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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
Mar
20
comment Product of exponential of operators
This shows the if but not the only if...
Mar
17
comment A very elementary question regarding force of friction
Good answer....
Mar
16
comment Most efficient type of heat pump
it's a fake. Not even Maxwell's daemon can beat the second law.
Mar
11
comment Most efficient type of heat pump
If we assume that the second law is true (which all patent examiners do) then it is not possible to beat the Carnot efficiency.