# All Questions

154 views

### Derivative with respect to ${\not}{p}$

When studying renormalization of QED in standard textbooks, we typically encounter derivatives with respect to ${\not}{p}=p^\mu \gamma_\mu$, i.e., $\partial/\partial{\not}p$. As far as I understand, ...
132 views

### Does radio use virtual photons?

In radio communication each accelerated electron in the transmitter antenna interacts with an electron in the receiver antenna by exchanging a photon. Is that photon always a virtual photon as ...
22 views

### High Young's Modulus and Tensile Strength of Carbon Nanotubes

I was recently reading about Carbon Nanotubes having extremely high Young's moduli, as well as high Tensile Strength, making them very interesting fibers. However, when I read this I wondered what was ...
78 views

### Separation of variables in various PDEs, physical meaning

The method of separation of variables produces an undetermined separation constant and a family of solutions indexed by the values of this constant. For instance, in the case of an infinitely long ...
84 views

### How to interpret band structures

I'm currently taking a Solid State Physics class, and is currently reading about the quantum mechanical description of solids. I then came across the following figure: It's supposed to be the band ...
105 views

### Quantum barrier for photons [closed]

In quantum mechanics, a particle may tunnel through a barrier it would not be able to surmount in a classical sense. My question is this: What are all the factors that may prevent a photon from ...
45 views

### Understanding Wikipedia's “Semiconductor Band Structure” diagram where the bandgap appears to increase with increasing density of states

I'm having a bit of trouble understanding the semiconductor band gap diagram on Wikipedia: (from Band gap article). Why is the size of the band gap increasing with the Density of States (DOS) in the ...
34 views

### shear and normal stresses

When we talk about shear stresses in a fluid, we find that the shear stress is given by $$\tau_{xy} = \mu(\partial_y u + \partial_x v) = \tau_{yx}$$ This relation we get when only looking at one side ...
1k views

### Tension in a massless string around a pulley [on hold]

A pulley having some mass has a massless string around it, with two unequal masses at the end of the string. Sufficient friction is present between pulley and the string such that the string does not ...
16 views

### How is the probability of intersystem crossing influenced by small differences in energy levels?

Say I have some dye molecule in some low level triplet state, e.g. $T_1$, and it's decaying slowly to the ground state via phosphorescence. However, there are also events where the dye in the $T_1$ ...
111 views

### Non-isolated universe and arrow of space

Could you please help me to understand two statements in physics: 1) Universe by definition is an isolated system. But universe expands, space pours into universe, space (vacuum) has zero-point ...
61 views

### What is *uplift* in respect to extra dimensions and their stability?

What is uplift in respect to extra dimensions and their stability? It's notoriously hard to find something on this, as all possible keyword combinations pull up plethora of unrelated Google hits.
33 views

### The story about pre-big bang scenario [on hold]

I came across this subject today while reading about BKL singularity. Doesn't seem to be explained concisely elsewhere, would someone kindly provide a quick summary of the story here?
133 views

### $2\pi$ and Feynman Rules

I notice a $2\pi$ term in the $\delta$-function when trying to construct an amplitude using the Feynman Rules. The $2\pi$ also appears as an integration measure to enforce normalisation in the phase ...