Pliny and Rømer on the speed of light

Factual Questions
1

Pliny the Elder notes in an offhand comment that light travels faster than sound (II.142). But in Denmark school children are taught that when the Danish astronomer Rømer observed the moons of Jupiter he noticed that light “delayed” and (as the first) proceeded to measure this speed of light. And that prior to that it had been a common assumption that light propagated instantaneously, which does not seem to agree with Pliny’s sentence that light travels. Or perhaps Pliny meant it travels, but at an infinite speed?

Was it accepted knowledge at the time (end 17th century) that light travels, or was Rømer first with this thought?

2

Check out the history section here:

Basically no one knew whether light travelled at a finite or an infinite speed until the Rømer experiments. There had been much speculation either way. There had been some experiments, but they weren’t exact enough to prove anything.

3

You don’t really get anything like a modern concept of light, as something that comes from the Sun (or whatever), bounces off objects and then into your eyes, until the work of the Muslim philosopher Ibn al-Haytham (a.k.a. Alhacen or, more commonly, but incorrectly, Alhazen) in the 10th or 11th century. Before that, light was not really considered as something that traveled at all. For Aristotle, for example, light was the power that the Sun, or a flame, had to make the air transparent.

It may well be true that no-one had much idea about the actual speed at which light travels before Rømer. However, once you have the idea of it as something that travels at all, you can certainly infer that it travels faster than sound by noticing that you can sometimes see a distant event, such as a lightening flash, or an avalanche on a distant mountainside, well before the sound that it makes reaches your ears.

But as Pliny lived well before the notion of traveling light, I am not sure what he might have been on about. Is it possible we are dealing with a mistranslation or an editorial “improvement” by some medieval scribe? (I cannot locate the passage you mention. Perseus seems to be telling me that book 2 of the Natural History only has 113 chapters.) Once your head is infected with a modern conception of light, it can become hard to imagine that people have not always thought of it this way, and it seems quite possible that some scribe or translator has somehow fixed the text to make it say what they think Pliny must obviously have meant to say.

In Pliny’s time, the most widespread ideas about how vision works involved either atoms from the object’s surface entering the eyes (these travel, but they are not light), or else, more commonly, some sort of influence (that was, perhaps, fiery, but not light as we think of it, despite the fact that some modern sources insist on calling it light) reaching out from the eyes towards the object to be seen. In the hands of mathematical theorists like Euclid, Ptolemy, and the Muslim Al-Kindi, this is analyzed in terms of rectilinear rays that reach out from the eyes towards visible objects (this allowed them to give quite accurate mathematical accounts of various aspects of perspective), but, to the best of my knowledge, they did not really think of these in a dynamic way, as traveling out from the eyes. Plato does talk about fire coming out from the eyes when we see, however, and Galen (and the Stoics) talk about visual pneuma emanating from the eyes, so I suppose it is possible that Pliny thought of these visual rays as traveling outwards. (I sort of doubt it, though.)

4

Empedocles had “the notion of traveling light” before Pliny – or Aristotle – arrived on the scene.

5

Yes, that was what I thought. That Rømer’s main accomplishment was the idea that light moved rather than the actual calculation of the speed (which although is a remarkable accomplishment all the same). But as Wendell Wagner showed, apparently this was not so, as there had been much discussion on the nature and speed of light back from ancient Greece.

I have a copy from Penguin Classics, which for some strange reason has a different numbering system than Perseus and the one on archive.org. Penguin give it number 142. Perseus/Archive number 54.

CHAPTER LIV. General Rules of Lightning.

Perseus:

Here is archive:

6

Er, no, that is the opposite of what I said. The idea of light traveling goes back to Alhacen. Rømer’s achievement was to actually measure the speed for the first time. Also, debates about the speed of light do not go back to ancient times (and Wendell Wagner does not say so). There is plenty of time, however, in the middle ages and after, between Alhacen in the 11th century and Rømer’s time, for such debates to have occured.

My Latin is not good enough to confirm this, but, at a guess, what he might really have meant is that seeing is faster than hearing. It does look as though he has a conception of sound traveling though, and being carried by the air. If Pliny did have a conception of light traveling from a source, like a lightening flash, to the eyes, this must have been pretty much his own idea, and it never got much attention from other ancient thinkers. Up to Alhacen’s time the visual rays from the eyes theory was the idea that was taken most seriously and was developed in mathematical detail, and Alhacen’s own, more modern conception of light was basically developed out of his critique of that theory.

All that we know that we know about Empedocles’ opinions about light or vision (or anything, actually) comes from a very small handful of comments or brief quotations in the works of later authors. About two or three of these so called fragments seem to be concerned with vision. They are difficult to interpret and not obviously consistent with one another. One seems to be about fire coming out of the eyes. One or two others seem to be about “effluences” (perhaps particles) entering the eyes. He has thus been credited as the originator both of the theory of “visual rays” issuing from the eyes, and the rival theory, found later in the atomist tradition, of vision being caused by particles entering the eyes after thrown off the surfaces of objects. Neither of these theories involves anything that bears much resemblance to our modern conception of light. Any interpretation of Empedocles that implies he believed that light traveled is almost certainly misleading and anachronistic. As I said, the modern conception of light as something that travels and bounces is so much a part of common sense for most people now, that it is all to easy to assume, quite wrongly, that when ancient authors talk about vision and related topics they must actually be talking about light as we understand it, and to twist their words to fit that assumption.

Anyway, in the unlikely event that Empedocles, through some strange prescience (unaware, as he was, of the sophisticated mathematical theories of the visual rays that laid the groundwork for Alhacen’s insight), did have a conception of light traveling in something like the modern sense, no-one in the ancient world understood it or took it seriously.

7

It occurs to me that Pliny may have been thinking, here, in terms of the Atomist theory of light, which was certainly around (as a minority view) in his time. In this theory, “atoms” (little particles of matter) were certainly conceived to travel to the eyes, but they were not light particles. In the case of seeing a dog, for instance, the idea was that some of the atoms comprising the dog were spontaneously thrown its surface and entered your eyes. In the special case of something like lightening (or a flame, or whatever) Pliny may have thought that we see it because lightening atoms travel to our eyes, and he might even have thought it natural to call these atoms “light” - and this, of course, sounds quite similar to what we now believe. It is not a general view about the nature of light however. The Atomist theory did NOT maintain that we see things when atoms of sunlight (or fire, or whatever) bounce off objects into our eyes. (Whereas it was the sunlight bouncing off the satellites that Rømer measured.)

[I am speculating here, because I do not know what Pliny’s general views about vision were. But in any case, he is not the sort of author from whom you expect much consistency, so I would not be much surprised if he says inconsistent things about light and vision in different parts of his writings.]

8

Well, how about Aristotle himself? "Empedocles says that the light from the Sun arrives first in the intervening space before it comes to the eye, or reaches the Earth. This might plausibly seem to be the case. For whatever is moved through space, is moved from one place to another; hence, there must be a corresponding interval of time also in which it is moved from the one place to the other. But any given time is divisible into parts; so that we should assume a time when the sun’s ray was not as yet seen, but was still travelling in the middle space. "

Aristotle went on to reject the idea, but not because he couldn’t take it seriously on its own terms; it does seem plausible, given an atomist starting point.

9

Well, fair enough, I missed that passage. :smack: I guess that Empedocles did think of light as traveling in a sense. However, I still very much doubt that what he (or Aristotle) had in mind was anything much like the notion of traveling light that came down to Rømer (and modern science in general) from Alhacen, and that Pliny superficially appears to be talking about. This passage does not seem to be about light rays traveling from reflective objects or from light sources (like Pliny’s lightening flash) to the eyes. (I think you have interpolated the bit about it coming to the eye. It is not in the version I am looking at.) It appears to be about sunlight spreading through and thereby illuminating (i.e., making transparent) the space between the Earth and the Sun. Apparently, Empedocles thought this took some (very brief) time (like a drop of dye diffusing through water, I imagine), whereas Aristotle thought it happened absolutely instantaneously. This sort of traveling light was not what Rømer sought to measure. It may be, of course, that Aristotle is misreading Empedocles through the lens of his own conception of light, but the fact remains that the fragments of Empedocles that deal with vision do not suggest that he had any notion of it as being caused by light rays (or light particles) traveling to the eyes.

I am not sure what you mean by “given an atomist starting point.” Neither Empedocles nor Aristotle were Atomists.

10

It is not in Smith’s translation, nor Hett’s, nor Lawson-Tancred’s. They all have something to the effect that Empedocles maintained that we do not notice the light spreading through and illuminating the space (presumably because it happens so quickly, even though not instantaneously.)

11

Folks, it’s lightning, if you’re talking about a bright flash during a thunderstorm. Lightening is what happens if you bleach something or reduce its weight.

12

Well, I’m not interpolating it; I was just relaying a translation by J. I. Beare. Arthur Fairbanks translates it only slightly differently: “Empedokles says that the light from the sun first enters the intermediate space before it comes to vision or to the earth.”

Wait – are you looking at the De Sensu, or the De Anima? Because you’re apparently right that the De Anima glosses over it: “So it is evident that light is the presence of this (fire). And Empedokles was wrong, and any one else who may have agreed with him, in saying that the light moves and arises between earth and what surrounds the earth, though it escapes our notice.” But the De Sensu doesn’t merely have something to that effect; it actually spells out the bit I was quoting in that other post.

Well, yes, I know he’s not classed with the (Big-A) Atomists. But I think calling him a (little-a) atomist makes sense – rather like calling Obama a republican, even though he’s of course not a Republican.

13

Of course, even if one interprets light (or at least, vision) as pieces of the object entering the eye, rather than being reflected from a fire or the Sun, one can still discuss the “speed of light” in that context.