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In 1905, a violin-playing patent clerk compensated for an unremarkable student career by publishing four papers that would change physics forever. Albert Einstein’s scribblings variously revealed the particulate nature of light, the equivalence of mass and energy — captured in the equation E=mc² — and the special theory of relativity. He showed the laws of physics, including the speed of light in a vacuum, to be the same for all non-accelerating observers.
And yet these last calculations felt, to him, like a blot on his annus mirabilis: how could he make his theory of relativity apply to accelerating observers, not just to the “special” situation in which they moved at constant speed? In November 1915 Einstein, still not yet 40, announced to the Prussian Academy of Sciences that he had cracked it. This month marks the centenary of that moment, when the physicist revealed his general theory of relativity. His triumph was to add gravity to special relativity and so create a more complete framework for understanding how the universe works. It ended Sir Isaac Newton’s reign as the emperor of physics.
General relativity has had its predictions borne out, and its continued supremacy as a way of understanding the universe is reflected in the number of events being held to mark the anniversary. The celebrations look forward as well as back, to upcoming experiments that will continue to search for chinks in Einstein’s master work.
Einstein’s insight was to see that objects exist in time, as well as in three spatial dimensions. His equations showed space and time to be knitted together in a continuum — and that gravity was a warp in the space-time fabric rather than the instantaneous force envisioned by Newton (Einstein saw that gravity could not act instantaneously across space because nothing can outrun light). Warps or curves in space-time, Einstein contended, were caused by objects such as stars and planets. While Newton explained the earth’s orbit around the sun in terms of a massive object tugging on a smaller one, Einstein pictured our closest star as a heavy ball on a mattress, with earth rolling around the contours like a ball whirling around a roulette wheel.
The first confirmation of Einstein’s tour de force came with Sir Arthur Eddington’s 1919 expedition to the west African island of Príncipe, to see a solar eclipse and study the position of stars lying nearby. Eddington had previously plotted the stars’ positions from England; he used the eclipse, which extinguished the blinding brightness of the solar disc, to show that those same stars were not where they were expected to be. The sun was bending the starlight, making the stars themselves appear shifted and distorted in the sky.
This phenomenon of massive objects distorting nearby light, called gravitational lensing, was a key prediction of general relativity. News of the expedition’s confirmation spread swiftly — and ensured that Einstein’s reputation would soon exceed that of Eddington, an early science populariser.
One forthcoming test for general relativity focuses on the supermassive black hole — the size of 4m suns — rumoured to lurk in the middle of the Milky Way. Black holes, predicted by Einstein, devour everything that comes near, even light, so they cannot be seen conventionally. But there is now a linked global network of radio telescopes scanning this region — effectively turning our planet into a huge collecting dish. Should this network find a tell-tale circular shadow amid galactic radio emissions, it will be another feather in Einstein’s heavily ornamented cap. Confirmation, or otherwise, could come as early as next year.
Perhaps the biggest challenge is to directly detect a gravitational wave — a ripple that would vindicate Einstein’s intuition that space is not an empty container but more like an invisible jelly studded with galaxies, stars and planets. A cosmic event such as the collision of two stars would make the jelly wobble. It is hoped that a US experiment called Ligo — conceived 20 years ago, decommissioned and then rebooted this year — will pick up traces of these wobbles.
Scientists seem confident that non-detection would be due to insensitive instruments rather than a flaw in general relativity. Just as nothing can outrun light, they remain convinced that nobody has yet outsmarted Einstein.
The writer is a science commentator
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