What Is 0 Kelvin

Content on WhatAnswers is provided "as is" for informational purposes. While we strive for accuracy, we make no guarantees. Content is AI-assisted and should not be used as professional advice.

Last updated: April 11, 2026

Quick Answer: 0 Kelvin, also known as absolute zero, is the lowest possible temperature in the universe at −273.15°C (−459.67°F), where matter theoretically has minimal internal energy. William Thomson (Lord Kelvin) calculated this value in 1848 based on gas expansion laws, establishing the foundation for the absolute temperature scale. According to the Third Law of Thermodynamics, absolute zero cannot be physically achieved, as it would require infinite cooling steps—scientists have only reached temperatures within billionths of a degree of absolute zero.

Key Facts

0 Kelvin equals −273.15°C and −459.67°F, officially established by the International Bureau of Weights and Measures in 1954 as the starting point for the Kelvin scale
William Thomson (Lord Kelvin) developed the Kelvin scale in 1848 at age 24 by calculating infinite cold from the thermal expansion coefficient of ideal gases
At absolute zero, molecular motion theoretically ceases and particles retain only quantum mechanical zero-point energy according to kinetic theory of matter
The Third Law of Thermodynamics, mathematically proven in modern physics, declares that absolute zero cannot be reached in any finite number of cooling steps
Scientists have cooled matter to within billionths of a degree (nanokelvin range) of absolute zero using laser cooling and dilution refrigeration techniques

Overview

0 Kelvin, also called absolute zero, represents the lowest possible temperature in the universe. It is defined as −273.15°C (−459.67°F) on the Celsius and Fahrenheit scales respectively. At this theoretical temperature, the internal energy of a system reaches its minimum value, and molecular motion would theoretically cease entirely.

The Kelvin scale was developed by William Thomson (later ennobled as Lord Kelvin) in 1848 when he was just 24 years old. Thomson derived absolute zero by extrapolating from Charles's Law, which had been established between 1787 and 1802 by Jacques Charles, John Dalton, and Joseph Louis Gay-Lussac. These scientists discovered that ideal gases contracted by approximately 1/273 parts per degree Celsius, suggesting that a gas cooled to −273°C would occupy zero volume. Thomson's calculation proved remarkably accurate, differing only slightly from the currently accepted value established by the International Bureau of Weights and Measures in 1954.

How It Works

Understanding 0 Kelvin requires examining the fundamental properties of temperature and matter at extreme conditions:

Molecular Motion Stops: At 0 Kelvin, the thermal motion of atoms and molecules theoretically ceases. In classical physics, this means no kinetic energy from thermal movement is available for transfer to other systems.
Quantum Zero-Point Energy: Even at absolute zero, quantum mechanics reveals that particles retain what is called zero-point energy. This irreducible quantum energy means particles never truly become completely stationary, maintaining minimal vibrational motion governed by Heisenberg's Uncertainty Principle.
Entropy Approaches Zero: The entropy (disorder) of a system approaches its minimum value as temperature drops toward absolute zero. This principle is central to the Third Law of Thermodynamics, which governs why absolute zero cannot be practically achieved.
Absolute Temperature Reference: The Kelvin scale uses absolute zero as its starting point (0 K), eliminating the need for negative temperatures when measuring thermodynamic properties. This makes it the preferred scale in physics, chemistry, and engineering for temperature calculations involving energy and heat transfer.
Cooling Efficiency Decreases: As a system approaches absolute zero, removing additional heat becomes exponentially more difficult. The efficiency of cooling processes decreases, requiring increasingly sophisticated equipment and techniques to reach lower temperatures.

Key Comparisons

Temperature Scale	Absolute Zero Value	Use in Science
Kelvin (K)	0 K (exact definition)	Primary scale for thermodynamics, physics, and international standards
Celsius (°C)	−273.15°C	Common use in everyday life and chemistry; relative to water freezing point
Fahrenheit (°F)	−459.67°F	Primarily used in the United States for weather and practical applications
Rankine (°R)	0°R	Absolute scale alternative used in some engineering applications, primarily in the U.S.

Why It Matters

Foundation of Thermodynamics: Absolute zero anchors the Third Law of Thermodynamics, which states that no physical process can cool a system to absolute zero in a finite number of steps. This principle governs energy efficiency, heat transfer, and the limits of refrigeration technology across all sciences.
Scientific Research: Understanding the behavior of matter near absolute zero has led to revolutionary discoveries in quantum mechanics, superconductivity, and Bose-Einstein condensates. Researchers push toward these extreme temperatures to study fundamental properties of matter that are invisible at normal conditions.
Technological Applications: Extreme cooling techniques developed to approach absolute zero have enabled breakthrough technologies. Superconductors function only at temperatures near absolute zero, allowing for applications in medical imaging (MRI machines), particle accelerators, and power transmission systems with virtually no energy loss.
Astrophysics Reference: Cosmic microwave background radiation measures approximately 2.73 Kelvin, close to absolute zero. This residual heat from the Big Bang serves as a natural temperature reference for understanding the universe's expansion and early formation.

The quest to understand and approach absolute zero has fundamentally shaped modern physics and engineering. While 0 Kelvin remains theoretically unattainable, scientists have achieved temperatures of mere nanokelvin (billionths of a Kelvin) using advanced techniques like laser cooling and dilution refrigeration. Each advance in reaching lower temperatures opens new windows into quantum behavior and enables technologies that improve human life. The principle of absolute zero demonstrates nature's fundamental limits while inspiring endless scientific innovation.