How The Total Solar Eclipse Of 1919 Helped To Confirm Einstein's General Theory Of Relativity

The total solar eclipse of April 8 2024 has generated a great deal of excitement all over the world. It covered some states in North America, Canada, and Mexico according to NASA. in a solar eclipse, the sun is completely blocked by the moon. This occurs only when the size of the moon is larger than the sun; when the moon is very close to the sun (nearer to perigee). Also, note that a variant of the total eclipse called the “annular” eclipse can also occur. However, this is when the moon is farther away and may not block the sun completely. In such an occurrence, the sun can be completely obscured from view. Total eclipses are rare because the sun, moon, and earth are not perfectly aligned most of the time ( you remember that the earth has a 23° tilt relative to the sun). Total eclipses happen when there is a coincidental alignment of these three celestial bodies such that they are on the same plane in the celestial sphere. The total eclipse is a rare occurrence ( it happens once in two or three years and can only be observed in very small regions of the earth meaning not many people can see it). The next total eclipse will happen on August 12, 2026. Due to its rare occurrence, many astronomers and researchers make expeditions to the locations where these eclipses are predicted to happen to capture the spectacular and rare sight and also to make experiments that may be valuable to physical science. This was how the total eclipse that occurred in 1919 catapulted Albert Einstein to fame and made him known as the greatest scientist ever. In the spirit of excitement of this solar eclipse, I decided to share a bit of history on how the phenomenon helped to confirm the general theory of relativity. 

General relativity and gravitational lensing

The  curvature of space-time in general relativity

The general theory of relativity is a beautiful theory of relativity that helps to describe the universe as a whole. It was put forward by Albert Einstein in 1911 and completed in 1915. It has accurately described the motion of objects in space and time, showing us how matter curves space-time. The concept of spacetime has been around for over a century. It brings a new understanding of how gravity works, better explaining the nature of attractive forces between astronomical objects. Classical explanations of gravitation, as put forward by Isaac Newton were accepted for over 200 years, explaining that there is a mysterious force that causes an attraction between two or more objects. Even Newton believed that light could be attracted by massive bodies but could not provide a perfect description of it. Einstein transformed the understanding of this gravitational effect by proposing that massive objects will warp the fabric of spacetime. In Einstein’s view, massive bodies instead create a curvature in spacetime such that light bends when it passes around it. This light may come from a star, galaxy, or any luminous body. This phenomenon is known as gravitational lensing.

The total solar eclipse of 1919

Principé Island where the experiment was conducted

On May 29, 1919, a total eclipse occurred through a narrow path off the west coast of Africa, across Central Africa and East Africa, and several regions in South America. After several attempts by scientists to use the phenomenon to test Einstein's theory, this eclipse gave a major milestone to the advancement of physics.

Einstein predicted that the general theory of relativity would imply that when light comes from distant objects, they are bent by the mass of the sun. One may ask: why do we need an eclipse to confirm it? The answer is thatyou can not observe the effect at night due to the absence of the sun. We need a situation where the sun is present, but prevented from shining, so we can have the darkness required to observe the light from the distant stars bending. This will help to test if the bending effect occurs.

The experiment was conducted in two separate regions where the eclipse was bound to occur. Sir Frank Watson Dyson, an influential astronomer in 1917, shared that a predicted total eclipse which will occur on May 29, 1919, would help to create the perfect conditions to put the theory to test. When the sun crosses the Hyades star cluster in the Taurus constellation, the light from the stars will have to pass through the sun's gravitational field and will be bent according to the curvature of spacetime. As there will be darkness, even in the presence of the sun, it will be possible to observe the effect with which the photographs obtained can be used to predict whether this effect occurs as predicted by Einstein. 

Arthur Eddington took up the idea and planned the Eddington experiments. The actual positions of these stars were first measured, then waited for the day of the eclipse to observe what would happen during an eclipse. To observe the effect, Eddington and his crew traveled to the island of Principé, a small island in the Gulf of Guinea where the total eclipse will be observed in full course. To ensure that results will still be obtained in case of unpredictable events (for example rain or clouds which may obscure the view) which may occur in Principé island, he sent another crew led by Andrew Claude de la Cherois Crommelin to Sobral, brazil, to take measurements. 

During the eclipse which lasted for about 6 minutes, several photographs were taken at the two locations to confirm the theory.

Results

Albert Einstein became famous after the experimental confirmation of his general theory of relativity

The results of the expedition were announced by Eddington on November 6 1919 at the Royal Society. This caused a great thrill in the astronomical society and has proved how a rigorous mathematical framework can accurately predict the behavior of the universe. This made Einstein the popular figure we know today. Several experiments have proven the theory of relativity since then including repeating the experiment at the subsequent total eclipse in 1922. So when you think about total eclipses, remember how they helped to transform our understanding of astronomy and physics.