Five ideas that built modern science

Science is a huge pile of results, but a few ideas carry most of the weight. This is my list of five: a personal choice, not a ranking, built on Newton's mechanics and mathematics as the foundation. For each idea I give the claim, the evidence, the people and what it made possible.

They are not separate; the map shows how they hold each other up. The dating in The universe in one year: a cosmic calendar rests on atoms and energy, and Plate tectonics: the moving ground shows how slowly even a good idea can win.

The idea. Everything is built from a small set of kinds of atoms, which combine in fixed patterns into molecules.

The evidence. Greek thinkers such as Leucippus and Democritus guessed at atoms in the 5th century BCE, but evidence began with chemistry. John Dalton (1803, published 1808) noticed that elements combine in small whole-number ratios by weight: 12 g of carbon takes either 16 g of oxygen (carbon monoxide) or 32 g (carbon dioxide), a ratio of 1 to 2, just what lumps would do. In 1869 Dmitri Mendeleev arranged the elements in a table with gaps and predicted the missing ones; gallium (1875) and germanium (1886) turned up with the predicted properties.

Doubters remained: around 1900 Ernst Mach still treated atoms as bookkeeping. The turning point was the jiggling of pollen grains in water, seen by Robert Brown in 1827. Einstein showed in 1905 that unseen molecules bumping a grain would produce exactly that motion, with a testable formula. Jean Perrin tested it in 1908, and the leading sceptic Wilhelm Ostwald conceded the next year. Then the atom was opened: J. J. Thomson found the electron (1897) and Ernest Rutherford's team the nucleus (1909 to 1911), which is roughly a hundred-thousandth of the atom's width.

Made possible. Chemistry and materials, semiconductors, nuclear power, medicine. In 1989 IBM researchers pushed 35 xenon atoms into place, one by one, to spell their company's name.

The idea. Living things are made of cells, and cells come only from cells. Heredity travels in discrete units, genes, made of DNA and read out as a four-letter code.

The evidence. Robert Hooke coined "cell" in 1665 looking at cork. Schleiden and Schwann (1838-39) extended it to all plants and animals, and Rudolf Virchow put it in a sentence in 1855: every cell comes from a cell. Heredity needed counting. Gregor Mendel crossed pea plants in the 1850s and 1860s and found that traits do not blend: they come in units that reshuffle in fixed proportions.

His second-generation counts, 5,474 round to 1,850 wrinkled seeds, give 2.96 to 1. Nobody took notice until 1900. Thomas Hunt Morgan's fruit flies tied genes to chromosomes; in 1944 Avery, MacLeod and McCarty showed that DNA carries them; in 1953 Watson and Crick built the double helix, drawing on X-ray images by Rosalind Franklin and Raymond Gosling. The code was broken by the mid-1960s, and the first gapless human genome appeared in 2022.

Made possible. Insulin made by bacteria, DNA testing, gene editing with CRISPR (Doudna and Charpentier, 2012) and mRNA vaccines. The human genome has about 3.1 billion letters: at two bits each, roughly 775 megabytes, small enough for a memory stick.

The idea. Species descend with modification from common ancestors, mainly through natural selection: individuals vary, some variation is inherited, more are born than survive, and the variants that fit their surroundings leave more descendants.

The evidence. Charles Darwin and Alfred Russel Wallace reached the idea independently; their joint paper was read to the Linnean Society on 1 July 1858 and Darwin's On the Origin of Species appeared on 24 November 1859. Since then the evidence has come from every direction. Fossils appear in a sensible order; in 2004 a team searched 375-million-year-old rock on Ellesmere Island, because the theory said a fish with limbs should be there, and found Tiktaalik. The same bones sit in a bat's wing and your arm. And DNA gave the strongest test: all life shares one code, and humans and other primates carry the same broken copy of a gene for making vitamin C, a scar easy to explain by descent and hard to explain otherwise.

You can also watch it. After the drought of 1977 on the Galápagos island Daphne Major, the surviving finches studied by Peter and Rosemary Grant had measurably deeper beaks. Richard Lenski's bacteria experiment, started in 1988, has run for tens of thousands of generations.

In the 1930s and 40s Fisher, Haldane, Wright and Theodosius Dobzhansky joined evolution to genetics. Dobzhansky wrote in 1973 that "nothing in biology makes sense except in the light of evolution".

Made possible. Tracking antibiotic resistance and the family tree of a virus, conservation genetics, and directed evolution, in which enzymes are improved by mutation and selection in a test tube (Frances Arnold, Nobel Prize 2018). Evolution explains what happens once there is heredity. It does not by itself say how the first copying system arose; that is the subject of Life's origin: what we know and what we don't. And it is the design process behind every example in Nature as engineer: what nature teaches us.

The idea. Energy changes form but its total never changes. In every process some of it becomes less useful: entropy does not decrease. Together these two laws forbid perpetual motion and put a hard ceiling on turning heat into work.

The evidence. In 1775 the Paris Academy of Sciences decided to stop examining perpetual-motion machines, because none had ever worked. The measurements came in the 1840s. James Joule let falling weights turn a paddle in water and measured a tiny rise in temperature, and found that the same work always gave the same heat, however it was done. (Water falling 100 m warms by only 0.23 degrees Celsius, so he needed extraordinary thermometers.) Mayer and Helmholtz made it a general law. Sadi Carnot had already shown in 1824 that an engine's best efficiency depends only on two temperatures, and Clausius named entropy in 1865. Ludwig Boltzmann then explained it by counting the arrangements of atoms, which welded idea 4 to idea 1.

Real coal plants reach about a third to 40%, and the best gas plants approach 60%. A beautiful footnote: Emmy Noether proved in 1918 that energy conservation follows from the laws of physics not changing with time.

Energy also played detective: using the new laws, Kelvin argued that the Sun was only tens of millions of years old, far too young for geology and Darwin. He was wrong only because fusion was still unknown.

Made possible. Engines, power stations, refrigeration, and the chemistry of why reactions go.

The idea. Space is not empty. It holds fields, electric, magnetic, gravitational and others, and light is a wave in them. Two further ideas sit on top. Relativity says space and time form one fabric that energy bends. Quantum theory says energy comes in packets and nature deals in probabilities. In modern physics the two merge: the world is made of quantum fields.

The evidence. Michael Faraday found in 1831 that a changing magnetic field drives a current, and imagined invisible lines of force. James Clerk Maxwell wrote the equations in the 1860s and calculated the speed of their waves from electric and magnetic constants alone. It came out as the speed of light: light is an electromagnetic wave. Heinrich Hertz made radio waves in 1887-88. Einstein published special relativity in 1905, and E = mc^2 says that one gram of mass is about 9 x 10^13 joules, around 21 kilotons of TNT. In 1919 an eclipse expedition led by Arthur Eddington measured starlight bending near the Sun as his 1915 general relativity predicted, and in September 2015 the LIGO detectors heard gravitational waves from two black holes that merged about 1.3 billion years ago.

Max Planck found in December 1900 that glowing bodies emit energy in packets. Einstein, Bohr, Heisenberg, Schrödinger and Dirac turned that into a theory that predicted antimatter (found in 1932). Its quantum-electrodynamics form predicts the electron's magnetic moment to about one part in a trillion, among the best-tested predictions in science.

You use relativity daily. A GPS satellite's clock runs fast by about 45 microseconds a day because gravity is weaker up there, and slow by about 7 because it is moving: net +38. Left uncorrected, positions would drift by roughly 11 km a day (38 microseconds times the speed of light).

Made possible. Electric power, radio, radar, the transistor (Bardeen, Brattain and Shockley, 1947) and so every computer, lasers, LEDs, MRI scanners and GPS.

Three patterns stand out. Each idea needed an instrument or a measurement before it won: Joule's thermometer, Perrin's microscope, Eddington's eclipse plates. They fuse in pairs: atoms plus energy is statistical mechanics, atoms plus quanta is chemistry, genes plus evolution is the modern synthesis, genes plus atoms is molecular biology. And acceptance was never one experiment. It came when independent lines of evidence, such as fossils and DNA, pointed the same way.

Other ideas could sit on this list: germ theory, plate tectonics, the expanding universe and computation each have a case. If you want to argue with my five, good. Arguing from evidence is the habit they all have in common.