Just what is it that contracts?

There are three basic possibilities:

(I)   The whole universe contracts, parallel to the
      line of the moving object's direction.

(II) The whole universe contained in a cylinder centered on
     the moving object and extending forwards and rearwards
     contracts.

(III) The moving object only contracts.

The really 'cute' one is (II).
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(III) The moving object only.

Let there be two markers in space at a constant distance
of 10^10 kilometers from each other, as measured by an
observer at rest wrt the markers.

Let a spaceship exist that is measured by the observer as
one kilometer long while it is a rest wrt the observer.

The distance between the markers is thus 10^10 spaceship
lengths.

Let the spaceship depart the observer and eventually pass
one marker at .7071c. The observer sees the spaceship now
as being .5 kilometers in length at t=0, and the moving
clock to be ticking only half as fast as his own.

The spaceship does not see his length as having changed,
and if the distance between the objects didn't also
change, then its perceived distance to the second marker
is now 2*10^10 kilometers, so it takes twice the time
to get to the second marker as one might have supposed,
so according to both the stationary and moving clocks,
the transit time from one marker to the other will be
the same.

QED: if only the object contracts, there is no transit
time difference between the two systems at a given
velocity.
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(I)  The whole universe contracts.

(a) Is the contraction instantaneous throughout the universe?

    How could you tell? And what possible difference
    could it make? Those are not rhtorical questions.
    There would be no way SR could have a meaningful
    application, right? If you suggest that time would
    not similarly be dilated throughout the universe,
    you are suggesting an apparent change in v, for
    v is constant in SR only because  d/t=d'/t', and
    in this case we have no possible d'<>d because
    all the universe's measuring sticks contract sim-
    ilarly. Similarly? Identically!


(b) Does the contraction propagate through the universe at
the speed of light from the location of the moving object?

    Except for questions like "speed of light from any
    viewpoint?" this might not be different that the
    instantaneous model. Hmm. Or maybe a number of widely
    distributed observers in one frame could tell that
    something had happened? Again, none of these are
    rhetorical questions.

(c) Does the contraction propagate through the universe at
less than the speed of light from the location of the
moving object?

    One could see that parts of the universe had contracted.
    Your own measuring stick wouldn't contract until after
    it had measured the distant contraction.


Whole Universe Summary: who knows what effect could be
eventually discovered; what is knowable is that there would
be no simple(ton) visible contraction.

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(II) The whole universe contained in a cylinder centered on
     the moving object, and extending forwards and rearward,
     contracts.

This is compatible with standard SR; elsewise a transit time
between two markers would show the same elapsed time as for
an observer at rest wrt the markers, as we saw in the dis-
cussion of the 'object only' case.

Let there be a spaceship be at rest between two stars, and
with its axis of incipient motion passing through both stars.

When it accelerates to any appreciable velocity, is it the
center cylinder of the forward star that is snatched from
its guts and hurtles toward the spaceship, or the rearward
star's guts?  Or both?

That assumes the center of contraction is at least somewhere
from the rearward star almost to the forward star. If the
center of contraction were somewhere very distant from the
ship,  it could be that both star centers and the spaceship
would all be yanked instaneously through the center of one
start to a point that could be light years distant. Unless
the contraction wasn't instantaneous, and then we'd have
some mess indeed, figuring out how much and how far the
contraction had taken effect before the ship once again
changed velocity.

At a simpler level, of course, contraction along the line
of movement implies faster than light transit of information
if the contraction is instantaneous, or at least faster than
light.

In any case, we'd certainly see some calamitous effects were
objects other than light moving at high v anywhere in the
near universe, wouldn't we?  If SR were correct.
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Summary.
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For our three possibilities in the contraction circus,

(I)   The whole universe, parallel to the line of a
      moving object's direction contracts, and why
      wouldn't time also dilate universally?


(II) The whole universe contained in a cylinder centered on
     the moving object and extending forewards and rearwards
     contracts, and would yield stellar catastrophes we'd
     almost surely have seen by now.

(III) The moving object only contracts, and SR's claim
      about transit time differences would be invalid.

The less unlikely possibility seems to be the one where
you not only couldn't tell there had been contraction,
but you'd be darn silly saying there had been.

Then again, that last is the standard SR position, isn't
it? The contractions don't really occur, they're just
observational differences (which you couln't see in the
whole universe case). That's what SRians on these newsgroups
say; and they also say the time differences are real and
lasting, except when they aren't. <g>