The history
From a warm glass bulb in Padua to atoms a few trillionths of a degree above absolute zero: 400 years of learning what heat is and where it can go.
For centuries people could feel hot and cold but could not measure them, and most thought heat was an invisible fluid. Engineers trying to build better steam engines, and a brewer with a paddle wheel, showed that heat is energy on the move and that it only flows one way by itself. Then physicists found a coldest possible cold, explained entropy by counting atoms, and discovered that even forgetting information gives off heat.
c. 1593
Instrument that shows hot and cold
Galileo Galilei, Padua, Italy
1714
Reliable mercury thermometer
Daniel Gabriel Fahrenheit, Amsterdam
1824
Theory of the ideal heat engine
Sadi Carnot, France
1850
First and second laws stated together
Rudolf Clausius, Germany
1877
Liquid oxygen
Louis Paul Cailletet and Raoul Pictet, France and Switzerland
1908
Liquid helium
Heike Kamerlingh Onnes, Leiden, Netherlands
1933
Temperature below 1 kelvin
William Giauque and Duncan MacDougall, Berkeley, USA
1995
Bose–Einstein condensate
Eric Cornell and Carl Wieman, Boulder, USA
1593 – 1765
People learn to see heat with glass tubes, agree on scales with fixed points, and discover that heat and temperature are not the same thing.
1593
c. 1593 (Viviani says 1597)
Galileo warmed a glass bulb with a long neck in his hands and stood it upside down in water. As the air inside cooled, water crept up the neck. The rise and fall showed that something was getting hotter or colder, but it had no numbers on it.
Why it mattered. It was one of the first instruments to make heat visible, the ancestor of every thermometer.
1714
1714 and 1724
Fahrenheit, a glassblower born in Danzig, made reliable thermometers, first with alcohol and by 1714 with mercury. In 1724 he described his scale to the Royal Society in London. Two of his thermometers made in the same way gave the same reading.
Why it mattered. For the first time, people in different places could compare temperatures as numbers.
1742
Celsius showed that melting snow always sits at the same temperature, and so does boiling water at a fixed air pressure. He split the gap into 100 steps. But he put 0 at boiling and 100 at freezing. Soon after his death in 1744 the scale was flipped the right way up.
Why it mattered. Two fixed points and 100 steps gave the world the scale most people use today.
1761
c. 1761–1762
Black noticed that ice melting in a warm room stays at the same temperature for a long time, even though it keeps soaking up heat. He called this hidden heat latent heat. He also found that the same amount of heat warms different materials by different amounts, which we now call specific heat.
Why it mattered. It showed that heat and temperature are two different things.
1798 – 1848
Cannons, steam engines and paddle wheels show that heat can be made from work and work from heat, and that there is a coldest possible cold.
1798
Read 25 January 1798
Rumford was in charge of drilling out brass cannons. He saw that the blunt drill made heat for as long as the horses kept turning it, enough to boil water in a few hours. Heat that never runs out could not be a fluid stored in the metal, he argued. It had to come from the motion.
Why it mattered. It was strong evidence that heat is a kind of motion, not a substance.
1824
A young French engineer wrote a short book called Reflections on the Motive Power of Fire. He imagined a perfect engine and showed that the work it can give depends only on how hot its hot side is and how cold its cold side is. Few people read it at the time, and Carnot died of cholera in 1832.
Why it mattered. It set a limit on every engine ever built, and it became the seed of the second law.
1834
Clapeyron rewrote Carnot's ideas in mathematics. He drew the engine's cycle as a closed loop on a graph of pressure against volume, and the area inside the loop is the work done. That is how Carnot's forgotten book reached Kelvin and Clausius.
Why it mattered. The pressure-volume diagram is still how engineers picture engines today.
1845
1843–1850
Joule, from a brewing family, let falling weights spin a paddle wheel in water and measured how much warmer the water got. Over years of careful work he showed that the same amount of work always makes the same amount of heat. In Germany, Julius Robert Mayer had argued for the same idea in 1842.
Why it mattered. Heat and work turned out to be one thing, energy, which led straight to the first law.
1848
Thomson, aged 24, used Carnot's ideas to propose a temperature scale that does not depend on any one liquid or gas. It starts at an absolute zero, the coldest anything can ever be, which works out to about minus 273 degrees Celsius. We now measure it in kelvins, named after him.
Why it mattered. Temperature got a true zero, which later made the third law possible.
1850 – 1878
Clausius and Kelvin write down the first and second laws, entropy gets its name, and Maxwell, Boltzmann and Gibbs explain it by counting molecules.
1850
Clausius showed that Carnot and Joule were both right. Energy is never lost, only changed, which is the first law. And heat does not flow by itself from something cold to something hot, which is the heart of the second law.
Why it mattered. It is widely seen as the birth of thermodynamics as a science.
1851
March 1851
Thomson gave his own version of the second law. No engine working in a cycle can take heat from one place and turn all of it into work. Some heat must always be passed to something colder.
Why it mattered. It explains why every car engine and power station has to throw heat away.
1865
Clausius needed a name for the quantity that always grows when heat spreads out. He built the word entropy from the Greek for transformation, and chose it to sound like energy. He summed up: the energy of the universe stays the same, and its entropy tends to a maximum.
Why it mattered. Entropy gave the second law a number you can calculate.
1867
11 December 1867
In a letter to his friend Peter Guthrie Tait, Maxwell imagined a tiny being guarding a door between two boxes of gas. It lets fast molecules through one way and slow ones the other, so one side gets hot and the other cold without any work. Maxwell made it public in his book Theory of Heat in the early 1870s, and Kelvin later nicknamed it a demon.
Why it mattered. It showed that the second law is about probability, and the puzzle took about a century to solve.
1876
1875–1878
Gibbs, a quiet professor at Yale, published a very long paper called On the Equilibrium of Heterogeneous Substances in a small local journal. It used energy and entropy to predict when chemicals react, when ice melts and when mixtures separate. It took Europe years to notice.
Why it mattered. It turned thermodynamics into a tool for chemistry and materials science.
1877
Boltzmann showed that entropy measures how many ways the tiny particles of a thing can be arranged and still look the same from outside. Messy arrangements vastly outnumber tidy ones, so things drift towards mess. Max Planck later wrote it as S = k log W, around 1900, and that formula is carved on Boltzmann's grave in Vienna.
Why it mattered. It explained the second law using atoms, before most scientists believed atoms were real.
1906 – 1939
Helium is turned into a liquid, Nernst finds the third law about absolute zero, and the most basic law of all finally gets a name.
1906
1906–1912
Nernst studied chemical reactions at lower and lower temperatures. In 1906 he proposed his heat theorem: as things approach absolute zero, their entropy changes shrink towards nothing. By 1912 he argued it also means no process can ever reach absolute zero in a finite number of steps.
Why it mattered. It became the third law, and it won Nernst the 1920 Nobel Prize in Chemistry.
1908
10 July 1908
After years of building ever better coolers, Onnes turned helium gas into a liquid at about 4.2 kelvin. By pumping on the liquid he got down to about 1.5 kelvin, the coldest place on Earth at the time. Three years later his team found that mercury loses all electrical resistance when it is this cold.
Why it mattered. It opened the door to the science of the very cold, and won him the 1913 Nobel Prize in Physics.
1939
1930s (in print 1939)
If A is as warm as B, and B is as warm as C, then A is as warm as C. This is why thermometers work, but nobody had called it a law. Fowler did in the 1930s, and it appeared in his 1939 book with Guggenheim. The other laws already had numbers, so it became the zeroth law.
Why it mattered. It made the idea of temperature itself part of the rules.
1961 – 2012
Wiping out a bit of information turns out to cost heat, and atoms are cooled to billionths of a degree above absolute zero.
1961
1961 (tested 2012)
Landauer, a physicist at IBM, argued that erasing one bit of information must release a tiny minimum amount of heat. That link between information and entropy helped Charles Bennett explain in 1982 why Maxwell's demon cannot break the second law. In 2012 a team in Lyon trapped a single tiny bead with a laser and measured the heat, and it matched Landauer's limit.
Why it mattered. It showed that information is physical, and it sets a floor on how little energy computers can use.
1995
5 June 1995
Cornell and Wieman cooled a cloud of rubidium atoms to about 170 billionths of a degree above absolute zero. The atoms merged into one quantum blob, a Bose–Einstein condensate. It had been predicted 70 years earlier by Satyendra Nath Bose in India and Albert Einstein. Wolfgang Ketterle made one soon after, and all three shared the 2001 Nobel Prize in Physics.
Why it mattered. It showed how close to absolute zero we can get, while never quite reaching it.
By the numbers
Each step is a new way of getting colder. The line keeps falling but can never touch zero, just as the third law says.
Did you know?
Celsius's first scale ran backwards: water boiled at 0 and froze at 100.
Maxwell never called his tiny being a demon. Kelvin gave it that nickname in 1874.
The zeroth law was named after the first, second and third laws, so it had to take the number before one.
Boltzmann's grave in Vienna carries S = k log W, though it was Planck who first wrote it in that form.
Since 20 May 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin.
The coldest atoms made in a lab, at 38 trillionths of a kelvin, were about 70 billion times colder than deep space.
The people
1728 – 1799 · Chemist and physician · Scotland
Showed that melting and boiling soak up hidden heat without getting hotter.
1796 – 1832 · Engineer · France
Wrote one short book about steam engines that founded a whole science.
1818 – 1889 · Brewer and physicist · England
Measured how much work makes how much heat, and the unit of energy is named after him.
1822 – 1888 · Physicist · Prussia (now Germany and Poland)
Stated the first and second laws and invented the word entropy.
1824 – 1907 · Physicist and engineer · Belfast, Ireland, then Glasgow
Gave us absolute zero and the temperature scale that starts there.
1839 – 1903 · Physicist and chemist · USA
Turned thermodynamics into the working rules of chemistry.
1844 – 1906 · Physicist · Austria
Explained entropy by counting the ways atoms can be arranged.
1864 – 1941 · Chemist · Germany
Found the third law about the approach to absolute zero.
1853 – 1926 · Physicist · Netherlands
Made helium liquid and discovered superconductivity.
1894 – 1974 · Physicist · India
His 1924 counting of light particles led to the prediction of the Bose–Einstein condensate.
Where it happened
Sources
Dates marked “c.” are approximate, and historians sometimes disagree about who was first. If you spot a mistake, tell us.
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