The “luminiferous aether”
An aether of some sort was postulated in both the particle and wave theories of light.
Isaac Newton introduced an “aethereal medium” in an attempt to explain diffraction, which could not otherwise occur in his particle theory. And Augustin Fresnel included it in his wave theory as the medium through which waves of light propagate.
Of the two competing theories, the wave explanation won out when James Clerk Maxwell combined four equations of electricity and magnetism to produce a wave equation in which the propagation speed was the speed of light. Visible, infrared, and ultraviolet light were seen to be electromagnetic waves with different frequencies.
Maxwell later suggested that the motion of the Earth through the aether could be worked out by measuring changes in the speed of light received from a distance source when the Earth is moving toward it at one time of year compared to when the Earth is moving away from it at another time of year.
Several experiments along these lines were carried out, the most famous of which was run by Albert Michelson and Edward Morley – and they failed to detect any difference in the speed of light.
Their result was one of the motivations for Albert Einstein to develop the special theory of relativity, in which the speed of light is constant and time and length are relative and depend on motion. There was no longer a reason to assume the existence of a luminiferous aether.
Relativity also holds that nothing can travel faster than light. But in 2011, neutrinos produced in the OPERA experiment at CERN appeared to do just that.
Muon neutrinos produced in the Super Proton Synchrotron particle accelerator on the border between France and Switzerland and detected in a lab in Gran Sasso, Italy, seemed to have made their journey in less time than light would.
This would not only violate the principle of special relativity, which had held up in experimental tests for a century, but would also contradict observations of supernovae, in which neutrinos arrive at the same time as light does from across the vast gulf of space.
After further investigation failed to repeat the anomaly, it became apparent that it was due not to new physics but to equipment malfunction.
The expanding universe
Special relativity is limited in scope, and gravity wasn’t accounted for until Einstein developed what became known as the general theory of relativity, completing it in 1915.
When applied to the whole universe, the theory showed that it should be either expanding or contracting. But since the prevailing view of the time was that it was static, Einstein introduced a term to the equations that became known as the “cosmological constant” – a fudge factor that balanced against gravity and kept the universe static and stable.
But in 1929, Edwin Hubble discovered that the galaxies are receding from one another – the universe is expanding. Einstein called the cosmological constant his “biggest blunder”.
A constantly expanding universe is equivalent to having a cosmological constant equal to zero, which it was taken to be for a long time following Hubble’s discovery. But this proved to be inaccurate too. In 1998 it was discovered that the expansion is accelerating, and the cosmological constant must have a small positive value – but it’s uncertain where this comes from.
Age of the Earth
Long before physicists were pondering the shape of the universe, the age of the Earth was causing scientists to scratch their heads.
Lord Kelvin attempted to work it out by assuming that the Earth had started off molten and cooled to its present temperature – estimating how long this would take would give a rough figure for the age of the Earth, which Kelvin calculate to be about 20–40 m years old.
But this was at odds with both geology and biology – it was far too short a time for evolution to have occurred.
While Kelvin’s calculations may have been accurate, his assumptions were not – he was unaware of convection within the Earth, which can keep the outer layers warm, and with radioactive decay, which provides an additional source of heat.
The development of radiometric dating in the early 20th century resolved the issue, placing the age of the Earth at a few billion years. (The currently accepted figure is 4.5 bn.)