Liquid Helium

Copyright © Karl Dahlke, 2023

There are certain quantum effects that you can see with your eyes; no special detectors required, no electron microscope, no mysterious changes in voltage or current. However, a cryogenic lab is required to produce these effects, so you're not going to be doing this experiment at home. Still, you can watch the effects on youtube; they are extremely counterintuitive.

Helium is the second element in the periodic table, with atomic number 2. It has two protons in its nucleus, and two electrons buzzing about. There are two isotopes of helium, He3 and He4. These have one and two neutrons in the nucleus, respectively. The vast majority of helium in the universe is He4, and throughout this chapter, I will assume He4. In fact I will simply say helium, when I mean helium 4 - since some of these quantum effects do not occur with helium 3.

Almost every substance has a melting point and a boiling point. At very cold temperatures it exists as a solid, then it melts to become a liquid, then it boils to become a gas. The familiar example is ice to water to steam. Sometimes the transition temperatures are very low, sometimes they are very high. The smiths of old were barely able to melt iron, at 1535 °C, and only a modern day furnace can bring it to its boiling point, 2750 °C. At the other extreme, hydrogen melts at -259 °C, and boils at -253 °C. Because we live at room temperature, we tend to view it from the other side. Hydrogen liquifies at -253 °C, and freezes at -259 °C. That is really saying the same thing.

There are some substances that don't pass through all three phases. Carbon dioxide changes from a solid to a gas, without becoming a liquid, at -78.5 °C. This is known as dry ice. It keeps food cold, without making a watery mess as it melts.

Glass doesn't have a specific melting point; it gets pliable, and soft, and gradually becomes a liquid. There are other exceptions as well, but most substances transition from solid to liquid to gas.

The quantum effects we are looking for happen when helium is very cold. I will give temperatures in Kelvin, that is, degrees above absolute 0. These are unthinkably cold. Remember that 0 kelvin, absolute 0, is -273 °C, or -459 °F. This is the temperature where atomic motion stops; this is as cold as an object can get. 1 °K is just a whisper above absolute 0, the atoms barely quiver within their framework.

As mentioned earlier, hydrogen liquifies at 20 °K, and freezes at 14 °K. The hydrogen molecule, H2, is so light, and small, and symmetric, that it has almost no attraction to its neighbors. We have to bring molecular motion to a crawl, to turn it into a liquid, and then a solid.

Helium is a tad heavier than hydrogen, with an atomic weight of 4 instead of 2, however, the helium atom is smaller, and it is a perfect sphere. The attraction between adjacent helium atoms is almost nonexistent. You have to bring it down to 4 °K, just 4 degrees above absolute 0, to turn it into a liquid. And here is the first amazing thing about helium, (that is, helium 4), it doesn't freeze. No matter how cold it gets, it remains a liquid. We can never reach absolute 0, that is an unattainable goal, but if we could, our calculations tell us helium would still be a liquid. How is that possible? Doesn't all atomic motion stop? Isn't everything frozen in place? Doesn't it have to be a solid? Quantum mechanics tells us each atom is both a particle and a wave. That means it doesn't have a locked-in location, even at absolute 0. This, combined with the incredibly weak attraction between atoms, allows it to remain a liquid, even when it has no heat energy. we can't bring it all the way down to 0, true, but we have brought helium down to fractions of a degree above absolute 0, and it remains a liquid.

Below 2 degrees, something amazing happens; helium becomes a superfluid. This is nothing like the fluids you have experienced before. There are several ways in which a superfluid is unique. Indeed, three nobel prizes have already been awarded for work on superfluid helium, and more could be forthcoming.

As usual, I wish I could experience these phenomena first-hand. What if I had a sink full of liquid helium, and swished my hand about? Obviously my skin would flash-freeze, and then my flesh soon thereafter, but let's pretend like it didn't, or I was wearing a super-insulated glove. Also let's pretend like the helium isn't creeping out of my sink and onto the floor. As long as I don't move my hand too quickly, there is no friction. I'd say it is just like moving through air, but even air has resistance. This is more like moving my hand through the vacuum of space. There is really nothing there. I can see the liquid parting in front of my hand, to make way, and then filling back in behind, but there is no drag on my hand.

What if I had a swimming pool full of liquid helium, and a super insulated space suit? There is not the slightest microscopic leak in the rubber gaskets that allow my joints to move, for if there was, the helium would leak in and freeze my flesh. I can walk around the shallow end with no resistance at all. You know how hard it is to push through water; this is like moving through the vacuum of space, like moving through nothing at all. However, I can't go swimming in the deep end, because the density of liquid helium is about one eighth that of water. I would sink to the bottom of the pool.

Suppose we have a sea of liquid helium, and a boat to keep us afloat. The liquid would climb the walls of the boat and pour inside, so let's put a cover on the boat, an enclosed craft. How would it move? A paddle wheel, a propeller, a screw, these all pass through the liquid with no friction. They impart no thrust. The only way to move forward is to scoop up some helium in a cup of some sort, and hurl it behind us. The cup has a lid, so the helium doesn't splash out without resistance, then the lid opens on the backstroke and sends the liquid flying. This is an awkward motion to be sure, but it might work.

It's fun to think about a planet between the galaxies, a million light years from the nearest star. Such rogue planets do exist. It is so cold that it has a liquid helium ocean all around it. Don't get excited though, empty space is 3 °K, because of the cosmic microwave background, and helium doesn't become superfluid until 2 °K. It is just an ordinary liquid. Also, even earth doesn't have enough gravity to hold onto helium; it simply floats off into space. This would happen much slower if the helium were cold, and in a liquid state, but it would still happen over time.