Re: SR's velocity addition -- ANY Experimental Evidence?

From: Jim Greenfield (greenfield_7_at_hotmail.com)
Date: 07/16/04 

Date: 15 Jul 2004 18:09:01 -0700

Timo Nieminen <timo@physics.uq.edu.au> wrote in message news:<Pine.LNX.4.50.0407151628570.13757-100000@localhost>...
> On Thu, 14 Jul 2004, Jim Greenfield wrote:
>
> > Timo Nieminen <timo@physics.uq.edu.au> wrote:
> > > On Wed, 14 Jul 2004, Androcles wrote:
> > >
> > > > "Timo Nieminen" <timo@physics.uq.edu.au> wrote:
> > > > | On Tue, 13 Jul 2004, Sam Wormley wrote:
> > > > |
> > > > | > Jim Greenfield wrote:
> > > > | > >
> > > > | > > Like the man said: 1c+1c=2c if this formula doesn't hold in
> > > > | > > standard math (classical mechanics), then please put me right. :-)
> > > > | >
> > > > | > [Physics FAQ]
> > > > | > How Do You Add Velocities in Special Relativity?
> > > > | >
> > > > http://hermes.physics.adelaide.edu.au/~dkoks/Faq/Relativity/SR/velocity.html
> > > >
> > > > yeah, it says:
> > > > Suppose an object A is moving with a velocity v relative to an object B and
> > > > B is moving with a velocity u (in the same direction) relative to an object
> > > > C. What is the velocity of A relative to C?
> >
> > In case T N (deliberately) missed it:
> > see here that A, B & C are all in the same frame?
>
> In case J G (deliberately) missed it:
> See here that A is measured to have velocity v in a coordinate system in
> which B is at rest, and B is measured to have velocity u in a coordinate
> system in which C is at rest.

You can do this if you choose, but you are mistaken! B is clearly in
motion;
A has > motion than B, which is u+v. Ignoring B's motion doesn't make
it "go away".

Important Q here: can observer see C,B& A simultaneously? Can C see
both A& B?
If not, why not?
>
> Whatever do you mean by "A, B & C are all in the same frame"? A, B, and C
> are physical objects, and exist independently of any coordinate system.

Exactly! Because there is only ONE coordinate system in nature! (and
you'd better beleive they are in it)
>
> > > > w = (u + v)/(1 + uv/c2)
> > > >
> > > > Let object A be the "tip of the ray" moving relative to B, the initial point
> > > > of k.
> > > > Let object B be moving relative to C, the "stationary" frame. What is the
> > > > velocity of A relative to C?
> > > > Einstein can do it, he says c = (c+v)/(1+v/c), right?
> > > >
> > > > Quote:
> > > > "It follows, further, that the velocity of light c cannot be altered by
> > > > composition with a velocity less than that of light. For this case we obtain
> > > > V = (c+w)/(1+w/c) = c."
> > >
> > > Straightforward use of Lorentz transformations to determine what the
> > > measured value of the velocity would be in a second coordinate system,
> > > given the measured value in the first coordinate system.
> >
> > .......and now they are not! How is that, for sleight of hand? (and
> > brain)
>
> ??? Translation, please!

(sigh) A,B,C were ALL measured relative to each other = ONE frame
Now has been introduced ANOTHER frame........
>
> > > > So how in the blue blazes does he come up with
> > > > "But the ray [call it A] moves relatively to the initial point of k [call it
> > > > B], when measured in the stationary system [call it C], with the velocity
> > > > c-v ..."
> > > > Reference :
> > > > http://www.fourmilab.ch/etexts/einstein/specrel/www/
> > > >
> > > > Seems to me that someone can't think straight, and it sure ain't me.
> > >
> > > This actually well illustrates the difference between (1) the addition of
> > > velocities, and (2) the transformation of velocities when changing
> > > coordinate systems.
> > >
> > > In coordinate system C, the ray of light moves at a velocity c, the
> > > original moving point from which the ray of light was emitted is moving at
> > > v in the same direction. So, as measured by coordinate system C, the
> > > velocity of light relative to the moving point is c - v. What's so
> > > mysterious about that? It's simple Galilean physics. Lorentz
> > > transformations only required when changing coordinate systems.
> >
> > (from faq "adding velocities in Relativity")
> > ***Notice that the only case with velocities less than or equal to c
> > which is singular is w = u = c which gives the indeterminate zero
> > divided by zero. In other words it is meaningless to ask the relative
> > velocity of two photons going in the same direction.
>
> A simple consequence of there being no inertial coordinate system in which
> either of the two photons is at rest.

Inertial means "not accellerating" ? So if I match the speed of my
"selected" coordinates to that of a photon, that photon is at rest.
>
> The FAQ is using "relative velocity" restricted to the special case of
> "relative velocity of A relative to B as measured in an inertial
> coordinate system where B is at rest." Without this restriction, it is no
> longer meaningless to ask the relative velocity of two photons going in
> the same direction (and the relative velocity is zero).
>
> > No worries! I just happen to have here a mirror which lets one photon
> > through, and reverses the other. Now what is the problem?
>
> There still isn't any inertial coordinate system where either photon is at
> rest. So, with the restricted defintion as above, there is still exactly
> the same problem. Without the restriction, there is no problem, and this
> situation was already addressed in the following paragraph:
>
> > > In exactly the same way, the relative speed between two pulses of light
> > > emitted in opposite directions in coordinate system C is 2c. Simple
> > > vector addition. Do you think that calculations of relative speed in
> > > coordinate system C, using velocities measured in coordinate system C,
> > > should use something other than straightforward vector addition?

Do you not see here, that (-c) pulse going west, + (+c) pulse going
east
equals 2c.
The photons are separating at 2c, agreed?
The distance between them after a certain time is 2c (units of length,
agreed?
That distance is NOT < 2c, agreed?
So how in hell is a STICK shorter, when these two pulses were used to
measure it?

 
> > Choosy, choosy! You wish to add light pulses using "ordinary" vector
> > addition, but refuse to allow it for the sources of the photons.
> > Please play fair :-(
>
> Exactly the same rules apply to both. Use Lorentz transformations when
> changing coordinate systems, and use vector addition when adding
> velocities within a coordinate system.

.....and DHR's reserve the right to choose when coordinate systems are
changed, and whether 3 objects may be all compared in one frame, or
not.
Tell it to the students who will have to suffer through this crap or
be failed; don't try to fool me with such pathetic logic
>
> Compare with classical mechanics: use Galilean transformations when
> changing coordinate systems, and use vector addition when adding
> velocities within a coordinate system.

as above!

Jim G

Relevant Pages

  • Re: SRs velocity addition -- ANY Experimental Evidence?
    ... I add in the source velocity of the light which conveys the ... The observer is mistaken. ... > measurements are made in a coordinate system. ... > consequences of the way that measurements are made in coordinate systems. ...
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  • Re: SRs velocity addition -- ANY Experimental Evidence?
    ... I add in the source velocity of the light which conveys the ... The observer is mistaken. ... > measurements are made in a coordinate system. ... > consequences of the way that measurements are made in coordinate systems. ...
    (sci.physics.relativity)
  • Re: SRs velocity addition -- ANY Experimental Evidence?
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  • Re: SRs velocity addition -- ANY Experimental Evidence?
    ... > which B is at rest, and B is measured to have velocity u in a coordinate ... and exist independently of any coordinate system. ... >> velocity of two photons going in the same direction. ... > "relative velocity of A relative to B as measured in an inertial ...
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  • Re: SRs velocity addition -- ANY Experimental Evidence?
    ... I add in the source velocity of the light which conveys the ... Measurements made by a coordinate system don't care about the propagation ... The location of the observer is ...
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