An example of a metaphor used to explain a scientific idea:
"…new material appears in space to compensate for the background material that is constantly being condensed into galaxies. … I find myself forced to assume that the nature of the Universe requires continuous creation – the perpetual bringing into being of new background material.
…Matter that already exists causes new matter to appear. Matter chases its own tail.…
I do not agree that continuous creation is an additional assumption. It is certainly a new hypothesis, but it only replaces a hypothesis that lies concealed in the older theories, which assume…that the whole of the matter in the Universe was created in one big bang at a particular time in the remote past. On scientific grounds this big bang assumption is much the less palatable of the two. For it is an irrational process that cannot be described in scientific terms. Continuous creation, on the other hand, can be represented by mathematical equations whose consequences can be worked out and compared with observation. On philosophical grounds, too, I cannot see any good reason for preferring the big bang idea. Indeed, it seems to me in the philosophical sense to be a distinctly unsatisfactory notion, since it puts the basic notion out of sight where it can never be challenged by a direct appeal to observation…
The average rate of appearance of matter amounts to no more than the creation of one atom in the course of a year in a volume equal to that of a skyscraper…
It is a simple consequence of all this that the total amount of energy that can be observed at any one time must be equal to the amount observed at any other time. This means that energy is conserved. So continuous creation does not lead to non-conservation of energy as one or two critics have suggested. The reverse is the case, for without continuous creation the total energy observed must decrease with time."
Fred Hoyle (1960) The Nature of the Universe (Revised ed.)
The reference to a skyscraper offers an everyday comparison to give a sense of scale of an abstract process
Read about quotidian comparisons
Hoyle is using the metaphor of matter chasing its own tail to describe his theory of continuous creation, which leads to a steady state universe: that is, the gross appearance of the universe remains constant over time: in contrast to the idea that the universe was formed in a singularity (the 'big bang') and is continually expanding such that groups of galaxies will eventually move apart, and eventually be too far apart to be detected from each other. On this scenario, all the stars eventually' 'burn out' and dim and cool, and eventually all the energy in the universe is so evenly spread that it is in effect a 'dead' dark (and very cold) place.
Hoyle's alternative to the big bang did not persuade most astronomers, and today the Big Bang theory (ironically known by the term Hoyle used to denigrate it) is widely accepted as the likely origin of the universe.
Note that Hoyle's version of the law of conservation of energy does not refer to all the energy in the universe being conserved (the usual account – which of course can never be 'proved!), but rather that the energy in the observable universe is conserved (while the total energy increases with the creation of new material). In effect, it is conservation of mean energy density, not total energy.
Nowadays, Hoyle's theory is largely regarded as a historical alternative conception.
