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Results tagged with quantum-mechanics
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user 42937
Quantum mechanics describes the microscopic properties of nature in a regime where classical mechanics no longer applies. It explains phenomena such as the wave-particle duality, quantization of energy, and the uncertainty principle and is generally used in single-body systems. Use the quantum-field-theory tag for the theory of many-body quantum-mechanical systems.
3
votes
Accepted
Explicitly prepare a particle in a state other than the eigenstate
Yes. Take a particle and measure its spin along some axis. Then take a second detector and put it after at some angle $\alpha$ compared to the first one. The second detector will detect $\cos^2 \alpha …
- 2,194
3
votes
"In"- and "out"-states in scattering theory
You are right, in the sense that, if you consider the Hamiltonian eigenstates, the time evolution of the state just yields a meaningless phase factor. The true definition of the asymptotic states star …
- 2,194
6
votes
Kaon oscillations
You say that $K_0$ and $\bar{K}_0$ oscillate because while you create a state with definite strangeness (either $K_0$ or $\bar{K}_0$ from a strong decay) what you observe decaying are mass eigenstates …
- 2,194
3
votes
Accepted
Electron's spin Angular Momentum numeric value
In SI units the electron spin is:
$$S = \frac{1}{2}\hbar \approx 5.272859\times 10^{-35} \text{J}\cdot\text{s}$$
The thing is that you can't use classical mechanics to describe spin and you can't sa …
- 2,194
2
votes
Two-nucleon coupling: symmetrization postulate
Protons and neutrons are not identical, but they would be if electromagnetic interactions would not exists. Since you are studying the nucleus and the strong force is strong, electromagnetic interacti …
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0
votes
Color factor in Breit-Wigner formula
I think the extra $1/N_c$ might come from the definition of $\Gamma_{ud}$.
If $\Gamma_{ud}$ is the decay width of the $W$ into $u$ and $d$ then it is proportional to $N_c$. In your computation you wan …
- 2,194
5
votes
Accepted
Term in Lagrangian Invariant under $SO(n)$ but not $O(n)$?
Take a field theory with a scalar field $\phi_i$ as a multiplet of $O(N)$, where $i$ is the $O(N)$ index.
You can construct two scalar combinations:
$\phi_i \phi^i$ that is both $O(N)$ and $SO(N)$ …
- 2,194
1
vote
Why is Thomson scattering the low energy limit of Compton scattering?
The limit you have to take to go from the Klein-Nishina computation to the Thomson case is the non-relativistic limit.
The KN computation "is "quantum" in the sense that you are treating the photon as …
- 2,194
6
votes
Accepted
Relationship Between Magnetic Dipole Moment and Spin Angular Momentum
$\mathbf{S}$ is the spin operator. It is a vector operator that acts on spinors. It will have three components $(S_x, S_y, S_z)$ and for example if you take the $z$ axis as your spin measurement axis, …
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1
vote
Observables commuting with the Hamiltonian
The Hamiltonian gives you the time evolution operator. So a general observable evolves (in the Heisenberg picture) as
$$ O(t) = e^{i H (t-t_0)} O(t_0) e^{- i H (t-t_0)} \,.$$
If $O$ and $H$ commute, y …
- 2,194