Absolute Zero

Absolute zero is the lowest possible temperature at which matter theoretically possesses minimum thermal energy. It represents the point at which the motion of particles ceases completely in a perfect crystalline substance. Absolute zero forms the foundation of the Kelvin temperature scale, corresponding to 0 Kelvin (K), which is equal to –273.15°C or –459.67°F. It is a fundamental concept in thermodynamics, physics, and chemistry, as it defines the limit beyond which no further decrease in temperature is possible.

Concept and Definition

At the molecular level, temperature is a measure of the average kinetic energy of particles in a substance. As temperature decreases, the motion of atoms and molecules slows down. At absolute zero, this motion theoretically comes to a complete halt, meaning that the particles retain only their zero-point energy—the minimal energy required by quantum mechanics.
Therefore, absolute zero can be defined as:

“The temperature at which the kinetic energy of the particles in a system is at its minimum possible value.”

It serves as the zero point of the Kelvin scale, where temperature values directly represent the internal energy of a system.

Equivalent Values

Historical Background

The concept of absolute zero was developed through studies of gas behaviour in the 17th and 18th centuries.

• In 1665, Irish scientist Robert Boyle observed that gases contracted when cooled, suggesting a limit to how cold a gas could become.
• In the early 19th century, French physicist Guillaume Amontons and later Lord Kelvin (William Thomson) extended these findings mathematically.
• Kelvin (1848) proposed a thermodynamic temperature scale beginning at absolute zero, marking it as the temperature at which an ideal gas would theoretically have zero volume and pressure.

Theoretical Basis

The relationship between temperature and pressure of a fixed mass of gas at constant volume is described by Gay-Lussac’s Law, which shows that pressure decreases linearly with temperature. Extrapolating this line to zero pressure indicates a theoretical limit at –273.15°C, corresponding to absolute zero.
In thermodynamic terms:

• At absolute zero, entropy (a measure of disorder) approaches a constant minimum value.
• According to the Third Law of Thermodynamics, it is impossible to reach absolute zero through any finite series of physical processes.

Physical Interpretation

1. Atomic and Molecular Motion:
   • At higher temperatures, atoms and molecules vibrate, rotate, and move freely.
   • As temperature decreases, these motions reduce.
   • At absolute zero, only quantum mechanical vibrations (zero-point motion) remain.
2. Energy State:
   • The internal energy of the substance is at its lowest possible level.
   • The substance cannot transfer thermal energy to another body because it contains none to give.
3. Volume and Pressure:
   • For an ideal gas, both volume and pressure approach zero as temperature approaches absolute zero.

Experimental Approaches

Although absolute zero can never be reached exactly, scientists have come extremely close to it using cryogenic techniques and laser cooling.

• Cryogenic Cooling: Utilises liquefied gases such as liquid helium to achieve temperatures close to 1 K.
• Magnetic Refrigeration: Employs magnetic fields to align electron spins and reduce thermal energy, reaching microkelvin levels.
• Laser Cooling (Doppler Cooling): Uses lasers to slow atoms, reducing their motion to a few billionths of a Kelvin (nano-K range).

In 2019, physicists achieved temperatures as low as 38 picokelvin (0.000000000038 K) in laboratory conditions, the closest approach to absolute zero ever recorded.

Effects on Matter

As substances approach absolute zero, their physical properties change dramatically:

• Electrical Resistance: In some materials, resistance drops to zero, resulting in superconductivity.
• Viscosity: Liquids such as helium exhibit superfluidity, flowing without friction.
• Thermal Expansion: Most materials contract and reach near-zero expansion coefficients.
• Magnetic Properties: Substances can exhibit unusual magnetic ordering, such as Bose–Einstein condensation, where atoms act as a single quantum entity.

The Kelvin Scale

The Kelvin temperature scale, proposed by Lord Kelvin, is an absolute thermodynamic scale beginning at absolute zero.

• It uses the same increments as the Celsius scale but starts at 0 K instead of –273.15°C.
• Hence, the conversion formula is:
  • K = °C + 273.15
  • °C = K – 273.15

The Kelvin scale is used universally in scientific measurements because it provides a direct relationship between temperature and molecular energy.

Absolute Zero and the Third Law of Thermodynamics

The Third Law of Thermodynamics states that:

“The entropy of a perfect crystal at absolute zero is exactly zero.”

This implies that all molecular motion ceases, and the atoms are arranged in their most ordered possible state. However, quantum mechanics shows that complete stillness cannot occur due to zero-point energy, meaning absolute zero cannot be physically attained.

Importance and Applications

1. In Physics:
   • Defines the lower limit of the thermodynamic temperature scale.
   • Used in the study of quantum mechanics and superconductivity.
2. In Chemistry:
   • Essential for calculations involving entropy, enthalpy, and Gibbs free energy.
   • Provides a reference point for determining absolute enthalpies and reaction spontaneity.
3. In Space Science:
   • The temperature of outer space is approximately 2.7 K, slightly above absolute zero, due to the cosmic microwave background radiation.
4. In Technology:
   • Ultra-low temperatures are utilised in quantum computing, cryogenics, MRI technology, and superconducting materials.

Summary