Talk:Superfluid helium-4

message: An encyclopedia is written for the educated layman, not the specialist. This article is terrible in that regard. Sentences like "Landau thought that vorticity entered superfluid 4He by vortex sheets, but such sheets were shown to be unstable" is totally devoid of context or meaning for anyone but the professional scientist. After reading this article it is hard to understand anything about what superfluidity actually is, which I only understood after checking popular science writing on the matter.
59.93.184.227 (talk) 06:08, 6 August 2009 (UTC)
replies: text: This is still a major problem -24.130.65.122 (talk) 21:49, 11 January 2010 (UTC)
replies: text: Yeah, well, the theory behind the mechanics of superfluidity? Kind of a technical subject. But I was able to comprehend the majority of the article, including, most importantly, the introduction, and I have no expert knowledge on this subject. I'm not going to remove the flag at the top of the page, but I disagree with your views. --75.111.25.142 (talk) 02:55, 28 May 2011 (UTC)
thread_title: Poorly Written for Encyclopedia

message: "(It is thus impossible to set up a temperature gradient in a superfluid, much as it is impossible to set up a voltage difference in a superconductor.)"
This sentence is misleading. For example, if you put an ice cube and a hot iron on either side of a container with a superfluid, that is by definition a temperature gradient (whether or not there is a temperature gradient *inside* the fluid). However, heat conduction is limited by the speed of light, and thus temperature can't change instantaneously, and so temperature gradients must also exist inside superfluids - if only for an unmeasurable amount of time. 
Similarly for a superconductor, put a battery's leads on either end of a superconductor. Again the speed of light is a limitation. Fresheneesz 03:37, 30 May 2006 (UTC)
replies: text: I would say that's a trivial distinction.::Although I could be wrong, the quote in dispute seems to contradict Superconductor#Superconductors_in_popular_culture which states "superfluid helium has immense but finite heat conductivity" as well as helium which says that heat flows at a rate of 20 m/s through helium II. The article also states that helium's thermal conductivity is non-infinite referring to it instead as: "several hundred times that of copper". Unless I've made a mistake it seems that the section in question should be changed immediately.-Fiber B 00:35, 22 August 2007 (UTC)
thread_title:  temperature gradient 

message: Slashdot recently linked to an article that prominently linked to this page on Wikipedia so keep your eye out for vandals. --⁪frothT C 07:43, 4 January 2007 (UTC)
thread_title:  Possible vandalism alert 

message: I noticed that this article mentions Lene Hau slowing light to 17 meters per second with a superfluid the article for Lene Hau reads by 17 meters per second. Slowing a light beam to 17 meters per second would be much more extraordinary than by 17 meters per second but I am not sure which statement is true.162.84.60.14 00:48, 12 September 2007 (UTC)
thread_title:  slowing a beam of light 

message: The statement about slow light in this entry is misleading.  The experiment mentioned (Lene Hau) did slow light in a superfluid medium (Bose-Einstein condensate) but the superfluidity of BEC plays no role in why the light slows down.  Light slowing has been performed in non-superfluid media, and the two phenomena are not linked, they just happen to both be possible in a BEC.  I'm new to editing wiki, and so won't make any changes to the entry until I understand how to go through the process and can make a good edit, but this is a heads up to anybody watching this page.  Maybe more detail in under a new heading (slow light) would be appropriate?
Lazerbrane 15:08, 3 October 2007 (UTC)
thread_title:  Error in statement about superfluidity and slow light 

message: The figure references the Rollin film article which says liquid helium forms a film 30 nm thick, but this article says it forms a layer one atom thick.  Obviously one atom of helium is nowhere near 30 nm.  I'm inclined to believe the 30 nm figure, but I don't understand why such a thick film is "needed" (energetically favorable). 70.15.116.59 03:38, 23 October 2007 (UTC)
thread_title:  one atom or 30 nm 

message: regarding a rotating container full of superfluid, what if the fluid is already rotating with the container as it is cooled below the point where it becomes superfluid? does it stop rotating? of so then where does its angular momentum go? also how exactly is the rotation of the superfluid measured? just-emery (talk) 00:23, 11 May 2009 (UTC)
thread_title:  rotating container of superfluid 

message: Perfect vacuums are impossible to create here on earth but I seem to recall reading that a molecule floating in superfluid behaved exactly as it would in a perfect vacuum. If true then doesnt this imply that atoms in a superfluid behave as though they had zero effective mass? just-emery (talk) 04:15, 3 July 2009 (UTC)
thread_title:  Zero effective mass 

message: IRAS page mentions about 475 liters of superfluid helium, not 750. Which is correct?  —Preceding unsigned comment added by 81.219.148.15 (talk) 22:22, 30 July 2009 (UTC)
thread_title:  IRAS 

message: The simple "helium-4" notation is far easier to read in narrative.  The superscript notation may be good for concise formulas, but not for narrative.  I don't have time to fix this right now, but if someone else can it would be helpful.  Thanks!  -- DBooth (talk) 17:48, 7 July 2010 (UTC)
thread_title:  Ouch!  Prefixed-superscript notation like "{{SimpleNuclide|Helium|4}}" is too hard to read! 

message: suggests that electrically conducting superfluids (as in a neutron star) are superconductors. Could some one say something about that in the article ? Are there any electrically conducting superfluids we can use on earth ? Rod57 (talk) 03:38, 25 February 2011 (UTC)
thread_title:  Conducting superfluids are superconductors 

message: For example, a paddle or propeller would meet no resistance in superfluid helium, and would fail to push a boat. However, once in motion (for example, when pushed from the side of a pool), a hypothetical boat would continue to move, without slowing, until it reached the other side.
:I'm pretty sure this is false. A propellor doesn't depend on any kind of dissipation in order to work, so it would work just as well in a perfect superfluid. You turn the propellor blades, the blades push the superfluid (because the superfluid can't actually penetrate the blades), and the boat moves because of the reaction force. There's no friction or dissipation involved, just normal force. Superfluids still feel the normal force.
:If I'm totally wrong about this, there should be a reliable source that says so. —Keenan Pepper 06:11, 1 June 2011 (UTC)
replies: text: Well, now you've got me. I know that lift vanishes in He-II in the limit of low velocity (we neglect the normal non-superfluid component), but this may be classic aerodynamic lift that doesn't just rely on brute angle of attack. The experiment is a well known one done in 1957 by Craig and supervised by Feynman . You can make an argument that a simple flat object dragged though He-II must have some drag because it pushes He-II out of the way. However, if that were true, all those experiments with He-II circulating around small objects in in a ring wouldn't work-- the simple drag of this kind would move the objects and stop the helium. Would it not? There are discussion on the web about whether or not your could swim in He-II (in the limit of perfect superfluidity) and the consensus is that you can by jet-siphon effect (grab some in a closed container and push it behind you) but with surfaces that the He=II can "get around" perhaps not. Probably I should remove this statement until I find something definitive. SBHarris 18:50, 1 June 2011 (UTC)
thread_title:  Propellor example 

message: The article mentions super-fluids as having infinite thermal conductance, but this should be seemingly impossible as it would nesecitate thermal transfer at faster than c. can someone please explain how it is possible to have infinite thermal conductance without violating the laws of physics?  — Preceding unsigned comment added by 68.48.169.133 (talk) 21:45, 31 October 2011 (UTC)
replies: text: Actually, heat conductivity is defined to be thermal diffusivity × heat capicity × density. Thermal diffusivity has the units cm2/s rather than cm/s so if you have a glass sheet with a certain temperature difference between one side and the other, then a glass sheet twice as thick with the same temperature difference will actually take 4 times as long to conduct heat from one side to the other, not twice as long. In the case of a superfluid conducting heat at the speed of light, the larger the size of the superfluid, the larger is the speed of conductance times the distance the heat is conducted so the heat conductivity approaches infinity as the size of the superfluid approaches infinity. Blackbombchu (talk) 23:21, 31 July 2013 (UTC)
replies: text:  Not buying your handwaving. The glass sheet with the constant temperature difference on one side to the other is subject to Fourier's law. At steady state (that is what we're talking about, no?) it will have a linear and constant temperature gradient across it, and if it is twice as thick it will conduct heat half has fast (per unit area). The units of diffusivity are irrelevant. Conductivity is also diffusivity times volumetric heat capacity, which is just 1/V times dQ/dT. You see immediately that the units for k come out to dQ/dt per meter per K, as advertized. Heat transfer per time is proportional to temp difference and inversely proportional to a distance, which is the area divided by the insulation thickness. SBHarris 03:54, 1 August 2013 (UTC)

text: Thermal conductance has nothing to do with how fast, in terms of m/s, i.e. a speed, heat is transfered. The common heat equation is not Lorentz invariant and as such is not compatible with relativity. The speed of heat is infinite according to that equation, regardless of the value of the thermal conductance. But it doesn't matter, it's still a good approximation for describing the temperature in most systems. Lastly, thermal conductance has to do with the rate of how the temperature stabilizes throughough the material. If one heats a spot of a material, the temperature in the whole material is affected instantly once the heat has been applied. Thermal conductance gives us a measure of how quickly (in terms of time), the rest of the material will achieve a homogeneous temperature (strickly speaking it should never be reached, but we can use reasonable reasonable thresholds to consider what a homogeneous temperature distribution is.)  — Preceding unsigned comment added by 176.163.163.86 (talk) 10:46, 14 August 2018 (UTC)
thread_title:  infinite thermal conductance. 

message: Under the heading Film Flow, by critical speed, did the article mean the speed limited by quadratic resistance? Blackbombchu (talk) 23:20, 31 July 2013 (UTC)
thread_title:  What critical speed means 

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