Re: SR cannot determine Contraction

On Feb 24, 11:13 pm, Dono <sa...@xxxxxxxxxxx> wrote:
On Feb 24, 8:45 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:




Correct.
-This is why one can't close the doors simultaneously in the barn
frame without hitting the pole.

In the frame in which the barn is at rest, you can.

No , you can't. This would be equivalent with a physical contraction
of the rod and this is not what is going on.


I'm sorry, I still don't follow. The simultaneity of the doors closing
can be settled unambiguously by setting clocks at the two doors, and
then synchronizing the clocks. In the frame in which the barn and the
clocks are at rest, this synchronization procedure is well defined.

Furthermore, I don't see how this is equivalent with a physical
contraction of the rod.

As Feynman says, even with thought experiments it's important to lay
down the essential details of what's actually done. If the doors
aren't actually closed and they are merely open doors with photocells
that record the passage of the pole, then the criterion is very
simple: Is the passage of the rear of the pole through the rear door
of the barn sooner than the passage of the front of the pole through
the front door of the barn, according to the synchronized clocks. SR
says yes it is possible this can happen. The question then is how is
that possible in the frame in which the pole is at rest. The answer is
that the clocks that determined that state are not synchronized.

If the front door of the barn is actually closed, then the accounting
of the events is somewhat different, and here the pole becomes
*stopped* by the door. Then the way to resolve the events as seen in
the two frames has to do with the communication of the pole's stopping
from the front of the pole to the back of the pole.

-This is why the pole in the barn, being a thought experiment, is a
very poor illustration for length contraction.

I don't follow this.

There is a large class of thought experiments (the pole in the barn
being one of them) that rely on the misguided idea that length
contraction allows larger objects

Larger in what frame?? You make it sound like its "largerness" is an
inherent property of the pole. It's not.

(like the pole) to fit inside
smaller enclosures (like the barn) as a byproduct ofrapid relative
motion. This is not the case.

-This is why, to date, we have no experimental test for length
contraction (http://www.edu-observatory.org/physics-faq/Relativity/SR/
experiments.html#Length_Contraction)

I don't follow that, either. What does the frame-dependence of length
do to prohibit measuring that effect, when the frame-dependence of
just about any other frame-dependent quantity has certainly been
measured? (E.g. muon lifetime in g-2, angular distributions of
secondary particles in hadron-hadron collisions, etc.)

Length contraction is a measurement artifact that comes about when we
attempt to measure lengths of moving objects. It is easy to prove that
the effects are of the second order in v/c, something well outside the
current precision of measuring devices.

Oh, BS. The momentum effects in a cyclotron are of second order in v/c
as well, and it led to a serious problem getting the momentum desired
out of the cyclotron -- which led to the synchrotron instead.

Take a look at a "4pi" calorimeter surrounding a vertex in a fixed-
target hadron-hadron collision experiment, and compare it with a 4pi
calorimeter in a collider. One is a boost map of the other. Then stand
there with the two blueprints and tell me that one is immeasurably
different from the other because the effect is second order in v/c.

If, by contrast, length contraction were a physical effect,
compressing objects such that

No, let's recap. Length contraction is physical because it is defined
by a physical measurement prescription. There is NO definition of
physical length that bypasses this prescription. This measurement
prescription is as physical as length gets. The fact that it is a
frame-dependent property does NOT imply that there is a physical
process physically altering the rod, any more than the frame
dependence of momentum implies that a different observer adds more
momentum to the object by interacting with it somehow.

, at relativistic speeds they would fit
into much smaller enclosures (like in the case of the paradox in
discussion) we would have been able to measure it through tension/
stress effects. This is not the case, so we dont' have any tests of
length contraction to date, and probably none for the forseable
future:

http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#...- Hide quoted text -

- Show quoted text -

.

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