The technique of inference that has been discussed in the preceding
chapter calls for a survey of all the forces that could conceivably have an
influence on the course of events. The following list seems to be exhaustive:
gravitational forces, centrifugal forces, forces of impact, pressure of radiation, electrostatic forces, magnetic forces. But a moment's thought shows
that some of these cannot materially influence the shape or behaviour of
an extragalactic cloud.
Electrostatic and magnetic forces, for instance, can have a big influence
on small-scale events. They can cause turbulence in a gas. There is little
doubt that they play a large part in terrestrial and solar phenomena of
various kinds. But the known processes by which such forces are produced
set a limit to the distance over which they act.
An electrostatic force occurs only when electric charges have been
separated, and the region within which the force acts is the region between
the places to which the charges have been removed. If an electrostatic
field is to cause hydrogen to move through a great distance, the field must
extend over at least the same distance. For this to happen charges must
have been torn asunder and removed to the extremities of the field. This
must have been done against the force of their mutual attraction. So the
charges must have been subjected for a long period of time to a unidirectional force before the field could be produced. In the process vast
quantities of energy must have been employed. Now from established
knowledge, and without the help of some quite fantastic ad hoc hypothesis,
it is not possible to infer a process that would separate charges by the
requisite distance.
It is by the same reasoning that one is obliged to dismiss magnetic
forces from the list of those that could cause the unidirectional movement
of matter over any distance that was an appreciable fraction of the diameter of a galaxy. I am aware that at the time when this is being written
the hypothesis has considerable backing that large-scale movements within
galaxies are controlled by magnetic forces. It cannot be proved that they
are not, but I should be more inclined to accept this hypothesis if its
authors showed more awareness of the difficulty there is in finding a
cause of the hypothetical magnetic forces. Magnetic forces are known only
as the result of the movement of electric charges. They operate only over
such distances as may form the limits of an electrostatic field. It is not so
difficult to believe that electric charges must circle the earth and cause
opposite places on it to have, respectively, north and south polarity. But
one would have to go a very long way in search of an hypothesis by which
charges would encircle a noticeable part of a galaxy and give it a particular
magnetic polarity.
That external pressure of radiation has to be ruled out has been
explained already in Chapter 11. Such pressure, it will be remembered,
must operate generally in the same direction as gravitation and diminish
according to the same inverse square law. But its intensity must everywhere
be negligibly small in comparison with the intensity of the gravitational
field.
Internal pressure of radiation must, however, have a significant effect
on any body hot enough to generate much radiant energy in its interior. It
is known to be so for the stars and must also be so for the hot central core
of a spiral nebula. However, we are now considering the early cloud and
we have not discovered any reason why this should be hot. When a cloud
begins to form on an astronomical summit, its component molecules can
be moving only under the influence of the faint gravitational fields in
which they find themselves. Their velocity must be so slow to begin with
that the cloud must be quite near the absolute zero of temperature. It
should not be overlooked, however, that internal pressure of radiation
on the very large core of the cloud may have a significant effect at some later
stage.
Thus inference (as distinct from speculation) only permits three kinds
of force for the construction of a cosmological model based on Symmetrical Impermanence. They are: gravitational forces, forces of impact,
and centrifugal forces. If these alone do not serve for inferring a model
that resembles actuality. Symmetrical Impermanence will have to be
replaced by some other basic hypothesis.
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