Absolute zero

There is a point that is very cold.

It is the coldest thing possible. We call it absolute zero.

At this point, things have the least energy. It is like a machine that has stopped.

Scientists can get very close to it. But they can never truly reach it.

As things get colder, it is harder to cool them more. Even at this low point, tiny bits of matter still move a little. This is called zero-point energy. It is a very strange and cool part of our world.

Imagine the coldest place in the universe. This limit is called absolute zero.

At absolute zero, a system has its lowest possible energy. We can measure this on the Kelvin scale. On this scale, absolute zero is 0 K. This is the same as -273.15 °C.

Scientists can get very close to this cold point. In 2018, experts reached 38 picokelvin. That is a tiny fraction above zero. But they cannot actually reach it. The third law of thermodynamics says we cannot get there. Cooling things down becomes harder and harder as they get colder.

Even at this limit, tiny bits of matter are not still. They have a small amount of motion. This is called zero-point energy.

At these low temperatures, matter acts in strange ways. It can show things like superfluidity. This is when a liquid flows without any friction. Matter can also become a Bose–Einstein condensate. This is a special state where particles act together.

Absolute zero is the coldest temperature possible in our universe. It is a special limit where a system reaches its lowest possible internal energy.

Scientists use the Kelvin scale to measure this extreme cold. On this scale, absolute zero is exactly 0 K. This is the same as -273.15 °C on the Celsius scale. It is also -459.67 °F on the Fahrenheit scale. Understanding this limit helps us learn how energy and matter behave.

We can understand this limit by looking at how gases act. In an ideal gas, the pressure and volume change with temperature. If you cool a gas, its pressure and volume drop in a straight line. If you follow that line down, it hits zero at -273.15 °C. This suggests there is a floor to how cold things can get. Below this point, a gas would have impossible negative pressure or volume. Therefore, absolute zero is the point where these values would vanish.

People have wondered about the coldest possible temperature for a long time. In 1665, Robert Boyle wrote about the idea of a minimum cold.

Later, in 1703, Guillaume Amontons used an air thermometer to study this. He thought the zero point was where the air's pressure became nothing. Other scientists like John Dalton had different ideas for a long time. Some even thought the zero point was much lower, around -3,000 °C. It took many years to agree on the true value.

Even though we can get close, we can never actually reach absolute zero. This is because of the third law of thermodynamics. This law says that as we get closer to 0 K, entropy reaches a minimum. Entropy is a way to measure the order of a system. Removing heat becomes harder and harder as the temperature drops. No physical process can reach absolute zero in a finite number of steps. In 2018, scientists at the University of Bremen reached 38 picokelvin.

This is incredibly close, but it is still not zero.

When matter gets this cold, it acts in very strange ways. Particles can show things like superconductivity or superfluidity. In superfluidity, a liquid can flow without any friction at all.

Matter can also form a Bose–Einstein condensate. This is a state where particles act together in a special way. Even at this limit, particles are not perfectly still. They have something called zero-point energy. This tiny bit of motion is required by the Heisenberg uncertainty principle.

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.