The Ascent of Gravity by Marcus Chown

Author:Marcus Chown
Language: eng
Format: epub
Publisher: Pegasus Books

The anomalous motion of Mercury

On Christmas Eve 1907, just after he had completed his review of special relativity, Einstein had written to his Zurich friend Conrad Habicht: I hope to explain the still unexplained secular changes in the perihelion distance of Mercury.’27 Back then, he had failed. Nevertheless, the letter reveals Einstein’s prescience in recognising that such a tiny effect was a subtle symptom of a fundamental failure of Newton’s theory of gravity.

Mercury is the Sun’s closest companion. Being so near the most massive body of all, the planet is forced to negotiate the most grossly warped space-time in the Solar System. It makes it the planet on which the effects of warped space-time leave their biggest mark.

In 1905, Einstein had discovered that all forms of energy have an effective mass. It follows that all forms of energy must exert gravity. And one of those forms of energy is gravitational energy – the energy of warped space-time itself. Remarkably, this means that warped space-time is not only gravity but a source of more gravity. Gravity creates more gravity!

Close to the Sun, therefore, gravity is stronger than predicted by Newton. It departs from a force described by an inverse-square law.

Newton’s great triumph was of course his demonstration that a body experiencing a centrally directed inverse-square law of force travels in an ellipse. It obviously follows that, if a body does not experience an inverse-square law of force, it does not travel in an ellipse. The trajectory is instead an ellipse that ‘pre-cesses’, continually changing its orientation in space, and tracing out a rosette-like pattern.

Einstein calculated Mercury’s orbit. His theory predicted that the effect of warped space-time near the Sun should indeed cause an anomalous precession of Mercury’s orbit. The amount was 43 arc seconds per century.

It was exactly the anomalous precession that had mystified astronomers for half a century. It was exactly the precession that had prompted Urbain Le Verrier to postulate the existence of the planet Vulcan.

There was, of course, no Vulcan. The anomalous motion of Mercury was not telling astronomers of the existence of an unknown planet skirting the fires of the Sun. It was signalling something far more fundamental and astonishing – something that nobody before had ever suspected: that Isaac Newton was wrong.

‘The theory agrees completely with the observations,’ Einstein concluded on presenting his Mercury result to the Prussian Academy. He had overturned 200 years of physics and shown that the greatest scientist ever to have lived had been wrong but he had managed to disguise what he was actually feeling. Inside he was in utter turmoil. He was beside himself with excitement.28 In fact, he was having palpitations.29

Physicists scrawl arcane mathematical equations across blackboards but it is an enormous leap of faith to believe that nature really obeys those equations. It invariably comes as a massive shock when it turns out that nature really does.

After an eight-year struggle, Einstein had finally reached the summit of a towering mountain. The fog that had enshrouded him every step of the climb had cleared.

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