In Section 2.2 of their QFT textbook, Peskin & Schroeder introduce the Lagrangian and Hamiltonian field theories of a classical scalar field. While defining the action $S[\phi]$ and deriving the Euler-Lagrange equation for the classical scalar field $\phi$, the classical scalar field $\phi$ is co...

In cosmology, we speak of galaxies racing away from us due to the expansion of the universe. The light from these galaxies appears red-shifted. If the galaxies are far enough, then the space between us actually expands faster than light, and light from these galaxies never reaches us. My question...

At the center of neutron stars are tightly packed neutrons with almost no space between them. Also, proton and electron have all merged (electron capture) forming as much neutrons as possible. When this star exceeds the Tolman–Oppenheimer–Volkoff limit, the result would be a black hole. Out of cu...

How much force do I do apply when I lift my leg above the ground? The same amount as gravity does on my leg (mg)? Or MORE than it (greater than mg)? If the displacement from the ground to my lifted leg is h meters, then what's the work done? mgh or more than mgh? Basically I wanna know if I have ...

In classical mechanics, we know that laws of physics are invariant in Galilean transformations of the form: $$ x' = x -vt$$ My question is does shifting to rotating frame also count as a Galilean transformation? If it is not so, then are laws of physics invariant when shifting between rotating fr...

Uncertainty of an operator $\hat{A}$ when observing a state $|\psi\rangle$ is defined as \begin{equation} \Delta A_{\psi} = |(\hat{A} - \langle\hat{A}\rangle)\psi| \end{equation} Now assume that there is some uncertainty for an operator $\hat{A}$ i.e., when I operate it on a state $|\psi\rangle$ ...