In 1905, Albert Einstein found that the laws of physics are similar for all non-accelerating spectators, and that the speed of light in a vacuum was independent of the motion of all viewers. This was the theory of special relativity. It presented a new framework for all of physics and suggested new ideas of space and time.

Einstein then spent ten years trying to add acceleration in the theory and issued his theory of general relativity in 1915. In it, he described that massive objects cause a distortion in space-time, which is sensed as gravity.

## The tug of gravity

Two objects employ a force of attraction on one another known as "gravity." Even as the center of the Earth is dragging you toward it (keeping you firmly lodged on the ground), your center of mass is dragging back at the Earth, although with much less force. Sir Isaac Newton quantified the gravity among two objects when he formulated his three laws of motion. Yet Newton's laws suppose that gravity is an inborn 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 similar for all non-accelerating observers, and he disclosed that the speed of light inside a vacuum is identical no matter the speed at which a spectator travels. As a result, he discovered that space and time were linked into a single continuum acknowledged as space-time. Events that happen at the same time for one observer could happen at different times for another.

As he worked out the equations for his general theory of relativity, Einstein comprehended that massive objects produced a distortion in space-time. Visualize setting a large body in the center of a trampoline. The body would press down into the fabric, affecting it to dimple. A marble rolled about the edge would spiral inward in the direction of the body, dragged in much the similar way that the gravity of a planet pulls at rocks in space.

## Experimental evidence:

Though instruments can neither see nor measure space-time, numerous of the phenomena expected by its warping have been confirmed.

Gravitational lensing: Light about a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lay behind it. Astrophysicists regularly use this method to study stars and galaxies behind huge objects.

Einstein's Cross: A quasar in the Pegasus constellation, is a superb illustration of gravitational lensing. The quasar is approximately 8 billion light-years from Earth, and sits behind a galaxy that is 400 million light-years away. Four pictures of the quasar appear around the galaxy as the strong gravity of the galaxy bends the light coming from the quasar.

 Einstein's Cross is an example of gravitational lensing.Credit: NASA and European Space Agency (ESA)
Alterations in the orbit of Mercury: The orbit of Mercury is shifting very slowly over time, due to the curving of space-time about the massive sun. In a few billion years, it could even crash into the Earth.

Frame-dragging of space-time around revolving bodies: The spin of a massive object, such as Earth, should bend and distort the space-time about it. In 2004, NASA launched the Gravity Probe B. The accurately calibrated satellite caused the axes of gyroscopes inside to drift very faintly over time, a outcome that agrees with Einstein's theory.

Gravitational redshift: The electromagnetic radiation of an object is stretched out somewhat inside a gravitational field. Think of the sound waves that originate from a siren on an emergency vehicle; as the vehicle travels toward an spectator, sound waves are compressed, but as it moves away, they are stretched out, or redshifted. Known as the Doppler Effect, the same phenomenon happens 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 discovered them to be minutely less than their natural frequency due to alterations produced by gravity.

Gravitational waves: Violent events, such as the crash of two black holes, are supposed to be able to generate ripples in space-time known as gravitational waves. The Laser Interferometer Gravitational Wave Observatory is currently searching for the first signs of these tell-tale indicators.

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