Wikimedia Commons // Public Domain
Wikimedia Commons // Public Domain

12 Illuminating Facts About General Relativity

Wikimedia Commons // Public Domain
Wikimedia Commons // Public Domain

This year marks the 100th anniversary of a scientific breakthrough that fundamentally changed our world. 

In 1915, Albert Einstein presented his theory of general relativity, which proposed that gravity itself was the result of a warping of spacetime by massive objects like stars and planets. He was 36 years old and already quite famous in the world of theoretical physics, most notably for his theory of special relativity, which proposed that the laws of nature are the same for all nonaccelerating observers everywhere—and that the speed of light is constant (also, E=mc2!). At the time, these ideas rocketed Einstein to worldwide fame. Today, they're the basis for much of our understanding of the universe.

At the World Science Festival last week, the premiere of the stage performance Light Falls: Space, Time and an Obsession of Einstein shed new—well, you know—on Einstein's historic 1915 discovery. Led by physicist Brian Greene, the show featured a dramatic (and historically accurate) account of Einstein’s journey toward the incredible breakthrough. In celebration, here are a few things we learned.

1. A Compass Provided Early Inspiration.

When he was 5 years old, Einstein’s father gave him a compass. The instrument enthralled his curious young mind, as the needle always pointed north regardless of its position. The boy asked himself, "How?" And thus began Einstein's lifelong journey to understand unseen forces. "That experience made a deep and lasting impression on me," he later wrote. "Something deeper had to be hidden behind things."

2. So Did Clocks.

Another common instrument inspired Einstein too. At the turn of the 20th century, while young Albert was a clerk at a patent office in Bern, the world was becoming more technologically advanced and connected. It became increasingly important for clocks in faraway cities to agree on the time. Figuring out a way to synchronize the world’s timepieces led to many proposals that likely passed through Einstein’s hands. His own take on the problem was inspired by his lifelong fascination with light. He reasoned that if you could used light signals to coordinate and account for the infinitesimal travel time for the light to deliver the message, you could synch clocks pretty easily. But Einstein realized that two clocks moving at two different speeds—say, on two moving trains—wouldn't be able to precisely synchronize. This understanding of the relativity of time was an integral step in the development of his later theories.   

3. The Constancy of the Speed of Light Was a Huge Breakthrough.

While clocks can travel at different speeds, light can't. That's what Einstein postulated in 1905 with the special theory of relativity, which says the speed of light is constant. We take it for granted now, but at the time, this theory was radical. While supported by James Maxwell’s equations, the idea flew in the face of Newtonian physics. The concept that anyone in the universe, regardless of their own speed, would measure the speed of light as 300,000 km/s, meant that light behaves unlike anything else we know of. This core insight took him a step closer to the theory of general relativity, which essentially simply adds gravity to the equation. Special relativity put the burgeoning scientist on the map.

4. He Found Happiness in Strange Things.

In 1907, just two years after Einstein published the special theory of relativity, he had the “happiest thought of his life.” It wasn’t about a loved one, a remembrance, some sense of self satisfaction, or even the poetry of the cosmos. It was about a man falling from a building. Einstein realized that a man falling alongside a ball would not be able to recognize the effects of gravity on the ball. Again, it’s all relative. This connection between gravity and acceleration became known as the equivalence principle.

5. His General Relativity Drafts Are Contained in a Notebook.

When Einstein died in 1955, a small, brown notebook was found among his papers. It contained within it the notes he was taking while working through the ideas of general relativity from the winter of 1912 when he moved from Prague to Zurich. The Zurich notebook contains amazing bits like a modified four-dimensional Pythagorean theorem to account for the curvature of spacetime. The notebook also contains traces of Einstein’s mistakes (yes, even he made them). Wrong assumptions and dead ends are all contained in the pieces of aged graph paper. All were part of the path to greatness.

6. He Had Friends Who Helped Him Refine the Theory …

Marcel Grossmann and Einstein met in school, and they remained friends for the rest of their lives. Grossmann helped Einstein get hired at the patent office, and Einstein later called on him to help through some ideas. Grossmann was a mathematics professor at the Swiss Polytechnic when Einstein visited him in 1912, and the academic helped his old classmate with the math that would prove this new take on gravity. When the theory of general relativity was finally published, Einstein praised his collaborator: “Grossmann supported me through his help, not only in sparing me the study of the relevant mathematical literature, but also in the search for the gravitational field equations.”

7. ... and a Frenemy Who Accused Him of Stealing It.

David Hilbert was a fellow scientist and friend of Einstein’s—until their relationship took a negative turn leading up to the publication of the theory of general relativity. Hilbert too developed a theory of general relativity—and even published it five days before Einstein. What started as camaraderie and a supportive exchange of ideas turned into a bitter rivalry that included accusations of plagiarism. Since then, historians have examined the proofs and say that Hilbert’s lack certain key ingredients to make the theory work. In other words, history got it right: the cred belongs to Einstein. Oddly, a portion of Hilbert’s proofs are missing, with no indication of what they might have held.

8. The Introduction of the Theory Was Huge. 

In November 1915 Einstein presented his masterwork to the Prussian Academy of Science, wherein he introduced general relativity and what are now known as the Einstein field equations. The paper was published the following year, and while the man and the concepts received great attention (after all, Einstein was already a well-regarded figure), it wasn't until he was able to confirm the predictions that he became a towering figure in scientific achievement and a worldwide celebrity. It was a big moment for Einstein. He'd synthesized the ideas he'd been working on for 10 long years. Now he had to show the world he was right.

9. The Sun Helped Prove Him Right. 

As any good scientist knows, an unproven theory isn’t science, it’s philosophy. Einstein needed his equations to make accurate predictions about the behavior of objects in space. One of his conjectures held that light traveling near a large gravitational field should curve. To test it, Einstein needed the help of a solar eclipse, which would facilitate the view of starlight passing through the sun’s gravitational field. On May 29, 1919, in a test conceived by astronomer Sir Frank Watson Dyson, and with the help of Sir Arthur Eddington, astronomers were able to take pictures to compare with their "true" location and measure the bend of light of 1.75 arcseconds—the very number Einstein’s theories predicated.  “LIGHTS ALL ASKEW IN THE HEAVENS” read the November New York Times headline. From that moment on, Einstein was a superstar.

10. General Relativity Explained Mercury's Weird Behavior.

“The discovery was, I believe, by far the strongest emotional experience in Einstein’s scientific life, perhaps in all his life. Nature had spoken to him.”

-Abraham Pais

The general theory of relativity’s ability to explain the precession of the perihelion of Mercury—the change in orbital orientation the planet experienced when closest to the sun—gave Einstein another opportunity to test his theory. When it neared the sun, Mercury didn't behave as Newtonian physics predicted it should. The problem had baffled scientists for years. The behavior of gravity as laid out in the general theory explained these discrepancies. His understanding of how mass warps space ended a 200-year-old mystery about our celestial neighbor. 

11. His Scientific Papers Became Front Page News.

Once general relativity theory had been proven, Einstein skyrocketed to fame in a way that’s hard to imagine today. His papers were published in their entirety on the front page of newspapers like the Herald Tribune and pasted in department store windows where people would clamor to read them.

12. The Discovery Made So Much More Possible.

One hundred years later, the impact of the general theory of relativity is almost too massive to quantify. It’s why we have GPS, and it’s paved the way for our understanding of black holes and dark matter, the Big Bang and its immediate aftermath, and the discovery of our expanding (and accelerating) universe. It doesn’t stop there: we’re still waiting to see things like gravitational waves—little ripples in the fabric of spacetime—predicted by general relativity. Perhaps most importantly, the theory was a step that may one day lead to a grand unified theory that will complete the picture of the universe that humans have been trying to piece together since the beginning of our existence. Einstein’s one small step was a giant leap that we’ll spend perhaps another 100 years trying to match.

Why Your Radiator is So Loud, According to Science

Even if you live alone, it’s easy to believe that a tiny man with a hammer takes up residence in your radiator all winter long, happily banging away. What else could explain those sudden, low-pitched clangs, which are as unpredictable as they are annoying? But as SciShow’s latest video explains, tiny drops of water are to blame for the phenomenon.

“You have a central boiler that boils water, and the steam that the boiler makes takes up way more space than it did as a liquid,” host Stefan Chin explains. “As it expands, it pushes itself through a series of pipes. Now, assuming you've opened the intake valve to let the steam into the radiator, it will rush in and push the air out through a small air vent on the opposite end. And when there's enough steam inside the radiator, the pressure causes the vent to close, trapping the steam inside.”

As this steam cools, the radiator’s metal grows warmer. Some of the vapor converts back into water and drips down the pipes, but the steam can also fling droplets of condensed water against the radiator’s inner walls—and that's what causes those startling sounds.

Watch SciShow’s full video below to learn more about why your radiator becomes louder than a mechanic’s garage during the winter.

Hulton Archive/Getty Images
6 Priceless Documents That Reveal Key Moments Early in Einstein's Career
Hulton Archive/Getty Images
Hulton Archive/Getty Images

You've probably seen it before on coffee mugs, crocheted pillows, or personal journals. It's one of Albert Einstein's most famous quotes: "I have no special talent, but am only passionately curious."

Einstein wrote this self-effacing description on March 11, 1952 in a letter—seen below—to his biographer Carl Seelig. (The original German: Ich habe keine besondere Begabung, sondern bin nur leidenschaftlich neugierig.) The letter is one of some 200 priceless documents of Einstein's that are held in the library archives at ETH Zurich, the university where the scientist got his undergraduate degree in 1900.

einstein letter to seelig 1952
ETH Zurich

As was directed in his will, Einstein's papers went to Hebrew University in Jerusalem, which holds tens of thousands of his documents. In conjunction with Caltech and Princeton University, Einstein's professional home for 20 years, Hebrew University has made some of these documents searchable (and some viewable) online.

The collection at ETH Zurich is composed of letters and postcards he wrote to friends and colleagues, which were either donated or acquired from private collections, along with university papers associated with his days as a student and teacher there. These papers give us an intimate look at some seminal moments of his famed life long before he became fixed in the public mind as a wild-haired genius.

Mental Floss got to see some of these documents firsthand at the ETH Zurich Library. They're almost never on display, but are kept in a vault under lock and key. You can, however, see much of the collection online.

We've chosen six documents to highlight. For insight about each, we spoke to Michael Gasser, the library's director of archives.


letter about einstein from hertog to maier
ETH Zurich

When Einstein was 16, his family moved from Munich, Germany to Milan, Italy to start a business, and he dropped out of school. "He was just living in Milan for a year," Gasser says. "He didn't go to school there, he studied at home."

He then decided he wanted to go to college at the Federal Polytechnic Institute in Zurich—now known as ETH Zurich. But he wasn't 18 and lacked a diploma; both were required by the university. A well-connected friend of the Einstein family, a banker named Gustav Maier, wrote a letter on his behalf to Albin Herzog, the university director, asking that Herzog let Einstein, whom Maier called a wunderkind, take the entrance exam anyway. His plea worked: In the September 25, 1985 reply to Maier, above, Herzog writes that despite his misgivings about a wunderkind, Einstein can take the exam.

Einstein picked up his pencil in October 1895—and failed. He did fine on the mathematics and natural sciences sections but was deemed "insufficient" on language and history. Back to high school Einstein went. He got his diploma a year later at a school in Aargau, near Zurich. There was one upside: While Einstein was still in high school, Friedrich Weber, a physics professor at the university, let Einstein attend his lectures.


einstein's failing grade
ETH Zurich

Einstein did eventually get into Polytechnic/ETH Zurich, attending from 1896–1900. He did not impress his professors. "He was a strong-headed student in the sense that he attended some courses and skipped others. He was interested in some subjects and fields—especially [theoretical] physics—that were not taught at ETH Zurich at the time. He preferred to read papers at home," Gasser says. "This is clearly reflected in the student file he has. In his third term, he got the worst mark he could get in Switzerland: a 1, for a course on practical physics, from Jean Pernet. He was reprimanded by the head of the school."

Who wrote that thick black 1, above, is a mystery; Gasser says it likely wasn't Pernet himself but someone in the registrar's office. But whoever marked the grade seems to have had strong feelings about it. "It does look like an angry 1," Gasser says. "It stands out. It's not something you find often in such files."

There's also a remark about Einstein's scholastic habits written in his student file that Gasser says is hard to translate, but it essentially accuses him of "laziness."


einstein's failing grade
ETH Zurich

In Einstein's department, there were five students (above). Of the four who passed the final exams, Einstein had the lowest mark and was the only one who wasn't offered a job as an assistant teacher at ETH Zurich. The fifth student, and only woman, was his girlfriend (later wife) Mileva Maric, who failed.

When it came to cramming for tests, the diffident student Einstein often leaned on the meticulous notes kept by his classmate and close friend Marcel Grossman, who got the second highest exam score. After graduation, as Einstein struggled to find teaching work, Grossman, with the help of his father, hooked him up with a job as a clerk in the Swiss patent office in Bern in 1902. Grossman became a renowned mathematician. Einstein turned to his friend again when refining the math of one of his seminal works. "Grossman helped Einstein with some mathematical problems in the General Theory of Relativity," Gasser says.

Grossman died young, in 1936, after a slow and painful deterioration, likely from multiple sclerosis. "It was kind of a sad story," Gasser says. "Einstein kept in touch with some of his friends and former fellow students till the very end. He was a very loyal friend."


einstein letter to harbicht
ETH Zurich

"This is probably the most famous letter in all of ETH Zurich," Gasser says. It dates to May 15, 1905, when Einstein was employed at the Swiss patent office but in his spare time was plugging away at "very high-level work," including his doctoral thesis for the University of Zurich (which he dedicated to his pal Grossman). This letter is to mathematician Conrad Habicht, a close friend with whom he'd formed a small group called Akademie Olympia that discussed physics and philosophy over food and drink, usually in Einstein's Bern apartment. In the letter, Einstein is in high spirits, teasing Habicht about missing him on Easter, asking for Habicht's dissertation, and mentioning that he is working on four papers.

"Dear Habicht,

"Such a solemn air of silence has descended between us that I almost feel as if I am committing a sacrilege when I break it now with some inconsequential babble. But is this not always the fate of the exalted ones of the world? So what are you up to, you frozen whale, you smoked, dried, canned piece of soul, or whatever else I would like to hurl at your head, filled as I am with 70% anger and 30% pity! You have only the latter 30% to thank for my not having sent you a can full of minced onions and garlic after you so cravenly did not show up at Easter.

"But why have you still not sent me your dissertation? Don’t you know that I am one of the 1.5 fellows who would read it with interest and pleasure, you wretched man? I promise you four papers in return. The first deals with radiation and the energy properties of light and is very revolutionary, as you will see if you send me your work first. The second paper is a determination of the true sizes of atoms …

"The third proves that bodies on the order of magnitude 1/1000 mm, suspended in liquids, must already perform an observable random motion that is produced by thermal motion. Such movement of suspended bodies has actually been observed by physiologists who call it Brownian molecular motion. The fourth paper is only a rough draft at this point, and is on the electrodynamics of moving bodies which employs a modification of the theory of space and time."

What Einstein so casually refers to as a "rough draft" featuring a "modification" of the theory of space and time we know by a different name: the Theory of Special Relativity. He also got his Ph.D. in 1905, which would go down in history as Einstein's annus mirabilis, or miracle year.


classes einstein taught at ETH zurich
ETH Zurich

After 1905, Einstein became famous in his field virtually overnight, Gasser says. In 1909, the University of Zurich created a new professorship for theoretical physics, and Einstein was its inaugural professor. Other universities competed for him, including the German University of Prague.

Einstein was a good teacher. When his students at the University of Zurich learned that he was being lured away to Prague, they signed petitions to raise his salary, hoping to keep the rising star. "I think he had a good relationship with his students," Gasser says, but "he didn't want to invest much time in teaching."

After a couple years in Prague, he returned to Zurich in 1912 as a full professor at ETH. Above are some of the course offerings in the math and physics department for the winter term of 1912–1913. Einstein taught analytical mechanics, thermodynamics, and a seminar in physics. "It was seven hours per week," Gasser says. "That was a normal teaching load for the time."

But research remained his main interest. At the time he was working on the problem of gravitation; once again he collaborated with Grossman, now his fellow professor at ETH. This work would eventually play a role in his General Theory of Relativity.

When Berlin's Friedrich Wilhelm University offered him a professorship with no teaching obligations, Einstein couldn't resist, and in 1913 he left Zurich for Germany.


einstein letter to weyl 1916
ETH Zurich

In 1915, Einstein published The Formal Foundation of the General Theory of Relativity. One of its earliest and most enthusiastic proponents was a geometry professor and former colleague of Einstein's at ETH Zurich named Hermann Weyl, who sought to express the theory using mathematical formulas different from Einstein's. The letter above, dating to November 23, 1916, is Einstein's take on a lecture Weyl gave in which he proposed these other formulas. Einstein says his ideas are interesting and plays around with the equations. "He’s working out the math as he’s writing," Gasser says. "It’s very technical."

For us non-geniuses, one appeal of this letter lies not in its far-reaching intellect but in its scribbles and crossouts. It's consoling, somehow, to know that even Einstein made mistakes.

That notion wouldn't be lost on him, Gasser says: "He doesn’t describe himself as a solitary genius. He really believed in cooperation and was actively seeking help at some stages. He relied on excellent mathematicians, and this letter really illustrates this."

Two years later, in 1918, Weyl published his seminal work Raum, Zeit, Materie (Space, Time, Matter), which explained general relativity in more elegant mathematical terms than Einstein himself ever had. Einstein was greatly impressed. 


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