In the beginning of twentieth century, Albert Einstein published the most elegant physical theory known to mankind, the theory of general relativity. General relativity is the geometric theory of gravitation. It explains how gravity works through the notion of mass, energy and *space-time*.
According to Einstein, the force of gravitation is a *curvature* in *space-time* due to the presence of mass and energy (mass and energy are interconvertable). The more the mass the more the curvature of space-time. The *curved* space-time affects the trajectory of other massive particles and this results in gravity.
This curvature provides various intriguing phenomenon such as gravitational time dilation (extensively used in Interstellar), expanding universe (fundamental for Big Bang), Possibility for black holes etc.
Another significant result from relativity is that light curves with the curvature of space-time. So a very heavy object can bend light passing near it. If someone could show this experimentally then it would be the conclusive evidence for the theory of relativity.
The problem with this is that the mass needs to be immense to show even a slight change in trajectory of light. Hence normal lab experiments of early 20th century could not be used to prove or disprove relativity.
Therefore Einstein's Marvelous theory became very hard to test. But here, solar eclipse came to the rescue. The most massive object near earth is the sun and it can produce significant and observable amount of deviation to light. The problem is that the light from the stars near the sun is several times fainter than the Sun. So we can't normally observe them.
This situation changes at the time of solar eclipse, when most of sun’s light is blocked by moon. And we can thus photograph stars nearby sun, then compare it with the images without sun and observe the distortion due to sun’s gravitation.
The approximate deviation in Radians is given by the equation given below.
Where, theta = deviation in radians, r = radius of sun, M = mass of the sun, G = Universal gravitational constant, c = speed of light.
This equation can be used to match the observation with the values predicted by relativity. All modern measurements have been found to be correct to a very high degree of precision.
The first time solar eclipse was used to provide evidence for relativity was in 1919. And with the conclusive evidence from the experiment, Einstein became the house hold name for everyone, here is a news clipping:
The US Eclipse was a wonderful opportunity for many amamture astronomers to photograph stars near the sun during Eclipse and use simple mathematics to verify the most beautiful physical theory ever.
The bending of light due to massive heavenly bodies such as black holes can be further utilised as telescopes to view darter into the depths of our universes. The possibilities of astronomy are endless.
