The history
From a pendulum that always swings back to its starting height to a ghostly particle caught at a nuclear reactor: 300 years of learning that energy is never made or destroyed, only moved and changed.
In the 1600s, Galileo, Huygens and Leibniz noticed that something about moving things seemed to be kept, never lost. For a long time nobody could say where it went when a ball stopped or a wheel rubbed and got hot. In the 1840s a ship's doctor, a brewer and an army surgeon showed that heat, motion, electricity and food energy are all one thing, and in the 1900s Einstein, Noether and Pauli showed how deep the law really goes.
1669
Mass times speed squared kept in collisions
Christiaan Huygens, Netherlands
1807
'Energy' used in its modern sense
Thomas Young, England
1829
Work defined and kinetic energy written as ½mv²
Gaspard-Gustave de Coriolis, France
1842
Heat and work declared equivalent
Julius Robert Mayer, Germany
1847
Energy conservation as one general law
Hermann von Helmholtz, Germany
1853
The term 'potential energy'
William Rankine, Scotland
1918
Energy conservation linked to time symmetry
Emmy Noether, Germany
1956
Neutrino detected
Clyde Cowan and Frederick Reines, USA
1638 – 1775
Galileo's pendulum, Huygens's collisions and Leibniz's 'living force' hint that motion carries a hidden quantity that is never lost.
1638
In Two New Sciences, Galileo described a pendulum whose string catches on a nail partway through its swing. Even though the swing changes shape, the bob rises back to almost exactly the height it started from. He argued that a ball rolling down a ramp gains a speed that depends only on how far it drops, not on the slope.
Why it mattered. It was an early hint that height and speed can be traded back and forth without anything being lost.
1669
March 1669
Huygens had solved the puzzle of hard balls colliding back in the 1650s. In 1669 he published his rules and pointed out that if you multiply each ball's mass by its speed squared and add them up, the total is the same before and after a bouncy collision. The Royal Society in London printed a Latin version soon after.
Why it mattered. It was the first time the quantity mass times speed squared was shown to be kept in a collision.
1686
March 1686
Descartes had said the world keeps a fixed amount of 'motion', measured as mass times speed. In a short paper called the Brevis demonstratio, Leibniz argued that this was a mistake and that the true measure of a moving body's force goes with its speed squared. He later called it vis viva, Latin for 'living force'.
Why it mattered. It started a famous argument that lasted decades, and vis viva became what we now call kinetic energy.
1740
In 1722 the Dutch scientist 's Gravesande dropped brass balls into soft clay from different heights. A ball going twice as fast made a dent about four times as deep, so the effect grew with speed squared. Du Châtelet used these results in her book Institutions de physique to back Leibniz's living force.
Why it mattered. She helped show that mass times speed squared, not just mass times speed, measures what a moving body can do.
1775
So many inventors sent in machines that were supposed to run for ever that the Academy got fed up. It decided to stop examining perpetual motion machines, along with attempts to square the circle. Nobody could yet explain why such machines always fail, but experience said they did.
Why it mattered. It showed scientists already trusted that you cannot get work out of nothing, long before the law was written down.
1798 – 1846
Rumford boils water by boring cannons, Young names 'energy', and Mayer, Joule and Colding show that work turns into heat at a fixed rate.
1798
Read 25 January 1798
Most scientists thought heat was an invisible fluid called caloric. Rumford watched brass cannons being drilled and saw that the rubbing made heat without end. He put a cannon in water, used a blunt borer, and made the water boil in about two and a half hours without any fire.
Why it mattered. Heat that never runs out cannot be a stored fluid, so it pointed to heat being a kind of motion.
1807
1807 (lectures given 1802–1803)
In his lectures on natural philosophy, Young suggested that mass times speed squared could properly be called a body's 'energy'. The word comes from ancient Greek and used to mean activity. Young was probably the first to use it in something like its modern scientific sense.
Why it mattered. It gave the idea the name we still use.
1824
Carnot, a 28-year-old army engineer, wrote a short book asking how much useful work a steam engine can get out of heat. He showed that an engine works because heat flows from something hot to something cold, and that there is a best possible efficiency. He still believed in caloric, and only 600 copies were printed.
Why it mattered. His ideas became the starting point for thermodynamics, the science of heat and energy.
1829
In his book on calculating what machines do, Coriolis defined 'work' as a force times the distance it moves something. He put a half in front of Leibniz's living force, giving ½mv², so that the work done on a body exactly equals the energy of motion it gains.
Why it mattered. Engineers finally had a clean way to count energy going into and out of machines.
1841
1840 (abstract), 1841 (full paper)
The young Joule dipped wires in water and passed electric currents through them for half an hour at a time. He found the heat made grows with the wire's resistance and with the square of the current. This is Joule heating, the same effect that warms the element in an electric water heater.
Why it mattered. It showed electricity turns into heat by a precise rule, one more sign that different forms of energy are linked.
1842
1840 (voyage), May 1842 (paper)
In 1840 Mayer sailed to Java as a ship's doctor. When he bled sick sailors in the tropics, their vein blood was surprisingly bright red, and he reasoned that the body burns less food to stay warm in a hot place. Back home he argued that heat and motion are two forms of the same thing, and in 1842 estimated how much work equals a given amount of heat.
Why it mattered. He was the first to state the equivalence of heat and work as a general law of nature.
1845
1845–1850
Joule let a falling weight turn a paddle wheel inside a can of water and measured the tiny rise in temperature. After years of careful repeats, in 1849 he reported that 772 foot-pounds of work warm one pound of water by one degree Fahrenheit. That is within about 1 percent of the modern value.
Why it mattered. It turned the link between work and heat into a precise, trusted number.
1847 – 1900
Helmholtz and Clausius turn the idea into a law of nature, Rankine and Thomson give us 'potential' and 'kinetic' energy, and the joule becomes a unit.
1847
23 July 1847
Helmholtz was a 26-year-old army surgeon when he read his paper Über die Erhaltung der Kraft to the Physical Society in Berlin. Using mathematics, he showed that the same rule covers falling weights, heat, electricity, magnetism and living muscles. The main physics journal turned it down as too speculative, so he printed it as a booklet.
Why it mattered. It stated the conservation of energy as one general law for all of nature.
1850
18 February 1850
Clausius showed how Carnot's ideas about engines could fit with Joule's discovery that heat and work are the same kind of thing. He wrote that the heat you add to a gas either raises its internal energy or does work. Today this is called the first law of thermodynamics.
Why it mattered. Energy conservation became the first law of the new science of heat.
1853
1853 (potential), 1862 (kinetic)
In 1853 the Scottish engineer Rankine wrote about 'the law of the conservation of energy' and coined 'potential energy' for stored energy, like a raised weight. He called the energy of motion 'actual energy'. Thomson and Tait swapped that for 'kinetic energy', which appeared in their 1862 article titled Energy.
Why it mattered. These are the two words every student now uses to describe energy stored and energy in motion.
1889
31 August 1889
Following an idea put forward by the engineer William Siemens in 1882, an international congress adopted the joule as a unit of energy, along with the watt. One joule is roughly the energy needed to lift an apple one metre. Joule himself died that October.
Why it mattered. Every kind of energy, from food to lightning, can now be measured in the same unit.
1905 – 1960
Einstein shows mass is energy, Noether shows why energy is conserved, and Pauli saves the law by inventing the neutrino, which is finally caught in 1956.
1905
21 November 1905
In a three-page paper, Einstein showed that when a body gives out energy as light, its mass goes down by that energy divided by the speed of light squared. We now write this as E = mc². Mass and energy turned out to be two faces of one conserved quantity.
Why it mattered. It explained where the Sun's energy and nuclear energy come from, without breaking the conservation law.
1918
26 July 1918
Noether was helping David Hilbert and Felix Klein with Einstein's new theory of gravity. She proved a theorem showing that every smooth symmetry in the laws of physics comes with something conserved. Because the laws are the same today as tomorrow, energy is conserved. Klein presented her paper to the Göttingen science society.
Why it mattered. It explained, for the first time, why conservation of energy is true.
1930
4 December 1930
In beta decay, electrons shot out of atoms with less energy than they should have. Niels Bohr was even willing to give up energy conservation. Pauli wrote an open letter to physicists meeting in Tübingen suggesting that an invisible, uncharged particle carries away the missing energy. Enrico Fermi built a full theory around it in 1933 to 1934 and named it the neutrino.
Why it mattered. Instead of breaking the law, physicists trusted it enough to predict a new particle.
1956
June 1956 (published 20 July 1956)
Cowan and Reines placed big tanks of liquid next to a nuclear reactor, which sends out huge numbers of neutrinos. They picked up about three neutrino hits an hour and sent Pauli a telegram with the news. Reines shared the 1995 Nobel Prize in Physics for the discovery.
Why it mattered. Pauli's rescue of energy conservation was proved right, 26 years after his letter.
Did you know?
In 1775 the Paris Academy of Sciences decided to stop examining perpetual motion machines, together with attempts to square the circle.
Only 600 copies of Sadi Carnot's 1824 book on heat engines were printed, and it was largely ignored for years.
Rumford's cannon-boring experiment boiled water in about two and a half hours with no fire at all.
Helmholtz's famous 1847 paper was rejected by the leading physics journal of the day as too speculative.
Joule died on 11 October 1889, just weeks after the unit named after him was adopted.
Pauli skipped the 1930 Tübingen meeting where his neutrino letter was read, because he wanted to go to a ball in Zurich.
Joule's 1849 figure of 772 foot-pounds is close to the modern value of about 778, and one food calorie is about 4,184 joules.
The people
1564 – 1642 · Physicist and astronomer · Italy
Showed that a pendulum or rolling ball rises back to the height it fell from.
1629 – 1695 · Physicist and mathematician · Netherlands
Found that mass times speed squared is kept in bouncy collisions.
1646 – 1716 · Philosopher and mathematician · Germany
Named vis viva, the living force, the ancestor of kinetic energy.
1706 – 1749 · Mathematician and physicist · France
Used 's Gravesande's clay experiments to argue that energy of motion grows with speed squared.
1753 – 1814 · Physicist and inventor · Massachusetts (worked in Bavaria and England)
Boiled water by boring cannons, casting doubt on heat as a fluid.
1796 – 1832 · Military engineer · France
Worked out the limits of heat engines and launched thermodynamics.
1814 – 1878 · Doctor and physicist · Germany
First stated that heat and work are equivalent, inspired by bright red blood in the tropics.
1815 – 1888 · Engineer · Denmark
Independently linked friction, lost motion and heat in 1843, but was little known outside Denmark.
1818 – 1889 · Brewer and physicist · England
Measured how much work makes a given amount of heat, and gave his name to the unit of energy.
1821 – 1894 · Physician and physicist · Germany
Stated conservation of energy as one law covering all of physics and life.
1882 – 1935 · Mathematician · Germany
Proved that energy is conserved because the laws of physics do not change with time.
1900 – 1958 · Theoretical physicist · Austria
Predicted the neutrino to save energy conservation in beta decay.
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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