Gravity, Light and Solar Eclipses

In 1915, when Einstein published his Theory Of General Relativity, he was hardly known to the public, but scientists across countries were fighting his predictions head-on. This is understandable, as at that time Newton’s views and principals of gravity ruled the scientific sphere. Newton believed that time is absolute and that space cannot change. In his view, time and space were the stage setting within which physical events occurred. In contrast to this, Einstein not only believed but later on proved, that space is not static and that gravity is a manifestation of the curvature of space and time. He was challenging a 200 year old theory. What a rebel!

Because space-time is curved, objects moving through space will follow the “straightest” path along the curve, which explains the motions of the planets in our Solar System. We follow a curved path around the Sun because it bends the space-time around it. Same laws of physics apply to light. Photons of light are not technically affected by large gravitational fields, light, simply follows the distorted curvature of space. Easy enough to understand, right? However, without being able to experimentally test his theory, Einstein’s work would have never been taken seriously.

Luckily enough, the British astronomer, Sir Arthur Eddington was following Einstein’s work very closely at the time and thought of a way to test General Relativity and help the scientist change history. As the Sun is the biggest object in our solar system, its curvature of space-time would be the most noticeable and closest example we could potentially observe. However, to test Einstein’s theories, astronomers will have to study the positions of background stars, close to the Sun’s edge. It is impossible to make such an observation because the Sun is too bright – unless there is an eclipse.

During a total solar eclipse, the moon orbits directly in front of the Sun, blocking the light coming from the Sun’s disk. The basic idea here is that you need to compare the positions of the stars before the Sun arrives in its position of totality, and during an eclipse when the Sun is present. The star positions should appear to shift outwards from their normal sky position. Two years prior to the eclipse of 1919, Sir Frank Watson Dyson, Astronomer Royal of Britain, led an experiment that plotted the positions of the background stars close to the Sun’s limb during an eclipse. During the eclipse of 1919, the Sun was crossing the bright Hyades star cluster. The light from these stars would have to pass through the Sun’s gravitational field on its way to Earth, but it will be still visible due to the eclipse. If the positions of the stars can be measured during the eclipse and then compared to their normal positions in the sky, the effect of warped space-time could be observed and proven.


Hyades star cluster


Sir Arthur Eddington organised and led an expedition to Brazil, from where the eclipse was visible. Early in the year, he measured the “true” positions of the stars and then in May he went to the island of Principe (off the West coast of Africa) to measure the positions of the stars during the eclipse. There was a second team who traveled to Brazil to also take pictures of the eclipse, in case the cloud Gods decided to make an appearance. Fortunately, the period of totality was six minutes, the longest so far, which gave the astronomers more than enough time to document the relative locations of the stars in the Hyades cluster.

When Eddington returned to England and compared the data gathered before and during the total eclipse, he confirmed Einstein’s predictions and turned the German-born scientist into a sensation overnight. The Times published the news on November 7, 1919, and Einstein became known to people across the world as the poster boy of genius.

Albert Einstein and Sir Arthur Eddington

Since then, General Relativity has been tested in different ways, proving that Einstein’s view of warped space-time is very much the universe we live in.

The bending of light by massive objects is now known as gravitational lensing and physicists around the world use this knowledge to observe distant phenomena and better understand the expansion of our universe, but more on this another time.


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