Re: Is "malfunctioning" absolute or relative?

On Oct 13, 7:43 pm, shuba <tim.sh...@xxxxxxxxxxxxx> wrote:
Uncle Ben wrote:

[re: Bell's spaceship paradox]

W.r.t. the launch frame of reference, the distance between the ships
is constant.
W.r.t. a comoving frame of reference momentarily coinciding in
velocity with the lead ship, the distance between the ships is
increasing. W.r.t. this same frame at the time of launch, the firings
of the motors of the two ships are not simultaneous. Thus the
explanation of the breakage is different when observed in the two
frames of reference.  In one, the ships are pulling apart;  in the
other, the string is trying to contract, if you will excuse the
anthropomorphism. But the phenomenon is the same.

Anthropomorphism aside, it is completely incorrect to claim that
relativity says the string is trying to contract as determined by
measurements in the launch frame.  That may in fact be true, but
if so it does not have to do with relativistic effects.  What we
can say is that certain stresses are introduced in the string (a
frame-independent statement, the measurement of those induced
stresses will depend on the frame).  Whether or not these
stresses would naturally result in contraction depends on the
physical properties of the string (elastic limit, tensile
strength, etc.).  For example, let's cut the string at a certain
velocity.  Will it contract?  Maybe.  Perhaps not if the string
is made out of some putty-like material.  In this case, it seems
a bit hard to claim that the string is trying to contract when
given the opportunity to do so, it doesn't.  The problem is the
same as I've already pointed out.  You are not applying the idea
of relativistic length contraction appropriately, that is to the
same object with respect to different frames. Relativistic length
contraction is a geometric relationship between an object and two
frames, not a thing that "happens" to an object. It is a fact,
not a process.

By "real", I intend to mean what the ordinary man in the street would
mean:  The bending of the stick in a glass of water is not real.  The
shrinkage of the house in the distance is not real, in regard to the
linear size of the house.  Many authors of relativistic pedagogy
insist on saying that "thc clock appears to run slow" or "the ruler
appears to shrink" as if the relativistic effects are illusions
created by the finite speed of propagation of light.

Even if technically feasible, the man in the street isn't likely
to place a clock in a rocket in order to calculate its rate.  The
bending of a stick in water is precisely as real as relativistic
length contraction or time dilation.  It is measurable, it is
based on geometry, and may have real consequences, as mentioned
by Jerry.  Using the word "appear" is considerably better than
your preference about relative realities, but even better would
be to use "projected proper length and time" or something.

Unfortunately, many instructors introduce relativity as "physics
that makes clock run slow and rods get short" in an attempt to
sound profound or something.  Some students rightly wonder what
this "witch doctor physics" is based on and find the subject much
less intuitive than it might be if the instructors told the truth
from the start.  All of relativity is based on geometry, and the
basis for the geometry is the symmetries of space and time.  The
student may still struggle with the idea, but when the student
figures it out, he or she will even be able to detect the kind of
fundamentally incorrect statements such as the ones you have made
regarding what relativity says about a stressed string.

         ---Tim Shuba---

Well, thanks for this exchange of views.

I guess we have to leave it that in your view the "bending of the
stick" immersed in a glass of water is real. And in my view, the
bending of the stick is a misinterpretation of what we see in the real
bending of light rays from the scene.

If we cannot agree on this little demonstration, then we are doomed to
disagree.
That's OK.

(But it still bothers me a little that if the bending of the stick is
real, why doesn't it break?)

Uncle Ben
.

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