This hypothetical planet, imagined to be even closer to the Sun than Mercury, he named Vulcan, after the Roman god of fire.
They sought to catch a glimpse of Vulcan as it transited across the Sun’s disc or during a solar eclipse, when objects near the Sun might be more easily observed.
As such, there was a lack of cohesive and compelling evidence to vouch for the existence of planet Vulcan.
Einstein’s radical reimagining of gravity as a curvature of spacetime made specific, testable predictions that would compellingly validate its revolutionary claims.
The Sun’s brightness would ordinarily obscure such observations, so we would have to wait for a solar eclipse.
In the 1850s, Urbain Le Verrier, fresh from his triumph in predicting the existence of Neptune based on the wobbles in Uranus’s orbit, turned his attention to the closest known planet to the Sun, Mercury. He noted that the orientation of Mercury’s orbit, when it was closest to the Sun, was advancing more than expected by a tiny angle each century. Though seemingly small, this shift could not be accounted for by what was understood of Newtonian mechanics and the influence of other planetary bodies on Mercury.
Le Verrier hypothesised that perhaps Mercury, too, was receiving a tug from an undiscovered planet that was closer to the Sun and so luminously bathed in sunlight that it remained hidden in its glare. This hypothetical planet, imagined to be even closer to the Sun than Mercury, he named Vulcan, after the Roman god of fire.
In the following decades, numerous sightings of this new planet were reported by both amateur and professional astronomers. They sought to catch a glimpse of Vulcan as it transited across the Sun’s disc or during a solar eclipse, when objects near the Sun might be more easily observed. Still, the intense glare of the Sun made such observations extremely challenging; it was difficult to distinguish between a true planetary transit and sunspots or other celestial bodies. Some of the reported sightings were even contradictory. As such, there was a lack of cohesive and compelling evidence to vouch for the existence of planet Vulcan.
In 1915, as Einstein was finalising his theory of general relativity, he sought to test its validity by seeing if it could explain the observed anomaly in Mercury’s orbit. After applying the complex calculations of general relativity to the problem, Einstein realised that the small but persistent discrepancy unresolved for some decades could be precisely explained by his theory. In a letter to his friend Paul Ehrenfest, a jubilant Einstein wrote, “Imagine my joy at the result that the equations correctly yield the motion of Mercury. For a few days, I was beside myself with joyous excitement.” Indeed, this vindication of his theory gave a significant boost to its early acceptance.
Einstein’s radical reimagining of gravity as a curvature of spacetime made specific, testable predictions that would compellingly validate its revolutionary claims. General relativity predicted that light, despite being massless, would follow the contours of warped spacetime − such that a beam of light passing a massive star would bend ever so slightly. How could we test such a prediction? We could take the largest gravitational object near us − the giant star in our cosmic backyard, the Sun − and check whether the light from distant stars is deflected by it. The Sun’s brightness would ordinarily obscure such observations, so we would have to wait for a solar eclipse.
But in 1914, the world went to war and scientists who had previously worked across national boundaries suddenly found themselves on opposing sides; communication channels were disrupted, there was heightened suspicion and the flow of information so critical for the scientific enterprise was painfully stifled. German astronomer Erwin Freundlich planned an expedition to Crimea to observe an eclipse, but was promptly arrested by the Russian authorities and his equipment seized.
The turbulence of the war years made it difficult to proceed, despite several opportunities for observing eclipses. Still, the dramatic new theory had caught the attention of the British astrophysicist Sir Arthur Eddington, and, after the war ended, the total solar eclipse of May 29, 1919, presented a perfect opportunity to finally put it to the test. A British expedition was organised under the direction of Astronomer Royal Frank Dyson. Eddington led a team to the island of Príncipe off the coast of West Africa, while another team travelled to Sobral in Brazil. Both locations were on the path of totality, logistically feasible and had agreeable climatic conditions, and sending two teams increased the chances that at least one group would be graced by favourable skies.
As the Moon cast a shadow on the solar disc, blotting out its light, the objective was to photograph the positions of stars in the vicinity of the Sun. According to Einstein’s theory, a massive object like the Sun should cause a curvature in spacetime, which will bend the path of light from nearby stars as it passes. The apparent positions of the stars will thus shift by a specific amount when the Sun is in the path of their light – versus when it is not there.
Newtonian theory also predicted a shift, but by a different and much smaller amount. By comparing the precise positions of the stars during the eclipse to their known positions in the sky, the teams could also verify which of the two theories the results aligned with. Where, many millennia earlier, the Babylonians had anxiously anticipated eclipses to see what fate had in store for their kings, we now eagerly awaited the eclipse to determine what it meant for how we understood gravity.
Excerpted with permission from Brief History of the Universe (and Our Place In It), Sarah Alam Malik, Simon and Schuster UK.
