The jewel in the crown of Einstein’s achievements is his theory of gravity, known as General Relativity. Published in 1915 and so complex even Einstein needed help with the maths, General Relativity was the biggest advance in understanding gravity since Isaac Newton pondered the fall of that apple in his garden 250 years earlier.
Put simply, Einstein showed that what we think of as the ‘force of attraction’ between two masses is actually the result of warping of the very fabric of space and time around them. In everyday situations, this so-called spacetime curvature is so subtle that Newton’s far simpler law of gravity works perfectly well. But on cosmic scales, General Relativity should show its mettle.
The good news for Einstein is that earlier this year, his theory saw off a challenge from a rival theory, which was expected to do a better job of describing clusters of galaxies – the biggest-known lumps of mass in the Universe. According to an international team of astronomers, the results show that Einstein’s theory reigns supreme over distances of at least 130 million light-years.
The bad news is that other studies have shown that while General Relativity may work well on relatively modest distances and timescales, it’s looking increasingly creaky on truly cosmic scales. According to results announced in January, its equations need modification to explain how the Universe has behaved over the last six or seven billion years. It’s acting as if propelled by a force with its origins in sub-atomic effects, which Einstein refused to countenance.
Everyone knows Einstein’s equation showing that mass and energy are really just the same thing. Yet it’s taken over a century to prove it’s true. In 2008, a team of European researchers proved that the mass of protons and neutrons can only be explained by the energy of particles inside them – just as Einstein predicted.
As a teenager, Einstein wondered what the world would look like if he could travel on a light-beam. He came up with the answer in 1905 – Special Relativity. At its heart is so-called Lorentz Invariance, which implies that the speed of light in a vacuum must be the same for everyone, no matter what speed they’re moving at. The only way to achieve that is for measurements of space and time to be affected by how we’re moving.
At everyday speeds, these effects are negligible, but for sub-atomic particles and even orbiting satellites they become important, which is why they’re incorporated into the design of the giant Large Hadron Collider near Geneva, and GPS satellites. Last year, astronomers gave Special Relativity its toughest test yet, checking the Lorentz Invariance of light from a star that exploded over seven billion light-years away. Einstein’s theory passed – for now.
Which of Einstein’s theories saw him barking up the wrong tree?
Einstein thought his biggest mistake was refusing to believe his own equations that predicted the expansion of the Universe. Yet we now know he actually missed out on predicting something even bigger: Dark Energy.
The trouble began when he first applied General Relativity, to the entire Universe. Like everyone else, Einstein believed the Universe was static and unchanging, and was horrified when his mathematically beautiful equations predicted a dynamic Universe. So he forced himself to introduce an ugly fiddle-factor to force his equations into line with the ‘facts’. Yet at that same moment, astronomers were discovering that the facts were wrong – and that distant galaxies are racing away from each other in an expanding Universe. Once Einstein had got over the shock of failing to make the most amazing scientific prediction of all time, he declared that refusing to keep faith in his beautiful equations was “the biggest blunder of my career”.
But it didn’t stop there. In the mid 1990s, 40 years after his death, astronomers showed that his faith in his beautiful equations had been misplaced. Studies of exploding stars in distant galaxies had revealed that the Universe isn’t just expanding, it’s expanding at an ever-faster rate. The cause: a force even stronger than gravity, but acting in the opposite direction – and with no obvious source. This is the now-notorious Dark Energy, and Einstein’s theory can accommodate it. But at the price of reintroducing the same kind of ugly fiddle-factor Einstein loathed.
Most theorists believe Dark Energy has its origins in the quantum laws of the sub-atomic world, which allow even apparently empty space to be seething with energy. Given his loathing for quantum theory, it’s unlikely Einstein would be celebrating its incorporation into his most cherished work.
Einstein had to be dragged screaming to the idea that the Universe began in a Big Bang. After learning from the Belgian mathematician Georges Lemaître that General Relativity (GR) predicts the creation of the Universe, he dismissively replied: “Your calculations are correct, but your physics is abominable”. And it’s not hard to see why: the equations of GR go haywire at the moment of the Big Bang, giving literally infinite results. What’s needed is something extra to bring the theory back under control.
Theorists now believe they know what that something extra is – and, once again, it’s the very thing Einstein wouldn’t accept: quantum theory. Recent calculations by theorists in the US and Europe have shown that if GR is combined with quantum theory, the resulting theory of ‘quantum gravity’ gives insights not only into the Big Bang, but also what came before it. And early results suggest that today’s Universe is just the latest in an infinite cosmic cycle of Big Bangs.
His most famous equation might be E = mc2, but Einstein himself doubted how important it was. He dismissed the notion that it might one day be at the heart of a new energy source, declaring in 1934 that “there is not the slightest indication” that atomic energy will ever be possible. Now his equation is at the heart of over 400 nuclear power stations, which are about to become the world’s leading source of non-carbon-based energy.
Einstein played a major role in developing quantum theory, which describes the behaviour of atoms. But he became increasingly suspicious of its fuzzy, ‘probabilistic’ view of particles, which seemed to prevent even their position and speed being known with complete precision. Einstein summed up his view of quantum theory by saying “God does not throw dice”. But most physicists believe Einstein was wrong.
In April, an international team of scientists proved that by using quantum effects to create truly random ‘dice’ from particles of light. Despite appearances, conventional dice aren’t truly random. In principle at least, it’s possible to predict what number they’ll give when rolled. But if blasted by lasers, atoms emit particles of light that quantum theory predicts will vibrate in completely random directions or ‘polarisations’.
The team created light particles whose polarisations generated numbers like the toss of dice, but whose values were truly random – something Einstein insisted was impossible.
The jury’s still out
Maybe he wasn’t always right, but nobody knows for sure with these.
THEORY OF EVERYTHING
Throughout his career, Einstein had an almost religious belief in the fundamental unity of the Universe, and spent decades searching for the one true Theory of Everything (ToE), which described the Universe and everything in it. His failure has not deterred others, however, and over half a century after his death, the quest for the ToE continues.
It’s long been clear that Einstein’s antipathy towards quantum theory ruined his hopes of succeeding, as it gives the best account of all the particles making up the Universe and the forces that act on them with the sole exception of gravity. But theorists hoping to succeed where Einstein failed are finding their own problems.
For the last 25 years, they’ve pinned their hopes for finding the ToE on something called superstring theory. Put simply, this holds that all particles and forces are vibrations of small multi-dimensional entities called ‘strings’. The good news is that the theory has revealed glimmerings of the cosmic unity that Einstein sought. Better still, some of its predictions are about to be tested using the Large Hadron Collider at CERN.
The bad news is that there’s not just one candidate ToE. According to some estimates, there may be at least 10500, and no obvious way of deciding between them. There’s now a growing suspicion that the whole idea of just one true ToE may be a mistake – and that Einstein was naïve to spend his life looking for it.
In the 1930s, Einstein and his collaborator Nathan Rosen found that different parts of the Universe could be connected via tube-like ‘wormholes’. In theory, this opens up the possibility of faster-than-light travel. But such wormholes are incredibly unstable, and collapse unless held apart by a kind of force field. Suitable fields are theoretically possible, but no-one knows how to make them strong enough for the job. One thing’s for sure, though: as they’re quantum effects, Einstein would never have believed in them.
If there’s one thing Einstein is still likely to be proved right about, it’s the existence of gravitational waves. According to General Relativity, any sudden motion of mass creates waves in the fabric of space and time, which spread out across the Universe like ripples on a pond.
Calculations show, however, even the explosion of stars would create ripples equivalent to the entire Solar System wobbling by barely the width of an atom. Even so, during the 1980s astronomers found hints of the existence of gravitational waves in studies of collapsed stars orbiting each other. As they spiralled around one another, they appeared to lose energy – and the rate of energy loss was within 0.5 per cent of that expected if the cause was the emission of gravitational waves. Dedicated gravitational wave detectors have now been built in the US and Europe, and some theorists believe they could confirm Einstein’s prediction in just a few years.
Edit: since time of writing gravitational waves have been confirmed – get the full details here.