This question came to mind while reading about Weinberg's folk theorem that any quantum theory that is Poincare covariant and satisfies cluster decomposition will look like a quantum field theory at low energies, the caveat being that at high energies, string theory stands as a counterexample. In...

Based on the Maxwell equations we know that A time-varying magnetic field induces an electric field A time-varying electric field induces a magnetic field Suppose that an electric field, which is induced by a time-varying magnetic field, is itself time-varying (not stationary). Then the induc...

It is often stated that QED makes predictions correct up to many decimal places. What are the experiments that show this? And all have to provide measurements up to 12 decimal places? To my understanding you need at least two experiments with that precision, because first you need to determine th...

Why is the minimum amount of energy a photon has is equal to the rest energy of one of the particle ? I know that the the minimum possible energy a photon may have is only the rest energy of particles is converted. But if that's the case then the particle and antiparticle pair must be stationary ...

I was taught that Internal Energy $U$ is a relative quantity: only changes in $U$ are meaningful. It doesn't have an absolute value, since it always comes with an arbitrary constant (for example $U = nT +c$). Entropy $S$, on the other hand, has an absolute value thanks to the Third Postulate of ...

In QM we work with $H=L_2(\mathbb{R}^3)$ as a Hilbert space of square-integrable complex-valued functions. Now we define a special set of three operators $L_x, L_y, L_z$ by $L_i = \varepsilon_{ijk} \hat{x}_j \hat{p}_k$, where $\hat{x}_j$ is the operator that multiplies by $x_j$ and $\hat{p}_k$ is...