Dne 25/06/2017 v 20:04 Peter Percival napsal(a):
>>
> There is no point in measuring c, it is 299792458 m/s by definition.
>
It depends. c is defined for a sort of standard conditions.
But it can be still measured
if light behaves in various conditions differently.
By other words, if light at different conditions had propagated
by different speed, the metre would not have been defined by both.
It can be measured,
using a distance defined by the standard light,
if light passes the distance in different than expected time,
or if at expected time it passed different distance.
E.g. a specially taylored light beam
with nonzero orbital angular momentum and twisted wavefront
has the beam propagation speed slightly lower than c.
It is because the wavefront does not propagate linearly,
but on screwed trajectory of very tiny diameter.
So while its phase speed is still c,
its group and propagation speed is less than c.
--
Poutnik ( The Pilgrim, Der Wanderer )
A wise man guards words he says,
as they say about him more,
than he says about the subject.