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.
> Just as there are even some esteemed members of this list who think that
> c - a is a meaningful operation
> c is speed of light
> a is speed of an automobile
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:
The important part is the paragraph titled "How can that be right?" and
ending "In this sense velocities add according to ordinary vector
As I wanted to confirm my understanding of the situation: