Re: SR cannot determine Contraction


"PD" <TheDraperFamily@xxxxxxxxx> wrote in message
news:e68e3fde-5785-49ba-af67-08653fd954b6@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Feb 29, 11:40 am, Dono <sa...@xxxxxxxxxxx> wrote:
On Feb 29, 8:38 am, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:





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?

Then you are stuck with answering the following:

Would you care to show how?

-while in the proper frame S_0 : \ Sigma (F_internal) = 0

-in a frame moving with v wrt S_0 \Sigma (F'_internal)=0 (where F'_i
is the Lorentz transformation of F_i) and that , somehow, magically,
the atom sizes or the lattice spaces between atoms have Lorentz
contracted. I think Lorentz spent a good
10 years of his life trying to prove that and it amounted to nothing.

It's not that complicated. One *measures* the rod to be of shorter
length. One *measures* the rod to have no fewer atoms strung end-to-
end -- or at least finds no evidence of some atoms being ejected from
the length. One *measures* that there is no stress in the rod -- or at
least finds that there is no evidence of any stress. One therefore
concludes that the interatomic spacing has also contracted. Then one
slaps oneself on the forehead when it occurs that there is no reason
why an atom should be any more inherently absolute in dimensions than
a rod and somehow resist the effect of contraction.


I know that you can calculate very well, so , I'd appreciate some
math

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.

You are missing the point (perhaps intentionally), I was showing you
that the notion of contraction is tied with the measurement
methodology, so, it is not INTRINSIC to the relative motion as YOU
claim.

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>

All right , let's make it even less ambigous, look here:

http://en.wikipedia.org/wiki/Length_contraction#A_trigonometric_effec...

What in the Roberto Torretti statement " Relative motion will not make
a solid body shorter in the way that, say, heat
makes it larger." don't you understand, Tom?

That's right, it isn't the same process. The shortening of the rod
should in no way be confused with what happens when a rod is cooled.


What in the paper by Redzic don't you understand, Tom?

I know that these references are kind of new but you need to keep up.

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 technique I mentioned shows that "length contraction" is not an
INTRINSIC property, it depends on the choice in simultaneity.

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.

But this is not (and has not been) the point of the discussion. The
point of the discussion has been whether or not the barn doors will
clip the rod.

And that is a *measurable* result. The answer that observers in *all*
frames will agree is that, no, the barn doors will not clip the rod.
Different observers will have different accounting for *why* that is
the case, but they will all agree on that.

It is not a measurable result. It is a predicted result. Physical length
contraction never been measured. In fact it is impossible to do so.....Why?
because you have to mark the ends of a moving rod simultaneously in the
observer's frame.




.

Relevant Pages

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    (sci.physics.relativity)
  • Re: Androcles and Draper resume Einstein 1905
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    (sci.physics)