Universal truths

worked out the laws of planetary motion. But Kepler could not explain why celestial bodies moved in that fashion.

Isaac Newton made a huge jump when he added an understanding of gravity, and formulated the generic laws of motion. Over the next two centuries, luminaries such as Pierre-Simon Laplace, Urbain Jean Joseph De Verrier, and sundry others built on these foundations.

Newton predicted planetary and cometary orbits. (Edmund Halley, of comet fame, edited Newton’s magnum opus, Principia Mathematica). Newton’s predictions matched astronomical observations with great accuracy. But some discrepancies remained.

As telescopes got more powerful and mathematicians learnt more tricks, those anomalies were picked apart. Over 200odd years, new discoveries were made by examining the astronomical data that did not match Newton’s predictions.

His Law of Gravitation indicated that massive objects influenced one another and quite a few of those anomalies could be explained by finding objects that were not easily visible. For example, discrepancies in the movements of Saturn and Jupiter indicated that some undiscovered celestial object (or objects) could be influencing those orbits. Similarly, anomalies in Mercury’s orbit led to the hypothesis that another planet was placed somewhere between Mercury and Sol.

The Saturn-Jupiter hypothesis was correct. Uranus and Neptune were found by focussing telescopes on those parts of the sky where 18th and 19th century astronomers guessed undiscovered objects could be. Once Pluto and other dwarf planets and asteroids were discovered in the outer reaches of the Solar System, the predicted orbits of the outer planets started to match observations more exactly.

But Mercury remained an enigma. The observed orbit differed from predictions by a tiny but significant amount. Observing Mercury is tough even using 21st century technology. It’s a small planet; it’s very close to the sun and hard to spot against background glare. More complications are introduced by sunspots — these relatively cool solar regions show up as dark spots which may be easily mistaken for planets. Eclipses help a lot. Mercury’s movement is mapped much more easily against the backdrop of a darkened sun.

The elusive inner planet between Mercury and the Sun was tentatively named Vulcan. But finding a small planet so close to the sun, or definitely proving that it did not exist, were both very hard tasks. Over a period of many decades, astronomers of all calibres lined up at eclipses to look for Vulcan and every so often, somebody would “find” it. But those observations were never replicated.

Einstein got into the act in the early 20th century. Sitting in Berlin during World War I, Einstein started to calculate multiple iterations of the General Theory of Relativity (GTR). By 1915, he could make two concrete quantitative predictions.

One was that the sun’s gravitational influence would bend starlight by much more than Newton’s laws predicted. The only way to check this was during an eclipse. Stars located behind the sun and normally obscured by sunlight could then be observed. Their apparent positions would differ from their known positions in the night sky due to the starlight being warped by the sun’s mass. Einstein also calculated Mercury’s orbit, using the GTR estimates to account for the influence of the sun’s mass. Again this could only be checked during an eclipse.

World War I delayed any attempts to look at eclipses until 1919 when Sir Arthur Eddington took photographs during an eclipse. Both the predictions proved to be correct. Vulcan, therefore, did not exist since Einstein’s calculations entirely matched Mercury’s orbit. A century later, the LIGO picked up gravity waves emitted during the merger of black holes and again, these matched Einstein’s GTR predictions.

Predicting and spotting planets has now become an art-form. Many exo-planets orbiting other stars have been discovered in the past five years, using new technology and techniques. But it all started with the 18th century discovery of Uranus.

There were other hypothetical planets. One was Phaeton — this assumed that the Asteroid Belt between Mars and Jupiter had been formed by the break-up of a large planet. (Most likely the Asteroid Belt never coalesced into a single planet.) Poseidon was thought to be another possible planet, somewhere out beyond Neptune. Eventually, Poseidon morphed into the Kuiper Belt, which includes Pluto and other dwarf planets and asteroids.

Newton’s model worked like clockwork. This book describes how that evolved into the integrated space-time model of Einstein’s thought experiments. This is territory that has been covered innumerable times. But Mr Levenson adopts a fresh approach that succeeds in being both lucid and entertaining. How Albert Einstein destroyed a planet and deciphered the universe Thomas Levenson Speaking Tiger 229 pages; ~695