Re: SR cannot determine Contraction

On Feb 26, 8:06 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
Dono wrote:
On Feb 25, 9:47 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
The "pole and barn" gedanken is valid. [...]

This is not what I asked you. What I asked is can the pole fit in the
barn with both doors closed simultaneously in the barn frame?

With the parameters I gave, or with the parameters you gave, yes.

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. All this, while the contraction is "stressless". 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. 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.

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

And if the "measurement apparatus" includes doors that
close simultaneously in their frame, then for appropriate values of the
parameters a moving object can fit between closed doors that are closer
together (in their rest frame) than the proper length of the object --
at least in principle.


"At_least_in_principle". No one has seen it in practice. Because it
comes with the "stressless contraction", a phenomenon never observed
and never 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. Kind of
disturbing, wouldn't you agree?




then the distance between the marks is measured with rulers at rest in
the measuring frame. But the order can be reversed: one can pre-position
doors an appropriate distance apart, and "mark" the ends of the object
by opening/closing them. 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 the effect were indeed "just a trigometric one" (http://
en.wikipedia.org/wiki/Length_contraction#A_trigonometric_effect.3F).
After all, the "physical" length contraction has escaped detection for
more than 100 years. Projection on the other hand, is much easier
justifiable.



As I have said before, IMHO the pole and barn gedanken is best
understood as displaying the relativity of simultaneity, rather than
length contraction.

Tom Roberts

We agree on this. The problem still stands: is length contraction a
physical or a geometric effect? You seem to believe that it can
magically shrink the 50 foot pole to fit inside the 30 foot barn. I
have serious doubts about it. Unfortunately the only way to decide is
via experiment, one that it is not readily achievable.

.

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: What should a theory give us?
    ... physically shorter .. ... except that it is somehow the ether doing it, ... (wrt the ether frame) ... actually different absolute lengths as a result of their absolute motiob, ...
    (sci.physics.relativity)
  • Re: What should a theory give us?
    ... physically shorter .. ... except that it is somehow the ether doing it, ... (wrt the ether frame) ... actually different absolute lengths as a result of their absolute motiob, ...
    (sci.physics.relativity)