On Fri, 13 Jun 2014 03:18:00 +1000, Chris Angelico wrote:

> On Fri, Jun 13, 2014 at 3:04 AM, Steven D'Aprano
> <steve+comp.lang.pyt...@pearwood.info> wrote:
>> Take three numbers, speeds in this case, s1, s2 and c, with c a strict
>> upper-bound. We can take:
>> s1 < s2 < c
>> without loss of generality. So in this case, we say that s2 is greater
>> than s1:
>> s2 > s1
>> Adding the constant c to both sides does not change the inequality:
>> c + s2 > c + s1
> As long as we accept that this is purely in a mathematical sense. Let's
> not get into the realm of actual speeds greater than c.

Well, yes, it is in the mathematical sense, and it doesn't require any 
actual physical thing to travel at faster than light speed. There is no 
implication here that there is something travelling at (c + s1). It's 
just a number.

But note that even in *real* (as opposed to science fiction, or 
hypothetical) physics, you can have superluminal speeds. Both the phase 
velocity and group velocity of a wave may exceed c; the closing velocity 
of two objects approaching each other is limited to 2c. Distant galaxies 
are receding from us at greater than c. There are other situations where 
some measurable effect can travel faster than c, e.g. the superluminal 
spotlight effect.


Steven D'Aprano

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