Absolute zero

Absolute zero is the theoretical limit of coldness in the universe. It represents the state where a system's internal energy reaches its minimum possible value. At this point, the entropy, or the level of disorder, in a system also reaches its minimum.

Scientists use the Kelvin scale to define this boundary. On this scale, absolute zero is exactly 0 K. This value is equivalent to -273.15 °C on the Celsius scale and -459.67 °F on the Fahrenheit scale. Understanding this limit is essential for studying how matter and energy behave under extreme conditions.

We can understand why this limit exists by observing the behavior of an ideal gas. In an ideal gas, the pressure and volume change in a predictable way as temperature changes. If you keep the volume constant, the pressure decreases linearly as the temperature drops. Similarly, if you keep the pressure constant, the volume decreases linearly as the temperature falls.

If you follow these mathematical lines downward, they both reach zero at -273.15 °C. This suggests a physical floor exists. Below this temperature, a gas would have negative pressure or negative volume, which is physically impossible. Therefore, absolute zero is the point where these properties would vanish.

While scientists can approach this limit, they can never actually reach it. This impossibility is explained by the third law of thermodynamics. This law states that as a system approaches 0 K, its entropy approaches a constant minimum. For a perfect crystal, this minimum entropy is exactly zero because the system reaches a state of perfect order.

The law implies that no physical process can reach absolute zero in a finite number of steps. As a system gets colder, removing the remaining heat becomes increasingly difficult. Every cooling attempt becomes less efficient as you get closer to the limit.

Despite the difficulty, researchers have reached incredibly low temperatures. In 2018, scientists at the University of Bremen achieved temperatures as low as 38 picokelvin (pK). A picokelvin is one trillionth of a kelvin.

Even at these extreme levels, matter behaves in strange, "exotic" ways due to quantum mechanics. For example, matter can exhibit superconductivity or superfluidity, where liquids flow without any friction. Particles can also form a Bose–Einstein condensate, a state where they act together in a unique way.

Even at absolute zero, particles are never perfectly still. This is due to a concept called zero-point energy. According to the Heisenberg uncertainty principle, we cannot know both the exact position and the exact momentum of a particle at the same time. Because of this uncertainty, particles must retain a tiny amount of kinetic energy.

This residual motion is a fundamental part of quantum systems. It even explains why liquid helium does not freeze into a solid at normal pressure, even near absolute zero. The zero-point motion of the helium atoms is strong enough to prevent them from settling into a solid structure.

Humans have debated the existence of a minimum temperature for centuries. In 1665, Robert Boyle discussed the possibility of a "supremely cold" state.

In 1703, Guillaume Amontons used an air thermometer to suggest a zero point. He argued that the zero of his thermometer was the temperature where the air's pressure, or "spring," became zero. Later, scientists like John Dalton had different estimates. Dalton even suggested a natural zero of -3,000 °C. It took many years of observation and mathematical refinement to arrive at the modern value we use today.

Absolute temperature is also a vital concept in statistical mechanics. In various mathematical distributions, such as the Maxwell–Boltzmann or Fermi–Dirac distributions, absolute temperature helps determine how particles occupy different energy states. The relative number of particles at a specific energy level depends on the temperature. This connection allows scientists to use temperature to predict how large groups of particles will behave. By studying these limits, we gain a deeper understanding of the fundamental rules that govern the physical world.