Inspired by this question.

Imagine a sea of liquid helium. Some people like to use submarines, but we've got a better plan.


Impeller pumps are used in jet-skis and can also be used to shift helium around, so we'll assume that the motive power of the craft is dealt with and the internal components are built for the cold. We're also assuming that our own bodies are protected from the ridiculously low temperatures by a MK4 Plot-Device.

As we're not professional helium jet-skiers and possibly in quite shallow helium it may be involved in collisions with the iron-hard water that forms the coastline of our frigid sea.

The question is what hull material is strong enough, light enough and flexible enough to avoid becoming brittle in the extreme cold and easily sustaining damage?

  • $\begingroup$ Relevant XKCD. But it's mainly about how you would die in a boat on such a sea $\endgroup$
    – Secespitus
    Commented Feb 10, 2017 at 10:27
  • $\begingroup$ @Secespitus Lucky I have my MK4 Plot-Device to keep me safe from superfluid creep! $\endgroup$
    – Joe Bloggs
    Commented Feb 10, 2017 at 10:31
  • $\begingroup$ Do you feel it's cheating to heat the hull? If not I think that is the solution. I'd also like to point out that your MK4 Plot-Device is probably a space suit considering every element on a 1 kelvin planet is going to be liquid or solid(AFAIK). $\endgroup$ Commented Feb 10, 2017 at 19:13
  • $\begingroup$ @DubberRucky if that's the only way, but I'd like to do it at as low a temperature as possible. $\endgroup$
    – Joe Bloggs
    Commented Feb 10, 2017 at 19:30
  • $\begingroup$ When designing your jet ski, remember that liquid helium is much less dense than water (about 1/8), so you are going to need a much larger/more buoyant hull in order to float. And if you fall off, you will sink to the bottom. $\endgroup$
    – jamesqf
    Commented Feb 10, 2017 at 20:46

4 Answers 4


I have several ideas on this.

Materials exist in the Cold Regime

Things we consider solid at our normal temperatures lose any elacticity and ductility before getting down to 4K.

But what about materials that could not exist at our temperature? As we found when exploring silicon-based life, direct analogs of organic molecules using Si for C would require cryogenic temperatures to exist. So it might be possible to design something that has enough bonding strength to hold together but not too much as to prevent all movement, at 4K. At significantly warmer temperatures the molecules would fly apart.

This could be true for both elastic (rubbery) and ductile (metal) materials.

Material Withstands Being Brittle

A material may be too brittle, but it can still be very hard. If there are no flaws in the microstructure, it might prove very difficult to chip at all, and any chip you do make will be small spallations (the force only goes down as it spreads out). So a perfect crystal may be just fine in terms of strength.

There are also composite materials. If we can’t combine strong and flexible because there is nothing flexible, you at least still have a combination. This will still provide the effect that cracks can’t propigate farther than the breaking of one fiber. The material boundary will interfere with breaking forces, and even reflect the force.

Avoiding the Brittle Regime?

The linked Physics post discusses the ductile-to-brittle transition

Temperature sort of maps to time and information transfer. At high temperatures, particles/dislocations travel quicker and with more ease than at lower temperatures. Thus information (stress, strain, ...) travels through the sample. There is more time to move around and shift to try to alleviate the applied stress or strain.

So... do you know why diamond is a better thermal conductor than copper, even though it’s not a metal? Phonons. If information about the stresses could be carried off by electron density waves which amplify the actual atom displacements of the material, you can avoid super-brittle behavior.

Maybe that’s not true—but it’s an awsome handwave for a science-based not-dumb story!

Phase Change

Ever hear of nitinol? A number of years ago, superelastic nitinol was all the rage for eyeglasses frames and watch bands.

How does solid metal seem to be rubbery? The stress causes a high pressure which causes a phase transition to a smaller crystal. When released, it pops back to the larger form!

So, the atoms don’t rip free of their bond positions and thus it is not damaged. Engineering this property into a material at 4K might be possible, not with a simple alloy, but with a complex material or even grains that act as a meta-material.


Remember this Answer?

Such low temperatures enable super effects like superconductivity, so maybe the solution is to take advantage of that. In my “plausible supermaterial”, tiny bits the size of mineral grains are held in place via flux pinning, overcoming the normal physical strengths of atomic bonds and making overextension reversable rather than damage.

The 4K temperature makes this easier to acheive, with today’s knowledge. You can get ductile behavior on the scale of individual grains, substituting the flux pinning for normal atomic bonds. Even if it doesn’t have the futuristic ability to fly the units back where they belong, a simple mass of this material will exhibit the ductile behavior of a room-temperature metal, with no accumulation of fracture growth or “work hardening”, even as the individual grains are very hard and brittle.

And that’s the answer I had in mind when I saw your initial comment on the subject: engineer the bulk properties using grain-sized units of normal matter, and superconductive effects between them to bypass the limitations in available atomic bonds.


Firstly, see: https://physics.stackexchange.com/questions/130803/does-extreme-cold-make-everything-extremely-brittle

A heated thermally conductive plastic would work, polymers can be designed to have a huge range of properties, including for cryogenic applications. They also tend to outperform metals in terms of density. I would expect this to still be more delicate than a normal jetski, but not to shatter if you tried to turn.

Bonus points, it will be warm enough to boil the helium and create a leidenfrost effect, which is actually going to be (fractionally) higher friction than helium, but also protect from capillary action covering the whole set up in a layer of helium. The downside of this is that in-taking helium is going to be a lot harder if most of your craft has to be above the boiling point of helium. I guess that's the right place to invoke another MK4 plot-device.

Either way, jetskis rely a lot on drag for steering, so you are going to slide around like an air hocky puck. I recommend directional jets for directional authority.

Source on plastics: https://www.curbellplastics.com/Research-Solutions/Industry-Solutions/Challenging-Environments/Plastics-for-Extreme-Temperature-Applications

  • $\begingroup$ Heating the hull until the whole assembly is a leidenfrost hovercraft? That's hilarious and awesome. $\endgroup$
    – Joe Bloggs
    Commented Feb 10, 2017 at 20:14

Flexibility, or better resilience, is due, at atomic level, to the ability of the molecules building the material to being displaced around their positions without breaking their bonds. This accounts why plastic materials (made of long flexible spaghetti-like chains of carbon atoms) are more flexible than ionic crystals.

When you lower your temperature close to 0 K (and liquid Helium is around there), you are pretty much nailing the molecules of any material to their position, with any displacement resulting in breaking the bond between neighbouring molecules.

Therefore I am afraid that the answer to your question is: no material can remain resilient in liquid Helium


It can't be done

According to this answer everything becomes brittle at 0K.

It also says dense materials tend to become very strong, needing a lot of force to break, however a dense material wouldn't help you as you'd have to build something larger. You would also need to make it even larger to compensate for the fact that liquid helium is one-eighth as dense as water.

It wouldn't look like a jet-ski at that point, and having said that I don't think the submarines would work either as they would most likely sink to the bottom.

The liquid helium would also flow up the jet-ski and freeze you to death and freezing any electrical equipment the jet-ski may have, resulting in you freezing in the middle of a sea with no way of coming back.

Long story short, don't venture into a helium sea.

  • $\begingroup$ Fun question: Is it still a submarine if it's crawling along the ocean floor? ;D $\endgroup$
    – Joe Bloggs
    Commented Feb 10, 2017 at 12:12
  • $\begingroup$ @JoeBloggs Well I guess so. But you might as well call it a submarine car. $\endgroup$
    – user31746
    Commented Feb 10, 2017 at 12:45

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