Galileo Galilei, Isaac Beeckman and René Descartes Discuss the Speed of Light
Does light travel instantaneously, or is it simply very fast? This debate is not new, and goes back to Antiquity; for example, Aristotle argued that light was transmitted instantaneously. During the seventeenth century, with the development of finer instruments, both for observing the heavens and for measuring time, the debate was revived. In this post, I will look at the positions of three important scientists, Galileo Galilei (1564-1642), Isaac Beeckman (1588-1637) and René Descartes (1596-1650).
Galileo initially believed that light is transmitted immediately. Here is a passage in The Assayer1 (Il Saggiatore, 1623):
The rubbing together and friction of two hard bodies, either by resolving their parts into very subtle flying particles or by opening an exit for the tiny fire-corpuscles within, finally set these in motion and, upon their encountering our bodies and penetrating and coursing through them, our conscious mind (anima sensitiva) feels that pleasant or obnoxious sensation which we have named heat, burning, or scalding. And perhaps when the thinning and attrition stop at or are confined within the tiniest particles (i minimi quanti), their motion is temporal and their action is calorific only, but, when their ultimate and highest resolution into truly indivisible atoms is reached, light is created which has instantaneous motion—or let us say instantaneous expansion and diffusion—and is capable of occupying immense spaces by its—I do not know whether to say by its subtlety, its rarity, its immateriality, or yet some other property different from all these, and nameless. [p.313. my emphasis]
However, over time, Galileo considered the possibility that light might in fact have finite speed. In his Two New Sciences2 (Discorsi e dimostrazioni matematiche intorno a due nuove scienze, 1638), he proposed an experiment with two men holding lanterns. It is Salviati who presents the experiment to Sagredo:
Salv. The inconclusiveness of these and like observations caused me once to think of some way in which we could determine without error whether illumination (that is, the expansion of light) is really instantaneous. The rapid motion of sound assures us that that of light must be very swift indeed, and the experiment that occurred to me was this. I would have two men each take one light, inside a dark lantern or other covering, which each could conceal and reveal by interposing his hand, directing this toward the vision of the other. Facing each other at a distance of a few braccia, they could practice revealing and concealing the light from each other’s view, so that when either man saw a light from the other, he would at once uncover his own. After some mutual exchanges, this signaling would become so adjusted that without any sensible variation, either would immediately reply to the other’s signal, so that when one man uncovered his light he would instantly see the other man’s light.
This practice having been perfected at a short distance, the same two companions could place themselves with similar lights at a distance of two or three miles and resume the experiment at night, observing carefully whether the replies to their showings and hidings followed in the same manner as near at hand. If so, they could surely conclude that the expansion of light is instantaneous, for if light required any time at a distance of three miles, which amounts to six miles for the going of one light and the coming of the other, the interval ought to be quite noticeable. And if it were desired to make such observations at yet greater distances, of eight or ten miles, we could make use of the telescope, focusing one for each observer at the places where the lights were to be put into use at night. Lights easy to cover and uncover are not very large, and hence are hardly visible to the naked eye at such distance, but by the aid of telescopes previously fixed and focused they could be comfortably seen.
Sagr. The experiment seems to me both sure and ingenious. But tell us what you concluded from its trial.
Salv. Actually, I have not tried it except at a small distance, less than one mile, from which [trial] I was unable to make sure whether the facing light appeared instantaneously. But if not instantaneous, light is very swift and, I may say, momentary; [pp.50-51]
Of course, as Salviati states, the distances proposed for this experiment were way too short, and so the experiment was inconclusive.
We can now move on to Beeckman, whose notes3 are in Latin and Dutch. The original quotations are too long for the text, so I have chosen to put only the translations in the text, placing the originals in the footnotes.
Beeckman believed that the speed of light was finite, and devised some experiments to verify his hypothesis. The first was to create an explosion, and measure the time it would take for someone far away on a mountain to see it4. Here is a Google translation from the Latin:
I once wrote before that men think that light requires no time to traverse any space, because there is no measure by which the speed of light can be measured, in the same way that light is measured the speed of sounds.
But today, which is the 19th of March, 1629, in Dort, I have come across a way in which this can be done.
A man is separated from another by as many miles as the light of an exploded bomb can be seen; and when this distance is greater, let each one stand on a high mountain, so that there is nothing in the middle so that the light or the flame of the lighted fire can be seen less. It is probable, however, that a great distance would be required to notice any difference in time, owing to the incredible speed at which light moves. Each man should have a very accurate portable clock, and each, both he who is standing when the bomb explodes and he who is so far removed from it, each, I say, at the moment he sees the light, marks some point on the clock's fastest wheel, either with ink or in some other way , by which one can know precisely how many teeth were touched while they met each other on the road. For each of them goes with his watch to his partner, and when they meet each other, each one counts how many teeth have passed in his watch, and this is done more often, as the watches are exchanged.
It seems to me very likely that the speed of light is not so great that they would have been caught by several teeth in the clock of him who stood by when the bomb exploded.
The second was to strike something with a hammer5: Here is a Google translation from the Dutch:
To prove that light flies in time, that is to say, that it must also travel from the place where it was born to another place, we should set someone behind a wall or fence with a hammer in the hand, and you should stand on the other side of the guard with good binoculars in hand. Half a mile away (the further the better) there will be a flat mirror, as if with your binoculars you could see the person standing behind the wall with the hammer in his hand in the mirror. When you look at them, you will shout to knock with the hammer, which you will hear as if you heard it in an instant; but the light coming from the moving hammer must fly half a mile after the mirror and from there another half a mile back to you; so you will hear the sound sooner than you will see the movement.
From these passages, we can understand that Beeckman believed that the speed of light was only just faster than the speed of sound.
Descartes, like Galileo, initially believed that light was transmitted instantaneously. Here is a passage from The World6 (Le Monde, written in 1629-1633, published posthumously in 1664):
The principal properties of light are: (1) that it extends circularly in all directions around those bodies one calls luminous; (2) to any distance whatever; (3) instantaneously; (4) and ordinarily in straight lines, which should be taken as rays of light; (5) and that several of these rays coming from different points can collect together at the same point; (6) or, coming from the same point, can go out toward different points; (7) or, coming from different points and going to different points, can pass through the same point without impeding one another; (8) and that they can sometimes impede one another, namely when they are of very unequal force, that of some rays being far greater than that of others; (9) and, finally, that they can be diverted by reflection; (10) or by refraction; (11) and that their force can be increased, (12) or diminished by the different dispositions or qualities of the matter that receives them. Here are the principal qualities observed in light, and all of them are in accord with this action, as you shall see. [pp.62-63, my emphasis]
Descartes’s position was confirmed in a letter he wrote to Beeckman7 dated 22 August 1634:
Dixi nuper, cum vna essemus, lumen in instanti non quidem moueri, vt seribis, sed (quod pro eodem habes) à corpore luminoso ad oculum peruenire, addidique etiam hoc mihi esse tam certum, vt si falsitatis argui posset, nil me prorsus scire in Philosophia confiteri paratus sim. [pp.307-308]
Here is my adaptation of a Google translation:
I said recently, when we were together, that light does not move in an instant, as you will see, but … instantly reaches from a luminous body to the eye. I add that this is so certain to me, that if it could be accused of falsity, I would confess to not know anything at all in philosophy.
So, in this quotation, Descartes makes it clear that the instantaneous nature of light is crucial for his philosophy.
Nevertheless, over time, Descartes changed his mind, as can be seen by comparing a specific passage in his Discours sur la Méthode8 (Discourse on Method), and comparing the French original (1637) and the Latin translation (1644). Here is the passage in the original French:
Et icy, m'estendant sur le suiet de la lumière, i'expliquay bien au long quelle estoit celle qui fe deuoit trouuer dans le Soleil & les Estoiles, & comment de la elle trauersoit en vn instant les immenses espaces des cieux, & comment elle se refleschissoit des Planètes & des Comètes vers la Terre. [p.43, my emphasis]
And here is the same passage in the Latin translation:
Et hoc loco in tractationem de Luce digressus, prolixe exposui quænam ea esse deberet quæ Solem & stellas componeret, & quomodo inde temporis momento immensa cœlorum spatia trajiceret, & à Planetis Cometisque ad terram reflecteret. [p.564, my emphasis]
In the original French, we read “en un instant” (“in an instant”), while in the Latin translation, we get “temporis momento” (“in the time of a moment”). The Latin gives the impression that a short moment of time passes for light to travel, while the French states that no time at all passes.
It is important to remember that the Latin translation was supervised by Descartes, so we cannot consider this to simply be a mistranslation. This subtle change of perception of Descartes’s was noted by Geneviève Rodis-Lewis, who was editor of the Discours sur la Méthode for the prestigious Bibliothèque de la Pléiade, in a paper entitled “Some comments about the question of the speed of light in Descartes's works”9. Here is the key paragraph of her article:
Or, en préparant l'édition du Discours de la Méthode pour la Bibliothèque de la Pléiade, et en contrôlant les modifications apportées par la traduction de 1644, j'ai eu la surprise de voir que la phrase de la cinquième partie sur la lumière du soleil et des étoiles qui « traversait en un instant les immenses espaces des cieux » [p.43] était traduite par « temporis momento » [p.564]. Cela m'a d'autant plus frappée que j'avais relevé dans la quatrième partie, à propos de l'âme substance pensante, l'addition répétée de « per momentum temporis », suivie de : « durante illo tempore » [p.558]; durée et continuité de la pensée étaient donc affirmées par Descartes bien avant sa vive réaction contre Burman lors de l'entretien de 1647, où Descartes nie que notre pensée se fasse en un instant : « Cela est faux, dit-il, toutes mes actions se font "in tempore" même si leur durée (durationem) est divisible » [p.148]; il y a ici une opposition radicale entre la temporalité avec durée et la stricte instantanéité. Alors, bien que je ne me sois occupée que de l'édition du Discours et non des Essais, j'ai contrôlé les mentions de l'instant dans la Dioptrique et dans les Météores et découvert ces modifications accordant à la lumière une vitesse extrême, ce qui paraît abandonner la stricte instantanéité.
Here is a slightly modified Deepl translation:
However, when I was preparing the edition of the Discours de la Méthode for the Bibliothèque de la Pléiade, and checking the changes made by the 1644 translation, I was surprised to see that the sentence in the fifth part about the light of the sun and stars ‘crossing the immense spaces of the heavens in an instant’ [p.43] had been translated as ‘temporis momento’ [p.564]. This struck me all the more as I had noticed in the fourth part, in connection with the thinking substance soul, the repeated addition of ‘per momentum temporis’, followed by : ‘durante illo tempore’ [p.558]; the duration and continuity of thought were thus affirmed by Descartes well before his sharp reaction against Burman in the interview of 1647, where Descartes denies that our thought takes place in an instant: ‘This is false, he says, all my actions take place “in tempore” even if their duration (durationem) is divisible’ [p.148]; there is here a radical opposition between temporality with duration and strict instantaneity. So, although I was only concerned with the edition of the Discours and not the Essais, I checked the mentions of the instant in the Dioptrique and the Météores and discovered these modifications granting light an extreme speed, which seems to abandon strict instantaneity.
So, as we have seen, in the early seventeenth century, it was common to consider that the transmission of light was instantaneous. Over time, the possibility that the transmission might take place at a finite speed was considered, but the proposed experiments could in no possible way determine this.
In my next post, I will write about Cristiaan Huygens’s (1629-1695) presentation of Ole Rømer’s (1644-1710) proof that the speed of transmission of light is in fact finite.
Galileo Galilei. The Assayer. Translated from the Italian by Stillman Drake. In Galileo Galilei, Horatio Grassi, Mario Guiducci, Johann Kepler. The Controversy on the Comets of 1618. Translated by Stillman Drake and C.D. O’Malley. Philadelphia: University of Pennsylvania Press, 1960.
Galileo Galilei. Two New Sciences, Including Centers of Gravity & Force of Percussion. Translated, with Introduction and Notes, by Stillman Drake. University of Wisconsin Press, 1974.
Isaac Beeckman. Journal tenu par Isaac Beeckman de 1604 à 1634. Tome 3: 1627-1634 (1635) (ed. Cornelis de Waard). Den Haag: Martinus Nijhoff, 1945.
Scripsi antè aliquando putare homines lumen nihil temporis requirere ad quodvis spacium peragrandum, quia nulla mensura est quâ tanta luminis celeritas potest metiri, eo modo quo lumen celeritatem sonituum metitur.
At hodiè, qui est 19 Martij 1629 te Dort, mihi incidit modus aliquis quo id fieri possit.
Distet homo ab alio per tot miliaria per quot bombardi explosi lumen potest videri; et quo spatium hoc sit majus, stet uterque in monte excelso, ne quid in medio obstet quo minus lux vel flamma ignis accensi videri possit. Verisimile autem est magnum spatium requiri ad differentiam aliquam notandam tempore, ob incredibilem luminis in movendo celeritatem. Uterque homo habeat exactissimum horologium portatile et uterque, tam is qui bombardo exploso astat quàm qui tam longè ab eo remotus est, uterque, inquam, eo momento quo lumen videt, in horologij celerrimâ rotâ notet punctum aliquod, vel atramento vel alio modo, quo exactè potest scire quot denticuli tacti fuerunt dum sibi invicem in viâ occurrerunt. Uterque enim, cum horologio suo ad socium proficiscatur, atque ubi sibi occurrerint, unusquisque numeret quot denticuli in suo horologio transierint, idque saepiùs fiat, permutatis horologijs.
Verisimilè mihi videtur, non tantam esse lucis celeritatem, quin illi deprehensuri sint plures dentes transijsse in horologio ejus, qui bombardo exploso adstiterat. [19 March 1629, p.112]
Om te bewysen dat het licht in tempore vlieght, dat is, dat het oock tyt hebben moet eert van de plaetse, daert geboren wort, tot een ander plaetse geraeckt, so salmen ymant achter een muer of schutsel setten met eenen hamer in de hant, ende ghyselve sult staen aen d'ander syde van het schutsel met een goede verrekycker in de handt. Een half myle vandaer (hoe verder hoe beter) sal een platte spiegel staen, also gestelt dat ghy met uwen verrekycker den persoon, met den hamer in de handt achter de muer staende, in de spiegel sien kondt. Also ghy hun dan terdeghen siet, sult ghy roepen dat hy met den hamer kloppe, twelck geluydt ghy als in eenen ooghenblick hooren sult; maer het licht, dat van den bewegenden hamer kompt, moet vlieghen een half myle weeghs na den spiegel ende vandaer noch een half myle na u verrekycker; so sult ghy het geluydt eer hooren dan ghy de beweginghe sien sult. Twelck so seker teecken sal syn dat het licht in tempore movetur, alshet een seeker teecken is dat het geluydt in tempore vlieght, doordien datmen op een toren de hamer al noch eens op siet eer men het geklop hoort. Hinc proportio utriusque elicietur. [2 May-7 July 1634, p.349]
René Descartes. The World and Other Writings. Translated and Edited by Stephen Gaukroger. Cambridge University Press, 2004.
René Descartes. Œuvres. Tome I: Correspondance Avril 1622 - Février 1638. Publiées par Charles Adam et Paul Tannery sous les auspices du Ministère de l’Instruction publique. Paris: Léopold Cerf, 1897.
René Descartes. Œuvres. Tome VI: Discours de la Méthode & Essais. Publiées par Charles Adam et Paul Tannery sous les auspices du Ministère de l’Instruction publique. Paris: Léopold Cerf, 1902.
Geneviève Rodis-Lewis. Quelques remarques sur la question de la vitesse de la lumière chez Descartes/Some comments about the question of the speed of light in Descartes's works. Revue d'histoire des sciences, 51(2-3):347-354, 1998.