I recently watched “Einstein and Eddington”. The storyline is about how the English scientist Eddington became interested in Einstein’s General Theory of Relativity, and how he helped “release it to the world”. This all happened around the time of World War I. The Great War provides a continuous backdrop to the story; the scandal that an English scientist was promoting the work of a German-born scientist.
I’m unsure how historically accurate the storyline is (for example, did Eddington really write to Einstein to ask if his theories could predict the orbit of Mercury that could not quite be explained by Newton’s Theory of Gravity?). Nevertheless, I enjoyed the film and would recommend it.
So, how did Eddington demonstrate that Einstein was right?
One of the things that General Relativity predicts is that light will bend around a massive object (such as the Sun). Newtonian gravity also predicts this. However, General Relativity predicts that light will bend twice as much as the value predicted by Newtonian gravity.
So, we have something that can be tested. If we can measure how much light bends around the Sun, then the value obtained will show which prediction was right: Einstein’s or Newton’s.
An experiment could be devised to test this.
There are plenty of light sources that can be used to see how much light bends when passing by a massive object; there are millions of stars that we can see in the night sky. The problem is the “night sky” bit of that last sentence. We see the stars at night when the Sun isn’t there. How can we check if the Sun is bending starlight when the Sun is too bright to allow the stars to be seen?
Simple: run the experiment during a total eclipse.
So, Eddington and his colleagues performed some calculations to find a suitable location to observe the next solar eclipse, and determined which stars would be close to the Sun during the eclipse.
They calculated that the tiny island of Príncipe off the coast of West Africa would be an excellent place from which to observe and photograph the next solar eclipse on 29 May 1919. Eddington himself would go to Príncipe and he also despatched one team to Sobral in Brazil.
The Hyades open cluster was selected as the stars that would be measured. This is how it looks in the night sky:
Eddington’s experiment would attempt to image this star cluster during the eclipse. An image taken during the eclipse would then be superimposed on top of an image taken of Hyades at night (i.e. when the light from stars in the cluster was nowhere near the Sun). Both Einstein and Newton’s gravitation theories predict that stars in the eclipse image that were close to the Sun would appear to be shifted away from the corresponding stars in the night time image – i.e. there would be a gap between stars on the eclipse image and the night image. The theories predict different values; the size of this gap would determine which prediction was right.
When the eclipse and night images were compared a gap was found. And when measured, it confirmed that Einstein’s prediction was right. Something that the General Theory of Relativity (and its explanations for gravity) had predicted had now been confirmed experimentally.
The results were reported by the worldwide press. The Illustrated London News explains the experiment well with a nice graphic:
The New York Times got a bit “tabloidy”:
Almost overnight, Eddington had turned Einstein into an international celebrity.
The rest is history.Follow @kashfarooq