Re: SR cannot determine Contraction

Dono wrote:
On Feb 26, 8:06 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
Dono wrote:
In other words, do you , Tom Roberts, believe that uniform relative
motion makes objects shorter?
That's your confusion. This is phrased so poorly that no answer is
possible. Your phrase "makes objects shorter" implies a change to the
object, but observations made from another frame cannot possibly affect
the object itself.

Correct. But this is exactly what the paradox states, that , from the
barn frame, the pole becomes magically shorter, by virtue of relative
motion.

You insist on using loaded words. This is not "magic", this is GEOMETRICAL PROJECTION. When you approach a doorway with a ladder, in some orientations the ladder fits through, and in other orientations it doesn't. THIS IS THE SAME PHENOMENON, but it occurs in the X-T plane, not the X-Y plane like the ladder's rotation. Relative velocity is a (hyperbolic) ROTATION in the X-T plane.

Actually the analogy would be better if one considered
moving the doorway around the ladder, leaving the ladder
fixed. But common experience does not include either
moving doorways or poles moving at 0.8 c.


All this, while the contraction is "stressless".

Of course there is no stress -- what stress is induced in the ladder when you rotate it? Or rather, when you look at it from another angle?


To make
things even more interesting, if the measurement methodology is
switched to marking the pole endpoints simultaneously in the pole
frame, this results into length dilation.

That's completely irrelevant -- no measurement does that. To measure the length of a moving object you CLEARLY must mark its endpoint simultaneously IN YOUR FRAME. No other approach can possibly be said to measure its length IN YOUR FRAME.


The whole point is the
interpretation of length contraction: real vs. imaginary. You seem to
believe it is real, Michael Janssen points the other way.

Stop using such loaded and ambiguous words -- they merely confuse you and your reader: "real" means many different things to many different people. Discuss MEASUREMENTS, not what someone happens to think is "real". There is no doubt that MEASURING the length of a moving pole will obtain a value smaller than its proper length. And if your measurement includes rapidly opening and closing doors, the pole can fit inside a barn that is shorter than the pole's proper length. <shrug>


Uniform motion makes objects be measured to be shorter than when they
are at rest.

Correction:
Depending_on_the_measurement_technique_uniform_motion_MAY_return_shorter_or_LONGER_length_for_
the _measured_object.

No. The "technique" you mentioned does not measure the length of a moving object.


The "model" predicts shorter, which is correct. But you don't know if
the object becomes shorter because there is no experimental
confirmation.

Again, stop using loaded and ambiguous words -- "becomes" has the same ambiguity as "real". What SR unambiguously predicts is that a MEASUREMENT of the length of a moving pole will give an answer shorter than the pole's proper length. Yes, at present there is no direct experimental confirmation of this prediction, but given all the other confirmations of SR predictions, it would be perverse to deny this one.


In order to become shorter you's have to ascribe to the
theory that the "atoms_are_becoming_shorter" or that
"the_interatomic_spaces_are_becoming_shorter". All this without any
increased internal stress.

Again, stop using such loaded words. All that is happening is that the "stationary" observer is LOOKING AT the moving pole differently from a co-moving observer. OF COURSE this induces no stress in the pole, OF COURSE the pole can look different, OF COURSE the effect on the atoms is the same as on the pole, OF COURSE the effect on interatomic spaces is the same as on the pole. Because this is simply geometric projection, not anything more complicated (and not anything simpler).


No one has seen it in practice. Because it
comes with the "stressless contraction", a phenomenon never observed
and never explained theoretically.

Nonsense. Looking at an object cannot possibly produce stresses in the object. This is COMPLETELY explained theoretically.


Remember how the length of a moving object is measured: its front and
rear locations are marked SIMULTANEOUSLY in the measuring frame,

Yes, I know that very well. Problem is, if you mark the ends of the
object in the object's frame you get length...dilation.

That is not any common type of measurement, and it does NOT measure the length of a moving object.


Kind of
disturbing, wouldn't you agree?

Only about your personal mental state. This is YOUR problem, not any problem with SR, or with anybody else.


See my description of the gedanken without
accelerations, and my solution to the problem you posed.


Yes, I saw it. What if the effect is not the "physical" contraction?

What if your aunt had wheels -- would she be a teacart?

You use loaded words like "real" and "physical", without specifying PRECISELY what you mean by them. Discuss MEASUREMENTS, not such loaded words. Then everything works out and the ambiguities and confusions disappear.


We agree on this. The problem still stands: is length contraction a
physical or a geometric effect?

It is most definitely a geometric effect. But geometric effects can affect physical results. The ladder fits through the doorway only in some orientations, because the GEOMETRIC PROJECTION of the ladder's length onto the plane of the doorway differs. The length of a pole is measured to be shorter in a frame relative to which the pole is moving because the GEOMETRIC PROJECTION of the pole's endpoints onto the frame is different.


You seem to believe that it can
magically shrink the 50 foot pole to fit inside the 30 foot barn

There's no "magic", just simple, basic, GEOMETRICAL PROJECTION.


The measurement of the length of moving objects is nothing more than
an exercise in projective geometry. Depending how you do the
projections, you get a DIFFERENT result.

Right. But there's only ONE WAY to project the length of a pole onto a frame -- mark its endpoints simultaneously in that frame. That method makes it be measured to be shorter in a frame relative to which it is moving, than in a frame relative to which it is at rest. <shrug>


Tom Roberts
.

Relevant Pages

  • Re: SR cannot determine Contraction
    ... Your phrase "makes objects shorter" implies a change to the ... but observations made from another frame cannot possibly affect ... barn frame, the pole becomes magically shorter, by virtue of relative ... That's completely irrelevant -- no measurement does that. ...
    (sci.physics.relativity)
  • Re: EXPERIMENTS THAT REFUTE RELATIVITY
    ... you ignore the evidence of light also arriving at the observer at ... The rod is shorter, yes, but not by the same amount. ... Initially the rod is moving wrt both A and B. ... but NO LONGER short in B's frame. ...
    (sci.physics.relativity)
  • Re: SR cannot determine Contraction
    ... Your phrase "makes objects shorter" implies a change to the ... but observations made from another frame cannot possibly affect ... barn frame, the pole becomes magically shorter, by virtue of relative ... if the measurement methodology is ...
    (sci.physics.relativity)
  • Re: Pole and Barn for Dummies
    ... frame of reference B such as a barn 10 m long, ... back door, fixed to the earth. ... the pole would be trapped. ... is moving leftward at 0.75 c.  Then the barn is much shorter than the ...
    (sci.physics.relativity)
  • Re: Pole and Barn for Dummies
    ... frame of reference B such as a barn 10 m long, ... back door, fixed to the earth. ... the pole would be trapped. ... is moving leftward at 0.75 c.  Then the barn is much shorter than the ...
    (sci.physics.relativity)