When replying to any post on any forum, one must use the "reply" function in order for it to reply correctly to the person one meant to reply to.
IF one simply uses the already there "quick reply box" to reply, even when in threaded mode, the post, when posted, will appear as a reply to the original post in the thread, and not the post the reply was meant for.
I have noticed quite a few replies that made no sense in context because of this; the posters apparently have not figured out that one must use the "reply" function to avoid having this happen.
Helium II is a superfluid, a quantum mechanical state of matter with strange properties .
The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and hundred of times that of copper. This is because heat conduction occurs via a quantum mechanism.
Second sound is a quantum mechanical phenomenon in which heat transfer occurs by wave-like motion, rather than by the usual mechanism of diffusion. Heat takes the place of pressure in normal sound waves. This leads to very high thermal conductivity. It's known as "second sound" because the wave motion of heat is similar to the propagation of sound in air.
Sound waves are fluctuations in the density of molecules in a substance; second sound waves are fluctuations in the density of phonons. Second sound can be observed in any system in which most phonon-phonon collisions conserve momentum. This occurs in superfluids and in dielectric crystals when Umklapp scattering is small.