Obviously, you couldn’t swim for very long in liquid He, but let’s say that we came up with a substance that was superfluid at reasonable temperature. Could we swim in it?
There was an experiment that suggests that viscosity does not affect swimming speed, but that had to do with a potential slowdown due to an increase in drag. In this case, I think the danger might be that you have no way to impart any momentum to the fluid and propel yourself forward. The fluid would just flow frictionlessly around you.
Thoughts? No, not really. I just saw your name and wanted to say hi. We met, kinda, at the Huntington Beach DopeFest last Sept. and I haven’t seen you around much.
Have a great summer up there in the land of warm days, cool breezes and brilliant sunsets!
Sorry. It’s late. I just repeated what you already knew back at you. :smack:
Since I now have the need to write something further to somehow make up for my lack of worth in this thread so far:
The wiki page says that a superfluid can flow without friction in a closed loop. Perhaps there would be friction if your were in a swimming pool. Not likely, however. Between the lack of friction/viscosity and the conditions needed for superfluidity (i.e., only 4 atoms of He, according to the link at the bottom of the wiki page), I’d say the pool’s closed till next summer.
A superfluid would still presumably have mass. If you displace it by doing the butterfly stroke or whatever, you are moving mass around. Given the old “for every action there is an equal and opposite reaction” it would seem that you would be able to move yourself forward.
In fact, thinking about it some more, it seems like you would be able to swim better since there would be no friction induced drag.
A superfluid does not have viscosity, but it does have pressure. So you could exert a net pressure force on a superfluid, and in turn receive a net pressure force from it, propelling you forward. The viscosity swimming experiments all dealt with fluids thicker than water, but I think it’s probably fair to extrapolate to low-viscosity fluids as well, in which case swimming speed in a superfluid would be the same as in any other fluid. However, I suspect (but am not certain) that it would be easier and less tiring than swimming in an ordinary fluid.
I’m not sure. this site (warning: really ugly background) says that superfluids do not transmit pressure waves in the normal sense. Part of the fluid does, but there are a number of other strange waves.
Furthermore, it says a superfluid can move through “the tiniest cracks” without friction. It seems to me that it would just flow through the cracks between my fingers, and I’d have no meaningful way to push off against it. It has mass, but it seems like you could never grab onto enough of it to get you anywhere.
I don’t know much about superfluids, but unless I’m misreading it, the article says they actually can transmit pressure waves. But in any case, the relevant question isn’t whether a superfluid can transmit pressure waves (meaning longitudinal compression waves – i.e., sound waves), but whether a superfluid can exert pressure. I assume Chronos knows what he’s talking about, so the answer is yes.
As far as this business of flowing between your fingers, I don’t think that matters. Some water probably flows between your fingers whenever you take a swimming stroke, but the small amount of friction involved in that process isn’t what propels you – it’s the water displaced by your hand’s motion that does it.
Look at it this way: suppose you’re floating vertically in a superfluid, and you raise your hand in front of you palm-outwards and then push straight away from your body. As you push, some of the fluid will flow between your fingers. But the fluid in front of your palm isn’t going to jump out of the way of your palm, pass through your fingers, and then jump back to where it was before your palm got there. Why would it? Instread, it will be displaced by your palm – and if you can displace the fluid, then I don’t see any reason in principle why you couldn’t be propelled by it.
I’m not so sure you can in a conventinal sense. You may be able to propel yourself by moving air. Or perhaps lifting the fluid above the fluid line and throwing it in the air.
If you can move fast enough to produce a shock wave as you approach the S.o.S. in that fluid you should be able to push against it.
Correct me if I’m wrong but superfluids are a single quantum state right? That means that if you try and swim in them, either you heat it up so much that it loses it’s quantum coherency and turns into a normal fluid or you cool down enough than you become part of it’s quantum state which means theres no meaningful way to say whether your swimming or not.
If you can lift the fluid and throw it in the air, then you could use the same means to throw it while still underneath the surface. Essentially what you do in swimming is grab a blob of fluid and throw it backward to propel yourself forward.
Hmmm… suppose you were floating in the middle of the room in large spacecraft under microgravity conditions. Could you swim to the walls? Air is a fluid too.
Yes, but more slowly, because there’s not much there to push off of.
True, but assume for the sake of argument that we have a suit of some sort that is sufficiently insulating. One of the properties of superfluids is that they will form very thin skins on any surface they are next to. I assume that in a pool of superfluid, there would be such a layer around a swimmer. Or, alternately, assume a substance that is superfluid at higher temperatures.
Superfluids are all very cold, but what exactly constitutes “very cold” depends on the context. For instance, the interior of a neutron star, at a few million degrees, can be very well approximated as zero temperature. As hot as it is, the Fermi energy is much higher yet, so the effect of the heat is negligible, and neutronium is superfluid.
Of course, there are plenty of other reasons you can’t swim in neutronium, but it’s at least conceivable that there might be some substance which would be superfluid under conditions survivable by a human.
Actually, no. Atoms in a Bose-Einstein condensate are in a single quantum state. Atoms in a superfluid are in a coherent many-body state. The two are related, but not the same. Even at zero temperature, superfluid helium only has a condensate fraction of about 8%.
However, your point remains. If you thrash around, the motion will cause local excitations in the fluid and decrease the superfluid fraction. All that means, though, is you might get a little bit of helium to wet your hand – it won’t stick to the superfluid any better than your hand.
I don’t actually know if swimming would be possible in a superfluid. As Chronos said, if you could generate pressure with your hands, you would be propelled forward. (I’m pretty sure kicking would be useless.) I suspect it would be very hard to get moving, though, as most of the fluid would flow right around your hands. Once you did get moving, though, you’d go until you hit a wall.
The bigger problem would be keeping yourself afloat. Your body is much denser than liquid helium, even with your lungs full of air, so you’d sink like a stone.
kanicbird answered this, but just to elaborate… You can swim in air. It’s just not as effective as swimming in water because the air is less dense. It’s a matter of conservation of momentum. If you throw a handful of air, it has very little mass (compared to water) so your body is not moved significantly the other direction. If you throw a handful of water at the same velocity, you move more because there’s more momentum to balance.
In an inviscid fluid, you can still move the fluid (and hence yourself) if you have an efficient paddle.