zero assoluto -273,15 °C

Absolute zero, because the lowest possible temperature is precisely –273.15 °C

The absolute zero it is the minimum temperature possible in principle: it corresponds to –273.15 degrees Celsius (°C) and represents the physical limit for cold, corresponding to the situation (unattainable in practice) in which the molecules, atoms or particles that make up a physical system are completely still and therefore have no vibrational motion. The concept was based on this Kelvin scale for temperatures, which has its zero right at absolute zero. The average temperature of the Universe is about 3 °C above absolute zero and is due to the energy carried by the cosmic microwave background. The record for the minimum temperature recorded on our planet is –89.2 °Cthat is, 184°C above absolute zero. Around the Earth, where for example the International Space Station orbits, the shaded areas compared to the Sun they reach –100 °C approximately, that is, 173°C above absolute zero.

The Kelvin Scale and the Theoretical Limit of Temperature

The temperatures we indicated before are all expressed in degrees Celsiusthe most commonly used unit. It is a measurement scale that takes as a reference the main physical phenomena of water, the most important substance for life on Earth: it defines 0 °C is the freezing point and 100 °C is the boiling point, thus obtaining the unit of measurement, the degree Celsius or centigrade degree. In some countries, such as the USA, temperature is measured in Fahrenheit °F, a scale with different references.

In the scientific field, however, the Kelvin scale is used. The scale has as its unit of measurement the Kelvinindicated by the symbol Kwhich has the same amplitude as a degree Celsius (1 K = 1 °C) but has zero corresponding to absolute zero: for this reason the temperatures indicated in kelvin are also called absolute temperatures. This is a scale invented by the scientist William Thomsonknown as Lord Kelvinalso influenced by the studies of the physicist James Prescott Joule.

Thomson intended to create a “absolute” systemnot related to the characteristics of the water but based on theoretical calculations, while maintaining theminimum unit of the degree bound to a value of “mechanical work” constant.

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William Thomson Baron of Kelvin, the inventor of the scale of the same name. Credit: T. & R. Annan & Sons; restored by Adam Cuerden – National Galleries of Scotland, via Wikimedia Commons

On the base of relationship Between temperature and volume and pressure of a gasKelvin was then able to calculate the absolute zero (0 K) corresponding to –273, 15 °C, and base a new measurement scale on this value.

Ideal Gases: Pressure, Volume and Temperature

To understand how Kelvin arrived at this number, we need to take a few steps back, prepare a little experiment, and tackle a bit of physics: we’ll try to keep it short.

If we inflate a balloon and then put it in the fridge, it will shrink: the lowering of temperature will in fact lead to a decrease in volume and internal pressure of the balloon. Bringing it back to room temperature, we will see it expand to its original condition (or almost, because in reality nothing is perfect, especially gases). Why does all this happen?

Centuries of studies, starting from the discoveries of the physicist Boyle in the seventeenth century to those of Gay-Lussac and the chemist Amedeo Avogadro in 1800, led the physicist Émile Clapeyron to formulate the ideal gas law:

pV = nRT

This law indicates how the product between pressure (p) And volume (V) of a gas depend directly on the temperature (T): these quantities are the only possible variablesgiven that R is a constant And n is the amount of substancein this case “closed” in the balloon, measured in moles (a unit used in chemistry, corresponding to 6.022 · 1023 atoms or molecules of substance).

There temperature of a bodyin our case a mass of gas, is actually a measure of the average kinetic energy possessed from the atoms or molecules of a substance. This energy is related to the movement of these particles in space, such as vibrations and rotations of bonds between one atom and another in the molecules and of theenergy possessed by electrons.

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The molecules of a gas constantly impact against the walls of the container: a higher temperature means more frequent impacts and with greater kinetic energy. Credit: Becarlson, via Wikimedia Commons

Lowering the temperature therefore means “slow down” the atoms: this lowers the pressurewhich simplifying can be seen as the “force” with which atoms or molecules push from inside the balloon outwards. For this reason, the volume of the balloon decreases with the cold.

The Meaning of Kelvin’s Absolute Zero

Kelvin’s brilliant idea was then this: calculate at what temperature the volume of a gas, progressively decreasing, could reach zero.

Since the substance cannot “disappear” into nothingness, and the volume cannot become negative, the temperature calculated at –273.15 °C or 0 K is therefore the absolute minimum value.

Considering the temperature like a measurement of the “movement” of atomsat the absolute zero these would reach ideally the immobilitya consequence indicated by Kelvin and Boltzmann following their studies.

This assumption, valid for “classical” physics, would however clash with the principles of modern quantum physics, and in particular with the Heisenberg’s uncertainty principle: of a “still” particle it would be possible to measure both the position and the velocity (zero) simultaneously. In quantum physicsat a temperature of 0 K we are therefore talking about a “minimum energy state” of wave-particles… But in this article we will not go into these details. Suffice it to say that in physics there is a principle, the third law of thermodynamicsaccording to which absolute zero is not attainable: you can get arbitrarily close, but nothing in the Universe can have that exact temperature, precisely because from a quantum point of view molecules cannot be perfectly still. Modern experiments allow us to reach temperatures that are truly close to absolute zerowith a record of 38 pK (picokelvin, i.e. thousandths of a billionth of a degree).