mu$ in one inertial system and by that the physical quantities $\vec{E}$ and $\vec{B}$, that an observer in another system precisely sees these quantities $\vec{E}'$ and $\vec{B}'$ as given by the lorentz transformation. The point is, this must not be the case in reality. Saying this is just how a tensor transforms ignores whether this is actually physically the case in reality. Commented Oct 24, 2019 at 23:59
• In essence; how do you know that $A=A^\mu \partial_\mu$ with the physical quantities $\phi$ and $\vec{A}$ can be represented as such. It could be possible that a more elaborate choice of basis vectors (e.g. $A=A^\mu b_\mu^\nu \partial_\nu$ with constants $b$) need to be used to get the correct transformation. Commented Oct 25, 2019 at 0:07
• First, the fields don't transform by Lorentz, that's why you need the strenght tensor. Secondly, I'm going to edit the answer so check it out when I post it Commented Oct 25, 2019 at 0:14
• Why is $\int {\rm d}t A^0=\int {\rm d}x_\mu A^\mu$ if $\vec{A} \neq 0$? Commented Oct 25, 2019 at 22:47
• It's not, a mistake using an equal instead of an arrow. Check edit Commented Oct 25, 2019 at 23:34