E = mc^2. General relativity. The photoelectric effect. Brownian motion.
Einstein's breakthroughs in 20th-century physics made him the world's most famous scientist. And the recognition is well-deserved — his work in general relativity, which was first published 100 years ago, is still inspiring new discoveries today.
But Einstein wasn't a one-hit wonder. He had a host of other ideas, big and small, throughout his life. From his work on a weird little weather vane to his novel refrigeration scheme, here are some examples of Einstein's lesser-known work. [ Creative Genius: The World's Greatest Minds ]
In the 1870s, British chemist Sir William Crookes developed a neat little curiosity called the radiometer, or the light mill. The contraption was made up of a glass bulb with most of the air sucked out, with several metal, rectangular pieces aligned inside, like a windmill. When the vanes were exposed to sunlight, they seemed to turn almost magically. When the lights went out, the vanes spun the other way.
The object intrigued Einstein, and he spent a lot of time trying to figure out how it worked. He even convinced his niece Edith Einstein to focus on the topic for her research, said Daniel Kennefick, a physicist at the University of Arkansas and author of "Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves" (Princeton University Press, 2007).
"He quite liked to work on things even if it wasn't a huge, really important subject in physics," Kennefick told Live Science.
Eventually, Einstein figured out one piece of the puzzle: Molecules hitting the warmer side of the vane will bounce off faster at its edges, producing slightly more pressure at the edge. Though this minute difference in pressure could move the radiometer, it wasn't enough to explain its top speed, which was eventually explained by a different type of effect at the edges of the plate.
Einstein is perhaps most famous as a theorist, and many iconic pictures show him writing at a chalkboard. But Einstein was more than that.
"He was very interested in practical matters," Kennefick said. "He had all these great theoretical ideas, but he liked to do experiments. He liked to tinker with things."
With Hungarian-born physicist Leo Szilard, Einstein developed a novel refrigeration scheme that involved no motors, moving parts or coolant. The idea exploits the fact that water boils at lower temperatures at lower pressure. (This is why water boils at a lower temperature at the top of Mount Everest than it does in Death Valley, in California's Mojave Desert.)
Einstein and Szilard had the idea to place a flask of butane on a flame burner and under a vapor of ammonia. Because the ammonia vapor had lower pressure, it lowered the boiling point of the butane, and as the butane boiled off, it sucked energy from its surroundings, cooling an adjacent compartment.
And all those years at the Swiss patent office weren't just good for dreaming up the general theory of relativity; Einstein also became an expert in the patent process and got the novel refrigeration idea patented in 1930. [ Einstein Quiz: Test Your Knowledge of the Famous Genius ]
One of Einstein's virtues was that he knew a good idea when he saw it — even if it didn't come from a prominent person. When amateur Czech engineer Rudy W. Mandl came to the eminent physicist in 1936 with an idea, Einstein paid attention.
"Everybody else was telling this poor amateur scientist not to bother him, but Einstein very nicely gave him a whole afternoon," Kennefick told Live Science.
Mandl, who had studied Einstein's theory of general relativity, believed that if an object in space was big enough, it could bend light around it on all sides, creating a kind of gravitational lens that would focus the light that appears here on Earth.
Einstein himself had previously thought of the idea but had forgotten about it. Soon after, he did the calculations to show the process worked. The distinguished physicist hadn't thought the idea was all that important, and didn't want to take the trouble to publish it until Mandl pestered him into publishing the research in the journal Science.
But the "useless method" for calculating gravitational lensing turned out to be anything but. Several scientists went on to refine and build upon Einstein's work, and gravitational lensing is now a mainstay of modern astronomy, and is used to discover extrasolar planets.
That wasn't the only time Einstein took time to hear out an unknown scientist with an idea. He also took notice when Satyendra Nath Bose, an obscure physicist born in present-day Kolkata, India, contacted him with an alternative method for calculating statistics for photons (particles of light). Bose's work implied that, in a group of the same types of particles, all of the particles are fundamentally indistinguishable from one another.
Einstein quickly realized that if particles are cooled down to just a hair's breadth above absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius), they will all fall back to the same lowest energy state and will be indistinguishable. That, in essence, would mean that a collection of particles would act as if it were just one big particle, and would form an entirely new state of matter. This bizarre superblob of matter, now known as a Bose-Einstein condensate, has zero viscosity.
It took 70 years for scientists to prove that these superfluids exist, and the scientists who did so earned the Nobel Prize in physics in 2001.
Grand Unified Theory
Perhaps Einstein's most famous failure is that he never managed to create a theory that would unite all four fundamental forces (gravitation, electromagnetism, weak interaction and strong interaction). He spent his later years plugging away at the so-called Grand Unified Theory and died without cracking it. (To be fair, nobody has yet.) But that didn't faze Einstein.
"He himself very cheerfully said he kept changing his direction," Kennefick said.
And while Einstein could be very sure that he was right, he didn't mind being wrong. That resilience may have helped him keep working for days, weeks and years on the same problem without getting disheartened or distracted.
"It helped that he had all the genius insights to make progress, but I think a key factor was this focus and concentration and determination," Kennefick said.