Tuesday, April 29, 2014

Thomas Aquinas on the Speed of Light

On whether light can be a body:

Two bodies cannot occupy the same place simultaneously. But this is the case with light and air. Therefore light is not a body.

I answer that, Light cannot be a body, for three evident reasons. First, on the part of place. For
the place of any one body is different from that of any other, nor is it possible, naturally speaking,
for any two bodies of whatever nature, to exist simultaneously in the same place; since contiguity
requires distinction of place.

The second reason is from movement. For if light were a body, its diffusion would be the local
movement of a body. Now no local movement of a body can be instantaneous, as everything that
moves from one place to another must pass through the intervening space before reaching the end:
whereas the diffusion of light is instantaneous. Nor can it be argued that the time required is too
short to be perceived; for though this may be the case in short distances, it cannot be so in distances
so great as that which separates the East from the West. Yet as soon as the sun is at the horizon,
the whole hemisphere is illuminated from end to end. It must also be borne in mind on the part of
movement that whereas all bodies have their natural determinate movement, that of light is indifferent as regards direction, working equally in a circle as in a straight line. Hence it appears that the diffusion of light is not the local movement of a body.

The third reason is from generation and corruption. For if light were a body, it would follow
that whenever the air is darkened by the absence of the luminary, the body of light would be
corrupted, and its matter would receive a new form. But unless we are to say that darkness is a
body, this does not appear to be the case. Neither does it appear from what matter a body can be
daily generated large enough to fill the intervening hemisphere. Also it would be absurd to say that
a body of so great a bulk is corrupted by the mere absence of the luminary. And should anyone
reply that it is not corrupted, but approaches and moves around with the sun, we may ask why it is
that when a lighted candle is obscured by the intervening object the whole room is darkened? It is
not that the light is condensed round the candle when this is done, since it burns no more brightly
then than it burned before.

Since, therefore, these things are repugnant, not only to reason, but to common sense, we must
conclude that light cannot be a body.

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