Warp in Space

By means of prolonged processes of mathematics, entirely separate from the senses, astronomers are able to calculate when an eclipse will occur. They predict by pure reason that a black spot will pass across the Sun on a certain day. You go and look, and your sense of sight immediately tells you that their calculations are vindicated. So here you have the evidence of the senses reinforced by the entirely separate evidence of a vast independent process of mathematical reasoning.

Sir Winston Churchill

The Sun shines—but how? A hundred years ago this deceptively trivial question was causing great consternation not only to astronomers, but also to other scientists.

During the previous centuries, Western scientists discovered new phenomena that raised previously unsuspected quandaries. For example, before we realized that biological evolution occurs, producing new species from old, the avenue by which genetic change takes place was not a problem for consideration. Today, five or six generations after Charles Darwin, the mechanisms and processes of natural selection remain hotly debated within the academic community.

The question of how the Sun shines—that is, the source of its energy—did not become a matter of concern among scientists until the concept of geological deep time was established. Many will have heard of Archbishop James Ussher and his seemingly absurd statement that the world began in 4004 B.C. Those who mock Ussher do so from their own ignorance. One should not judge him by the standards of modern-day scientific knowledge, but rather from the perspective of the accepted wisdom in his own time, the mid-seventeenth century. In those days the age of the Earth was thought to number only a handful of millennia, and Ussher's conclusion was a respectable effort in the context of the scholarship of his era.

The realization that our planet is not just millions, but actually billions of years old was a long time coming. Edmond Halley enters our story again at this juncture: he suggested that the age of the Earth might be estimated by comparing the salinity of rivers with the salt content of the oceans, reasoning that the saltiness had built up over the eons. There are various shortfalls with this concept, but later experimenters did derive ages of many millions of years based on such measurements.

Another method was founded upon the observation that far below ground, deep down mine shafts, the rock is hotter than at the surface. Volcanoes provide unmistakable evidence that deeper yet it is hotter still. Eighteenth-century scientists reasoned that the elevated temperature below ground represents a gradual cooling of the planet since its formation, the heat still flowing upwards. They experimented with various-sized spheres of warmed rock and metal, and noted how long it took these to cool, scaling their results up to derive ages for the planet that were much longer than hitherto suspected.

Actually that basic technique is flawed, because it is tacitly assumed that the Earth has no internal heat generation, the temperature differential representing a fossil remnant from the planet's formation as a molten sphere. In fact energy is liberated deep within our globe through radioactive decay; but stepping back a century or two the phenomenon of radioactivity was yet unsuspected. This relates to the problem of the Sun's energy: it was assumed in that era that the Sun was glowing hot because, as a much larger body, it had cooled less than the Earth from its primordial state. The notion of nuclear reactions powering the Sun was unknown until early in the twentieth century.

The whole question was brought to a head when Darwin and his colleagues, studying geological strata such as limestone, showed that sedimentary rock sequences must be hundreds of millions of years old if laid down at a similar rate to those in production today. Up to that point the physicists, on the one side, who were measuring cooling rates and so on, had been able to reconcile their values with the age of the Earth according to geologists, biologists, and the like. However, such a vast planetary and solar age could not be accommodated by the physical theory of the time.

So physicists looked to other possible energy sources for the Sun. If the Sun were gradually shrinking, energy could be produced and the Sun heated. The process may be thought similar to a tennis ball warming as it is compressed whenever struck by a player. During a championship tennis match the balls heat up and this alters their bounce characteristics; cool ones are retrieved from the refrigerator every so often. The familiar phrase "New balls, please" is uttered by the umpire every seven games at Wimbledon. In the case of the Sun or some similar large object, as it contracts there is a decrease in its gravitational energy because the compos ite matter is moving closer to the middle, and that energy has to go somewhere. Half of it is converted into heat, which is then lost by radiation.

This shrinkage producing heating and hence radiation is a process that is known to occur in the Solar System. Although such a source is insufficient to explain the observed solar power output, we recognize that Jupiter is still settling after its formation so long ago. In consequence it emits two and a half times more energy than it receives from the Sun. Jupiter is not hot enough to emit visible light (we see it only by reflected sunlight), but it does radiate a huge flux of microwaves, making it quite bright to a radio telescope. Saturn and Neptune do likewise, although to lesser extents, whereas the data with respect to Uranus are ambiguous. For the Sun, there is no ambiguity: no such settling could explain the enormous radiated flux of light.

A suggested alternative solar energy source was that meteor-oids and other debris continually cascade down upon the Sun; although the individual particles could not be seen burning up, their combined contributions might power the solar furnace. Again, however, the sums would not add up, and the feasible age for the Sun calculated that way was much less than the geologists insisted upon.

A major confrontation over this matter therefore ensued late in the nineteenth century, the physicists seeing a relatively youthful Sun and Earth, the geologists requiring hundreds of millions of years of elapsed time to explain their data. In this argument some physicists acted rather arrogantly, with disregard for what they saw as "softer" scientific disciplines, and yet it was physics itself that threw up the solution and proved these earlier physicists wrong.

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