Re: The velocity of light going pass a moving train.

On Jun 13, 4:52 pm, "harry" <harald.vanlintelButNotT...@xxxxxxx>
wrote:
<luke.s...@xxxxxxxxx> wrote in message

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On Jun 13, 12:33 pm, "harry" <harald.vanlintelButNotT...@xxxxxxx>
wrote:
<luke.s...@xxxxxxxxx> wrote in message

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harry wrote:
<luke.s...@xxxxxxxxx> wrote in message
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On Jun 8, 6:31 am, "Gerald L. O'Barr" <glob...@xxxxxxxxx> wrote:
The velocity of light going pass a moving train.

********************************************************
When Einstein developed his SR theory, he used an
example of a moving train to do this. The speed of
light going past this train had to be exactly known,
in order for him to develop his SR theory. Einstein
said that the velocity of light relative to the train
that was moving at v on the fixed tracks was exactly:
c +/- v.

Not true.

Hi Luke,

You are right that that citation was not 100% accurate, but it's
roughly
correct nevertheless.
What Einstein said ws the following, and I copy-paste:
"But the ray moves relatively to the initial point of k, when measured
in
the stationary system, with the velocity c-v".
That is true of course.

Hi Harry, thanks for your reply.
What is the context here? Is this a ray of light? Is he describing
Galilean relativity?

Yes it's a ray of light and he is describing SRT:

http://www.fourmilab.ch/etexts/einstein/specrel/www/paragraph 3.

And there is nothing wrong with that: in standard physics text books,
"relative velocity" is defined as the vector subtraction of the
velocities
of objects in the used frame of reference.

[...]

OK, thanks I think I understand now. More pitfalls in language when
describing relativistic motion.

Just to double check, could you verify this statement for me? :

The "relative velocity" between two objects as measured in one other
frame, is not the same as the velocity of one of those objects as
measured in the frame of the other.

I think yes, but it's still a bit ambiguous. Here's an improvement:

The "relative velocity" between two objects as measured in an inertial frame
in which both objects are moving, differs from the velocity of one of those
objects as measured in the inertial rest-frame of the other object.

That disinguishes SRT from Galilean relativity.

Bravo

However I do not think that is true. I have been trying to understand
SRT for several years now, and I
well I understand it but cannot make sense of it..

For example: a light beam shot upwards in a moving train moves
relative to the source only.

come to think of it: why the need for several reference frames? We
need only the x, y, z axis and discuss
everything within that frame. Once you introduce another frame, a
moving one at that, it may lead to all sorts
of inconsistencies when observations of physical phenomena are assmued
to be the same measured in relative to each frame.

Why not use the International Celestial Reference Frame. That will
solve O'Barrs' problem

International Celestial Reference Frame
From Wikipedia, the free encyclopedia
Jump to: navigation, search

The International Celestial Reference Frame (ICRF) is a quasi-inertial
reference frame centered at the barycenter of the Solar System,
defined by the measured positions of 212 extragalactic sources (mainly
quasars). Although relativity implies that there is no true inertial
frame, the extragalactic sources used to define the ICRF are so far
away that any angular motion is essentially zero. The ICRF is now the
standard reference frame used to define the positions of the planets
(including the Earth) and other astronomical objects. Note that, in
astrometry, a reference frame is the physical realization of a
reference system, i.e., the coordinates of datum points. The ICRF is
the realization of the International Celestial Reference System, and
agrees with the orientation of the Fifth Fundamental Catalog (FK5)
"J2000.0" frame to within the ...






Therein lay my confusion.

Ciao -

Regards,
Harald


.

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