Absolute zero represents the coldest possible temperature, a fundamental limit in the universe where a system’s internal energy reaches its minimum value. The determination of this temperature was a significant scientific endeavor, providing crucial insights into the behavior of matter at extreme conditions.
Early Ideas of Temperature
Before the concept of absolute zero emerged, understanding temperature involved relative scales based on arbitrary reference points. Daniel Gabriel Fahrenheit developed a scale in 1714, setting the freezing point of water at 32°F and its boiling point at 212°F. Anders Celsius proposed a scale in 1742, defining 0°C as the freezing point of water and 100°C as its boiling point. These scales, while useful for everyday measurements, were based on the properties of specific substances like water and did not suggest a universal lowest temperature. They established relative degrees of hotness and coldness without identifying a true bottom limit.
Gases and the Path to Absolute Zero
Experiments with gases provided the initial key to understanding an absolute lower temperature limit. In the 17th century, Guillaume Amontons observed a consistent relationship: gas pressure decreased as temperature dropped when volume remained constant. This observation was further explored by Jacques Charles in 1787, who discovered that the volume of a gas decreases linearly as its temperature decreases, provided the pressure remains constant. This relationship, known as Charles’s Law, suggested that if a gas continued to cool, its volume would eventually shrink to zero.
Joseph Louis Gay-Lussac published his findings in 1802. He established that the pressure of a fixed mass of gas, held at a constant volume, is directly proportional to its absolute temperature. This principle showed a linear decrease in pressure with falling temperature. Across these experiments, a consistency emerged: different gases exhibited a similar linear behavior, indicating they would theoretically reach zero volume or zero pressure at the same extremely low temperature.
The Extrapolation Method
The consistent linear behavior of gases provided a powerful method for determining absolute zero through extrapolation. Scientists measured the volume or pressure of a gas at several temperatures, then plotted this data on a graph. Since the relationship was linear, a straight line could be drawn through the data points.
To find absolute zero, this line was extended backward, or “extrapolated,” to the point where the gas’s theoretical volume or pressure would become zero. This intersection on the temperature axis represented the temperature at which all molecular motion would theoretically cease. Experiments with different gases, when extrapolated, consistently converged on approximately -273.15 degrees Celsius. This convergence suggested that this temperature was a fundamental physical limit.
Kelvin’s Absolute Scale
The experimental observations and the extrapolated value for absolute zero laid the groundwork for a universal temperature scale. In 1848, William Thomson, Lord Kelvin, formalized this concept by proposing an absolute temperature scale, independent of any specific substance. This scale, now called the Kelvin scale, defines 0 Kelvin (0 K) as absolute zero, the point where a system possesses its minimum possible energy.
The Kelvin scale is directly related to the Celsius scale, with 0 K equivalent to -273.15 °C. A temperature change of one Kelvin is precisely the same magnitude as a change of one degree Celsius. This thermodynamic scale provided a scientific standard for temperature measurement, offering an absolute reference point not dependent on arbitrary substance-specific properties.
The Meaning of Absolute Zero
Absolute zero represents a theoretical state where a system’s particles possess the lowest possible energy. Classically, all molecular motion would cease. However, quantum mechanics states that even at absolute zero, particles retain a minimal vibrational motion known as zero-point energy.
The Third Law of Thermodynamics states that absolute zero cannot be fully reached through any physical process in a finite number of steps. While scientists have achieved temperatures close to absolute zero, reaching it perfectly remains practically unattainable. The concept of absolute zero is foundational to understanding matter at extremely low temperatures and the limits of cooling.