Re: On the physicality of length contraction

On Jul 22, 8:34 pm, Uncle Ben <b...@xxxxxxxxxxx> wrote:
On Jul 22, 1:14 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:



On Jul 22, 11:44 am, Uncle Ben <b...@xxxxxxxxxxx> wrote:

On Jul 22, 12:35 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 22, 11:15 am, Uncle Ben <b...@xxxxxxxxxxx> wrote:

On Jul 22, 11:21 am, eric gisse <jowr.pi.nos...@xxxxxxxxx> wrote:

Uncle Ben wrote:
The thread "There is no physical length contraction" is getting too
long to be coherent, IMHO, although very interesting.

No, it isn't. It is just a rehash of the same old ***.

Length contraction is as physical as the belief that a stick gets shorter
when you rotate it along your line of sight.

[...]

So, you don't believe the pole-in-barn story?

Well, in the pole-and-barn story note the enormous differences in the
physical accounts of what's going on, between the two frames.

In one frame, the explanation is that the pole really is shorter than
the barn, and the doors are shut simultaneously.

In another frame, the explanation is that the doors are shut
nonsimultaneously, and the pole is decidedly not shorter than the
barn.

But still, the fact that there is no impact between the pole ends and
the barn doors is experimentally verified in both frames.

Now, what's real and not real here?

PD

PD:
Suppose the doors remain shut.  In the barn frame, I don't see how to
deny the claim the the pole was really shorter. There will be a few
nanoseconds in which the pole is entirely within the barn and both
doors are shut.  That is observable.

In the barn frame, this is exactly what the accounting is, just as it
was in the barn frame when the doors were shut only briefly.

But in the pole frame, the accounting is different yet. It is STILL
true that the doors are not shut simultaneously, and it is STILL true
that the pole is NOT shorter than the barn. So how is it that the door
near the rear of the pole doesn't hit the pole? But now the physical
account is that when the front of the pole hits the closed barn door,
it stops but the rest of the pole doesn't know to stop yet. The signal
from the front of the pole takes a while to propagate to the back of
the pole, that speed certainly limited by c. So the back of the pole
proceeds forward still until sometime later when the second door shuts
and the rear of the pole is safely inside the barn.

Again, you see the *physical* accounts of the pole-and-barn puzzle are
still different in the two frames, though the observables are agreed
upon.

Darwin:
I have already argued the point about the term "phyical" Please note
that in the o.p.

Uncle Ben- Hide quoted text -

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Paul, I have told this same story often.  The question is, does
physics require that an explanation be frame-independent? Velocity is
frame dependent; so is momentum.  Yet we explain phenomena in these
terms all the time.

My basic point is that authors in relativity should not be so timid as
to suggest somehow to students that relativistic effects are mere
appearances or artifacts.  They are as real as other things we
casually accept as such.

So let us say that moving rods ARE shorter than the same rod at rest,
although frame-dependent.  To say that the rod "appears to be" shorter
or, curiously, "is measured to be" shorter, is ***-footing around,
as if the author is not quite sure whether it is real or not. (In the
hey-day of logical positivism, to be measured was the gold standard of
reality!)

Uncle Ben

I have reflected on why I disagree with that, and it turns out that
it's because of habits of phrasing in the context of physics.
For example, we all interpret "its speed has decreased" as merely from
the perspective of the used reference system. Thus I have also no
problem with the statement that "its length has decreased" (to be
interpreted just like speed). However, "the rod has become shorter"
sounds rather "absolute" to me; it's not common to attach to such
phrasings an observer-dependent meaning. That's all. :-)

Harald
.

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