Speed of Light

Variable Speed of Light
Effect of Gravity on Measured Speed of Light

An observer outside gravitational fields measures the speed of light locally (in his location) at 299792.458 km/s but when he looks towards a black hole he sees the speed of light there to be as slow as a few meters/s. At the same time an observer freefalling into that black hole measures the speed of light locally (in his location) at 299792.458 km/s; when he looks towards the black hole he sees the speed of light there much slower; when he looks away from the black hole he sees the speed of light there much faster. When he looks towards outside gravitational fields he sees the speed of light there a zillion km/s. In 1915 (10 years after Special Relativity) Einstein developed another theory called General Relativity that deals with gravitational fields and according to this latest theory the speed of light appears to vary with the intensity of the gravitational field.

Effective Speed of Light
Expansion of Universe & Speed of Light

Scientists just confirmed the existence of "Dark Energy", a mysterious repulsive force that acts in opposite to gravity. Scientists today do not know what this "Dark Energy" is but they know that it is causing the entire universe to expand at an increasing rate. This Dark Energy is even causing distant galaxies to recede from us faster than our local speed of light. This expansion is causing light en route to be dragged away from us. This makes observers to see distant inbound light at effective speeds less than 299792.458 km/s and to see distant outbound light at effective speeds greater than 299792.458 km/s. However every inertial observer measures the speed of light locally (in his location) at 299792.458 km/s in any direction.

Historical Measurements
Historical Measurements of Speed of Light

Before the 17th century scientists believed that there was no such thing as the "speed of light". They thought that light could travel any distance in no time at all. Later, several attempts were made to measure that speed. In 1667, Galileo is often credited with being the first scientist to try to determine the speed of light. His method was quite simple. He and an assistant each had lamps which could be covered and uncovered at will. Galileo would uncover his lamp, and as soon as his assistant saw the light he would uncover his. By measuring the elapsed time until Galileo saw his assistant's light and knowing how far apart the lamps were, Galileo reasoned he should be able to determine the speed of the light.

12000 Lunar Orbits/Earth Day
Inertial Earth-Moon System Gives: Speed of Light

But 1400 years ago it was stated in the Quran (Koran, the book of Islam) that angels travel in one day the same distance that the moon travels in 1000 lunar years, that is, 12000 Lunar Orbits / Earth Day. Outside the gravitational field of the sun 12000 Lunar Orbits/Earth Day turned out to be the local speed of light!!! This definition is independent of direction and common to all observers: An observer near a black hole, for example, sees the speed of light outside gravitational fields a zillion km/s but still equal to 12000 Lunar Orbits/Earth Day!!!

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