How do scientists cool objects to absolute zero?

Category: Physics
Published: May 31, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University

The Concept of Absolute Zero

Absolute zero is defined as the point at which all atomic motion stops. It is represented as $0 \, \text{K}$ on the Kelvin scale. However, reaching a temperature of exactly absolute zero is fundamentally impossible due to several physical principles.

The Nature of Temperature

Temperature is fundamentally a measure of the average kinetic energy of atoms in a substance. As temperature decreases, the random motion of these atoms diminishes. At absolute zero, theoretically, all atomic motion would cease, meaning that the atoms would be perfectly still relative to each other. However, this is more of a theoretical concept than a practical reality.

Quantum Mechanics and the Heisenberg Uncertainty Principle

One of the key reasons that absolute zero cannot be reached is rooted in quantum mechanics. According to the Heisenberg Uncertainty Principle, it is impossible to simultaneously know both the exact position and momentum of a particle. Thus, trying to perfectly stop every atom at a specific point would violate this principle. As a result, every quantum system has a non-zero ground state energy, which means that even at absolute zero, atoms retain some minimal energy.

Thermodynamic Limitations

From a thermodynamic perspective, cooling an object to absolute zero would require an infinite number of steps. This presents a significant barrier to achieving this temperature. The laws of thermodynamics dictate that we can never achieve a state of perfect order where all atomic motion ceases.

Approaching Absolute Zero

Despite the theoretical limitations, scientists have developed methods to cool materials to temperatures incredibly close to absolute zero. For instance, the Low Temperature Laboratory at Aalto University successfully cooled a piece of rhodium metal to a temperature of $0.0000000001 \, \text{K}$ using a technique known as nuclear demagnetization refrigeration.

Nuclear Demagnetization Refrigeration

Nuclear demagnetization refrigeration involves cooling a material by aligning the spins of its nuclei in a strong magnetic field. As the magnetic field is slowly reduced, the thermal energy of the material is transferred to the magnetic field, allowing the material to cool significantly. This method is one of the most effective ways of achieving temperatures close to absolute zero.

Conclusion

In summary, while scientists can reach temperatures extremely close to absolute zero, the laws of physics, particularly the Heisenberg Uncertainty Principle and thermodynamic constraints, ensure that absolute zero itself remains an unattainable limit. Nonetheless, advancements in low-temperature physics continue to push the boundaries of what is possible, allowing researchers to explore the unique properties of matter in these extreme conditions.