How Temperature and Absolute Zero Were Discovered

1. The Deception of Human Senses

Touch a piece of wood on your desk. Now touch the metal leg of your chair.

The metal feels significantly colder than the wood. Your brain immediately informs you: "The metal is at a lower temperature than the wood."

Your brain is wrong. Both objects have been sitting in the same room for hours; both are at the exact same temperature (roughly 21°C).

The metal feels colder solely because steel has a thermal conductivity forty times higher than wood: it siphons thermal energy away from your 37°C finger forty times faster! Human skin possesses zero biological sensors for measuring absolute temperature. Our nerve endings (thermoreceptors) measure only the transient rate of heat transfer (dQdt) into or out of our flesh.

For thousands of years, this sensory deception blinded human thought. Natural philosophers believed heat was a weightless, invisible fluid called caloric that soaked into porous bodies like water into a sponge. Hot objects were thought to be saturated with caloric fluid; cold was simply the absence of caloric.

To discover what temperature actually was, science had to invent an artificial eye that was completely immune to human tactile subjectivity: the thermometer.

2. From Galileo's Air Bulb to Mercury Capillaries

In 1592, Galileo Galilei constructed the first device capable of indicating differences in hotness: the thermoscope.

                  GALILEO'S AIR THERMOSCOPE (1592)
                  
                           Glass Flask (Air Bulb)
                               ┌─────────┐
                               │  AIR    │
                               │ TRAPPED │
                               └────┬────┘
                                    │ Glass Stem
                                    │
                                    │ Liquid Level Rises when COLD
                                    │ (Air contracts!)
                                    ▼
                          ┌──────────────────┐
                          │ Open Water Basin │
                          └──────────────────┘
                          
       Fatal Flaw: The bottom basin is open to the room!
       When atmospheric barometric pressure changes, the water level
       moves up or down EVEN IF TEMPERATURE STAYS THE SAME!

Galileo took a small glass flask with a long, thin neck, warmed the bulb in his hands to expand the air inside, and inverted the open tube into a vessel of colored water. As the bulb cooled, the trapped air contracted, drawing water up the tube.

When the room grew warmer, the air inside the bulb expanded, pushing the water column downward. When the room cooled, the air contracted, pulling the water column upward.

The Barometric Flaw

Galileo's thermoscope had a fatal flaw: it was an open system.

Because the water basin was open to the atmosphere, changes in barometric air weather pressure pushed down on the water basin, moving the liquid column even if the temperature had not changed a fraction of a degree! The device was half thermometer, half barometer.

To build a true thermometer, Italian glassblowers in Florence (the Accademia del Cimento) took two revolutionary steps in the 1650s:

1. They replaced air with liquid (wine alcohol or spirits), which expands and contracts with temperature.
2. They sealed the top of the glass tube hermetically with a blowtorch flame, isolating the liquid inside a pure vacuum and banishing atmospheric pressure interference forever.

3. Fahrenheit and Celsius: The Search for Fixed Calibration Points

A sealed tube of expanding liquid shows that something is getting hotter or colder, but it cannot give you a number.

To turn an expansion tube into a scientific measuring tool, you need fixed, reproducible calibration points—physical events in nature that always occur at the exact same degree of hotness.

                  FAHRENHEIT VS. CELSIUS CALIBRATION POINTS
                  
   Scale          Low Fixed Point                 High Fixed Point            Degrees Between
  ────────────────────────────────────────────────────────────────────────────────────────────
   Fahrenheit     **0°F**: Ice + Water +          **96°F**: Human body        **180°** between
   (1724)         Ammonium Chloride Salt Brine    temperature (armpit/mouth)   freezing & boiling
                  *(32°F: Pure Water Freezes)*    *(212°F: Pure Water Boils)*  of pure water
  ────────────────────────────────────────────────────────────────────────────────────────────
   Celsius        **0°C**: Pure Water             **100°C**: Pure Water       **100°** equal
   (1742/1744)    Freezing Point                  Boiling Point at 1 atm       decimal divisions

Fahrenheit's Precision Mercury Engineering (1724)

In Amsterdam, Polish-German instrument maker Daniel Gabriel Fahrenheit revolutionized the field with two breakthroughs:

1. Purified Liquid Mercury: Alcohol boils at 78°C and wets glass walls, leaving clinging droplets that corrupt readings. Fahrenheit perfected the distillation of liquid mercury. Mercury remains a liquid from −39^∘C to +357^∘C, expands with remarkable mathematical linearity, and does not stick to glass.
2. The Eutectic Salt Brine Standard: Fahrenheit wanted a zero point that would eliminate negative numbers in northern European winters. He created an ice-water-ammonium chloride salt eutectic brine mixture: this was the coldest reproducible temperature that 18th-century laboratory chemistry could create, which he designated 0^∘F. He then set the freezing point of pure water at 32^∘F and the core human body temperature (measured in the mouth of a healthy human) at 96^∘F (later refined to 98.6°F). On this scale, the boiling point of water at sea-level atmospheric pressure fell neatly at 212^∘F—exactly 180 degrees above the freezing point.

Celsius and the Centigrade Scale (1742)

In 1742, Swedish astronomer Anders Celsius proposed a simpler, decimal scale based entirely on the phase transitions of pure water at standard atmospheric pressure (1,013.25 hPa): the freezing point of water and the boiling point of water.

In a bizarre historical quirk, Celsius originally designed his scale backwards:

• He set the boiling point of water at 0^∘!
• He set the freezing point of water at 100^∘!

Celsius did this deliberately to prevent negative numbers during freezing Scandinavian winters: the colder the winter grew, the higher the thermometer reading rose!

Two years later, following Celsius's death, Swedish botanist Carl Linnaeus (the father of biological taxonomy) ordered custom thermometers from instrument maker Daniel Ekström with the scale inverted to our modern convention: 0^∘C for freezing water and 100^∘C for boiling water.

4. The Discovery of Absolute Zero: Gas Law Geometry

Mercury thermometers allowed science to measure daily temperatures between −30^∘C and +300^∘C. But they raised an immense, unanswered question: is there a bottom limit to cold?

Can an object get infinitely cold, just as it can theoretically get infinitely hot? Or is cold a dead end?

The answer came not from freezing liquids, but from the physics of expanding and contracting gases.

                  THE CHARLES & GAY-LUSSAC GAS EXTRAPOLATION
                  
   Volume (V)
       ▲
       │                                     Gas Volume expands with heat
       │                                  /
       │                              /
    V₀ ┼───────────────────────────/────── (0°C Reference Volume)
       │                       /
       │                   /
       │               /
       │           /
       │       /
       │   /
       ▼/
   ────┼───────────────────────────────────┼───────────────────────────► Temperature (°C)
   **-273.15°C**                           0°C                        100°C
   (Absolute Zero:
    Theoretical Volume = 0!)

In 1802, French physicists Jacques Charles and Joseph Louis Gay-Lussac conducted high-precision experiments on the thermal volumetric expansion of gases (air, hydrogen, oxygen, nitrogen).

They placed a sample of gas inside a glass bulb sealed with a droplet of mercury, maintained at a strictly constant atmospheric pressure (isobaric conditions). They discovered a startling, universal law:

For every single degree Celsius you heat any gas, its volume expands by a constant fraction of its volume at zero degrees Celsius.
And for every single degree Celsius you cool any gas, its volume contracts by that exact same fraction!

That fraction was found to be:

The isobaric gas equation took the form:

Where V₀ is the gas volume at 0^∘C, and T is temperature in degrees Celsius.

The Geometric Intercept

Now perform a thought experiment: what happens if you cool the gas down?

• Cool it to −100^∘C: The volume shrinks by 100273.15 of its original size.
• Cool it to −200^∘C: The volume shrinks by 200273.15.
• Cool it to −273.15^∘C: The volume shrinks by 273.15273.15 = 1.000!

At −273.15^∘C, the theoretical volume of the gas contracts to precisely ZERO!

No matter which gas Gay-Lussac tested—whether light hydrogen, atmospheric nitrogen, or heavy carbon dioxide—when the volume lines were graphed on paper and extrapolated backward, every single gas line converged at the exact same point on the temperature axis: minus 273.15 degrees Celsius!

Matter cannot have negative volume. A gas cannot occupy less than zero space.

This meant that −273.15^∘C was not merely a property of hydrogen or air: it was a fundamental physical boundary of nature itself. You could not go colder than −273.15^∘C because below that point, the geometry of matter ceased to exist!

5. Lord Kelvin, Thermodynamic Temperature, and the Kinetic Theory of Gases

Even after Gay-Lussac's gas experiments pointed toward −273.15^∘C, a profound theoretical problem remained: defining temperature by the expansion of mercury, alcohol, or even hydrogen gas tied the scale to the mechanical quirks of specific chemical substances—and every real gas eventually condenses into a liquid before reaching −273.15^∘C.

From Carnot Cycles to the Absolute Thermodynamic Scale (1848–1854)

In June 1848, 24-year-old Scottish physicist William Thomson (later Lord Kelvin) published On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat. Thomson realized that Sadi Carnot's ideal reversible heat engine—whose efficiency depends only on the temperatures of its hot and cold reservoirs and not on the working substance inside the cylinder—offered a way to define temperature purely from the laws of nature.

In his initial 1848 paper, Thomson still worked within Carnot's older assumption that heat ("caloric") was conserved during work production, which yielded a logarithmic temperature scale relative to air thermometers. Over the next six years, however, James Prescott Joule demonstrated experimentally that heat and mechanical work are interconvertible forms of energy (the First Law of Thermodynamics), and Rudolf Clausius reconciled Carnot's theorem with energy conservation (as detailed in How Thermodynamics Dictates the Arrow of Time).

In 1854, Thomson formulated our modern linear Absolute Thermodynamic Temperature Scale: for any reversible Carnot cycle operating between a hot reservoir at temperature T_H and a cold reservoir at T_C, the ratio of absolute temperatures is defined as the exact ratio of heat absorbed (Q_H) to heat rejected (Q_C):

In statistical mechanics, this macroscopic definition is expressed universally across all states of matter—solids, liquids, gases, and plasmas—as the inverse rate at which entropy (S) increases with internal energy (U) at constant volume and particle number:

Maxwell, Boltzmann, and Ideal-Gas Kinetic Energy

What does this thermodynamic temperature mean at the atomic scale for a gas?

According to the Kinetic Molecular Theory of Gases developed by James Clerk Maxwell and Ludwig Boltzmann, an ideal dilute gas consists of point-like particles in ceaseless random motion, colliding elastically with each other and with the container walls.

                  TEMPERATURE IN AN IDEAL MONATOMIC GAS
                  
    High Temperature (Hot):                 Low Temperature (Cold):
    High root-mean-square speed             Lower root-mean-square speed
    
        O           O                            O        O
         \         /                              \      /
          \       /                                \    /
       O───► O ◄───O                                O──O
          /       \                                /    \
         /         \                              /      \
        O           O                            O        O
        
    CLASSICAL EXTRAPOLATION TO 0 KELVIN (-273.15°C):
    Classical thermal translational kinetic energy approaches zero
    (while quantum zero-point motion remains).

By the classical Equipartition Theorem (which assigns 12k_B T of average thermal energy to each independent quadratic degree of freedom), the mean translational kinetic energy (⟨ E_trans ⟩) of an ideal gas particle in three-dimensional space is directly proportional to its absolute temperature T:

Where:

• m is the mass of a single molecule.
• ⟨ v² ⟩ is the mean squared translational speed.
• k_B is the Boltzmann Constant (1.380649 × 10⁻²³ J·K⁻¹).
• T is the absolute thermodynamic temperature in kelvins (K).

Kelvin aligned the size of one kelvin degree with one degree Celsius by shifting the zero point down to absolute zero:

On the Kelvin scale:

• 0 K: Absolute Zero (−273.15^∘C)
• 273.15 K: Standard Freezing Point of Pure Water at 1 atm (0^∘C)
• 273.16 K: Triple Point of Pure Water (0.01^∘C at 611.657 Pa)
• 373.15 K: Standard Boiling Point of Water (100^∘C at 1 atm)

Important Scope Qualification — Ideal Gases vs. Cryogenic Solids: Because Kelvin is a true ratio scale with an absolute origin at 0 K, 20 K is twice the thermodynamic temperature of 10 K, and an ideal monatomic gas at 20 K has exactly twice the mean translational kinetic energy (32k_B T) of an ideal gas at 10 K. In real crystalline solids and quantum fluids at cryogenic temperatures, however, classical equipartition breaks down: lattice vibrations (phonons) freeze out quantum-mechanically according to Debye's T³ heat-capacity law (so thermal internal energy above the ground state scales as U − U₀ ∝ T⁴), and atoms retain a large baseline of non-thermal quantum zero-point energy.

6. Quantum Mechanics and Why Absolute Zero is Unreachable

What actually happens if you cool a physical sample toward Absolute Zero (0 K)?

In classical 19th-century physics, it was assumed that at 0 K all atomic motion would cease completely and every atom would sit motionless at a fixed lattice coordinate.

In the 20th century, quantum mechanics proved that complete atomic stillness is physically impossible.

                  THE HEISENBERG QUANTUM FLOOR
                  
               Heisenberg Uncertainty Principle:
                      Δx · Δp ≥ ℏ / 2
                      
     If an atom sat completely motionless at a fixed point:
     1. Velocity = 0 and fixed position  ──►  Δp = 0 and Δx = 0
     2. This violates Δx · Δp ≥ ℏ / 2!
     
     RESOLUTION: Confined matter CANNOT stop moving!
     At 0 Kelvin, particles retain irreducible Quantum Zero-Point Energy.

According to Werner Heisenberg's Uncertainty Principle:

A localized particle confined within a crystal lattice or potential well (finite Δ x) cannot simultaneously have zero momentum spread (Δ p = 0). Even in its lowest possible quantum mechanical state (n = 0), a harmonic oscillator retains an irreducible minimum energy called Quantum Zero-Point Energy:

At Absolute Zero, a system has not lost its zero-point quantum motion; rather, it has settled into its quantum ground state, where zero further thermal energy can be extracted from it.

The Third Law of Thermodynamics: The Asymptotic Wall

Can a cryogenic cooling cycle (see How Refrigerators and Heat Pumps Work) ever cool a physical sample to exactly 0 K?

The Third Law of Thermodynamics (formulated by Walther Nernst in 1906–1912 as the Unattainability Principle) states that it is impossible by any procedure, no matter how idealized, to reduce the temperature of any thermodynamic system to absolute zero in a finite number of operations.

As temperature T → 0 K, the entropy difference between adjacent thermodynamic states vanishes (Δ S → 0), so each adiabatic demagnetization or laser-cooling stage removes a smaller and smaller fraction of remaining thermal energy:

   Milestone Low Temperatures Achieved in Physics:
   ─────────────────────────────────────────────────────────────────────────────
   Liquid Nitrogen Boiling Point (1 atm)             77.36 K
   Liquid Helium-4 Boiling Point (1 atm)              4.22 K
   Cosmic Microwave Background (Deep Space)           2.725 K
   Helium-3 / Helium-4 Dilution Refrigerator          0.002 K (2 millikelvin)
   Bose-Einstein Condensate Matter-Wave Cooling       ~38–50 picokelvin (5 × 10⁻¹¹ K)

Using laser Doppler cooling, magnetic trapping, and matter-wave lensing of rubidium Bose-Einstein condensates, physicists have reached effective kinetic temperatures below 50 picokelvin (5 × 10⁻¹¹ K)—orders of magnitude colder than intergalactic space, yet still finitely above 0 K.

From the Triple Point of Water (1954) to the 2019 SI Boltzmann Redefinition

For 65 years—from the 10th General Conference on Weights and Measures (CGPM) in 1954 until May 20, 2019—the international definition of the kelvin was anchored to a single physical artifact state: the triple point of water (T_TPW = 273.16 K, or 0.01^∘C at a vapor pressure of 611.657 Pa), the unique temperature and pressure at which solid ice, liquid water, and water vapor coexist in thermodynamic equilibrium. One kelvin was defined as exactly 1/273.16 of T_TPW.

However, even ultra-pure triple-point cells varied slightly with the isotopic ratios of hydrogen (²H/¹H) and oxygen (¹⁸O/¹⁶O) in water, and extrapolating from 273.16 K down to millikelvins or up to thousands of kelvins introduced metrological uncertainty.

As part of the historic 2019 Redefinition of the SI Base Units (detailed in How the Metric System and SI Units Were Invented), the BIPM detached the kelvin from water entirely by fixing the exact numerical value of the Boltzmann Constant (determined via high-precision Acoustic Gas Thermometry in argon/helium resonators):

Today, the kelvin is defined directly in terms of microscopic thermal energy k_B T—uniting practical thermometry with fundamental statistical physics across every scale of temperature in the universe.