Helium freezes at absolute zero

Science in dialogue

Why can it never get colder than minus 274 degrees Celsius on earth?

Because that's the natural lower limit for the temperature. Strictly speaking, it is minus 273.15 degrees Celsius. Nowhere in the universe can it get colder. To understand this, you first have to know what temperature is.

The temperature of an object is caused by the disordered movement of its particles. The lower the kinetic energy of the particles, the lower the temperature of the object. Theoretically, absolute zero is reached when the kinetic energy is equal to zero. Quantum theory shows that there is still a zero point energy that is independent of temperature.

The absolute zero point also forms the zero point of the Kelvin temperature scale and corresponds to minus 273.15 degrees Celsius. In practice it would be possible to come as close as desired to this absolute zero point. However, according to the third law of thermodynamics, it can never be exactly reached due to quantum physical phenomena. A law of quantum physics (Heisenberg's uncertainty principle) says that it is fundamentally impossible to precisely measure the location and the speed of a particle at the same time. But that is precisely what would be possible if a particle no longer had any kinetic energy at all.

In the laboratory, researchers can now reach temperatures of one nanokelvin, i.e. only one billionth of a Kelvin above absolute zero. So far, however, this has only been successful with samples that consist of a few particles. Larger samples that are suitable for practical use can, however, be cooled down to a millikelvin (one thousandth of a Kelvin).

In basic research, the goal of using the cold records is to investigate the properties of matter at these temperatures. For example, some materials conduct electricity without resistance at low temperatures. Physicists refer to this phenomenon as superconductivity. In addition to the loss-free conduction and storage of electrical energy, extremely low temperatures enable a variety of other applications. In airplanes or radio telescopes, for example, sensors are cooled so that the disordered heat movement of the particles does not interfere with the measurement. Also on the rise are electrical circuits with individual electrons (individual electronics), which are faster and consume less power than current semiconductor circuits. However, such a circuit can only work if the individual electrons are protected from collisions with other particles by cooling.

The cooling down to just above absolute zero is a multi-stage process and takes place in so-called cryostats. First, the sample is cooled to 77 Kelvin with liquid nitrogen. With the help of liquid helium, four Kelvin can be achieved in the next step. The gases helium and nitrogen are each cooled down to their liquefaction using a method that works on the principle of an ordinary refrigerator (refrigeration machine with gas circuit cooling). During a subsequent separation cooling, helium-3 and helium-4 - both differ only in the number of their neutral core particles - are separated from each other, whereby the heat of the solution is removed. The temperature of the sample drops to a few millikelvin. In order to finally penetrate into the nanokelvin range, the physicists use magnetic fields to change the magnetic order in the sample. This removes further energy from it (adiabatic demagnetization).

Compared to the temperatures achieved in the laboratory, even the coldest areas on earth are still very warm. The lowest temperature ever recorded was minus 89.2 degrees Celsius - that is, 183.95 Kelvin. The value was measured in 1983 in the Russian Vostok station in Antarctica. This place is known as the global cold pole.

It can't get much colder on earth because the sun warms up the earth and the earth's core and unfortunately also our civilization radiate heat and currently even a civilization-related greenhouse effect is additionally heating the earth. But even in places in space that are far away from warmth-giving stars, it cannot be colder than three Kelvin. This is how “warm” the so-called background radiation is, which is a remnant of the Big Bang and permeates the entire universe.

This question was answered by Prof. Dr. Paul Seidel, head of the low-temperature physics working group at the University of Jena.