# Re: From 1905 the SRT doesn’t give sleep.

```On Sat, Apr 21, 2012 at 10:40 AM, socra...@bezeqint.net <
socra...@bezeqint.net> wrote:```
```
> From 1905 the SRT  doesn’t give sleep.
> 1.
> One postulate of SRT takes vacuum as reference frame.
> Another postulate of SRT takes inertial reference frame (s).
>

No, none of the postulates take the vacuum as a reference frame, which
doesn't make sense since a vacuum doesn't have a measurable rest frame
(there are no landmarks in a vacuum that could be used to measure the
"velocity of the vacuum" relative to anything else). One postulate does
talk about the speed of light in a vacuum, but they're still talking about
the speed of light as measured in an inertial frame--"in a vacuum" is just
there to specify that it's not talking about a light beam moving through
some measurable medium like water or air.

In one reference frame speed of ‘Electrodynamics Bodies’ is constant.
> In another reference frame speed of ‘Electrodynamics Bodies’ is
> relative.
>

Nope, all speeds are measured relative to a particular frame. But in
relativity it works out that if you and I are riding in spaceships at rest
in different inertial frames (so we are moving relative to each other), and
we each measure the speed of the *same* light ray using our own rulers and
clocks, we will each find that the ray travels at a speed of 299792458
meters per second relative to ourselves (i.e. as measured in terms of
distance/time by rulers and clocks at rest relative to ourselves). This in
spite of the fact that in my frame, according to my rulers and clocks, the
distance between your spaceship and the light ray is changing at a rate
different than 299792458 meters per second (and you will say the same thing
about me when you measure with your own rulers and clocks); I will explain
the fact that you nevertheless measure the ray to be traveling at exactly
299792458 meters per second in terms of the fact that your rulers and
clocks appear to be distorted relative to mine, with your meter-stick
appearing shrunk relative to mine, your clock ticking slower than mine, and
your "synchronized" clocks appearing out-of-sync in my frame (and again you
will say exactly the same thing about my rulers and clocks relative to
yours)

So, in this sense the speed of light is "constant", because it has the same
measured speed of 299792458 meters per second relative to all inertial
frames. But the speed can still only be measured relative to a particular
frame, and if you make use of a *non* inertial frame (an accelerating
coordinate system like "Rindler coordinates", for example), the speed of
light relative to that frame's coordinates may be quite different.

Jesse

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