Ah, quantum mechanics: endlessly interesting and, despite the fact that we have been working on the subject for about a hundred years, still largely a collection of riddles and questions. One of those issues was the question of what it actually feels like to put your hand in a superfluid. During an experimental study, scientists ‘accidentally’ discovered what it feels like to touch the superfluid Helium-3.

Samuli Autti is the scientist who made the discovery. His research has been published in the journal Nature Communications. “We originally planned to investigate something completely different,” Autti says Scientias.nl. “The original plan was to see what happens when you hit a superfluid very hard with a rod. Traditionally, quantum mechanics has thought that the superfluid should respond to this. What we saw, however, was that the superfluid Helium-3 remained completely passive no matter how hard we tried.”

What is a superfluid?
To properly explain a superfluid, we must first look at what a ‘normal’ fluid is. Let’s look at a glass of water as an example. On a molecular level, the water molecules are attracted to each other enough that individual molecules cannot simply run away. At the same time, there is enough space for each molecule to do its own thing individually. Some water molecules shoot to the left, some to the right and some bump into each other. In the case of a superfluid this is not the case. In a superfluid, the molecules ‘know’ each other where the others are and they behave more as a whole. This is also called a ‘system’.

Extremely cold
One of the special properties of liquid Helium-3 is that it behaves very differently from a ‘traditional’ liquid when you make it very cold. For example, the experiment was carried out at a temperature of one thousandth of a Kelvin degree above absolute zero. At this temperature, Helium-3 turns into a superfluid, with the molecules behaving more like waves than ‘traditional’ particles. For example, in a superfluid all molecules move in the same direction at the same time, instead of all going more or less their own way. This modified behavior makes it possible for these superfluids to move completely without friction.

“So, as we found in our previous experiment, it is not possible to break the quantum state of the superfluid. This means that you experience no pressure when you put your hand in. It’s like your hand is in a vacuum,” Autti explains. “Just like a vacuum, the superfluid Helium-3 cannot conduct heat.”

However, that is not the whole story. Because although the research did not go as predicted, Autti did make another discovery that came as a complete surprise. “What we learned during the experiment was that there seemed to be a very thin layer along the edges of the superfluid.” The researchers therefore conclude that the superfluid Helium-3 is, as it were, packed in a subsystem: a very thin layer of Helium-3 that reacts differently than the rest of the superfluid and can therefore conduct heat.

“Imagine that there is a ‘warm’ surface in the superfluid Helium-3. You feel the heat the moment you touch it. But the moment you don’t touch it you don’t feel anything at all,” Autti explains. By this ‘warm’ surface he refers to the very thin layer around the Helium-3. “Since touching an object involves sensing pressure and heat, we can now say what it feels like to touch Helium-3.” Autti describes the entire experience as follows: “The superfluid would feel two-dimensional. The bulk of the Helium-3 would feel empty, like a vacuum, while heat in a two-dimensional subsystem runs along the edges of the superfluid, and therefore also along your finger.”

Possible consequences
Although touching the superfluid Helium-3 is not a daily occurrence for most people, the conclusion of the study can still be of great value. For example, Helium-3 is one of the most versatile macroscopic quantum systems used in laboratories. The superfluid features in studies of the Higgs mechanism, the Kibble mechanism and quantum information processing research. Autti said: “This research redefines our understanding of the superfluid Helium-3. This could have far-reaching consequences for science.”

What is quantum mechanics?
Fully understanding quantum mechanics is currently not possible – even for scientists. In order to explain quantum mechanics in a somewhat understandable way, The sciences spoken in the past with professor Ton van Leeuwen. He describes quantum mechanics mainly as a set of equations that we use to describe reality. It is the best model we have for this, but the consequences of this model are often counterintuitive. Central to this, among other things, is the idea that particles can behave not only as particles, but also as waves. We see this behavior, among other things, in superfluids, where all particles move in the same direction at the same time. Would you like to know more about quantum mechanics? Then read the interview with Van Leeuwen here.

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