An Eclipse Confirms General Relativity

Albert Einstein’s general theory of relativity, first published in 1915, proved to be a revolutionary way of looking at the universe.  The three dimensions of space were combined with time to create a unified whole; and this space-time grid, instead of being fixed and static, could be warped in certain circumstances.

Textbooks on physics employ a very useful analogy when explaining how this space-time grid operates.  We are to imagine a flexible rubber sheet stretched out tightly, suspended in the air.  If we then place heavy balls on the surface of this sheet, it depresses under the weight of the ball, forming a cavity; and if a smaller ball were rolled on this rubber sheet, it would roll towards the heavier ball’s depression.  This, we are told, is how gravity operates.  The gravitational forces of celestial bodies like planets and suns warp or “bend” the fabric of space-time that surrounds them.  The larger the body, the greater the distortion of space-time around it.

This warping could even affect light rays, Einstein explained.  Faint light from a distant star, for example, would be deflected as that light passed close to a massive body.  This deflection could even be measured.  Einstein’s theories were so strange to scientists at the time that they were greeted with a mixture of excitement, apprehension, and skepticism.  The fires of the First World War were raging across Europe; and Einstein, then a professor in Berlin, could hardly have picked a less favorable time to propose a new theory of physical reality.  The passions of war tended to cloud judgment and inflame prejudices against the perceived enemy; and in the English-speaking nations at least, there were some who saw Einstein’s theories as little more than ponderous Germanic turgidities.

An opportunity to test the theory of general relativity finally came in 1919, one year after the conclusion of the First World War.  Astronomers knew that a solar eclipse would block the sun’s overpowering rays, and allow measurements of distant stars whose light passed near the sun.  If Einstein was correct, the light rays of these distant stars would be bent as they passed by the sun; the sun’s warping of space-time would affect light rays as surely as physical objects.  In other words, the actual location of a distant star would be different from its apparent location.  (This phenomenon is now known as “gravitational lensing”).  The only practical way to verify Einstein’s predictions was to wait for an eclipse.  During daylight, the sun’s brightness prevented observation of distant stars; and during the night, there was no massive body in the sky that might deflect the light from distant stars.

However, performing such an experiment in the field was not a simple matter.  When Einstein was refining his theory in the years before 1915, he had asked other scientists to try to measure the light from distant stars during eclipses.  One unfortunate astronomer, Erwin Finlay-Freundlic, incurred great expense to transport himself and crates of specialized equipment to the Ukraine in 1914 in order to observe an expected eclipse.  When war broke out, he was arrested by the czar’s police, detained as a spy, and had all of his precious instruments confiscated.  Other efforts were made to photograph eclipses, but no one was able to obtain images that were clear enough to allow precise measurements.

Sir Arthur Eddington, director of the Cambridge Observatory in England, was a believer in general relativity and was determined to prove it experimentally.  A total solar eclipse in 1919 would provide an opportunity; but Eddington wanted not just one expedition, but two.  Eddington would supervise a team on the island of Príncipe, near Africa’s western coast; Dyson would command an expedition to Sobral in Brazil.  Their goal was to obtain the best possible photographs of the eclipse.  By comparing images of distant stars during the eclipse with images of these same stars in the normal nighttime sky, they hoped to measure the deflection of the light rays of the distant stars as they passed by the edge of the sun.  We should note that the deflection of the light rays is extremely slight (less than two arcseconds); and this fact highlighted the importance of taking clear, crisp photographic images of the eclipse.  Although Newtonian physics also predicted such bending, it predicted much less of a deflection than did general relativity.

Bad weather on the day of the eclipse nearly ruined all of Eddington’s photographic plates on Príncipe.  He was able to get two images, and this proved to be sufficient.  He was forced to leave the island in a hurry due to an intervening strike by workers of a shipping line.  In Brazil, Dyson’s 16-inch telescope encountered problems and was able to produce only some fuzzy images.  Yet he had had the presence of mind to set up a “backup” telescope, a small 4-inch instrument lent to him by a Jesuit priest who was a science enthusiast.  How often in history, it seems, have the Jesuits played an indirect role in the advancement of learning!  The tribulations faced by these expeditions remind us how difficult it can be to collect data under actual experimental conditions.  Taken together, the deflection coefficients measured by Eddington and Dyson agreed more with Einstein’s predictions than with what Newtonian physics would have suggested.  General relativity, it appeared, had been experimentally proven for the first time.

On November 9, 1919, at a joint meeting of the Royal Society and the Royal Astronomical Society, Eddington and Dyson announced their sensational results.  They were greeted with joyous applause and exuberant public acclaim.  Einstein suddenly found himself on the front pages of newspapers around the world.  Measurements made during other eclipses in the 1920s in Mexico and Australia provided further confirmation of Einstein’s theory.  While the theory would still take years to find universal acceptance, few could now doubt its essential veracity.  (It is a tragedy that Eddington’s original photographic plates were lost or disappeared; one historian speculated that they may have been accidentally discarded after his death in 1944).

One of the more moving results of the 1919 experiment was the role it played in the postwar reconciliation between the wartime antagonists, Britain and Germany.  As Europe lay in ruins, with millions dead, there was finally at least a sense that the two nations could collaborate amicably on that noblest of goals, the advancement of knowledge.  The quest for truth might, in some small way, provide a vehicle for high-minded common endeavors that elevated, rather than debased, the world’s restless and hopeful masses.  As Arthur Eddington himself once said,

Whether in the intellectual pursuits of science or in the mystical pursuits of the spirit, the light beckons ahead, and the purpose surging in our nature responds.



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