Absolute zero is the theoretical lowest possible temperature, defined as 0 Kelvin (K) or −273.15°C (−459.67°F). At this temperature, a system reaches its minimum thermal energy, and the motion of its constituent particles is reduced to the lowest possible level allowed by quantum mechanics.

What Happens at Absolute Zero?

• Minimum Particle Motion: At absolute zero, atoms and molecules do not move in the traditional sense. Their kinetic energy, which is a measure of motion, is at its lowest. However, due to the Heisenberg Uncertainty Principle, they still retain a tiny amount of zero-point energy — meaning they can never be completely at rest.
• No Heat Transfer: Since heat flows from hotter to colder objects, and there is nothing colder than absolute zero, no heat can be extracted from a substance at this point.

Why Is It Important?

Absolute zero serves as the foundation of the Kelvin temperature scale, which is used in scientific measurements because it starts from a true zero point of thermal energy. It also defines the third law of thermodynamics, which states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.

Can Absolute Zero Be Reached?

In practice, absolute zero can never be achieved, but scientists have come extremely close using laser cooling and magnetic evaporative cooling techniques. Researchers have cooled atoms to within a billionth of a Kelvin above absolute zero, where exotic quantum behaviors like Bose-Einstein condensation occur.

Absolute zero is a cornerstone of thermodynamics, quantum physics, and cryogenics — revealing the limits of temperature and the strange behavior of matter in the ultra-cold.