Speed and nature of light

There was a young lady named Bright,
Who travelled much faster than light.
She started one day
In a relative way,
And returned on the previous night.
— Anon. and trad.

Gin a body meet a body
Flyin through the air,
Gin a body hit a body,
Will it fly? And where?

Ilka impact has its measure,
Ne’er a ane hae I,
Yet a’ the lads they measure me,
Or, at least, they try.

Gin a body meet a body,
Altogether free,
How they travel afterwards
We do not always see.
Ilka problem has its method
By analytics high;
For me, I ken na ane of them,
But what the waur am I?
— James Clerk Maxwell (1831 – 1879)

… the object in motion strives to move over the shortest possible distance, since it has not the time for slower motion, that is, for the motion over a longer trajectory. The impelling force does not permit such retardation. And so, by reason of its speed, the object tends to move over the shortest path. But the shortest of all lines having the same endpoints is the straight line. That the rays proceeding from our eyes move with infinite velocity may be gathered from the following consideration. For when, after our eyes have been closed, we open them and look up at the sky, no interval of time is required for the visual rays to reach the sky. Indeed we see the stars as soon as we look up, though the distance is, as we may say, infinite.
— Hero of Alexandria (1st century CE), on the speed of light.

To see then whether the spreading of light takes time, let us consider first whether there are any facts of experience which can be made here on the Earth, by striking lights at great distances, although they prove that light takes no sensible time to pass over these distances, one may say with good reason that they are too small, and that the only conclusion to be drawn from them is that the passage of light is extremely rapid. Mr. Des Cartes, who was of the opinion that it is instantaneous, founded his views, not without reason, upon a better basis of experience, drawn from the Eclipses of the Moon …

… one has always observed, we are told, that the eclipsed Moon appears at the point of the Ecliptic opposite to the Sun … [so that the shadow of the Earth does not lag behind the Sun].

But it must be noted that the speed of light in this argument has been assumed such that it takes a time of one hour to make the passage from here to the Moon. If … it requires only ten seconds of time, the angle will be less than six minutes. And then it will not be possible to perceive anything of it in observations of the Eclipse; nor, consequently, will it be permissible to deduce from it that the movement of light is instantaneous.

It is true that we are here supposing a strange velocity that would be a hundred thousand times greater than that of Sound. For Sound, according to what I have observed, travels at about 180 Toises [a toise is 6 French feet: about 1.949 metres] in the time of one Second, or in about one beat of the pulse. But this supposition ought not to seem an impossibility; since it is not a question of the transport of a body with so great a speed, but of a successive movement which is passed on from some bodies to others … .

But that which I employed only as a hypothesis, has recently received great seemingness as an established truth by the ingenious proof of Mr. Roemer which I am going here to relate, expecting him himself to give all that is needed for its confirmation. It is founded as is the preceding argument upon celestial observations, and proves not only that Light takes time for its passage, but also demonstrates how much time it takes, and that its velocity is even at least six times greater than that which I have just stated. For this he makes use of the Eclipses suffered by the little planets which revolve around Jupiter, and which often enter his shadow …
— Christiaan Huygens (1629 – 1695), Traité de la Lumiere, chapter 1.

From the agreement of the measures, and from the similarity of the phenomena, we may conclude that these intervals are the same as are concerned in the production of the colours of thin plates; but these are shown, by the experiments of Newton, to be the smaller, the denser the medium; and since it may be presumed that their number must necessarily remain unaltered in a given quantity of light, it follows of course, that light moves more slowly in a denser … medium: and this being granted, it must be allowed, that refraction is not the effect of an attractive force directed to a denser medium. The advocates for the projectile hypothesis of light, must consider which link in this chain of reasoning they may judge to be the most feeble; for, hitherto, I have advanced in this paper no general hypothesis whatever. But, since we know that sound diverges in concentric superficies [surfaces], and that musical sounds consist of opposite qualities [compressions and rarefactions], capable of neutralising each other, and succeeding at certain intervals, which are different according to the difference of the note, we are fully authorised to conclude, that there must be some strong resemblance between the nature of sound and that of light.
— Thomas Young (1773 – 1829) on using Newton’s rings to measure the wavelength of light, Bakerian lecture, 24 November 1803.

The influence of the crucial Michelson-Morley experiment upon my own efforts has been rather indirect. I learned of it through H. A. Lorentz’s decisive investigation of the electrodynamics of moving bodies, with which I was acquainted before developing the special theory of relativity. Lorentz’ basic assumption of an ether at rest seemed to me not convincing in itself and also for the reason that it was leading to an interpretation of the result of the Michelson-Morley experiment which seemed to me artificial. What led me more or less directly to the special theory of relativity was the conviction that the electromotive force acting on a body in motion in a magnetic field was nothing else but an electric field … There is, of course, no logical way leading to the establishment of a theory but only groping constructive attempts controlled by careful consideration of factual knowledge.
— Albert Einstein (1879 – 1955), Scientific American, November 1964, 114.

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