11 votes
Accepted

Why is it that, when light travels in a medium, we say it's made of "quasiparticles"?

They're essentially the superposition of photons and the other particles in the medium the photons are interacting with, yes? So clearly they're different from photons. This is true. In a medium, ...
  • 27k
5 votes

What actually causes the electromagnetic field to be quantized?

You're confusing two things: The discreteness or "quantization" of available physical states, and the quantization of excitations in those states. The first thing exists in classical ...
  • 23.5k
4 votes
Accepted

Can the Dirac equation describe the production the of electromagnetic field?

This is something often overlooked in discussions about Electromagnetism: technically, the correct way of solving an E&M problem is to solve the coupled system of equations given by the Maxwell ...
3 votes
Accepted

Does single electron interact with its own electromagnetic field?

This is known as the self-energy problem in quantum electrodynamics. Computing higher order corrections to the two-point function of the electron field (the electron propagator) generates divergent ...
  • 1,470
3 votes

Does an electron's wave function collapse when it emits a photon?

$\newcommand\ket[1]{\left|{#1}\right>} $Electrons do not emit or absorb photons: this would violate conservation of momentum in the electron’s rest frame. Electrons can scatter from photons. And ...
  • 79.4k
2 votes
Accepted

Is there a physical interpretation of the quadrature operators of the quantised EM field in a cavity?

Thanks to a comment from KF Gauss, and by thinking of the comparison with the classical picture, I managed to work out what is happening. $q(t)$ and $p(t)$ evolve according to a classical harmonic ...
1 vote

Can we think of spontaneous emission of a photon from an excited atom as a driven harmonic oscillator problem?

However, I don't see how we get something like the coupling Hamiltonian $\hbar\omega_0(\hat{\sigma}^{+}\hat{a}+\hat{\sigma}^{-}\hat{a}^{\dagger})$ directly from the time evolution picture. We ...
  • 12.5k
1 vote

Feynman's QED 36000 revolutions per inch

Feynman's stopwatch is the phase of a complex number, $e^{i k x}$, where $k=2\pi/\lambda$ is the wavenumber and $\lambda$ is the wavelength of the wave. One rotation of the complex number occurs when $...
  • 41.1k
1 vote
Accepted

Is the QED equation of motion for the wavefunction just the Dirac equation?

Isn't that just the Dirac equation? Or if not, how is it different? Yes, it is the Dirac equation for a charged fermion interacting with the electromagnetic field. So then, are the Dirac and QED ...
1 vote
Accepted

Electron–positron annihilation (Quantum electrodynamics)

Yes, both examples are correct. Do notice that there is another difference between these two examples, though: in the first example, the single photon occurs only inside of the diagram (it is an ...
1 vote

Can the QED action be expressed using geometric algebra?

The problem is, this leads to a Lagrangian that is a multivector rather than a scalar. According Hestenes, traditional expressions like $$ \bar{\psi}\psi = \psi^{\dagger}\gamma^0\psi $$ are ...
  • 3,331
1 vote
Accepted

What is photon blockade?

Definition Photon Blockade is an effect that was brought up in analogy to Coulomb blockade (which refers to the suppression of electric transport due to electronic repulsion). In the photonic case, it ...
1 vote

What is the physical explanation of why a stationary fermion is experiencing zero force from an external static magnetic field?

I think the confusion here is that you intuitively know that a small rare-earth magnet, released between the poles of a large horseshoe magnet, will be pulled linearly (and will snap into contact with ...
  • 805

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