St. Thomas Aquinas: Is Light a Body?
February 2, 2009 Leave a comment
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.
Reference: St. Thomas Aquinas, ”Is light a body,” in Summa Theologiae, part I, question 67, article 2. In Christian Classics Ethereal Library.
 We may translate Aquinas’s “body” as “matter”. Matter occupies space; light does not. Matter has mass; light is massless. But Aquinas does not talk about mass of light. A closer translation of “body” would be Newton’s corpuscles (corpus = body): light is a stream of particles.
 The speed of light is now known to be finite: c = 299,792,458 m/s or about 3×108 m/s. The old definition of the meter is 1/10,000,000 of the distance from the equator to the northpole. This means that the distance between the poles through a longitudinal line is about a = 20,000,000 m or 2×107. The time for light to traverse this path, possibly through an an optical fiber link between the poles, is c/a = (2/3)×107-8 = 0.67×10-1 = 0.067 s = 1/15 s. The standard frame rate for old cartoon films is 16 frames per second, so that the pictures will appear continuous. If we consider pole-to-pole (or East to West) travel of light as a single frame, then the corresponding movie has 15 frames per second. Thus, to an human observer on the equator, the speed of light is nearly instantaneous, as Aquinas argued: “the sun is at the horizon, the whole hemisphere is illuminated from end to end.”
 Light is also known to be a wave. A simple model of light is a plane wave, like waves in a seashore. Another useful model is that light is a spherical wave, which move out like ripples caused by a coin tossed in a still pond. This model is still used in the design of optical optical systems like lenses and mirrors.
 The accepted description of light today is the so-called wave-particle duality: light sometimes behave like a particle and sometimes like a wave, depending on how you measure it.