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From: valls on 25 Jun 2010 08:00
Let be any body set with a material point modelling each one. If we
want to describe the movements of the bodies in an inertial frame, we
have a unique alternative: to use the centre of mass inertial frame
corresponding to that body set. Once the Newton’s absolute frame is
rejected by 1905 Einstein (and then rejected also all the others
moving with any uniform velocity with respect to it), remain only the
bodies themselves to determine inertial frames.

RVHG (Rafael Valls Hidalgo-Gato)
From: Androcles on 25 Jun 2010 13:46

<valls(a)icmf.inf.cu> wrote in message
news:0c8d6a3b-d037-4c19-b77b-0661b4f82531(a)d37g2000yqm.googlegroups.com...
Let be any body set with a material point modelling each one.
==========================================
WHAT?


From: BURT on 25 Jun 2010 14:41
On Jun 25, 10:46 am, "Androcles" <Headmas...(a)Hogwarts.physics_z>
wrote:
> <va...(a)icmf.inf.cu> wrote in message
>
> news:0c8d6a3b-d037-4c19-b77b-0661b4f82531(a)d37g2000yqm.googlegroups.com...
> Let be any body set with a material point modelling each one.
> ==========================================
> WHAT?

Center of mass is the center of the geometry of energy. There are
space-energy centers for every gravitational field.

Mitch Raemsch
From: Tom Roberts on 26 Jun 2010 11:43
valls(a)icmf.inf.cu wrote:
> Let be any body set with a material point modelling each one. If we
> want to describe the movements of the bodies in an inertial frame, we
> have a unique alternative: to use the centre of mass inertial frame
> corresponding to that body set. Once the Newton�s absolute frame is
> rejected by 1905 Einstein (and then rejected also all the others
> moving with any uniform velocity with respect to it), remain only the
> bodies themselves to determine inertial frames.

This is just plain not true. An observer can also determine an inertial frame.
So it is easy to discuss what (say) an observer moving with speed 0.5 c
(relative to the frame you mention) would measure in her inertial frame.

Logically, SR includes all possible inertial frames, so it is
OK to use any of them in one's analysis.

Indeed, for multi-particle systems it is more common for observers to determine
the frame. Yes, one might well choose the c-o-m frame of the particles, but that
is the OBSERVER'S choice, and is not dictated by either the particles themselves
or by the theory.

Indeed, a "frame" really means a collection of rulers and clocks associated with
an observer, not with any particles or objects.



Also: you seem to think that "1905 Relativity" is somehow different from Special
Relativity today. This is also not true. We have learned a lot about the theory
since then, but there has been no change in the theory itself (i.e. the set of
theorems and the meanings of the symbols in them).


Tom Roberts
From: valls on 28 Jun 2010 07:56
On 26 jun, 10:43, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote:
> va...(a)icmf.inf.cu wrote:
> > Let be any body set with a material point modelling each one. If we
> > want to describe the movements of the bodies in an inertial frame, we
> > have a unique alternative: to use the centre of mass inertial frame
> > corresponding to that body set. Once the Newton s absolute frame is
> > rejected by 1905 Einstein (and then rejected also all the others
> > moving with any uniform velocity with respect to it), remain only the
> > bodies themselves to determine inertial frames.
>
> This is just plain not true. An observer can also determine an inertial frame.
> So it is easy to discuss what (say) an observer moving with speed 0.5 c
> (relative to the frame you mention) would measure in her inertial frame.
>
You write “an observer can also determine an inertial frame”. Then,
without any doubt from my part, what you mean by “observer” is a very
important concept for the topic we are addressing here. What is an
“observer” for you? A human being? A God? A soul? What is very
important for me is if that “observer” has mass associated with (him,
her or it) or not, if it has massive clocks and rules (bodies)
associated with it or not. If you conceive an inertial frame without
massive bodies, totally empty, you are simply re-introducing the
frames that were put out by 1905 Einstein. If you conceive a clock
without mass measuring time, or a rod without mass measuring space,
you are simple re-introducing Newtonian absolute space and time (or
one of the also imaginary ones moving with a uniform velocity with
respect to the imaginary absolute one).
>         Logically, SR includes all possible inertial frames, so it is
>         OK to use any of them in one's analysis.
>
It depends of what are “possible” inertial frames for you. The
Newtonian absolute one (and all the other infinite derived empty ones
moving with all possible uniform velocities with respect to it) are no
more “possible” inertial frames after 1905 Einstein decision rejecting
them at the very start.
> Indeed, for multi-particle systems it is more common for observers to determine
> the frame. Yes, one might well choose the c-o-m frame of the particles, but that
> is the OBSERVER'S choice, and is not dictated by either the particles themselves
> or by the theory.
>
Totally non-sense in the 1905 Einstein’s view. The centre of mass of
some specific massive body set can’t be moving with respect to
nothing. It is OUR choice: to use the Newtonian view or the 1905
Einstein’s one. In this last one the centre of mass MUST be at rest.
You can’t put a centre of mass moving with respect to the absolute
frame (or any other moving with a uniform velocity with respect to
it). And if you have other massive bodies, you must change to a
different centre of mass frame that takes into account them. In any
case, you have as the unique possible choice to use the centre of mass
of all the massive bodies involved.
> Indeed, a "frame" really means a collection of rulers and clocks associated with
> an observer, not with any particles or objects.
>
Are not the clocks and rules massive bodies for you? They are not
“objects”? Only the absolute space and time can be measured with your
non-massive rules and clocks. And of course, in the real world you
will NEVER made a real measurement with those imaginary entities. All
your measurements will be also only imaginary ones without any
experimental value at all.
> Also: you seem to think that "1905 Relativity" is somehow different from Special
> Relativity today. This is also not true. We have learned a lot about the theory
> since then, but there has been no change in the theory itself (i.e. the set of
> theorems and the meanings of the symbols in them).
>
The “Special Relativity” denotation was introduced by first time by
1916 Einstein. He declared gravitation out of the scope of SR. In
1905 Relativity we have as a real example (at the end of paragraph 4
of the 30Jun1905 text) the rotating Earth as the stationary system and
a clock at the equator as the moving system. This one is moving
GRAVITATIONAL centripetal accelerated in a circular path. Why can 1905
Einstein consider the clock at the pole at rest? Because all the
rotating axis is at rest. The stationary system is the centre of mass
inertial system corresponding to all the massive bodies involved, the
UNIQUE possible alternative. That system is the today denoted GPS ECI.
To use an imaginary inertial system moving at a uniform velocity with
respect to the ECI is a total absurd. You NEVER can make a real
measurement in that totally imaginary system that has no value at all.
1905 Relativity is NOT Special Relativity. And 1905 Relativity has all
the huge experimental evidence of today GPS.
Try to describe the Sun’s trajectory in the ECI to see how many
contradictions and absurdities arise.

> Tom Roberts

RVHG (Rafael Valls Hidalgo-Gato)
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