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From: eklhad@ihnet.UUCP (K. A. Dahlke)
Newsgroups: net.physics
Subject: Swimming in Liquid Helium
Message-ID: <187@ihnet.UUCP>
Date: Mon, 28-Jan-85 15:46:34 EST
Article-I.D.: ihnet.187
Posted: Mon Jan 28 15:46:34 1985
Date-Received: Tue, 29-Jan-85 07:22:07 EST
Organization: AT&T Bell Labs, Naperville, IL
Lines: 46

<  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