Conference decwet::physics

    re .0
    
    Put it the other way...
    
    since the length contraction is an observational effect caused by
    relative motion between the observer and the observed, why would you
    expect contraction to be uniform in all directions?

>    since the length contraction is an observational effect caused by
>    relative motion between the observer and the observed, why would you
>    expect contraction to be uniform in all directions?
    
    	Then is the observational effect, just an illusion, such as we might 
    see looking at a ruler on nearly on edge vs straight on? ie
    
    
                 ] 
                /
              [/
                vs
     
              [======] 
    /Dean ?

    re .2
    
    I wouldn't use the word "illusion" but your analogy is reasonable. It
    is perhaps a question of philosophy as to how one describes the
    situation.

    Despite the fact that length, time and mass, as measured by a "moving"
    observer are all different from those as measured by a "stationary"
    observer (and indeed generally speaking different moving observers will
    also disagree in their measurements), the laws of physics (e.g.
    conservation laws) are observed to be obeyed by all observers. Thus it
    is a very carefully crafted illusion. (-: NB: here we are talking
    Special Relativity and all observers are in *uniform* relative motion
    i.e. no acceleration.

    re .3 >> Carefully crafted illusion. Not sure about this. The observed
    differences are real rather than illusory and are a result of the
    relative motion of the observer's and observed's inertial frames of
    reference.
    
                                   Dennis 

    re .4
    
    As I said, more a question of philosophy than physics.
    
    That the measured values vary depending on relative motion is well
    tested experimentally. How you interpret that is up to you.

.3
	I agree a carefully crafted illusion, with the respect to the
time dilation makes sense in moving objects, and that is another subject
which is bugging me, but ,,

.4    length contraction in one plane as actually seen would have to be real
wouldn't it??  I have a clue that I just read about magnetic fields, in that
they describe them as "photons" to carry the force.  I need help here as I
am no math wizard, but this concept really set off a light bulb!!

 If we our just atoms composed of electrons, protons, ie mainly the fields they
represent, then, if these fields are "photons" and if MOST IMPORTANTLY
photons are LIMITED AND FIXED to light speed just like real light photons! then
does some flavor below describe a real contraction?

	Say we consider one hydrogen atom electron with its electron 
spinning in a circle in the same plane contained with a forward motion.
Since its field is a "photon", and by defintion photons must move fixed at 
light speed.


Atom at with zero motion, photon at C speed in circle eg

      * 	
   *      *      A bad circle, I'll admit, but a steady round' C velocity..
 *          *
*            *    
 *          *    
   *      *
      * 	

Now, apply a velocity of say 1/2 light speed, like were on a rocket, and 
view from looking down from a fixed observer. We must agree on light speed,
and the photon must be a light speed for angular motion.

	      ((*     [ C speed in circle ]
	   ((*    ((*      
1/2 C	 ((*         ((*     1/2 C
---->  ((*            ((*    ---->
	 ((*         ((*    
	   ((*     ((*
	      ((* 	

	I don't know what the math would be to visulize what the heck this
atom would look like, but I'm thinking it would REALLY be contracted from
a remote observer! It could not hold its shape. - Am I all wet????

/Dean
    

    re .6                                                   
    
    There are problems with what you are discussing.
    
    1. The model of an atom as a nucleus with electrons in orbit around it
       simply doesn't work. It's good for a lot of Chemistry and OK for
       starting out in Physics but it has its limitations.
    
    2. An object in orbit is not in uniform motion and therefore there
       could be additional considerations pertaining to General Relativity
       that are way beyond me but which can't be ignored.
    
    Putting that aside, I can imagine that a system consisting of a central
    body and another object in a circular orbit around it could appear to
    an observer moving relative to the central body as if the orbit were
    elliptical - without however attempting any detailed analysis.
    
    However I have an objection to the basic idea that the length
    contraction is "real". Imagine n observers each in motion relative to 
    some much observed target object but with different relative motions (i.e.
    both different directions and different speeds). Each observer observes
    the size and shape of the target object to be different.
    
    I have the opinion therefore that it is better to regard the measured
    dimensions to be a property of the reference frame of the observer and
    to single out measurements made by an observer who is not moving
    relative to the target object as "real" i.e. the rest length, mass,
    etc.
    
    All the observers who are moving relative to the target object can
    determine this fact by experiment and know therefore that their
    measurements may need to be transformed to coincide with another
    observer's measurements. On the other hand the observer can choose to
    ignore the relative motion since the laws of physics are not affected by
    uniform motion.

    re .7 The way that I tend to look at it is that all the measurements
    which are taken from various frames of reference are real,***in the 
    frame of reference in which they are made***. One of the main
    components of SR is that there is no such thing as absolute time and
    space (hence length) but that everthing is relative to the observers
    frame of reference. 
    
                             Dennis 

.7 Replacing "objects" in .2 with "fields" then

o If magnetic fields use, or are in themselves photons, and
 o If electromagnetic forces hold electrons, (fields ARE the electrons?) and,
  o If photons or electromagnetic fields MUST and DO move at C speed -
   o Then a satisfactory description of real contraction could, in
     some form of the visualization of .6, might be made (guessing!)
    
.8
	I think your right, what shape is real in this sense, would look
different to observers at different speeds.  We here on the earth could be in a
contraction, but to our way of thinking we wouldn't know it.  The key seems to
be Light speed C, - for motion, AND the fields that make up matter - because
both Time and Length distort proportionaly.

  I've picked up Einsteins other book, the Meaning of Relativity, which
I think will lose me very rapidly! but covers fields and addition of velocites.
This stuff is hard work to ponder about!  Sometimes I don't know why I (or We?)
do this, but I sure am looking at the world and night skies m-u-c-h
differently every day.

/Dean

    

re .9
    
>o If magnetic fields use, or are in themselves photons, and
> o If electromagnetic forces hold electrons, (fields ARE the electrons?) and,
>  o If photons or electromagnetic fields MUST and DO move at C speed -
>   o Then a satisfactory description of real contraction could, in
>     some form of the visualization of .6, might be made (guessing!)
    
    It doesn't work for me.
    
    The fact that light always travels at the same speed regardless of
    motion between the relevant objects, and I include here the virtual
    photons that mediate the electromagnetic force, means that em
    interaction is completely unaffected by motion. I could speculate
    wildly that if light obeyed the normal speed addition rule then
    things would really happen to the structure of moving objects.
    But this is rampant speculation about a universe in which we do not
    apparently live.

>    The fact that light always travels at the same speed regardless of
>    motion between the relevant objects, and I include here the virtual
>    photons that mediate the electromagnetic force, means that em
>>>    interaction is completely unaffected by motion. I could speculate

	We're back where you were with the apparent contraction to the
remote observer!  My problem was why time dilation seemed to be real, better, 
linear (no differentiation as to the position of the clock with rotating
hands) for an object in motion, but length contraction was in one plane. This, 
to me said an object must really be "Crushed" to fit.  Describe an object 
as bunch of rotating photons, at it seems to me you are right - there is
no structural changes, just a remote observers perception.

	I'd still like math help, lets drop the complex rotating photon model,
make an simpler four mirrors clock out of a two mirror clock and visualize
what it would look like - (I can't do this). In "realm of the universe" they
described a light clock to demonstrate special relativity.

___ <--mirror2     time = duration of photon1 mirror1 to mirror2
 |  
 ^ <---photon1
 |
--- <--mirror1
 
	Apply motion of say v = 1/2 C

 v-->
        >>>___ <--mirror2     time(1) = duration photon1 mirror1 to mirror2
      >>> |                   
    >>> ^ <---photon1
  >>> |
>>>--- <--mirror1   
                           time = 1 (normalized)
    			time(1) = 1/ sqr(1 - v^2/c^2 )

	The diagonal progression represents the time dilation if photon speed
is fixed.  But What do I do with a 4 mirror clock!??


/-->--\ <---mirror 1 & 2
|     |
^     v <---photon
|     "
\--<--/ <---mirror 3 & 4  (what do I do here???)


	I draw a blank if I try to apply motion, and fix the speed of a photon
to a remote, like the picture as we see it (end on observer) - do I negate v? 
or time(1) on the lower piece, or both???)
    
    /Dean

    rep .9
    	Well it doesn't work for me either now!, I worked out what I think
    things would look like from viewing a rotating virtual photons perspective,
    using a 4-mirror photon clock.  An object might seem to look like viewing a
    drive-in movie screen, from near one edge across the other, ON edge!
    but that also would depend on the path of rotation one assumes the 
    virtual photons took!! guess below, dimensions in time T'
    
     _	   |\
    |_| vs | | in motion
     	   |/
    
    I give up for now! 
    /Dean

Is the very statement of the base question here (and the title of the topic)
"contraction in one plane" misleading?
It's not contraction in a PLANE, it's a linear contraction in one DIMENSION,
the dimension along the apparent direction of movement between the 
observer and the target.


So if the four mirror photon clock llooks like this at relative rest:

      /------\
      |      |
      |      |
      |      |
      \------/

it will look like this in relative motion:

      /--\
      |  |
      |  |    -----> direction of relative motion
      |  |
      \--/


If the direction of relative motion is not parallel to one of the edges
of the clock, then the rectangle will be skewed into a parallelogram not
easily drawn in character cell graphics.
Also, these pictures don't attempt to track the paths of the photons, which
are really the reason the length contraction occurs.  I just took a snapshot
of the whole clock at one instant of observation.

- tom]

rep .13

>So if the four mirror photon clock looks like this at relative rest:
>
>      /------\
>      |      |
>      |      |
>      |      |
>      \------/
>
>it will look like this in relative motion:
>
>      /--\
>      |  |
>      |  |    -----> direction of relative motion
>      |  |
>      \--/

   Thats what I thought I'd see.  If one thinks in terms of wavelengths (or
nodes), it looks different.  Let all sides between mirrors be 10 wavelengths
long of said photon.  This has to be a constant to any observer.  Assign the
photon path clockwise.  Assign C once again for photon speed to all observers.

   If one assumes the photon frequency (.aka. time) a constant, there is no 
contraction - if just wouldn't fit into the diagram - so end of that story.  

   If we assume the frequency (time) changes, we have to use the Doppler
equations.  It becomes clear that the top and bottom, because the photons are
traveling in opposite directions, cannot *both* be increased in frequency to
fit the number of wavelengths (node) count into the single dimension
contraction we expect, and as pictured above.  Further imagine the mirrors
switched out from the moving apparatus and we would see photons at a distance
of higher frequency to the right, (direction of motion), and lower frequency
photons to the left, corresponding to the Doppler shifts we'd calculate.

   I still don't get the one dimension contraction!

/Dean

    

>        <<< Note 420.14 by BULEAN::MCGORRILL "Its your turn anyway.." >>>
>                                -< wavelengths >-
>.....
>   If we assume the frequency (time) changes, we have to use the Doppler
>equations.  It becomes clear that the top and bottom, because the photons are
>traveling in opposite directions, cannot *both* be increased in frequency to
>fit the number of wavelengths (node) count into the single dimension
>contraction we expect, and as pictured above.  

Why not?  This is central to the SR premise that the speed of light 
is constant in all intertial frames of reference.
If you're in a space ship chasing another with a closing speed of 0.99C,
and you're firing communication lasers back and forth, a third observer
will see both of those lasered photons travelling at the same speed, over the 
same distance, therefore in the same elapsed time.


>Further imagine the mirrors
>switched out from the moving apparatus and we would see photons at a distance
>of higher frequency to the right, (direction of motion), and lower frequency
>photons to the left, corresponding to the Doppler shifts we'd calculate.

Well, no, not really.  YOu can't actually "see" the photons that 
have already passed you, because their presence can only be known
by other photons that would have to traverse the path back from "downstream"
to you.  Similarly, you can't see the photons coming at you untill they
get their.

This is a bit of trick in speaking of "observing" relativistic effects
when the obeserver is "off axis."

- tom]

       Yes, the constant speed of light is given above, the difference here    
is I'm using light as a clock, and a measuring rod.  Once in flight, photons
are, in a sense, not part of the apparatus.  Lets modify the test    
apparatus so we can see the photons. 

0....1....2....3....4....5....6....7....8  A "ruler" for Observer X

                    4 mirror clock,    
 /                  [] -->--            \ 
/ Fixed mirror			         \Fixed mirror
  A                                       B



                    X = Observer Fixed   {Fixed above = coordinates rel to X }

  Ok, we startup our clock, photons bouncing around clockwise. we switch
off the mirrors and emit photons out each end. if we have our setup aligned
correctly, Observer X see will see the two photons, one higher and one lower in
frequency.  Since we agree on photon speed at C, the effect is described by
Doppler. 


zoom to our test jig.
>
>      /--\
>      |  |
>      |  |    -----> direction of relative motion
>      |  |
>      \--/
>
  The key here, is to

1. photons travel must = at C speed.
2. each side must      = 10 wavelengths per side!!!!  

	A Michelson-Morley interferometer running along side the test apparatus
would show light speed C, and distance in wavelengths to be 10.  However
observer X also has to agree as well!!  So X sees the contraction from the
result of frequency shift.  Higher frequency, the shorter distance per 10
wavelengths, lower the longer distance per 10 wavelengths.  If we unite the
first picture of Doppler shifted photons, to the above.  Equal contraction
as pictured above to the top and botton cannot resolve.  Am I in error? 

/Dean
    
    

    Dean,                          
    
    The "contraction" effect and the Doppler effect are not the same thing.
    
    In the classical (non-relativistic) model, you still get a Doppler
    effect - with sound waves, for instance. If an observer is moving
    towards a sound source, he encounters more wave maxima in unit time
    than he would if he stood still and waited for them to reach him. He
    undestands perfectly well that neither the wavelength nor the speed of
    sound are changed by his walking towards the source, even though he
    hears a sound of higher frequency.
    
    The relativistic case is similar. There is now a time dialtion/length
    contraction effect involved, but regardless of that effect, someone
    actually observing the photons will see a frequency shift dependent on
    their velocity relative to the _source_ of the photons. (This is the
    "red-shift" seen in stellar spectra, for instance.)
    
    The observer's velocity relative to the photons themselves is
    irrelevant, since it is, of course, C.
    
    So the equations you need to work out the contraction are just the
    normal Lorentz equations for transforming coordinates between intertial
    frames in uniform relative motion. The contraction depends only on the
    relative speed, not direction, so the contraction is the same for both
    the approaching and receding photon.
    
    As for the original question of trying to rationalise the contraction
    occuring in one direction and not in the other, you can look at it as
    arising from the two observers' disagreement over simultaneity. You
    note the position of one end of the object at time t, and of the other
    end at the same time. You subtract the two position measurements to get
    the length of the object.
    
    The other observer agrees that you measured the two positions
    accurately, but sees the two measurement events (separated purely
    spatially in your frame) as happening at two different times. So
    according to him, you noted the position of the front end, then the
    object moved, then you noted the position of the back end. Little
    wonder you got the "wrong" answer.
    
    On the other hand, there's no disagreement about simultaneity along a
    line perpendicular to the direction of motion, so no contraction
    effect.
    
    Yes, this is all handwaving. I'll draw a diagram and some equations if
    this doesn't make sense to you.
    
    Andy.

Hi Andy,
	I've worked some of this out, its visualizing it that doesn't make
sense! Its supposed to look like this right, Lorenz

x1 = (x - v*t) / sqr(1-(v^2/c^2))
y1 = y
z1 = z
t1 = (t- ((v/c�)*x)) / sqr(1-(v�/c�))

	So how did I get myself so confused!  I took the two mirror clock
out the realm of the universe, and their formula and solved for t'. They
normalize t to 1  so



   vt'

 A     B'        c�t� = c�t'� - v�t'� 
 ---  ---
ct|  / ct'       t' = 1 / (SQR(1 - (v�/c�))
  | /
 ---  ---
 B   B'

	But thats the special case of t=1, in the general case

                 t' = SQR((c�t�)/(c�-v�))

	This would be seen by our photons on the top going right, and bottom
going left or somewhere in between.  The apparent time dilation (and distance)
changes to the remote depending on photon direction - in this kind of clock -
and the formula solved the frequency changes of the photons escaping the 
apparatus. 

	Below we imagine the test appartus, speed adjusted to give a
+- 100nm shift to the photons we leak out.

0....1....2....3....4....5....6....7....8  A "ruler" for Observer X

   Real 500Nm photon source fixed
L1 .......................................>
                  /..\....................> 
                  |  |
                  |  |    -----> direction 
                  |  | Rotating photon = 600nm
  <. . . . . . . .\. /
  <. . . . . . . . . . . . . . . . . . . L2 
                                         Real 700Nm photon source fixed
    
    
	I know, I know, it must be me ;-), but this picture doesn't have 
the same nodes number of standing waves if you will, between the top and
bottom sets of mirrors, but the guy on board does count 10 on each side.
    
     The kicker is, I can fire a photon counter clockwise and reverse the
    the apparent lengths, so I know my visualization is wrong.
    
/Dean
    

Maybe the explanation here. Let's even simplify to two mirrors, facing each 
other. So mirror "A" emits a periodic wave towards "B", which reflects the wave 
to "A". Suppose also, for simplification, that at rest this system has been 
tuned so that "A" and "B" are separated by exactly one wavelength. I aim to 
prove now that, for any observer, we always get *one* wavelength exactly.  
                 *
               *   *   

             *       *             
            A        [*]        B 
                       *       *
                       
                         *   *
                           *
                   
So if you place a mirror at both "A" and "B" (reflecting the wave to each 
other) it appears a stationary interference wave, with one node in the middle:
                 *         +
          ||   *   *     +   +   ||
          ||                     ||
          || *       * +       + ||
          ||A        [*]        B||    ----------------> direction x
          || +       + *       * ||
          ||                     ||
          ||   +   +     *   *   ||
                 +         *


            <------------------>
                    L0

Fine, this has been tuned in a rest laboratory. L0 is the wavelength, it's also
the distance between mirrors "A" and "B".

Now just put this device in a rocket (with guy "Remy" flying in that rocket), 
travelling at constant speed v in direction x. So, if "Remy" follows the 
device in the rocket, he keeps on seeing one single node of interference. So
Remy deduces that points "A" and "B" are exactly "in phase" (in our case, "A"
and "B" are separated by just one wavelength). This is the rocket laboratory.

Now just consider the other guy "Bob" at rest on earth. When that device
passes near Bob, he must agree with Remy on the nodes count (so: just one node,
or one wavelength exactly). So, for Bob, the points "A" and "B" must also be
in phase (separated by one wavelength exactly). Let's prove this fact that
for Bob, points "A" and "B" have one wavelength separation:

	. t'1 = time of emission at point A (photon #1) as measured by Remy
	. t'2 = time of reception at point B (same photon) as measured by Remy
        . x'1 = abscissa of point A at time t'1, as measured by Remy
	. x'2 = abscissa of point B at time t'2, as measured by Remy

              => for sure, we have x'2 - x'1 = L0 [ the wavelength at rest ]

And now for Bob, we have to apply Lorentz tranforms. With notations t1, t2,
x1 and x2 (same events as above, measured by Bob).

	x = (x' + v t')   / a
	t = (t' + vx'/c�) / a     [ with a == sqrt ( 1 - v� / c� ) ]

We get:

	t2 - t1 = [( t'2 - t'1 ) + (x'2 - x'1) v / c� ] / a

Or, introducing the wavelength L0 [ with L0 == (x'2 - x'1) == c ( t'2 - t'1) ]

	t2 - t1 = a  L0 / (c - v)  !!!!! I skipped easy algebra there

Great: (a L0) is the wavelength as measured by Bob (see Fitzgerald contraction).
And (c-v) is the relative speed of photon #1 with respect to mirror B. So
according to Bob, points A and B are also in phase, due to the fact
that (t2-t1) == wavelength / wavespeed.


Reversed problem : so now, how can a photon emitted by "B" arrive in phase at
point "A" ? With same notations (indice '1' for events at point A, and '2' for 
point 'B'). Well in this case, t'1 is "later" than t'2, so we start from:

	t1 - t2 = [( t'1 - t'2 ) + (x'1 - x'2) v / c� ] / a	

Watching that x'1 - x'2 = - L0, we get:

        t1 - t2 = a L0 / (c + v)

Same as above: (c+v) is the relative speed of my photon emitted by B in 
direction of A. So (t1 - t2) corresponds to one wave period.

In summary, the count of observed nodes is the same for any observer. Friendly.

##### Remy ######

Joke now : "everything is absolutely relative".

    rep .19
      thats great work!!!  I gave myself a dope slap last night, when
    watching tv, it occured to me.  Watching a car go left to right down a
    rode, looks different than watching the *car* and seeing the road go 
    right! So if I watch the test appartus, keep my eyes on it and follow it 
    along, I see the 10 nodes top and bottom!!
    
      I logically got to the 4 mirror clock, in an attempt to see how a
    "solid" object could distort.  If matter is more or less virtual
    photons or tied together by them, this could resolve how I can
    visualize a contraction. eg six little molecules of a salt crystal
    with virtual photons going any direction - like the 4 mirror clock
    or 2 setup you gave - 
    
         At Rest           In Motion
    	( )( )             ()()
        ( )( )             ()() ----->
        ( )( )             ()()
    	
    this make sense? I doubt my eyes these days! 
    
    /Dean 

Hi Dean, you are close to Lorentz's approach, when he wanted to explain the 
negative result of "Michelson-Morley" experiment. If I remember correctly, he 
tried to explain the "Fitzgerald contraction" by such mecanical compression of 
electromagnetic nodes [I mean: mecanical compression of equilibrium positions
in matter components]. He arrived to that conclusion that, in order to 
tell "Michelson-Morley" unexpected result, he could introduce such mecanical
contraction of the atoms of interferometer by a factor sqrt (1 - v�/c�), in the
direction of movement only. Fine, this could have a "mecanical" explanation at
this stage, but... but he had also to expand the time by this said factor (or
in other manner: arm compression was a mecanical fact, if the clocks of atoms
were slowed down by this said factor). But for such "time expansion" he could 
not give a satisfactory explanation, unless using the expression "virtual time" 
[<- or some funny thing like this, I read it long ago].

Problems with that: even though that Fitzgerald contraction could find a 
mecanical explanation, the time expansion was still a mystery. Moreover, he
was working from this assumption of an "absolute space", with Maxwell's waves
running through an ether. And all attempts to highlight such "mecanical
compression" of lengths ended up to failure. Moreover: Lorentz transforms happen
to constitute a "mathematical group": a Lorentz transform of a Lorentz 
transform *is* a Lorentz transform (provided the frames are in parallel motion).
So it gets impossible, from any experiment, to determine *the* absolute frame,
at rest in such absolute ether.

Then came Einstein: if *ether* is just a useless concept to live with, just 
get rid of it. French mathematician Poincare, and Einstein himself (and 
probably many other people) could explain these Lorentz transforms without 
relying on ether support. 

##### Remy #####