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General relativity grew out of Einstein’s theory of special relativity, which describes how the speed of light (in a vacuum) can always be constant. | Relativity describesIT
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19years of
relativity>> AN ASTRONAUT WAKES UP
in a spaceship, with no memory of how she got
there. Sitting alone in a chair, she wonders: “Where in
the universe am I?”
The ship has no windows. Its instruments are dead.
The only clue is the push of the chair against her body.
Phew, there’s gravity, she thinks. Her vessel must still
be on Earth.
But then a second possibility occurs to her. The ship
could be accelerating through space, pressing her into
the seat like a race car picking up speed. From inside the
vessel, there is — terrifyingly — no way to tell.
This spacefarer’s dilemma would have been familiar
to Albert Einstein. His 1915 general theory of relativity
built on the notion that gravity and acceleration are
not just easily confused, but are one and the same. This
equivalence, “the happiest thought” of Einstein’s life,
was his starting point for redefining gravity.
According to relativity, anything that can happen
inside of a box picking up speed — i.e., accelerating —
also happens in the presence of gravity. Imagine, for
example, a horizontal laser inside an elevator that’s
accelerating upward. As the light travels sideways,
the elevator rises, causing the beam to strike a spot
on the wall slightly lower than where it started. If the
elevator accelerates quickly enough, the beam visibly
bends toward the floor.
Einstein showed the same thing happens to a
beam inside a stationary elevator within a power-
ful gravitational field; the gravity bends the light.
Similarly, he expected a beam of starlight shouldbend when passing through the sun’s gravity. This predic-
tion proved correct when the stars moved during the
1919 solar eclipse. (See “Einstein’s Eclipse,” page 42.)
To link acceleration and gravity in this way, Einstein
overthrew one of his own heroes: Isaac Newton. You may
have learned that Newton described gravity as a force,
an invisible rubber band that pulls together objects with
mass. Newton’s math did a good job at predicting how
everything from projectiles to planets moved — but it
kept gravity separate from acceleration.
Einstein argued that gravity isn’t a force at all. He
described it as a curvature of time and space caused by
mass and energy. Confused? The German physicist was,
too, and he struggled with the theory for nearly a decade.
He got help from mathematician Marcel Grossmann,
an old friend who shared his notes when a young
Einstein skipped class.
Their math, laid down in 10 equations, explained
how gravity could move around objects via a
warped reality, accelerating without ever feel-
ing any mysterious Newtonian forces.^ DEin
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EVINPOWELLAn apple feeling no force usually stays in the same place (left). But when
gravity curves space and time (right), as Einstein’s general theory of
relativity predicts, the fruit winds up on the ground without feeling a force.
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