I am of opinion then that every Sun is surrounded with a Whirl-pool or Vortex of Matter in a very swift Motion; tho not in the least like Cartes’s either in their bulk, or manner of Motion. For Cartes makes his so large, as everyone of them to touch all the others round them, in a flat Surface, just as you have seen the Bladders that Boys blow up in Soap-suds do: and would have the whole Vortex to move round the same way. But the Angles of every Vortex will be no small hindrance to such a Motion....Against all which there are many Astronomical Objections, some of which I touch’d upon in my Essay of the Causes of Gravity. Where I gave another account of the Planets not deserting their own Orbs; which is their Gravitation towards the Sun. I show’d there the Causes of that Gravitation, and cannot but wonder that Cartes, the first man that ever began to talk reasonably of that matter, should never meddle with, or light on it. Plutarch in his Book of the Moon above mentioned says, that some of the Antients were of opinion, that the reason of the Moon’s keeping her Orbit was, that the force of her Circular Motion was exactly equal to her Gravity, the one of which pull’d her to, as much as the Other forc’d her off from the Centre. And in our Age Alphonsus Borellus, who was of this same opinion in the other Planets as well as the Moon, makes the Gravitation of the primary Planets to be towards the Sun, as that of the secondary is towards the Planets round which they move: Which Mr. Isaac Newton has more fully explained, with a great deal of pains and subtilty; and how from that cause proceeds the Ellipticity of the Orbs of the Planets, found out by Kepler. According to my Notion of the Gravitation of the Planets to the Sun, the matter of his Vortex must not all move the same way, but after such a manner as to have its parts carry’d different ways on all sides. And yet there is no fear of its being destroy’d by such an irregular motion, because the æther round it, which is at rest, keeps the parts of it from flying out. With the help of such a Vortex as this I have pretended in that Essay to explain the Gravity of Bodies on this Earth, and all the effects of it. And I suppose there may be the same cause as well of the Gravitation of the Planers, and of our Earth among the rest, towards the Sun, as of their Roundness: a thing so very hard to give an account of in Cartes’s System.
I must differ from him too in the bigness of the Vortices, for I cannot allow them to be so large as he would make them. I would have them dispers’d all about the immense space, like so many little Whirl-pools of Water, that one makes by the stirring of a stick in any large Pond or River, a great way distant from one another. And as their motions do not all intermix or communicate with one another; so in my opinion must the Vortices of Stars be plac’d as not to hinder one anothers free Circumrotations.
So that we may be secure, and never fear that they will swallow up or destroy one another; for that was a mere fancy of Cartes’s; when he was showing how a fix’d Star or Sun might be turn’d into a Planet. And ’tis plain, that when he writ it, he had no thoughts of the immense distance of the Stars from one another...For my part, I shall be very well contented, and shall count I have done a great matter, if I can but come to any knowledge of the nature of things, as they now are, never troubling my head about their beginning, or how they were made, knowing that to be out of the reach of human Knowlege, or even Conjecture.--Christiaan Huygens, The Cosmotheoros (1698). [HT Chris Smeenk]
Last week I suggested that there was no reason for Huygens, a well known and sophisticated relativist about space and time, to posit absolute time and (my real point) absolute simultaneity of events between 'different temporal frames.' Within a temporal frame there is simultaneity (let's call that local simultaneity). A temporal frame is governed by a local equation of time, which creates a mean day that can be used to calibrate and set local clocks. When widely adopted it ensures that astronomers are using the same temporal framework with which to interpret astronomical data (see here for explanation). It's a small step to posit a temporal frame for the solar system with its equation of time. As Newton writes in the original, suppressed version of the final part of the Principia, The System of the World: demonstrated in an easy and popular manner: “That the Planets, in respect of the fixed Stars, are revolved by equable motions about their proper aces. And that (perhaps) those motions are the most fit for the equation of time” Once the solar day is corrected by the (mathematical) equation of time, one obtains a shared temporal frame suitable for one’s physics. As Newton points out in his discussion, for the equation of time to work in practice, you need to assume that some of the background stars are fixed. So, you don't need to posit absolute time (as Newton does). So, I suggested, for Huygens the denial of absolute simultaneity is entirely possible within his physics, especially because there is no evidence he thought that motions in the universe were governed by a universal vortex which could provide the basis for a universal equation of time (which approximated absolute time).
There are really two kinds of issues at work here.
First, as I noted last week, to get the result that Huygens could have denied absolute simultaneity, one needs to posit some kind of causal isolation between different solar systems.If one understands Huygens as a Cartesian Plenist then there is no reason to assume he would allow such causal isolation (several people suggested this to me in private conversation and on facebook). Helpfully, my some-time co-author, Chris Smeenk [see this piece--I am very proud of it], called my attention to the closing lines of Huygens of the Cosmothereos. There Huygens clearly asserts the causal isolation among solar systems that are immensely apart. On his view there is a fixed æther that both holds them in place (so that the vortex does not come peeling off layer by layer)* and causally isolates them. Huygens explicitly rules out that the æther transmits motions among vortices. For present purposes we can ignore the obvious problems with positing such an æther; it shows that Huygens is committed to some significant causal isolation among solar systems and, thereby, has no reason to posit absolute simultaneity. One reason for my cautious expression in the previous sentence is that Huygens does seem to allow the transmission of light-waves through this æther. So, while Huygens would not have to be forced into positing absolute simultaneity, he could have been tempted to create a shared temporal framework by using (the finite speed of) light.
Even so, there is as second problem surrounding absolute simultaneity that needs to be distinguished from the one(s) I have been discussing. This involves what one may call absolute regularity. To quote Katherine Brading, "We can ask: Are equal intervals of time according to the equation of time for system A *regular* compared to equal intervals of time according to the equation of time for system B? Here we need a much lower degree of isolation for the problem to arise, at least in principle." Given Huygens's understanding of the equation of time, this is a very intelligible problem; it, too, could have made him cautious about positing absolute simultaneity.
Finally, I end with a two-fold observation on the closing lines of the Cosmotheoros quoted above. First, Huygens ends the book on a note of epistemic modesty: even speculative cosmogony is out of reach. Here Huygens rejects the known options: (a) chance (as favored by the Epicureans), (b) God as first cause that generates law-governed over-determination to the present (Descartes), (c) eternal necessatarianism (Spinoza), and (d) theistic design. As we know, in Newton's General Scholium (which was published after the Cosmotheoros), Newton argued forcefully for (d): ‘‘This most beautiful System of the Sun, Planets, and Comets, could only proceed from the counsel and dominion of an intelligent and powerful being’’ (Newton 1999: 940). But Newton also admitted that there is a gap in his account in the intermediary steps between God's plan and the secondary causes that governed the process leaving the door open to somebody to offer a law-governed cosmogony. (I have argued elsewhere that Kant takes up this challenge.)
Second, Huygens's note of epistemic modesty about cosmogony fits his general, mature methodological stance. But it is at odds with his the tenor of his claims earlier in the Cosmotheoros. For, throughout the book, Huygens suggests he accepts general providence and design arguments. For example:
The Stature and Shape of Men here does show forth the Divine Providence so much in its being so fitly adapted to its design’d Uses, that it is not without reason that all the Philosophers have taken notice of it nor without probability that the Planetarians have their Eyes and Countenance upright, like us, for the more convenient and easy Contemplation and Observations of the Stars. And the Wisdom of the Creator is so observable, so praiseworthy in the position of the other Members; in the convenient situation of the Eyes, as Watches in the higher Region of the Body; in the removing of the more uncomly parts out of sight as ’twere; that we cannot but think he has almost observed the same Method in the Bodies of those remote Inhabitants.
One would think then that Huygens also embraces (d) theistic design. Yet, here at the very close of the book, Huygens explicitly denies that we can know anything about the origins of things. So, while Huygens does not rule out theistic design, his final word on the matter is that we cannot know it.**
*I thank Marius Stan for this expression
**Some other time, I'll say more about the skeptical strain in the concluding pages of the Cosmotheoros. I thank Marius, Chris Smeenk, Katherine Brading, Erik Curiel, and Ori Belkind for very helpful discussion. They should be blamed for any remaining mistakes.