# Tag Info

### Why are physical states not eigenstates of BRST charge?

For starters, the BRST charge operator is Grassmann-odd, so an eigenvalue would be Grassmann-odd as well, which is unphysical.
Accepted

### Some question about the irreducible representation of Poincare group

You likely have already dealt with composite particles in non-relativistic quantum mechanics, under the guise of "addition of angular momentum." The upshot is that you can always choose to ...
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### Solution to Laplace equation in spherical coordinates and Legendre polyomials

Is there any physical reason that enforces us to use the polynomials instead of the general series solution? Yes. Legendre functions where $l$ is not an integer diverge at $x=1$ or $x=-1$ or both, ...

### Symmetry groups breaking for the lagrangian of two complex scalar fields

I'll actually not be focused on the explicit symmetry breaking you are asking about, but I'll summarize the full symmetry case (zero mass, or identity mass operator), and summarize the symmetry ...

### Particularity of symmetries generated by the action variables of a classically integrable system

My answer will be to try to be more intuitive than in-depth quantitative-wise. If the latter is what you seek, I would recommend Olver's Book "Applications of Lie Groups to Differential Equations&...
1 vote
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### Proper definition of spontaneously broken symmetry

We use uncharged operators like $\langle O^\dagger(x) O(y)\rangle$ where $O$ is charged under the relevant symmetry and $x$, $y$ are well separated to diagnose spontaneous symmetry breaking (this is ...
1 vote
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### Can the lattice of an element with face-centered cubic (FCC) crystal structure be regarded as simple cubic (SC)?

The fcc structure can be generated from a sc lattice with a four-atom basis. The sentence above is from lecture notes by Phil Ahrenkiel of South Dakota School of Mines and Technology. So, this means ...
1 vote

### Question on Gravity (Ring and Sphere) from JEE Mains 2021

The gravitational force from the sphere on any "chunk" $\Delta m$ of the ring is indeed $G M \Delta m / (3 R)^2$ (note the parentheses). However, this does not mean that the net force on ...
1 vote
Accepted

### What is the deeper relation between symmetry and the Hamiltonian?

If an operator (for example $\hat{p}$) commutes with the Hamiltonian, then the eigenstates of $\hat{H}$ are also eigenstates of $\hat{p}$ (this is a mathematical statement that can be formally proved ...

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