The sky has plentiful wonders. Among them, solar eclipse are one of the most popular. When we see the sun or the moon waning out of view in a matter of a few hours, a sense of wonder and curiosity is kindled. They constantly remind us of the enormousness and liveliness of our solar system. Eclipses are also among the oldest natural phenomena widely recorded, studied and predicted by different civilizations.

Apart from the undeniable visual pleasure that eclipses offer, they are also a source of understanding of the workings of the cosmos. For example, the fact that eclipses do not happen every month tells us that the moon’s orbit is tilted with respect to the earth-sun system, i.e. the earth, sun, and moon do not line up every month for eclipses to occur. But it turns out that in the year 1919, an eclipse helped us understand the cosmos on a much deeper level- one that is not obvious to the human senses.

The historic event that I am referring to is ‘The Great Total Solar Eclipse’ of May 29, 1919.

Even among eclipses, the total solar eclipses are the most spectacular. The entire moon moves in front of the sun, and during totality, i.e. when the entire sun is blocked out of view, you find that your surroundings have darkened. You are in the shadow of the moon. The day sky, for a few minutes of totality, is dark enough to reveal stars and planets that are bright enough to be visible. Words will always fail to describe this breathtaking sight.

Why is the 1919 total solar eclipse that happened nearly a hundred years ago, so special? Let us go back 4 years before the historic event.

The year was 1915. He was still a relatively unknown person in public but known in academic circles. He was known for his breakthrough work in fundamental physics through 4 of his papers that were published in 1905. One of these papers explained the photo-electric effect, establishing firmly, the particle nature of light. This would later win him the Nobel Prize in 1921. Another paper revealed The Special Theory of Relativity, the consequences of which also gave rise to the popular equation – Energy equals Mass multiplied by Speed of Light squared(E=mc2). Though his ideas were well known, they were hardly accepted, and to a much lesser extent understood. He instigated a revolution in our understanding of cosmos. The name was Albert Einstein.

As the scientific community was still trying to comprehend the path-breaking ideas, in 1915 he came up with another mind-bending theory called The General Theory of Relativity. One of the main ideas of the theory was that gravity bends space-time (for the relevance of this topic we will consider bending of space alone, not space-time). This led to an avalanche of crazy ideas, like the one that Gravity could bend light. The reasoning was this: If gravity can bend space, anything that moves in space will follow the bending (curved path) of space. For example, Earth moves around the Sun, because the space around the sun is curved by its own presence, and Earth is merely following the curved space as it moves. He viewed force of Gravity as no longer a force, but a mere consequence of the geometry of space-time. It follows from this, that light should also follow the curved path when it moves through space. In other words, gravity should bend light.
In science, evidence matters. More extraordinary the claim even more extraordinary the evidence must be.

The difficulty in this situation was multilayered. The first level was the difficulty in comprehending the idea, the second was arranging for an experimental set-up to test the idea, the third level was to try and do this in the middle of the First World War. The calculations showed that the amount of gravity needed to bend light to even observable levels is huge. Quite obviously, one cannot set up a lab.
When the nature of Nature cannot be tested in the labs, you turn Nature into labs. Scientists – as suggested by Einstein in his paper- knew that this particular idea could only be tested during a total solar eclipse. But how?

The stars in the sky are grouped into recognizable patterns called constellations. The relative distance between any 2 stars does not change from our point of view. Though in reality, the stars are moving at tremendous speeds, the effect of that motion is almost imperceptible on a time scale of centuries or even millennia. But given enough time, their relative positions would have changed in the sky. But since we are talking about a timeline of a few years, it does not make any difference.
When I say the relative distance between any two stars is fixed, I mean, for example, the pattern of the group of stars called ‘Orion the Hunter’ constellation would look the same from this year to next year or even to the next millennia. This fact comes in handy in testing the great idea.

On May 29, 1919, British astronomers Sir Arthur Stanley Eddington and Frank Watson Dyson, each led a team on an expedition, one to the West African island of Principe, and another to the Brazilian town of Sobral. They chose a particular group of bright stars called Hyades Cluster, part of Taurus Constellation. They knew this group of stars would be extremely close to the sun during the eclipse, and since they were bright they would become visible during the totality phase of the eclipse.

The objective of each team was simple: click as many pictures as possible during the totality phase. Both the teams were lucky to not have weather mess up their data collection. This was crucial, as had they not collected the data that day, the testing of the great idea would have had to wait until the next total solar eclipse which would also come with associated risks in weather conditions. Also, the totality phase of the 1919 eclipse happened to be the longest in nearly 500 years, nearly 7 minutes. The next longer eclipse was only in 1937.

Now, how can clicking pictures during the solar eclipse test the idea that light can be bent by gravity?

The relative positions of the stars in the Hyades cluster, when observed during the night (on some other day when the constellation would have appeared in the night sky), could be compared with the relative position of the same set of stars when they were close to the Sun. If in both these sets of pictures, the relative positions of stars were unchanged, then one could conclude that the presence of Sun had no effect on observing the stars.

how-a-solar-eclipse-made-Einstein-a-celebrity-bending-starlight
Image credits: NASA’s Cosmic Times

That is to say, the Gravity of the Sun has not bent the light from this group of stars. If it did, the position of the stars would appear to have changed. It is like when you put a spoon in a cup of water, it appears broken at the surface of the water. The reality is that from the surface of the water to the bottom of the cup, that part of the spoon merely appears to have changed its position because the light from that part of the spoon, as it exits the surface of the water, bends before it reaches our eyes.

The case here is similar, except that the light is from the stars and the act of bending is expected to be done by the gravity of the sun (the bending of space around the sun by its own presence).

The moment that the entire scientific community was waiting for had arrived. The post-May 29, 1919 solar eclipse analysis of the pictures revealed that relative positions of the stars had indeed changed. But even more remarkably, they had changed positions precisely by the amount predicted by Einstein in his General Theory of Relativity. Later in November the same year, the results were published. The genius of Albert Einstein made headlines across the world. Overnight, he became a rockstar in Science. Even today, Einstein is a household name.

The General Theory of Relativity had passed the first of several more rigorous tests that would eventually come. The theory has survived and in scientific terms, it has not yet been proven wrong. But scientists, by nature, want to prove the theory wrong; that’s how science progresses. Testing the theory in extreme regions like black holes and more might reveal a weak link in the theory.

It has stood the test of time and survived till date. More exciting discoveries are facing us. What a moment to be alive!

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