Timeline for What is the difference between a photon and a phonon?
Current License: CC BY-SA 3.0
9 events
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| Dec 31, 2012 at 20:49 | comment | added | emarti | @Slaviks, yes, I was just using a periodic crystal as an example of something that doesn't even have continuous translational symmetry. Air and liquids has no problem carrying sound. | |
| Dec 31, 2012 at 20:47 | history | edited | emarti | CC BY-SA 3.0 |
added 60 characters in body
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| Dec 31, 2012 at 18:34 | comment | added | argopulos | @Slviks: the existence of an ether is surely one of crucial differences, but it's not the only one. Surely the ether (+Galilean/Poincare' symm.) explains the existence of gapless scalar modes (the phonons)that are derivatively coupled; on the contrary, the existence of a massless (spin-1) excitation of the EM field doesn't come from either a background or gauge invariance per se' (gauge inv. is respected for massive vector bosons via the Stuckelberg trick that introduces an extra dofs). It rather comes from taking only two polarizations, Lorentz symmetry and locality. | |
| Dec 31, 2012 at 15:50 | comment | added | Slaviks | @argopulos +1; a nice perspective! Is that right that the difference essentially boils down to whether there is ether (hence, phonons) or not (hence, photons)? | |
| Dec 31, 2012 at 13:13 | comment | added | argopulos | @Slaviks: yes, the existence of photons doesn't require periodicity of the background (in fact they exist for fluids and other gelly solids too), but it does require instead the spontaneous breaking of 3 spacetime symmetries, namely the boosts (or translations). This is a very basic fact that boils down to the fact that a background breaks the invariance under Galilean or Poincare' boosts. Local boosts corresponds to gapless excitations because they cost no energy since the laws of physics are indeed Galilean (or Poincare') invariant. | |
| Dec 31, 2012 at 10:55 | comment | added | Slaviks | I beg your pardon, but the existence of phonons does not require periodicity or any other symmetry breaking. Every continuous solid possesses 3 branches of gapless acoustic phonons. | |
| Dec 31, 2012 at 10:51 | comment | added | argopulos | @emarti: I think phonons are NG bosons in periodic crystal too since spacetime boosts (and traslations, rotations) are spontaneously broken by the lattice. The residual discrete symmetries constraints the NG boson low-energy interactions. In fact, the theory of phonons is Galileo (or Poincare') invariant rather than be symmetric only under the discrete unbroken subgroup of the lattice. See my answer below. | |
| Dec 31, 2012 at 2:33 | comment | added | Freya Natasha Geneviève Paré | Wow! That's really cool thanks for taking the time to explain that. | |
| Dec 31, 2012 at 1:06 | history | answered | emarti | CC BY-SA 3.0 |