About Time by Paul Davies

Author:Paul Davies
Language: eng
Format: epub
ISBN: 9780141951980
Publisher: Penguin Books Ltd
Published: 2008-12-04T16:00:00+00:00

A REPULSIVE PROBLEM

Away with the cosmological term…

ALBERT EINSTEIN

In Chapter 5, I explained how Einstein, having produced his beautiful gravitational-field equations in 1915, then “sullied” them by adding an extra term—the cosmological or Λ term. He came to regret this step bitterly. First, it snatched away the chance for him to predict the expansion of the universe. Second, the extra term reeked of fudgery. Indeed, Λ has become known as “Einstein's fudge factor,” unworthy of a theory so breathtakingly elegant and awesomely powerful as general relativity, and unbecoming of a man of such purist tastes.

Taking their cue from the great man, scientists in general have tended to regard the Λ term as being as repulsive as the force it describes. Partly this is a reaction to Einstein's dramatic U-turn, partly it is because of Ockham's razor. Why add an extra term to a set of equations that is already demanding enough? It only serves to multiply the choice of cosmological models on offer and obscures the interpretation of the astronomical observations.

There is another reason why scientists would rather set Λ equal to zero. Cosmological observations limit the size of A to at most a very small value. As I described in Chapter 5, a A force is exceedingly feeble by any standards, orders of magnitude weaker than anything else known. Many physicists dream that the various forces of nature—gravitation, electromagnetism and the nuclear forces—will one day be combined in a unified field theory of the sort that Einstein heroically sought to construct in his latter years. It is hard to see how such a theory would predict one force so much weaker than all the rest.

Stephen Hawking has presented an elegant argument along these lines.3 To quantify just how weak the Λ force is, we must compare it with something. A convenient way to do this is in terms of the range at which the force makes itself felt. As explained, the Λ force is certainly negligible at distances of less than a few billion light years. The weaker the force, the greater the range at which it manifests itself. If the force is zero, the range is infinite. One can also discuss the range of the more familiar electromagnetic force, but in this case things are “back to front.” As I already pointed out, the Λ force is unusual because it grows stronger with distance rather than weaker. By contrast, the electromagnetic force declines with distance, so a measure of its range is the distance beyond which it becomes negligible.

Observations of the magnetic fields of galaxies suggest that electromagnetic effects extend over at least a million light-years, but little is known about distances greater than this. It is possible that the electromagnetic force abruptly vanishes at, say, a billion light-years from its source, but almost no physicists believe this. They argue that, as the range is already known to be so great, it really should be infinitely great, for it is hard to imagine how a fundamental distance as large as a billion light-years could enter into the basic laws of electromagnetism.

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