6.1 19th Century problems
The 19th century ended with several unresolved problems in physics. The Michelson-Morley experiment of 1887 (discussed in Book II) showed that space is not filled with Aristotle’s aether and that unlike everything else in the universe, the apparent speed of light is the same whether it’s travelling towards you or away from you.[1] This meant there was no explanation for how light can travel through space since waves usually need a medium to travel through. Olbers’ paradox and the shape of Mercury’s orbit remained unsolved problems (both discussed in Chapter 5), and people were increasingly worried about how long the Sun would continue to burn.
In the 17th century, Isaac Newton had suggested that the Sun is fuelled by comets,
fixed stars that have been gradually wasted by the light and vapours emitted from them for a long time, may be recruited by comets that fall upon them; and from this fresh supply of new fuel these old stars, acquiring new splendour, may pass for new stars.[2]
By the end of the 19th century, it had been suggested that the Sun could be fuelled by a chemical process, like combustion, yet even if the entire mass of the Sun was made of petrol, it would still only burn for about 7000 years.
The lifetime of the Sun if it were fuelled by combustion
Mass (m) = 2×1030 kg,
Energy emitted per second (L) = 3.9×1026 J/s, and
Energy released per kg burnt (HV) = 45×106 J/kg.
Rate of combustion (Rcom) = LHV = 3.9×102645×106 = 8.7×1018 kg/s.
Lifetime = mRcom = 2×10308.7×1018 = 2.3×1011 s = 7,300 years.
This was not considered a problem in the 18th century when the Earth was thought to be about 6000 years old. This was based on the Irish Bishop James Ussher’s argument that the world had been created on the 23rd of October 4004 BCE. Ussher had come to this conclusion by reading the Bible and adding the male lineage from Adam through to Solomon. Beyond the time of Solomon, he cross-referenced the lineage of Kings with known historical data.[3] Yet by the end of the 19th century, there was evidence that the Earth must be at least 35,000 times older than this.
People had been discovering the fossilised remains of unknown animals for thousands of years, and these were often only found in specific strata - layers of sedimentary rock or soil. In the 1790s, the British geologist William Smith suggested that different strata have different ages, where one layer of rock or soil forms over another. He argued that if the same fossils were found in strata at different locations, then the strata are probably the same age.[4]
Figure 6.1 |
Rock strata in the Petrified Forest National Park in Arizona. These were formed during the Triassic period over 200 million years ago. |
The British geologist John Phillips used Smith’s theory to show that there must have been at least three major geological eras: the Palaeozoic (‘Age of Fishes’), the Mesozoic (‘Age of Reptiles’), and the Cenozoic (‘Age of Mammals’). Phillips estimated that the Earth could be up to 96 million years old.[5]
The remains of extinct dinosaurs were first correctly identified in England in the early 19th century. The first belonged to the herbivorous Iguanodon, discovered in Sussex by the English palaeontologist Mary Ann Mantell in 1822.[6] The second was the carnivorous Megalosaurus, discovered in Oxford and identified by the English palaeontologist William Buckland in 1824,[7] and the third was the armoured Hylaeosaurus, also discovered in Sussex, and first identified by the English palaeontologist Gideon Mantell in 1833.[8]
The British biologist and palaeontologist Richard Owen coined the term Dinosauria to encompass all three genera in 1842. Dinosauria is derived from the Greek words deinos, which means ‘terrible, powerful or wondrous’ and the word sauros, which means ‘lizard’.[9]
Figure 6.2 |
An artist’s impression of an Iguanodon, 1859 (no longer considered accurate). |
Figure 6.3 |
An artist’s impression of a Megalosaurus, 1859 (no longer considered accurate). |
Figure 6.4 |
An artist’s impression of a Hylaeosaurus, 1871 (no longer considered accurate). |
In The Origin of Species, published in 1859, the British naturalist Charles Darwin suggested that our current stage of evolution would require hundreds of millions of years.[10] In 1897, William Thomson (Lord Kelvin) showed that it would take 20-40 million years for the Earth to cool to its current temperature from a molten state.[11]
In 1898, Darwin’s son, astronomer George Darwin, suggested that the Earth and Moon had once been part of the same molten mass but had broken apart. Astronomers now think that this occurred when it was hit by an object the size of Mars. George Darwin’s theory showed that it would take at least 56 million years for the Moon to become tidally locked with the Earth so that we only see one side.[12]
Finally, in 1899, the Irish physicist John Joly showed that it would take about 90 million years for the oceans to have accumulated their current salt content through mineral erosion.[13]
6.2 20th Century solutions
We now know that the Earth is about 4.54 billion years old. This was first determined by the American geochemist Clair Cameron Patterson using the radioactive dating of meteorites in 1953.[14]
Edwin Hubble estimated that the universe is about 9 billion years old in 1929.[15] Hubble’s calculations have since been refined to show that the universe is about 14 billion years old (discussed in Chapter 9).
The mystery of how the Sun is fuelled was solved in the early 20th century with the discovery of nuclear fusion, which can fuel the Sun for about 10 billion years (discussed in Chapter 11).
Olbers’ paradox was resolved with the discovery of the big bang (discussed in Chapter 9), and the shape of Mercury’s orbit was explained with Albert Einstein’s theory of general relativity (discussed in Chapter 8).
The problems produced by the Michelson-Morley experiment were resolved with quantum mechanics (discussed in Book II) and Einstein’s theory of special relativity (discussed in Chapter 7).