Big Bang. Simon Singh
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Название: Big Bang
Автор: Simon Singh
Издательство: HarperCollins
Жанр: Прочая образовательная литература
isbn: 9780007375509
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Galileo profited from his commercialisation of the telescope, but he realised that it also had a scientific value. When he pointed his telescope at the night sky, it enabled him to see farther, clearer and deeper into space than anyone ever before. When Herr Wackher told Kepler about Galileo’s telescope, the fellow astronomer immediately recognised its potential and wrote a eulogy: ‘O telescope, instrument of much knowledge, more precious than any sceptre! Is not he who holds thee in his hand made king and lord of the works of God?’ Galileo would become that king and lord.
First, Galileo studied the Moon and showed it to be ‘full of vast protuberances, deep chasms and sinuosities’, which was in direct contradiction to the Ptolemaic view that the heavenly bodies were flawless spheres. The imperfection of the heavens was later reinforced when Galileo pointed his telescope at the Sun and noticed blotches and blemishes, namely sunspots, which we now know to be cooler patches on the Sun’s surface up to 100,000 km across.
Figure 15 Galileo’s drawings of the Moon.
Then, during January 1610, Galileo made an even more momentous observation when he spotted what he initially thought were four stars loitering in the vicinity of Jupiter. Soon it became apparent that the objects were not stars, because they moved around Jupiter, which meant that they were Jovian moons. Never before had anybody seen a moon other than our own. Ptolemy had argued that the Earth was the centre of the universe, but here was indisputable evidence that not everything orbited the Earth.
Galileo, who was in correspondence with Kepler, was fully aware of the latest Keplerian version of the Copernican model, and he realised that his discovery of Jupiter’s moons was providing further support for the Sun-centred model of the universe. He had no doubt that Copernicus and Kepler were right, yet he continued to search for evidence in favour of this model in the hope of converting the establishment, which still clung to the traditional view of an Earth-centred universe. The only way to break the impasse would be to find a clear-cut prediction that differentiated between the two competing models. If such a prediction could be tested it would confirm one model and refute the other. Good science develops theories that are testable, and it is through testing that science progresses.
In fact, Copernicus had made just such a prediction, one which had been waiting to be tested as soon as the tools were available to make the appropriate observations. In De revolutionibus, he had stated that Mercury and Venus should exhibit a series of phases (e.g. full Venus, half Venus, crescent Venus) similar to the phases of the Moon, and the exact pattern of phases would depend on whether the Earth orbited the Sun, or vice versa. In the fifteenth century nobody could check the pattern of phases because the telescope had yet to be invented, but Copernicus was confident that it was just a matter of time before he would be proved correct: ‘If the sense of sight could ever be made sufficiently powerful, we could see phases in Mercury and Venus.’
Figure 16 Galileo’s sketches of the changing positions of Jupiter’s moons. The circles represent Jupiter, and the several dots either side show the changing positions of the moons. Each row represents one observation taken on a particular date and time, with one or more observations per night.
Leaving aside Mercury and concentrating on Venus, the significance of the phases is apparent in Figure 17. Venus always has one face illuminated by the Sun, but from our vantage point on the Earth this face is not always towards us, so we see Venus go through a series of phases. In Ptolemy’s Earth-centred model, the sequence of phases is determined by Venus’s path around the Earth, and its slavish obedience to its epicycle. However, in the Sun-centred model, the sequence of phases is different because it is determined by Venus’s path around the Sun without any epicycle. If somebody could identify the actual sequence of Venus’s waxing and waning, then it would prove beyond all reasonable doubt which model was correct.
In the autumn of 1610, Galileo became the first person ever to witness and chart the phases of Venus. As he expected, his observations perfectly fitted the predictions of the Sun-centred model, and provided further ammunition to support the Copernican revolution. He reported his results in a cryptic Latin note that read Haec immatura a me iam frustra leguntur oy (‘These are at present too young to be read by me’). He later revealed that this was a coded anagram that when unravelled read Cynthiæ figuras æmulatur Mater Amorum (‘Cynthia’s figures are imitated by the Mother of Love’). Cynthia was a reference to the Moon, whose phases were already familiar, and Mother of Love was an allusion to Venus, whose phases Galileo had discovered.
The case for a Sun-centred universe was becoming stronger with each new discovery. Table 2 (pp. 34—5) compared the Earth- and Sun-centred models based on pre-Copernican observations, showing why the Earth-centred model made more sense in the Middle Ages. Table 3 (overleaf) shows how Galileo’s observations made the Sun-centred model more compelling. The remaining weaknesses in the Sun-centred model would be removed later, once scientists had achieved a proper understanding of gravity and were able to appreciate why we do not sense the Earth’s motion around the Sun. And although the Sun-centred model did not chime with common sense, one of the criteria in the table, this was not really a weakness because common sense has little to do with science, as discussed earlier.
Figure 17 Galileo’s precise observations of the phases of Venus proved that Copernicus was right, and Ptolemy wrong. In the Sun-centred model of the universe, shown in diagram (a), both the Earth and Venus orbit the Sun. Although Venus is always half-lit by the Sun, from the Earth’s point of view it appears to go through a cycle of phases, turning from a crescent to a disc. The phase is shown next to each position of Venus.
In the Earth-centred model of the universe, both the Sun and Venus orbit the Earth, and in addition Venus moves round its own epicycle. The phases depend on where Venus is on its orbit and on its epicycle. In diagram (b), Venus’s orbit is such that it is roughly between the Earth and the Sun, which gives rise to the set of phases shown. By identifying the actual series of phases, Galileo could identify which model was correct.
At this point in history, every astronomer should have switched allegiance to the Sun-centred model, but no such major shift took place. Most astronomers had spent their entire lives convinced that the universe revolved around a static Earth, and they were unable to make the intellectual or emotional leap to a Sun-centred universe. When the astronomer Francesco Sizi heard about Galileo’s observation of Jupiter’s moons, which seemed to suggest that the Earth was not the hub of everything, he came up with a bizarre counter-argument: ‘The moons are invisible to the naked eye and therefore can have no influence on the Earth and therefore would be useless and therefore do not exist.’ The philosopher Giulio Libri took a similarly illogical stance and even refused to look through a telescope on a point of principle. When Libri died, Galileo suggested that he might at last see the sunspots, the moons of Jupiter and the phases of Venus on his way to heaven.
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