On Wed, 02 Jul 2014 21:06:52 -0700, Rustom Mody wrote: > On Thursday, July 3, 2014 7:49:30 AM UTC+5:30, Steven D'Aprano wrote: >> On Wed, 02 Jul 2014 23:00:15 +0300, Marko Rauhamaa wrote: > >> > On the other hand, floating-point numbers are perfect whenever you >> > deal with science and measurement. > >> /head-desk > > <wink> > > Just as there are even some esteemed members of this list who think that > c - a is a meaningful operation > where > c is speed of light > a is speed of an automobile > > > </wink>
You seem to be having some sort of nervous tic. Subtracting two numbers a and c *is* a meaningful operation, even if they are speeds, and even in special relativity. Consider an observer O in an inertial frame of reference. A car x is driving towards the observer at v metres per second, while a photon p travels away from the observer at c m/s: x --> v O p ----------> c According to the observer, the difference in speeds between x and p is just (c - v), the same as in classic mechanics. The technical term for it is "closing speed" (or "opening speed" as the case may be) as seen by O. Note that this is *not* the difference in speeds as observed by x, but I never said it was. You don't have to believe me. You can believe the Physics FAQs, maintained by John Baez: http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html The important part is the paragraph titled "How can that be right?" and ending "In this sense velocities add according to ordinary vector addition." As I wanted to confirm my understanding of the situation: https://groups.google.com/forum/#!topic/sci.physics/BqT0p_7tHYg -- Steven -- https://mail.python.org/mailman/listinfo/python-list