eklhad@ihnet.UUCP (K. A. Dahlke) (01/28/85)
< now that's cold > A friend told me a story yesterday, which seems unlikely. He loves to test my gullibility index. Tell me if this sounds reasonable. Yesterday morning, he was visiting the far side of the galaxy, when he came across a large planet that he hadn't seen before. It was traveling freely, orbiting no star, and its surface temperature was almost absolute 0. He approached the planet, and saw a large lake. He felt like taking a swim, so he left his hovering craft, and dove in. The very thought produces ice crystals in my blood, but he enjoys those brisk refreshing swims. Anyways, it turns out, some unusual physical process had separated the planet's helium isotopes, leaving a superfluid lake. So far, his story is completely plausible, but there is more. He said he couldn't swim in the stuff. His kicking and paddling tentacles slid effortlessly through the liquid, taking him no where. He was afraid he would die, floating forever in the helium bath. Fortunately, he had some initial momentum, which never dissipated. His drifting path lead to an island (which of course had a very thin coating of liquid helium). He pushed off from this island, drifted back to the mainland, and boarded his awaiting craft. Imagine, an island, just where and when he needed it, and pushing off in exactly the reverse direction. Picture a bullet moving through this lake, or anything that might exceed the speed of sound in liquid helium. The projectile has a vacuum behind it, and pressure ahead of it, and it must surely slow down. A similar thing would happen when he moved his appendages through the liquid, wouldn't it? Even if splashing were ineffective, he could use any container. Dip the container in helium, throw the contents behind, and repeat. By Newton's third law, you ought to be on your way. Of course this option might not occur to my friend, since it is very difficult to cup a tentacle, as you know. He could have taken mouthfuls of helium and spit them out, but the stuff might have spread throughout his entire insides. Is he making this story up, or did he really encounter these difficulties while swimming in superfluid helium yesterday? What can a swimmer expect from such an exercise? -- Karl Dahlke ihnp4!ihnet!eklhad
jin@hplabs.UUCP (Tai Jin) (02/01/85)
wouldn't the liquid helium flow out of the lake? anyway, it should be less dense than water (yes?), so i would imagine that your friend would have a difficult time staying afloat.
gvcormack@watdaisy.UUCP (Gordon V. Cormack) (02/06/85)
I don't buy the statement that nothing will slow down when moving in the lake. Even if the liquid has no viscosity, it has mass and must be accelerated to make room for an object moving through it. Thus, the kinetic energy of the object will he converted to kinetic energy in the liquid. -- Gordon V. Cormack CS Department, University of Waterloo gvcormack@watdaisy.uucp gvcormack%watdaisy@waterloo.csnet
merrill@rex.DEC (02/09/85)
> Thus, the kinetic energy of the object will he converted to kinetic > energy in the liquid. > Gordon V. Cormack CS Department, University of Waterloo Even in REAL water WAVES do not slow down noticably! An object in motion in a fluid without viscosity behaves like a wave. Assuming frictionless and perfectly elastic collisions, the balls (Neutonian "liquid") left behind have NO motion and hence have absorbed neither energy nor momentum. It DOES sound rediculous, doesn't it, but not too much more than swimming in liquid helium. Richard M. Merrill M.I.T.