At this stage of the present study the question may perhaps be raised
whether the pressure of radiation can significantly influence the movement
of ponderable matter in extragalactic space. Apart from gravitational
forces it is the only force known to act there over great distances. Its effect
is large enough to move metal discs in suitably constructed laboratory
apparatus. It serves as the most precise means of measuring the power
transmitted by a wave guide. It has recently become a major field of study
by physicists and engineers.
I cannot believe that any of those who have devoted their serious
attention to the pressure of radiation will regard it as having any
appreciable influence on the movement of any extragalactic particles,
great or small, or indeed on any interstellar particles within our own
galaxy. So I should hesitate to introduce the subject at all were it not
that some, who are non-specialists in this field, do seem to entertain such
The view has, for instance, received the backing of considerable
scientific authority that the pressure of radiation may have carried germs
of living substance away from a planet belonging to some distant constellation and deposited them on our earth. This notion has been urged to
account for the way terrestrial life began. On asking an astrophysicist of
repute, again, how the interstellar hydrogen of our galaxy can be accounted
for, he told me that, according to one view held seriously by himself and
others, this gas had been projected into outer space by the pressure of
radiation that emanates from bright stars.
So there seem to be some who believe that the pressure of radiation
can transport small particles of matter from outer space towards our
bright solar system and others who believe that the same pressure can
transport small particles into outer space away from a bright solar
system. The two views are hard to reconcile with each other. But the
effort to do so can be spared. For I shall show below that, when considering the movement of small particles in outer space, the pressure of
radiation can be disregarded. It would, however, seem that current opinion
about it cannot. Hence the subject must claim our attention for a short
Let the question take this form: When, at any place in extragalactic
space, the vector that represents pressure of radiation from a given star.
nebula or galaxy is added vectorially to the vector that represents the
gravitational field of the same star, nebula or galaxy what is the direction
and magnitude of the resulting vector?
The answer provides no difficulty. Both forces operate along the same
line, the one being a push away from the source and the other a pull
towards it. The resultant vector has therefore the same direction as the two
of which it is the sum and points in the direction of the preponderating
force. The effect of pressure of radiation is, in other words, to reduce
locally the effect of the gravitational constant, G, by a certain amount
but not to change the direction of the field. But should the pressure of
radiation preponderate anywhere, it would change the sign of the field.
If it did that, things would fall off a bright and massive body instead of
on to it.
Like gravity, pressure of radiation follows the inverse square law.
Both forces are therefore greatest in the vicinity of the body that constitutes their source and both diminish in the same ratio with increasing
distance. If the one force preponderates over the other near the source, it
does so everywhere else. If the pressure of radiation is not enough to
overcome gravitation at the surface of a star, it is not enough to overcome
The gravitational pull of a star is proportional to its mass and the
radiation push is proportional to its brightness. Those who believe that
pressure of radiation has a significant influence on the movement of
interstellar particles must therefore assume that the ratio of brightness to
mass of at least some stars suffices to cancel G. Does it?
We know that the pressure of radiation at the sun's surface is not
sufficient to overcome gravitation there. If it were, hydrogen would be
continuously rising against gravity from the whole of the sun's surface.
The sun would not hold together. In fact, any star bright enough to throw
its substance into outer space would gradually disappear like a drop
of water that evaporates into water vapour. And the sun is a star of average
brightness. So we may safely conclude that the pressure of radiation does
not significantly affect either the position or the steepness of any of the
astronomical features that have been discussed above. The astronomical
summits, the passes, the boundaries must be almost as they would be in a
world devoid of radiation.
We also have to reject the notion that the observed interstellar gas has
moved into outer space against the force of gravitation. It is but one of
those notions that seem attractive only so long as they are not subjected
to the test of quantitative thinking. The notion that the much heavier
germs of living substance have been moved by faint starlight not only
against the gravitational field of the planet on which they originated during
the early part of their journey but also against the sun's fierce brightness
during the latter part is, of course, quite untenable. Indeed I find it hard
to feel otherwise than flippant about those who tell us that, in all probability,
the pressure of radiation brought the germs of living substance to our
earth. Do they claim, perhaps, that it happened at night ? Can they have
forgotten that the sun is still shining when it is night time on the part of
the earth where they happen to be ?
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