A century ago, on May 29, 2019, a total solar eclipse occurred over West Africa and South America. During the event, two Bristish Astronomers, A.Eddington and F.Dyson could observe a fascinating phenomenon.
In this article, we are about to discover the noteworthy experiment which brought Albert Einstein to the fore and gave his very popular title of Genius!
★ Reminders on General Relativity
"The happiest idea of my life" were the words of the illustrious scientist known as Albert Einstein when he published in 1915 his theory of general relativity. The laws on which this theory is built were entirely brand-new ones, and needed an experimental validation that is to say a test of the theoretical predictions applied on well-know scientific phenomena.
The general relativity is based on a new approach of gravity. It reformulates the definition of gravity, the latter had been applied since 1687 with the Newton's law of universal gravitation. For instance, in the revolutionary Einstein's theory, space and time are warped altogether by gravity. In other words, the behavior of mass (matter) gives interaction to space and time ; something perfectly summed up by professor John Wheeler: "Space-time tells matter how to move; matter tells space-time how to curve".
The consequence of this concept brings us to visualize every massive celestial body as a disturbance of space-time. In the case of a massive star, the 3 dimensions of its surrounding space are warped, as well as time, both caused by the effect of gravity. We should no more see gravity as a force but rather as distortion (curvature) of space-time as a bowling ball lying on a trampoline deforms locally its elastic hessian.
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| Illustration of the bending of starlight close to a strong gravitational field (Sun) |
Within this situation, we can easily imagine a majority of scientists remained, at that time strongly skeptical regarding the idea of wobbling the Newton's gravitation model. They needed some proof and it was a legitimate argument from them! Indeed the point of every theory is to undergo series of tests based on real-life applications. In the opposite case, a theory will remain pointless and useless.
Despite the lack of enthusiasm from scientists, in 1915 Einstein had still drown up 3 observational experiments which could demonstrate he made the right predictions:
- the precession of Mercury perihelion
- the deflection of starlight passing close to the Solar disk
- or even the red-shift of light caused by gravity
The proposition of measuring the bending of starlight surrounding the solar disk during a total solar eclipse inspired two scientists: Arthur Eddington and Frank Dyson.
★How a Total Solar Eclipse can prove Eintein's General Relativity?
Gathering all the explanations we set our earlier in this article, we understood that massive bodies such as stars, star clusters, galaxies or even black holes are able to distort their nearby space. Since light respects physical laws, space itself dictates to light which path it should take to propagate. Consequently, light rays bend as they pass close to a heavy object, like a lens made of glass.
Light rays of a distant star can be considered as parallel. As soon as light travels in the vicinity of a massive celestial object like our Sun, light rays are curved inwards with a precise angle dependent of the distance between those rays and the Sun's gravitational field. This effect is just a bias which make us believe the background star is much further away of the sun that it actually is.
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| Drawing showing the gravitational lensing effect. Credit: ESA/NASA/Calçada. |
Remark: The curvature effect of space-time is regularly verified in modern astronomy with ground-based and space-based observations of dense objects (star and galaxies clusters...). It enables magnifying the image of a background target naturally by the so-called effect of gravitational lensing, and facilitates detection of new deep space objects.
Usually, it is entirely impossible to measure deviation of starlight close to the Sun since the starry sky is invisible by day. The solution is to find a moment when Sunlight is blocked. On Earth, we have the great opportunity to have a natural satellite, the Moon, that can fit and cover perfectly the solar disk: we know it as a Total Solar Eclipse! This coincidence can be explained very simply by the numbers. The Moon diameter is 400 times smaller than the Sun's, however the Moon is also 400 times closer to the Earth than the Sun is: this equilibrium between distances and diameter of the Sun and the Moon gives to the Moon the same apparent diameter as the Sun. Thus, our satellite can entirely recover the Sun as it passes in front of it.
★ The 1919 Solar Eclipse
It is this deflection of light rays Eddington and Dyson will try to quantify during two simultaneous experiments in different locations. Eddington chose the Principe island on the West coast of Africa whereas Dyson settled in the city of Sobral, in Brazil.
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| Equipment used for the 1919 experiment in Sobral, Brazil. |
The principle of the experiment was relatively simple in theory: equipped with telescopes and photographic plates the astronomers captured images showing the position of stars during the totality and then compared them with images of the same region of the sky without the eclipse. The difference between these 2 moments enabled to find back the deflection angle of the light rays close to the sun.
The 1919 Total Solar Eclipse (TSE) offered two important features:
- A 5-minute duration of totality gave to astronomers plenty of time to carry out their experiment and made sure their images were worthy. To put it into perspective, the 2017 TSE over the USA had just lasted 2 minutes. The 1919 TSE was the longest total eclipse ever observed within 5 centuries.
- A bright star cluster in the vicinity of the Sun (Hyades). This target was far easier to spot and to capture compared to the other faint stars in the sky. It contains lots of bright stars which helped to decrease the detection errors once the photographic plates were analyzed.
Nevertheless, both experiments did not smoothly succeed! In particular, Eddington had to handle with several problems: for example mosquitoes and bigger wild animals entering in the observation site. A more concerning issue arouse, dealing with the equipment: the whole series of 19 pictures taken during the eclipse were blurred because out-of-focus. Such a failure was caused by the high intensity of sunlight hitting the telescope's mirror and deforming its shape. Fortunately, Eddington was a devoted scientist and had foreseen the high-stake situation which could compromise his experiment. Therefore he brought a second setup, a smaller telescope reproducing the exact same process as the first one.
In any cases, the outcome of both experiments in Brazil and in Africa could show a light deflection of 1.6 arcsecs for Eddington against 1.98 arcsecs for Dyson. Theoretically, Einstein had predicted a deviation of 1.75 arcsecs. These angles are extremely narrow and are about the same as if you observe a 10" tablet from a distance of 30 km.
Right after the official publication of these stunning results, Einstein's theory of General Relativity was instantly popular and its related skepticism vanished. The 1919 Eclipse is the first ever observation which tested (and by the way verified) successfully his theory. Since then, the experiment became worldwide known as the Einstein's Eclipse, referring to the exciting experiment led by Sir Eddington.
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