This newscard is an excerpt from the original article published in the DownToEarth.

Einstein’s Eclipse

• Albert Einstein’s prediction of the bending of light by the gravity of the Sun, one of the components of his general theory of relativity, can be tested during a solar eclipse.
• Following an unsuccessful attempt to validate this prediction during the Solar eclipse of June 8, 1918, two expeditions were made to measure positions of stars during this eclipse.
• The eclipse presented a rare chance to verify one of the essential consequences of general relativity, the bending of light by gravity.
• Einstein’s theory predicted that rays of light passing near a massive body in space would be visibly bent as they followed the curve in space-time created by the body’s mass.
• In the case of a ray of light originating from a distant star and passing near the edge of the Sun, Einstein calculated a deflection of about 1.75 arc seconds.

Try this PYQ:

Q.Consider the following phenomena:

1. Light is affected by gravity.
2. The Universe is constantly expanding.
3. Matter warps its surrounding space-time.

Which of the above is/are the predictions of Albert Einstein’s General Theory of Relativity, often discussed in media?

(a) 1 and 2 only

(b) 3 only

(c) 1 and 3 only

(d) 1, 2 and 3

What was studied during the eclipse?

• Einstein published his theory and predictions in 1915, and in 1919 the British physicist Sir Arthur Eddington took advantage of a total solar eclipse to attempt to detect the shifting images of stars near the limb of the sun.
• The problem was that during totality the sky does not get perfectly dark, and only a handful of stars were visible near the sun from which to make the measurement.
• The darkness of an eclipse, though, would allow the astronomers to observe and photograph the field of stars around the Sun.
• By comparing the photographs with reference images taken at night, it would be possible to measure how much the presence of the Sun had bent the stars’ light.
• Conveniently, a cluster of bright stars known as the Hyades would appear near the Sun during the eclipse.

Significance

• After several months of analysis, researchers announced in November that their findings supported the theory of general relativity.
• Media coverage tended to dwell on the recondite nature of Einstein’s work, emphasizing that there were only a handful of people in the world who could understand it.
• It could be argued that 1919 was the year when Einstein’s name became a byword for superhuman intellectual ability—making possible the small industry of Einstein-themed merchandise that still exists today.

Back2Basics: General Relativity

• Einstein’s theory proposes that gravity is not an actual force, but is instead a geometric distortion of space-time not predicted by ordinary Newtonian physics.
• The more mass you have to produce the gravity in a body, the more distortion you get.
• This distortion changes the trajectories of objects moving through space, and even the paths of light rays, as they pass close-by the massive body.
• Even so, this effect is very feeble for an object as massive as our own sun, so it takes enormous care to even detect that it is occurring.
• General Relativity predicts how much of this bending of light you should see given the mass of the object.
• Called ‘gravitational lensing’ it has been detected on the cosmological scale as entire clusters of galaxies distort the light from more distant galaxies behind them as this image from the Hubble Space Telescope shows.

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