What the general theory of relativity doesn't explain
Einstein's errors
Einstein's general theory of relativity was a great leap forward for physics, but it couldn't explain some strange observations of reality made then, and others more recently.
The expanding universe
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Astronomer Edwin Hubble observed that the Universe was expanding, which general relativity predicted.
Einstein could not accept a surprising prediction made by his equations – that the Universe was expanding.
He thought the equations must be wrong, so he added a constant – a so-called 'fudge factor' – which he thought would fix this apparent flaw. However, in 1929 Edwin Hubble observed that the Universe actually is expanding, proving Einstein’s original work to be right. Einstein later called the cosmological constant his “greatest mistake.”
Unpredictable quantum mechanics
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Einstein (centre) at the Solvay Conference of Physics, 1927. He remained active in physics research after producing his general theory of relativity.
Einstein assumed that the movement of everything could, in theory, be accurately predicted by applying equations.
However, a new branch of physics, quantum mechanics, suggested that the Universe varied according to statistics, so cannot be neatly predicted. Einstein objected to this, saying: “God does not play dice.” However, experiments showed that quantum theory can be demonstrated with observable results. Einstein accepted these observations and spent much of his later life trying unsuccessfully to combine general relativity with quantum mechanics.
Black holes
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Artist’s depiction of a black hole. Black holes cannot be seen, but they do warp the space around them, meaning they can be detected.
Einstein’s equations predicted the existence of black holes, objects so massive and dense that nothing can escape from them, not even light.
Einstein didn’t believe the result from his own theory – first worked out by German physicist Karl Schwarzschild – and in a paper in 1939 he wrote that black holes "do not exist in physical reality." It wasn’t until after Einstein’s death that it was discovered that not only do black holes exist, they are common all over the Universe – we even have local ones in our own galaxy.
The gravity of tiny particles
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Einstein in 1951. He continued to work on the big problems in physics until his death in 1955.
General relativity works well on the scale it is usually applied, for example planets, but falls apart for very small and very large applications.
The equations of general relativity don’t work on very small scales. Here, quantum mechanics reigns, and searching for a quantum theory of gravity which supersedes general relativity is a holy grail of modern science. However on very large scales, general relativity has problems too. It can't account for the 1990s observation that the universe’s expansion seems to be accelerating. This is a problem that is still unsolved by modern physics.
