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IN SCIENCE, THEORIES MUST GIVE BIRTH TO TESTABLE PREDICTIONS
General Relativity (a theory) Predicts that Light is Affected by Gravity

Here are two examples of how scientists work with scientific theories, in this case the Theory of Relativity. (The images go with the second example.)

The curvature of space was originally proposed by Einstein in 1915, in the General Theory of Relativity.  A central prediction made from that theory predicts that light will bend in a gravitational field

GENERAL RELATIVITY
This theory generalizes Einstein’s Special Theory of Relativity and Newton’s law  of universal gravitation.  In so doing, it provides a unified description of gravity as a geometric property of space and time.  (Which therefore is more economically called spacetime.)  In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. [WP]

Less than five years later, this prediction was tested during the solar eclipse of May 1919, and Einstein’s calculations were shown to be correct.

During a solar eclipse expedition to Principe Island near the west coast of Africa led by the renowned British astronomer Arthur Eddington, positions of stars around the darkened solar disk were observed. It was found that the stars had moved almost 2 arc seconds (1/1800 of a degree) outwards on the sky, compared to when the Sun was not in the vicinity.

The expedition’s conclusions confirmed the prediction. You can read them first hand in the article in the 1920 article in which they were published:

  • Dyson, F.W.; Eddington, A.S., & Davidson, C.R. (1920). “A Determination of the Deflection of Light by the Sun’s Gravitational Field, from Observations Made at the Solar eclipse of May 29, 1919”. Phil. Trans. Roy. Soc. A 220 (571–581): 291–333. 

The article is freely available as a PDF here: 10.1098/rsta.1920.0009 
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General relativity also predicts the existence of gravitational waves and of phenomena like gravitational lensing due to the warping of spacetime by gravity.  The first image above shows what this looks like and the second, how it happens.

[1] Hubble Space Telescope image of a massive cluster of galaxies called Abell 2218, some 2 billion light-years from Earth.  
         Its enormous gravitational field deflects light rays passing through it, much as an optical lens bends light to form an image. This phenomenon, called gravitational lensing, magnifies, brightens, and distorts images from faraway objects.
         Here it has produced the arc-shaped patterns in the image. These "arcs" are the distorted images of very distant galaxies, which lie 5 to 10 times farther than the lensing cluster. [Hubble]

[2] The light rays (the grey arrows) from the distant galaxy (to the right in the image) are bent when passing a large mass — such as the galaxy cluster symbolised by the ball with blue glow in the centre.
         When the light finally arrives at the Earth (to the left),
Hubble observes it as coming from a slightly different direction (the red arrow). [Hubble]

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