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Apr
10
revised Do Standard Model Yukawa couplings depend on the gauge choice?
added 2 characters in body
Apr
8
comment Which came first the decay or the emission?
I don't think I understand your statement "...the state of the hydrogen atom evolves according to the unperturbed Hamiltonian and hence always remains in the excited state without emitting anything." Because the way I interpret it, it sounds false. When the hydrogen atom is coupled to the quantum electrodynamic field, the excited 2s state, say, is no longer a stationary state of the system. And so it will evolve to something else, even when there's nothing around initially. Maybe a clarification is in order?
Apr
7
comment Which came first the decay or the emission?
@L.Levrel In both cases, the form of the interaction $\hat{V}_0$ is fundamentally the full quantum electromagnetic field. Spontaneous emission is the process $|0 \gamma,\text{atom}^*\rangle \rightarrow |1 \gamma,\text{atom}\rangle$. Stimulated emission, on the other hand, is the process $|\text{many} \gamma,\text{atom}^*\rangle \rightarrow |\text{(many+1)} \gamma,\text{atom}\rangle$. The difference comes from the relevant matrix elements $\langle 1\gamma|\hat{V}_0 |0\gamma\rangle$ vs $\langle (\text{many}+1)\gamma|\hat{V}_0 |\text{many}\gamma\rangle$.
Apr
6
revised Which came first the decay or the emission?
deleted 9 characters in body
Apr
5
answered Which came first the decay or the emission?
Feb
20
awarded  Notable Question
Feb
18
asked What is the name of the basis that uses objects of definite parity?
Feb
5
awarded  Popular Question
Jan
17
awarded  Notable Question
Jan
17
awarded  Popular Question
Jan
13
revised What are anomalous threshold singularities
deleted 2 characters in body
Jan
13
answered What are anomalous threshold singularities
Jan
9
awarded  Nice Answer
Dec
16
comment Proof of Yang's theorem
@LubošMotl that's true. But I must be blind; which amplitude do you refer to that is bilinear in epsilon-V?
Dec
14
awarded  Popular Question
Dec
11
comment Different Signs in Yang Mills Gauge Transformations
In the end, I found out that this discrepancy amounts to a change of sign in the gauge coupling constant, or in the sign of the vector potential. It doesn't affect final results for computations of physical quantities.
Oct
2
accepted What is the chemical symbol for Mu-mesic atoms?
Sep
11
awarded  Nice Question
Aug
24
awarded  Revival
Aug
15
comment What are Maximally Helicity Violating (MHV) Amplitudes?
Neat. What is it that makes all amplitudes with all helicities the same sign (or with just one opposite sign) vanish?