9.1: Interstellar Gas must be Falling on to the Stars
Ponderable matter is very unevenly distributed within our own galaxy.
About one-half of it is concentrated in stars. The remainder is in the form
of a diffuse gas and occurs throughout the interstellar space. It contains
traces of many elements, but consists mostly of hydrogen. As the following
considerations will show, its mere existence does not seem to be compatible
with either the hypothesis of the past continuous existence of matter or
its past finite existence, labelled respectively (Al) and (A2) at the beginning
of Chapter 3. But the occurrence of interstellar gas is fully consistent with
(A3) and provides some evidence for it.
According to the inverse square law there are gravitational fields in all
parts of space, though they are very weak at great distances from any
star. Wherever the observed interstellar gas is, it finds itself in such fields
and, as it consists of ponderable matter, it must be falling everywhere down
the potential gradients of the fields. Every molecule of interstellar gas
must be falling with an acceleration that is proportional to the local
potential gradient and with a velocity that increases with the time that has
elapsed since the falling began.
Let us consider in turn what inferences one must draw about this gas
from each of the three hypotheses about the origin of matter: (Al), (A2)
9.2: Objections to (A1)
According to (A1), unaided by further hypotheses, the gas must have
been falling for an infinite time. If it had had an infinite distance through
which to fall it would have acquired an infinite velocity. But as the distance
between galaxies is finite and the farthest distance through which any
molecule can fall must be of the order of half the distance between
neighbouring galaxies, every molecule would have completed its fall an
infinite time ago. In other words, there would be no more interstellar or
Hence (A1) does not suffice to account for the present observation of
the interstellar gas without the aid of some additional hypothesis. The
falling of this gas on to stars is just one of those irreversible processes that
have already been discussed in Chapter 3 and that make it so difficult to
reconcile (A1) with observed facts. Here the task is made more difficult
by our knowledge of the history of the hydrogen gas after it has completed
its fall. When it has reached the star that has been attracting it, it does
not necessarily continue to be hydrogen. Some of it contributes to the
process known as the synthesis of helium and is converted into that
This process occurs when, under the influence of the high pressure and
temperature that pertain in a large star, hydrogen nuclei are brought into
close association. Some of them then combine to form nuclei of helium
and, with still greater pressure and temperature, those of heavier elements.
In this process the hydrogen nuclei lose some of their mass, which is con-
verted into radiation of a very high frequency. The radiation carries large
quantities of energy away with it into outer space.
In order to justify (A1) an hypothesis has to be invented that can
explain how the interstellar gas comes to be replenished. One might perhaps explore the notion that some so far unobserved forces are continuously pushing hydrogen out of stars and against gravity. But the
potential energy contained in the interstellar gas must be enormous and
one would need yet another hypothesis to account for the source
of such quantities of energy. Alternatively one might have recourse to
the hypothesis of a pulsating universe that has already been mentioned in
But whatever hypothesis ingenuity can devise to reconcile the inter-
stellar gas with infinite past duration, one must remember that more than
the mere movement of particles over great distances needs to be accounted
for. The existence of interstellar gas would imply, for instance, that the
helium synthesis was periodically reversed.
For this to occur the helium would first have to regain the energy that
had been radiated away when it was being formed. The protons and
neutrons of which the helium nuclei are composed would have to separate
and only then could the reconstituted hydrogen leave the star and rise
against gravity into outer space, there to distribute itself evenly prior to a
renewed fall. To believe that all this can happen is, perhaps, not to believe
the impossible; but it puts a severe strain on one's credulity.
9.3: Objections to (A2)
As a means of explaining the continued presence of the interstellar
gas, (A2) can hardly be called more successful than (Al). True, according
to this hypothesis, the gas has not had an infinite time in which to go on
falling. But it has been doing so for some thousands of millions of years.
During this long period it has been experiencing an acceleration proportional to the gravitational field in which it has found itself. There has been time enough for quite a small acceleration to result in quite a high
According to (A2), moreover, the universe was a more compact affair
at the beginning than it is now, for it is assumed to have been steadily
expanding ever since it began. The expansion, it will be remembered,
furnishes one of the clocks from which, it has been claimed, the age of the
universe can be read off. So the stars must have been closer together in
the past than they are now. According to the inverse square law the
average potential gradient in which the interstellar gas found itself at the
beginning must have been greater and therewith the acceleration that it
experienced also greater.
From (A2), unaided by any further hypothesis, one must thus infer
that the interstellar gas was falling with a high acceleration to begin with
and that, though the acceleration has diminished since then, the velocity
has continued to increase. If this were correct, a large proportion of the
gas, if not the whole of it, would have completed its fall ere now; and if
any were left in interstellar space it would be very unevenly distributed.
Regions remote from any star would have been entirely depleted long ago
and regions near stars would be surrounded by a comparatively dense
cloud of hydrogen of which a minute fraction might be rising in the form
of solar flares but most would be falling rapidly. This does not seem to be
the distribution of the gas that astronomers observe.
The above is, moreover, not the only difficulty that is presented to (A2)
by the uneven distribution of ponderable matter. One must infer from (A2)
that some thousands of millions of years ago interstellar space contained
a great deal of hydrogen and that much of this has since fallen on to stars.
But this hydrogen was accommodated in a smaller universe and must
therefore have been at a much greater density than the interstellar gas that
we now observe. And as the stars, if smaller, must have been much closer
together, their mutual attractions must have been much greater. Why then
did they ever separate ? Why did they not fall on to each other? One should
expect a universe consisting of many stars in crowded formation with a
massive gas distributed between them to shrink and collapse on to itself
until it became one single, highly compact, sphere. It does not seem
possible to infer from the known laws of physics that the kind of universe
that (A2) postulates at the time of the Creation would break up into discrete concentrations, like the observed stars and nebulae. The explanatory
power of (A2) is not sufficient to account for the existence of separate
concentrations of matter any more than of the more or less even distribution of gas that we observe to exist between these concentrations. I fear
that measures devised to save (A2) will have to be as desperate as those
devised to save (Al). (A2) will, I think, lose such scientific support as it
may still enjoy at the time when this is being written, when scientists begin
to ask more questions about the physics of the great explosion that is
assumed in (A2) to have occurred once-upon-a-time x-thousand million
years ago and said to have begun the expansion of the universe.
9.4: The Explanatory Power of (A3)
Here (A3) fares better, both in combination with (Bl) and with (B3).
One must infer from this hypothesis that the interstellar gas is being
continuously replenished. Any gas that finds itself in a gravitational field
must be falling down the potential gradient, as the water in a brook flows
down the valley. But more hydrogen from a region of higher potential
must also help to replace what is lost and this must happen throughout
space, even in the most distant regions. What is lost to a region by falling
out of it is replaced, more or less as the case may be, by new origins. Hence
even the remotest region can never become entirely depleted.
If we had no means of detecting the hydrogen that occurs in outer
space, we should nevertheless predict from (A3) (and without forming any
additional hypothesis) that there would be some. The interstellar gas is, in
other words, evidence in support of (A3).
Thus, the inhabitants of a planet surrounded by cloud would, if they
adopted (A3) and learnt that we were a part of a galaxy of stars, predict
that the space between these stars would contain diffuse hydrogen. They
would construct a cosmological model that conformed to actuality in this
one respect at least.
They would, of course, also predict that the space between nebulae,
extragalactic space, also contained diffuse hydrogen. Observation has so
far not succeeded in either confirming or denying this. We can only note
the fact that extragalactic hydrogen is one of the inferences to be drawn
from (A3). To postulate it is not to adopt an additional hypothesis.
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