General theory of relativity

In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating
observers, and that the speed of light in a vacuum was independent of the motion of all observers.
This was the theory of special relativity. It introduced a new framework for all of physics and
proposed new concepts of space and time.
Einstein then spent ten years trying to include acceleration in the theory and published his theory of
general relativity in 1915. In it, he determined that massive objects cause a distortion in space-time,
which is felt as gravity.
The tug of gravity
Two objects exert a force of attraction on one another known as “gravity.” Even as the center of the
Earth is pulling you toward it (keeping you firmly lodged on the ground), your center of mass is pulling
back at the Earth, albeit with much less force. Sir Isaac Newton quantified the gravity between two
objects when he formulated his three laws of motion. Yet Newton’s laws assume that gravity is an
innate force of an object that can act over a distance.
Albert Einstein, in his theory of special relativity , determined that the laws of physics are the same for
all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no
matter the speed at which an observer travels. As a result, he found that space and time were
interwoven into a single continuum known as space-time. Events that occur at the same time for one
observer could occur at different times for another.
As he worked out the equations for his general theory of relativity, Einstein realized that massive
objects caused a distortion in space-time. Imagine setting a large body in the center of a trampoline.
The body would press down into the fabric, causing it to dimple. A marble rolled around the edge
would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at
rocks in space.
Experimental evidence
Although instruments can neither see nor measure space-time, several of the phenomena predicted
by its warping have been confirmed.
Gravitational lensing: Light around a massive object, such as a black
hole, is bent, causing it to act as a lens for the things that lay behind
it. Astronomers routinely use this method to study stars and
galaxies behind massive objects.
Einstein’s Cross, a quasar in the Pegasus constellation , is an
excellent example of gravitational lensing. The quasar is about 8
billion light-years from Earth, and sits behind a galaxy that is 400
million light-years away. Four images of the quasar appear around
the galaxy because the intense gravity of the galaxy bends the light
coming from the quasar.
Changes in the orbit of Mercury: The orbit of Mercury is shifting very
gradually over time, due to the curvature of space-time around the
massive sun. In a few billion years, it could even collide with the
Earth.
Frame-dragging of space-time around rotating bodies : The spin of a
heavy object, such as Earth, should twist and distort the space-time
around it. In 2004, NASA launched the Gravity Probe B . The
precisely calibrated satellite caused the axes of gyroscopes inside to drift very slightly over time, a
result that coincided with Einstein’s theory.
Gravitational redshift: The electromagnetic radiation of an object is stretched out slightly inside a
gravitational field. Think of the sound waves that emanate from a siren on an emergency vehicle; as
the vehicle moves toward an observer, sound waves are compressed, but as it moves away, they are
stretched out, or redshifted. Known as the Doppler Effect, the same phenomena occurs with waves of
light at all frequencies. In 1959, two physicists, Robert Pound and Glen Rebka, shot gamma rays of
radioactive iron up the side of a tower at Harvard University and found them to be minutely less than
their natural frequency due to distortions caused by gravity.
Gravitational waves: Violent events, such as the collision of two black holes, are thought to be able to
create ripples in space-time known as gravitational waves . The Laser Interferometer Gravitational
Wave Observatory is presently searching for the first signs of these tell-tale indicators.

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