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May
26
comment Addition of 3 angular momenta
You seem to be forgetting Pauli's principle...
Apr
22
comment Is the Landau Free Energy U-TS or βH?
No, I mean the fact that calculating the log of the partition function gives $U - T S$, so no just the mean energy $U$ but also the entropy term.
Apr
22
comment Is the Landau Free Energy U-TS or βH?
Do you understand why $F= - T \ln Z$, with $Z=\mathop{\rm tr} e^{-H/T}$?
Apr
6
awarded  Yearling
Jan
17
comment Klein-Gordon field commutator integral identity
@spitespike: if the integral is taken over $\mathbb{R}^3$ the result quoted is wrong...
Jan
16
comment Klein-Gordon field commutator integral identity
You posted the same question on math.stackexchange. You should only post it to one of the sites. As I have explained there, there is something seriously wrong with your first formula as the term in the bracket simply vanishes. Furthermore, the integral over $k$ should be in 2d.
Jan
11
comment Harmonic oscillator coherent state expected values
The energy is conserved...
Nov
21
comment Does the $\bf{1+3}$ representation of $SU(2)$ also represent $SU(2)\times SU(2)$?
@Lior: I'm not sure I understand your statement. Typically in quantum mechanics one looks at the action of $S= S_1 +S_2$ to determine which spin the tensor product states $|m_1,m_2\rangle$ have. This corresponds due to $\exp(S) = \exp(S_1)\otimes \exp(S_2)$ to simultaneously rotating both spins.
Nov
21
comment Does the $\bf{1+3}$ representation of $SU(2)$ also represent $SU(2)\times SU(2)$?
@Lior: if the two spins are unrelated the representation does not factor into 1+3 but it is irreducible itself.
Nov
21
comment Does the $\bf{1+3}$ representation of $SU(2)$ also represent $SU(2)\times SU(2)$?
Physically you are rotating both spins simultaneously and thus you have a representation of SU(2) and not of SU(2)$\times$SU(2)!
Nov
21
answered Does the $\bf{1+3}$ representation of $SU(2)$ also represent $SU(2)\times SU(2)$?
May
9
comment Nonzero ground state energy of the quantum harmonic oscillator
There are several physical effects due to the zero point motion. The most celebrate one is the Casimir force.
May
6
comment What is the mechanism for graphene to conduct so well?
In what sense graphene conducts well?
May
5
comment Finding current in a sphere while given a changing current density
To get from the current density to the current, you should integrate over an area. Your area is the surface of a sphere (as given in the task you have to follow). You should know how to integrate over the surface of the sphere?! Does this involve an integration over $R$???? The answer is no, in fact $R$ is a fixed parameter indicating the size of the sphere (that is why I gave it a different letter from $r$ which was the radial index of the vector field $J$.)
May
5
comment Bloch's theorem
Good question: the short answer is that $k$ is NOT the momentum of the electron. Momentum is not conserved in the presence of a lattice. You will find more information about that when you search for the term pseudomomentum. Also a good exercise is think how to describe a free electron in terms of Bloch momentum (as Bloch's theorem for sure also applies for free space with $V=0$).
May
5
answered Finding current in a sphere while given a changing current density
Apr
6
awarded  Yearling
Mar
12
revised Partition function of bosons vs fermions
added 1 characters in body
Nov
5
comment Are length contraction and time dilation physical?
Both, but it cannot explain this clearer than Einstein does, see here.
Apr
6
awarded  Yearling