Who was Albert Einstein? His big ideas, in plain words

Albert Einstein was a German-born physicist who lived from 1879 to 1955 and changed how scientists understand space, time, light and gravity. In one year, 1905, he showed that light comes in packets of energy, that mass and energy are the same thing, and that time passes differently for things in motion. In 1915 he explained gravity as the curving of space and time. He won the Nobel Prize in Physics for 1921.

In short

• In 1905, while working as a patent clerk, Einstein published papers on light, atoms, relativity and E=mc² that reshaped physics.
• His general theory of relativity (1915) explains gravity as curved space and time, and its predictions are still being confirmed.
• He helped found quantum physics and then spent decades doubting where it led.

Who was Einstein, in brief?

Einstein was born on 14 March 1879 in Ulm, in the German Empire, and grew up in Munich. He left school at 15, finished it in Switzerland, and graduated in 1900 from the Federal Polytechnic in Zurich as a physics and mathematics teacher. He couldn't find an academic post, so from 1902 he worked at the Swiss patent office in Bern, checking inventions. He did his most famous work there, in his spare time.

Fame came in stages. After 1909 he held professorships in Zurich and Prague and from 1914 in Berlin. In 1919, measurements during a solar eclipse matched his prediction for how much the Sun's gravity bends starlight, and newspapers around the world made him a celebrity. When the Nazis came to power in 1933, Einstein, who was Jewish, left Germany for good and joined the Institute for Advanced Study in Princeton, New Jersey. He became a US citizen in 1940 and died in Princeton on 18 April 1955.

What did Einstein discover in 1905?

1905 is called his miracle year, and the name is fair. A 26-year-old with no university job published four papers that each opened a field.

Paper (1905)	The idea, in plain words	What came of it
Light quanta	Light arrives in tiny packets of energy, later called photons, which explains why light knocks electrons out of metal only above a certain color	A founding idea of [[what-is-quantum-mechanics
Brownian motion	The jittering of pollen grains in water is caused by molecules hitting them	Strong evidence that atoms and molecules are real
Special relativity	The laws of physics, and [[what-is-the-speed-of-light	the speed of light]], are the same for everyone moving steadily; so moving clocks run slow and moving objects shorten
E=mc²	Mass is a form of energy: a little mass equals a great deal of energy	Explains where the energy of nuclear reactions, in power plants, bombs and the Sun, comes from

His Nobel Prize, the 1921 prize awarded in 1922, was given "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect", not for relativity, which was still seen as too contested.

What does E=mc² actually mean?

The equation says that the energy (E) locked up in a piece of matter equals its mass (m) times the speed of light (c) squared. Because the speed of light is about 300 million meters a second, c squared is a gigantic number, 90,000 trillion. Here is what that means for one gram of matter, about the mass of a paper clip, worked through:

• Energy = 0.001 kilograms × (300,000,000 meters a second)², which is 90 trillion joules.
• One kiloton of TNT releases about 4.2 trillion joules.
• So one gram of mass, turned wholly into energy, equals about 21 kilotons of TNT.

The bomb dropped on Hiroshima released about 15 kilotons. It converted less than one gram of its uranium into energy. Nothing in daily life turns mass into energy that completely; nuclear reactions convert only a small fraction, which is still millions of times more than burning fuel. That is how nuclear weapons and the Sun get their power.

What is general relativity?

Special relativity left gravity out. Einstein spent ten years putting it in, and in November 1915 he completed the general theory of relativity. Its central idea is that mass and energy curve spacetime, and objects moving through curved spacetime follow the curves. The Earth circles the Sun not because a force pulls it along but because it follows the straightest path it can through space that the Sun has bent. The theory is explained further in what the theory of relativity says.

General relativity keeps passing tests a century on. It explained a small wobble in Mercury's orbit that Newton's laws could not. It predicted gravitational waves, ripples in spacetime, which detectors called LIGO first picked up on 14 September 2015, from two black holes colliding 1.3 billion years ago. And it is in your pocket: the clocks on GPS satellites run faster than clocks on the ground by about 38 millionths of a second a day, mostly because gravity is weaker up there. Without correcting for it, GPS positions would drift by about 10 kilometers a day, as the astronomer Richard Pogge explains.

Why did Einstein doubt quantum mechanics?

Einstein helped start quantum physics, but he never accepted what it became in the 1920s. The new theory said that nature, at the smallest scale, is a matter of probabilities: you can predict how likely an outcome is, not which one will happen. In a 1926 letter to the physicist Max Born he wrote that he was convinced "He", meaning God, "does not throw dice", which is the source of the line usually quoted as "God does not play dice."

In 1935, with Boris Podolsky and Nathan Rosen, he argued that quantum mechanics must be incomplete, because it implied that two particles could stay linked across any distance, which he later called "spooky action at a distance." Experiments since the 1970s have come down against Einstein's hope for a hidden, more complete theory of the kind he expected, and the 2022 Nobel Prize in Physics went to three physicists for that work. Yet his objection drew attention to entanglement, which is now the basis of quantum computing research.

Still argued

What quantum mechanics says about reality is still argued. The predictions are not in doubt; how to read them is. Some physicists hold that measurement truly creates definite outcomes, others that every outcome happens in branching worlds, and others that hidden variables of a different kind than Einstein imagined still exist. Experiments have ruled out the simplest version of his view, not the question he asked.

What did Einstein not do or say?

Several stories about Einstein are wrong, and many popular quotes were never his.

Claim	What the record shows
He failed math at school	False. He mastered calculus before 15. Shown a 1935 Ripley's column claiming he had failed, he replied, "I never failed in mathematics," as his biographer Walter Isaacson recounts. He did fail a university entrance exam at 16, on the non-science parts
He built the atomic bomb	No. He signed a letter to President Franklin Roosevelt, dated 2 August 1939 and drafted with Leo Szilard, warning that Germany might build one. He did no work on the Manhattan Project
"Insanity is doing the same thing over and over and expecting different results"	No evidence he said it; it appears decades after his death

A quick check for any Einstein quote: if it can't be traced to a letter, a speech or a book with a date, treat it as doubtful. In his later years he was a public voice for civil rights, against nuclear arms, and for a Jewish homeland; in 1952 he turned down an offer to become president of Israel.

Einstein's way of working, imagining a situation precisely and following the logic wherever it led, then letting experiment decide, is a good picture of how science works at its best.

Check yourself

• Name the four ideas of 1905 and say in one line what each one changed.
• Without looking back, explain why one gram of mass holds so much energy.
• Why do GPS satellites need Einstein's theories to work?

Sources

• Albert Einstein, Facts — The Nobel Prize
• Albert Einstein — Wikipedia
• Gravitational Waves Detected 100 Years After Einstein's Prediction — LIGO Caltech, 2016
• Real-World Relativity: The GPS Navigation System — Richard W. Pogge, Ohio State University
• Making the Grade, from 20 Things You Need to Know About Einstein — Walter Isaacson, TIME, 2007
• Manhattan Project: Einstein's Letter to Roosevelt — U.S. Department of Energy, OSTI