In the remainder of this book the Principle of Minimum Assumption in
general, and the inference from it that I have called the Symmetrical
Impermanence of Matter, will be tested by the criterion of explanatory
power. It will be shown that one can infer from this principle the answers
to a number of questions about our galaxy and those extragalactic bodies
that resemble it closely enough also to be called galaxies. It will also be
shown in Part Four that one can infer the characteristic properties of
gravitation from the same basic principle.
Among the questions to be raised and partly answered are:
Why does the space within our galaxy contain very diffuse hydrogen?
Why does some of the matter in the universe occur in the form of
concentrations similar to our galaxy?
Why do the masses of all galaxies lie between finite upper and lower
Why do stars and galaxies rotate ?
Why do many galaxies resemble one another in having the charac-
teristic structure for which a dense central core is surrounded by spiral
arms all in the same plane ?
Why do the galaxies contain stars ?
(A similar list of the questions about gravitation will be provided in
Chapters 21 and 22.)
Not so very long ago the search for answers to such questions would
have appeared meaningless. It was thought that stars, nebulae and galaxies
just were, and it was taken for granted that they always had been like that.
Questions about their origin seemed idle and not worth asking.
A new outlook occurred when it was first appreciated that many
processes are irreversible, so that every physical system undergoes certain
unidirectional changes. Such a system must have one form or state at its
beginning and tend steadily towards another terminal form or state.
With this new understanding of the nature of the physical world
questions about the origin and evolution of the galaxies acquired a new
meaning. It was realized that these, like all other objects, must be in a
process of unidirectional change and so it became natural to want to
know as much as possible about the beginning of the process as well as
about its development and all subsequent states. In short, we are today
no longer satisfied merely to observe the objects that are revealed by our
telescopes; we want also to explain them and to discover their past and
By 'explain' we mean here 'account for in terms of basic physical
principles'. When we speak of explaining a phenomenon we mean that the
phenomenon can be inferred; that, given certain accepted facts and
principles one could predict the phenomenon without needing to observe
it. In this sense Dirac predicted the positron, it will be remembered from
Chapter 2, and would have been said to have explained it if its discovery
had preceded his deductive reasoning. Thus a satisfactory explanation of
the particular way in which ponderable matter is distributed must be based
on more than an ingenious ad hoc hypothesis. It must be based on a sound
philosophy, logical inference and a minimum of speculation.
Every one of the nine possible combinations from the (A) and (B) lists
that occur at the beginning of Chapter 3 is speculative, though some may
regard some of them as more speculative than others. Any such judgements
would, however, be merely subjective and depend on the temperament of
the person concerned. On an objective assessment one cannot grade the
nine combinations into those that are more and those that are less speculative. But the question arises whether, having made one's choice among
these nine possibilities, one will need to speculate further. Can one infer
from one of the combinations, without the need for any additional hypo-
theses, a cosmological model that resembles actuality?
In the rest of this book I shall develop the cosmological model that
one infers if one adopts the philosophy expressed by what I have called
the Principle of Minimum Assumption, the principle according to which
the minimum assumption is always the true generalization about the
physicist's world. To meet this principle the model must be based on the
combination of (A3) with (B3), on the Hypothesis of the Symmetrical
Impermanence of Matter.
It will be found that this model does seem to resemble actuality, where-
as every other one, including the model based on the combination of (A3)
with (Bl) fails completely.
I have to say 'it seems to', for I cannot exclude the possibility that there
may be errors in the reasoning, which will of course have to be checked for
these. But if there are errors, they will be of the kind that arises from
faulty logic and mathematics, not of the kind that arises from indulgence
in unjustifiable speculation.
Let me express the task in a different way. It is to show that all the
facts about which the above questions are asked could be discovered with
the help of a sound philosophy and accurate reasoning by the inhabitants
of a planet permanently enveloped in cloud.
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