This concept was a bit hard to grasp, because I'm bad at seeing the real-world implications of multiplication. Division makes sense, but multiplication is harder for me. Pressure-volume work is measured in $1$ Pa $\times$ m$^3$. It says that $1$ Pa $\times$ m$^3 = 1$ J. Does that mean that no mat...

I was looking at an old textbook (The Magnetic Circuit, 1911) that talks about magnetic flux as having a physical manifestation. It says that flux may be an actual flow of an incompressible ether along the lines of force. It says other physicists consider the magnetic circuit to be whirls or vort...

I understand that the way light takes through a pinhole creates an inverted image on a surface behind the pinhole. I remember this effect from school experiments, it's also described in this wikipedia article. I punctured a piece of paper and looked through it (instead of watching the reflection)...

I'm currently playing around with gauge transformations in the Fock space of photons. Let's consider a local gauge transformation with gauge function $f(\mathbf{r})$. So the magnetic vector potential transforms like $$\begin{align} \mathbf{A}'(\mathbf{r}) &= \mathbf{A}(\mathbf{r}) + \mathbf{\na...