Thursday, September 25, 2014

Is light a body? Aquinas, Newton, and Maxwell on the nature of light

A. Aquinas: Light is not a body 

In his discussion on the nature of light, Aquinas argued that light cannot be body:
Objection 1: It would seem that light is a body. For Augustine says (De Lib. Arb. iii, 5) that "light takes the first place among bodies."Therefore light is a body. Objection 2: Further, the Philosopher says (Topic. v, 2) that "light is a species of fire." But fire is a body, and therefore so is light. Objection 3: Further, the powers of movement, intersection, reflection, belong properly to bodies; and all these are attributes of light and its rays. Moreover, different rays of light, as Dionysius says (Div. Nom. ii) are united and separated, which seems impossible unless they are bodies. Therefore light is a body. On the contrary, 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. (Note: I'll discuss the Aquinas's other reasons in another blog post)
If we translate these in modern language, what Aquinas is saying is that light is not matter but a field. Matter is something that occupies space and has mass. Light, on the other hand, does not occupy space and has no mass.

B. Newton: Light is a body

In contrast to Aquinas, Newton believes that light is a body: a stream of particles called corpuscles (the word "corpus" is Latin for body).It is also true that light behaves like particles or corpuscles as stated in Objection 2. Newton himself used the corpuscular theory of light to describe the reflection, refraction, and dispersion of light into different colors :
In these Descriptions I have been the more particular, because it is not impossible but that Microscopes may at length be improved to the discovery of the Particles of Bodies on which their Colours depend, if they are not already in some measure arrived to that degree of perfection. For if those Instruments are or can be so far improved as with sufficient distinctness to represent Objects five or six hundred times bigger than at a Foot distance they appear to our naked Eyes, I should hope that we might be able to discover some of the greatest of those Corpuscles. And by one that would magnify three or four thousand times perhaps they might all be discover'd, but those which produce blackness. In the mean while I see nothing material in this Discourse that may rationally be doubted of, excepting this Position: That transparent Corpuscles of the same thickness and density with a Plate, do exhibit the same Colour. And this I would have understood not without some Latitude, as well because those Corpuscles may be of irregular Figures, and many Rays[Pg 262] must be obliquely incident on them, and so have a shorter way through them than the length of their Diameters, as because the straitness of the Medium put in on all sides within such Corpuscles may a little alter its Motions or other qualities on which the Reflexion depends. But yet I cannot much suspect the last, because I have observed of some small Plates of Muscovy Glass which were of an even thickness, that through a Microscope they have appeared of the same Colour at their edges and corners where the included Medium was terminated, which they appeared of in other places. However it will add much to our Satisfaction, if those Corpuscles can be discover'd with Microscopes; which if we shall at length attain to, I fear it will be the utmost improvement of this Sense. For it seems impossible to see the more secret and noble Works of Nature within the Corpuscles by reason of their transparency. (Isaac Newton, Opticks

C. Maxwell: Light is an oscillation of a body

 But the laws of reflection and refraction of light may also be described using the electromagnetic wave model using the four Maxwell's Laws of Electrodynamics. In this model, light is described as an electromagnetic wave. When light hits the interface, the boundary conditions are provided by Maxwell's equations, and these boundary conditions would predict the behavior of light after reflection or refraction, resulting to the Fresnel's equations for reflection and refraction of light:
 Around 1862, while lecturing at King's College, Maxwell calculated that the speed of propagation of an electromagnetic field is approximately that of the speed of light. He considered this to be more than just a coincidence, commenting, "We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."[54] Working on the problem further, Maxwell showed that the equations predict the existence of waves of oscillating electric and magnetic fields that travel through empty space at a speed that could be predicted from simple electrical experiments; using the data available at the time, Maxwell obtained a velocity of 310,740,000 metres per second (1.0195×109 ft/s).[87] In his 1864 paper A dynamical theory of the electromagnetic field, Maxwell wrote, "The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws".[6] (Wikipedia: James Clerk Maxwell)
The color of light that Newton sees actually depends on the wavelength (or frequency) of light. For example, in the electromagnetic spectrum red light has a wavelength of 700 nanometers while blue light is 400 nm. The separation of colors that Newton sees is called dispersion. The dispersion of light in a prism is due to the fact that different colors have different refractive indices for the same material, so that the blue light is bent more than the red. The same is true with waves on the seashore: the reason why waves appear regularly spaced is because of frequency dispersion--waves with different frequencies travel at different speeds, so that waves with the same frequencies arrive as a group.

Since light is an electromagnetic field, then superposition of several electromagnetic fields is possible even at one point-- something like the Medieval problem of determining how many angels can dance on the head of a pin. If we liken angels to electromagnetic waves, then we can say that an infinite number of angels can dance on the head of a pin, because an electromagnetic field can be expressed as an infinite Fourier series either in trigonometric sines and cosines with harmonic frequencies, with each harmonic frequency corresponding to one angel.  The analogy is interesting: we know that there are countably infinite integers, but angels, though there are a multitude of them like the stars in heavens or the sands in seashore, remains finite.

But we now ask: if light is an undulation of a medium, is this medium a body? This is the most perplexing problem in modern physics regarding the luminiferous aether.  We shall discuss this in future blog posts.

The Mystery of Matter: Nonlocality, Morphic Resonance, Synchronicity and the Philosophy of Nature of St. Thomas Aquinas
The Mystery of Matter: Nonlocality, Morphic Resonance, Synchronicity and the Philosophy of Nature of St. Thomas Aquinas
Commentary on Aristotle's Physics [Aristotelian Commentary Series]
Commentary on Aristotle's Physics [Aristotelian Commentary Series]
Opticks: Or a Treatise of the Reflections, Refractions, Inflections & Colours of Light-Based on the Fourth Edition London, 1730
Opticks: Or a Treatise of the Reflections, Refractions, Inflections & Colours of Light-Based on the Fourth Edition London, 1730
Maxwell on the Electromagnetic Field: A Guided Study (Masterworks of Discovery)
Maxwell on the Electromagnetic Field: A Guided Study (Masterworks of Discovery)
Icon Wall Decals Light Dispersion Illustration - 30 inches x 20 inches - Peel and Stick Removable Graphic
Icon Wall Decals Light Dispersion Illustration - 30 inches x 20 inches - Peel and Stick Removable Graphic
Who Is Fourier?: A Mathematical Adventure
Who Is Fourier?: A Mathematical Adventure
Einstein and the Ether
Einstein and the Ether