Harry Veeder
Thu, 26 Jun 2008 12:07:27 -0700
Re: [Vo]:Three Words That Could Overthrow Physics On 9/6/2008 3:51 PM, Stephen A. Lawrence wrote: > Tell me *why* momentum is conserved -- that would be an "explanation". > But Newton didn't tell us *why*, he merely told us that it *is* > conserved. It's like the following little convsersation: > > "Go to bed NOW!" > > "Why?" > > "Because I told you to!" In chapter seven of "The world", Descartes explains why motion is conserved using a theological argument. http://www.princeton.edu/~hos/mike/texts/descartes/world/worldfr.htm The whole chapter should be read but here are some passages: ----------------------- ...Know, then, first that by "nature" I do not here mean some deity or other sort of imaginary power. Rather, I use that word to signify matter itself, insofar as I consider it taken together with all the qualities that I have attributed to it, and under the condition that God continues to preserve it in the same way that He created it. For from that alone (i.e. that He continues thus to preserve it) it follows of necessity that there may be many changes in its parts that cannot, it seems to me, be properly attributed to the action of God (because that action does not change) and hence are to be attributed to nature. The rules according to which these changes take place I call the "laws of nature."... ...Finally, the motion of which they speak is of such a strange nature that, whereas all other things have as a goal their perfection and strive only to preserve themselves, it has no other end and no other goal than rest. Contrary to all the laws of nature, it strives on its own to destroy itself. By contrast, the motion I suppose follows the same laws of nature as do generally all the dispositions and all the qualities found in matter, as well those which the scholars call modos et entia rationis cum fundamento in re (modes and beings of thought with foundation in the thing) as qualitates reales (their real qualities), in which I frankly confess I can find no more reality than in the others.... ...Now it is the case that those two rules manifestly follow from this alone: that God is immutable and that, acting always in the same way, He always produces the same effect. For, supposing that He placed a certain quantity of motions in all matter in general at the first instant He created it, one must either avow that He always conserves as many of them there or not believe that He always acts in the same way. Supposing in addition that, from that first instant, the diverse parts of matter, in which these motions are found unequally dispersed began to retain them or to transfer them from one to another according as they had the force to do, one must of necessity think that He causes them always to continue the same thing. And that is what those two rules contain. I will add as a third rule that, when a body is moving, even if its motion most often takes place along a curved line and (as has been said above) can never take place along any line that is not in some way circular, nevertheless each of its individual parts tends always to continue its motion along a straight line. And thus their action, i.e. the inclination they have to move, is different from their motion... ----------------------- Harry