**12 Illuminating Facts About General Relativity**

By Caitlin Schneider in Mental Floss

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.”

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.

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