English translation of the original Chinese article. Publication dates and the extent of recovered text are preserved. Figures retain their original labels. Read the Chinese original.
Helium, the second element in the periodic table, is a familiar name to anyone who has studied chemistry—who has not memorized “hydrogen, helium, lithium, beryllium, boron”? Yet it occupies a surprisingly small place in most people’s knowledge. For many, helium is simply the lightweight gas used to fill helium balloons and airships, or the “unreactive” gas used to protect welding processes and crystal growth. Some may also know that helium is a product of fusion reactions in stars and radioactive decay on Earth, but these seem far removed from everyday life. Its important role in today’s cutting-edge scientific research is even less widely known.
This low profile is not because people deliberately overlook helium; it is simply very unobtrusive. It is scarce and reluctant to become “entangled” with other elements, a lone wanderer that comes and goes without a trace. Under ordinary conditions, helium is a colourless, odourless gas with a density much lower than that of air—about one seventh as dense. Like another lightweight contender, hydrogen, it can therefore easily escape Earth’s gravitational pull into the vastness of space. Its concentration in air is extremely low, just five parts per million by volume (5 ppm), hence its description as a rare gas. Meanwhile, because of its stable atomic structure, helium almost never reacts chemically with other elements, which is why it is also called an inert gas. This prevents it from remaining on Earth in compounds as hydrogen does. Together, these two factors make helium very scarce on Earth, even though it is the second most abundant element in the universe, accounting for 24% of the total mass of its elements, second only to hydrogen’s 75%.
The Discovery of Helium
Because helium is so unobtrusive, people discovered it quite late. In the mid-nineteenth century, scientists developed techniques for determining chemical composition through spectral analysis. Each kind of atom has a distinctive pattern of spectral lines, determined by its energy-level structure. Like human fingerprints, these patterns allow people to identify individual components within the complex spectrum of an unknown substance. In 1868, observations of the spectrum during a total solar eclipse revealed a completely new “fingerprint”, unlike the spectral lines of any element then known on Earth. This was clearly a new element discovered on the Sun, so it was named helium after Helios, the Sun god. Nearly 30 years later, in 1895, people finally detected its faint presence in radioactive minerals on Earth, marking its formal discovery. Only then did people learn that helium existed not just “in the heavens”, but also “in the human world”.
A Permanent Gas?
The discovery of helium coincided with the rapid development of thermodynamics in the second half of the nineteenth century. Thermodynamics and refrigeration technology drove each other forward, repeatedly breaking low-temperature records and advancing towards absolute zero, 0 K (zero kelvin, or −273.15 degrees Celsius; a temperature difference of 1 K equals a difference of 1 °C). One after another, the “permanent gases” once thought difficult to liquefy became quietly boiling liquids in complex experimental apparatus. In 1898, the newly invented Joule–Thomson throttling refrigeration technique was used to liquefy hydrogen at 20 K. Helium then became the last gas that had not been liquefied.
A fierce race to liquefy helium soon began between James Dewar’s team at the Royal Institution in Britain, Kamerlingh Onnes’s team at Leiden University in the Netherlands, and other groups. Dewar’s team had been the first to liquefy hydrogen and had invented advanced low-temperature equipment and techniques such as the Dewar flask and Joule–Thomson throttling valve, giving it a clear head start. Onnes’s team, however, devoted itself to upgrading equipment and improving procedures. After ten years of patient effort, it finally liquefied the last “permanent gas” in 1908 using throttling expansion, winning the race. Onnes himself consequently received the 1913 Nobel Prize in Physics.
The successful liquefaction of the last naturally occurring gas marked the beginning of a new era in experimental low-temperature physics. Helium liquefies at 4.2 K, allowing researchers to study the properties of all kinds of substances conveniently at a temperature just 4.2 degrees above absolute zero. This laid the foundations for pioneering work in many fields of cutting-edge fundamental science today. In 1947, Collins built the first commercial helium liquefier. Access to low temperatures was no longer a privilege reserved for a handful of advanced laboratories, and experimental research in low-temperature physics quickly spread around the world.
In fact,the largest single use of helium at the time of writing was neither festive balloons nor industrial shielding gas, but as a coolant. For example, magnetic resonance imaging scanners in hospitals need liquid helium for cooling. Inside the scanner’s large cylindrical housing is a superconducting electromagnet that supplies the strong magnetic field needed for magnetic resonance. The electromagnet is a coil wound from superconducting material. When immersed in liquid helium, the coil becomes superconducting and its electrical resistance disappears. It can then carry the enormous current needed to generate a strong magnetic field without producing any heat.
A Permanent Liquid?
After obtaining liquid helium, people immediately hoped to solidify it. What they did not know at the time was that the traditional method of cooling a liquid by continuously removing its vapour was destined to fail. This newly created liquid stubbornly refused to freeze, becoming the last “permanent liquid”.
Only many years later did people realize that the reason this method failed was precisely the quantum-mechanical behaviour of helium atoms. We now know that, because of the uncertainty principle of quantum mechanics, atoms and molecules are not completely motionless even at absolute zero, when they occupy their lowest-energy state. These particles still vibrate continuously and therefore retain some kinetic energy, known as zero-point energy. For most other substances, the influence of this zero-point energy is negligible. For the solitary helium atoms, however,the weak van der Waals forces between them cannot overcome their own zero-point energy. In other words, even with no thermal energy supplied from outside, the solid helium that might otherwise crystallize is melted by its own zero-point energy. Near absolute zero,liquid helium can solidify only when sufficient pressure—about 25 atmospheres—is applied to provide additional interaction. Precisely because helium can remain liquid indefinitely when the pressure is insufficient, its unexpected quantum properties become apparent at sufficiently low temperatures. A liquid in which quantum effects are significant is called a quantum liquid.
References
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[2] 章立源, Beyond Freedom: Remarkable Superconductors, Science Press, Beijing, 2005.
[3] A. M. Guénault, Basic Superfluids, Taylor & Francis,London and New York, 2003.
[4] T. M. Flynn, Cryogenic Engineering, Marcel Dekker,New York, 2005.
[5] F. Pobell, Matter and Methods at Low Temperatures,Springer, Berlin, 2007.
[6] E.R. Dobbs, Helium Three, Oxford, NewYork, 2000.
[7] D.Vollhardt and P. Wölfle, The SuperfluidPhases of Helium 3, Taylor & Francis, London, 1990.
[8] M.Tinkham, Introduction toSuperconductivity, 2nd ed, McGraw-Hill Book, 1996.
[9] M.R. Norman, The Challenge of Unconventional Superconductivity. Science, 332,196 (2011).
[10] T.Mizushima, Y. Tsutsumi, T. Kawakami, M. Sato, M. Ichioka and K. Machida,Symmetry-Protected Topological Superfluids and Superconductors – From the Basicto 3He. J. Phys. Soc. Jpn.,85,022001 (2016).
[11] Y.Okuda and R. Nomura, Surface Andreev bound states of superfluid 3Heand Majorana fermions. J. Phys.: Condens.Matter, 24, 343201 (2012).
[12] Theofficial website of the Nobel Prize. https://www.nobelprize.org/nobel_prizes/physics/
The body text has been restored from a preserved WeChat manuscript with the same title, retaining the original website’s publication record. The old WeChat promotional layout has been removed. Available original illustrations have been restored.
Editorial note: Restoration edited on 2026-10-10: Restored as a historical article from 2018. References to “today” retain the context of the original period; the scientific views have not been rewritten.
