by     Reginald O. Kapp


Chapter 11 - The Pressure of Radiation

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 a notion.

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 while.

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 it anywhere.

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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