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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.

Dean Mouhtaropoulos/Getty Images
Essential Science
What Is a Scientific Theory?
Dean Mouhtaropoulos/Getty Images
Dean Mouhtaropoulos/Getty Images

In casual conversation, people often use the word theory to mean "hunch" or "guess": If you see the same man riding the northbound bus every morning, you might theorize that he has a job in the north end of the city; if you forget to put the bread in the breadbox and discover chunks have been taken out of it the next morning, you might theorize that you have mice in your kitchen.

In science, a theory is a stronger assertion. Typically, it's a claim about the relationship between various facts; a way of providing a concise explanation for what's been observed. The American Museum of Natural History puts it this way: "A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts."

For example, Newton's theory of gravity—also known as his law of universal gravitation—says that every object, anywhere in the universe, responds to the force of gravity in the same way. Observational data from the Moon's motion around the Earth, the motion of Jupiter's moons around Jupiter, and the downward fall of a dropped hammer are all consistent with Newton's theory. So Newton's theory provides a concise way of summarizing what we know about the motion of these objects—indeed, of any object responding to the force of gravity.

A scientific theory "organizes experience," James Robert Brown, a philosopher of science at the University of Toronto, tells Mental Floss. "It puts it into some kind of systematic form."


A theory's ability to account for already known facts lays a solid foundation for its acceptance. Let's take a closer look at Newton's theory of gravity as an example.

In the late 17th century, the planets were known to move in elliptical orbits around the Sun, but no one had a clear idea of why the orbits had to be shaped like ellipses. Similarly, the movement of falling objects had been well understood since the work of Galileo a half-century earlier; the Italian scientist had worked out a mathematical formula that describes how the speed of a falling object increases over time. Newton's great breakthrough was to tie all of this together. According to legend, his moment of insight came as he gazed upon a falling apple in his native Lincolnshire.

In Newton's theory, every object is attracted to every other object with a force that’s proportional to the masses of the objects, but inversely proportional to the square of the distance between them. This is known as an “inverse square” law. For example, if the distance between the Sun and the Earth were doubled, the gravitational attraction between the Earth and the Sun would be cut to one-quarter of its current strength. Newton, using his theories and a bit of calculus, was able to show that the gravitational force between the Sun and the planets as they move through space meant that orbits had to be elliptical.

Newton's theory is powerful because it explains so much: the falling apple, the motion of the Moon around the Earth, and the motion of all of the planets—and even comets—around the Sun. All of it now made sense.


A theory gains even more support if it predicts new, observable phenomena. The English astronomer Edmond Halley used Newton's theory of gravity to calculate the orbit of the comet that now bears his name. Taking into account the gravitational pull of the Sun, Jupiter, and Saturn, in 1705, he predicted that the comet, which had last been seen in 1682, would return in 1758. Sure enough, it did, reappearing in December of that year. (Unfortunately, Halley didn't live to see it; he died in 1742.) The predicted return of Halley's Comet, Brown says, was "a spectacular triumph" of Newton's theory.

In the early 20th century, Newton's theory of gravity would itself be superseded—as physicists put it—by Einstein's, known as general relativity. (Where Newton envisioned gravity as a force acting between objects, Einstein described gravity as the result of a curving or warping of space itself.) General relativity was able to explain certain phenomena that Newton's theory couldn't account for, such as an anomaly in the orbit of Mercury, which slowly rotates—the technical term for this is "precession"—so that while each loop the planet takes around the Sun is an ellipse, over the years Mercury traces out a spiral path similar to one you may have made as a kid on a Spirograph.

Significantly, Einstein’s theory also made predictions that differed from Newton's. One was the idea that gravity can bend starlight, which was spectacularly confirmed during a solar eclipse in 1919 (and made Einstein an overnight celebrity). Nearly 100 years later, in 2016, the discovery of gravitational waves confirmed yet another prediction. In the century between, at least eight predictions of Einstein's theory have been confirmed.


And yet physicists believe that Einstein's theory will one day give way to a new, more complete theory. It already seems to conflict with quantum mechanics, the theory that provides our best description of the subatomic world. The way the two theories describe the world is very different. General relativity describes the universe as containing particles with definite positions and speeds, moving about in response to gravitational fields that permeate all of space. Quantum mechanics, in contrast, yields only the probability that each particle will be found in some particular location at some particular time.

What would a "unified theory of physics"—one that combines quantum mechanics and Einstein's theory of gravity—look like? Presumably it would combine the explanatory power of both theories, allowing scientists to make sense of both the very large and the very small in the universe.


Let's shift from physics to biology for a moment. It is precisely because of its vast explanatory power that biologists hold Darwin's theory of evolution—which allows scientists to make sense of data from genetics, physiology, biochemistry, paleontology, biogeography, and many other fields—in such high esteem. As the biologist Theodosius Dobzhansky put it in an influential essay in 1973, "Nothing in biology makes sense except in the light of evolution."

Interestingly, the word evolution can be used to refer to both a theory and a fact—something Darwin himself realized. "Darwin, when he was talking about evolution, distinguished between the fact of evolution and the theory of evolution," Brown says. "The fact of evolution was that species had, in fact, evolved [i.e. changed over time]—and he had all sorts of evidence for this. The theory of evolution is an attempt to explain this evolutionary process." The explanation that Darwin eventually came up with was the idea of natural selection—roughly, the idea that an organism's offspring will vary, and that those offspring with more favorable traits will be more likely to survive, thus passing those traits on to the next generation.


Many theories are rock-solid: Scientists have just as much confidence in the theories of relativity, quantum mechanics, evolution, plate tectonics, and thermodynamics as they do in the statement that the Earth revolves around the Sun.

Other theories, closer to the cutting-edge of current research, are more tentative, like string theory (the idea that everything in the universe is made up of tiny, vibrating strings or loops of pure energy) or the various multiverse theories (the idea that our entire universe is just one of many). String theory and multiverse theories remain controversial because of the lack of direct experimental evidence for them, and some critics claim that multiverse theories aren't even testable in principle. They argue that there's no conceivable experiment that one could perform that would reveal the existence of these other universes.

Sometimes more than one theory is put forward to explain observations of natural phenomena; these theories might be said to "compete," with scientists judging which one provides the best explanation for the observations.

"That's how it should ideally work," Brown says. "You put forward your theory, I put forward my theory; we accumulate a lot of evidence. Eventually, one of our theories might prove to obviously be better than the other, over some period of time. At that point, the losing theory sort of falls away. And the winning theory will probably fight battles in the future."

Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0
An Ancient Book Blasted with High-Powered X-Rays Reveals Text Erased Centuries Ago
Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0
Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0

A book of 10th-century psalms recovered from St. Catherine’s Monastery on Egypt's Sinai Peninsula is an impressive artifact in itself. But the scientists studying this text at the U.S. Department of Energy's SLAC National Accelerator Laboratory at Stanford University were less interested in the surface text than in what was hidden beneath it. As Gizmodo reports, the researchers were able to identify the remains of an ancient Greek medical text on the parchment using high-powered x-rays.

Unlike the Large Hadron Collider in Switzerland, the Stanford Synchrotron Radiation Lightsource (SSRL) used by the scientists is a much simpler and more common type of particle accelerator. In the SSRL, electrons accelerate to just below the speed of light while tracing a many-sided polygon. Using magnets to manipulate the electrons' path, the researchers can produce x-ray beams powerful enough to reveal the hidden histories of ancient documents.

Scanning an ancient text.
Mike Toth, R.B. Toth Associates, Flickr // CC BY-NC-SA 2.0

In the case of the 10th-century psalms, the team discovered that the same pages had held an entirely different text written five centuries earlier. The writing was a transcription of the words of the prominent Greek physician Galen, who lived from 130 CE to around 210 CE. His words were recorded on the pages in the ancient Syriac language by an unknown writer a few hundred years after Galen's death.

Several centuries after those words were transcribed, the ink was scraped off by someone else to make room for the psalms. The original text is no longer visible to the naked eye, but by blasting the parchment with x-rays, the scientists can see where the older writing had once marked the page. You can see it below—it's the writing in green.

X-ray scan of ancient text.
University of Manchester, SLAC National Accelerator Laboratory, Flickr // CC BY-NC-SA 2.0

Now that the researchers know the hidden text is there, their next step will be uncovering as many words as possible. They plan to do this by scanning the book in its entirety, a process that will take 10 hours for each of the 26 pages. Once they've been scanned and studied, the digital files will be shared online.

Particle accelerators are just one tool scientists use to decipher messages that were erased centuries ago. Recently, conservationists at the Library of Congress used multispectral imaging, a method that bounces different wavelengths of light off a page, to reveal the pigments of an old Alexander Hamilton letter someone had scrubbed out.

[h/t Gizmodo]


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