tetrahedron wrote:
Thanks Bjoern.
tetrahedron wrote:
Something both EM and gravitation agree
on is the speed of light c as the speed limit in the universe.
I'm so sorry about this. I meant to say that both EM and GR agree on
the speed of light c.
GR says nothing about the speed of light.
I was misled by this:
Uncle Al wrote:
Special Relativity was specifically configured to be consistent with
Maxwell's equations.
incorporating erroneously SR into EM.
Huh? Who did make that error, in your opinion? You or Uncle Al?
My questions, to be more presice
are the following. So far, c is just defined as the speed of EM
radiation in vacuum. Why is c the maximum speed in the universe, as
predicted by GR?
It is a postulate by SR that the speed of light is an
invariant. It then follows mathematically that this speed
has to be an upper limit on all speeds in the universe.
And since SR is a subset of GR, it includes automatically
that statement.
Is c measurable in any other way than considering EM
waves?
For startets: Measure epsilon0 and mu0 and calculate c from it.
;-)
How does GR refer to this speed?
It is essentially incorporated into the metrics.
Let's call v the speed limit
in the universes predicted by GR. How can you, inside GR state that
v = c
since c is not defined in GR?
Where did I say that "c is not defined in GR"? I'm not even
exactly sure what you mean by this.
Or is it? You offered a hint to this question:
OTOH, if you want to show that the speed of gravity waves is
c, then try looking into a textbook on GR. The calculation
is straightforward, but much too long for a newsgroup post.
At this point we would have two kinds of waves that move at c.
Indeed.
This is far from saying that c is the speed limit.
Indeed. Again, that follows from SR.
At any rate, I'm afraid
this would result by the initial assumption of SR that c is the speed
limit.
No, this is not initially assumed in SR. The only thing assumed
in SR is that c is an invariant.
Is the latter just a postulate of relativity or can it be
demonstrated? I guess not. Einstein just talked about the speed of
light. But now that EM and GR have further evolved and thanks to QFT
found bigger incompatibilities,
I'm not exactly sure what you mean with incompatibilities
between EM and GR.
I was not sure why c can be so easily
talked about in GR, since light and electricity are apparently not GR
phenomena.
Indeed, they aren't. But SR has shown that c is not only
the speed of light, but the ultimate speed limit of the
universe, part of the spacetime metric. And hence it automatically
appears also in GR.
I wanted to apologize about my sloppy terminology and presentation in
general.
No problem. As long as you try to clarify when asked for
clarification...
Actually I'm just learning EM at the moment and this is what
pushed me to wonder.
How are you actually learning EM? In a course? On what level?
What are you reading?
Bye,
Bjoern
.
