Re: TOM ROBERTS - Dono is confused, please help him out (was SR cannot determine Contraction)

On Feb 26, 12:45 pm, Dono <sa...@xxxxxxxxxxx> wrote:
On Feb 26, 10:07 am, PD <TheDraperFam...@xxxxxxxxx> wrote:

Think about it. How else would you define length? You could say,
"Well, the rod's composed of a number of atoms arranged in a lattice,
and the number of atoms from end to end is *certainly* frame
invariant." And I would say, "Yes."

Good, we agree.

And you would say, "And the
lattice spacing is frame-invariant unless something stresses the
lattice." And I would say "No."

Is there any experimental confirmation for the above? Looking at the
FAQ, there isn't.

Well, you have to go a bit deeper than the FAQ. I'll give you one I
know, that is only loosely connected with the properties of a solid,
but it very much illustrates the same principle. Hadron-hadron
collisions (say protons on protons) produces an angular distribution
of secondary particles that is well-known and is completely
reproducible in a statistical way for a given center-of-mass collision
energy. Let's consider this to be a lattice of secondary particles
with a certain spacing at a given radius from the collision point.
(And in fact, this is not so far removed from the solid case, because
the secondaries are NOT independent of each other, and in fact the
spacing is due to the interaction between the separating quarks and
gluons in the collision.) You can do this experiment in two different
venues: one where the two hadrons collide at the same momentum so that
the center-of-mass momentum is zero; and one where one of the hadrons
is at rest and the other bangs into it so that the center-of-mass
momentum is NOT zero. The center of mass energy of the collision is
the same in both cases. That is, you are looking at two statistical
samples of the *same* kind of collisions, just in two different
reference frames.

It turns out that the distribution of the secondaries in one case maps
to the the distribution of the secondaries in the other case by
exactly the Lorentz boost -- the very same boost that produces the
stressless lattice compression of a rod. We know in this case that
there is no stress involved, because the distributions are *exactly*
the same in every other respect, except for this compression of the
spacing between the particles. A stressed compression would have
other, easily distinguishable signals.

This is not the *direct* measurement of the compression of a solid rod
such as you described, but it is nevertheless solid evidence that
spatial distances compress without stress when changing reference
frames.


Judging by the fact that in the proper frame of the rod the lattice
spacing change is due to stress , I would question your statement.

Judging by the fact that you are effectively saying (correct me if I
am misinterpreting you) that relative uniform motion induces a
stressless lattice contraction, I would question your statement a
second time.

Yes, that's exactly what it does.


The dimensions of the *atoms
themselves* are not a frame-invariant or inherent property of the
atoms. The dimension of the atoms you know come with the *stipulation*
that they are measured when the atom is at rest (or at least slow
enough that the frame-dependence is not measurably significant). You
*cannot* insist that the width of an atom is a frame invariant
quantity.

I am not insisting that this is the case, I am just questioning how
relative motion can decrease such "atom dimension" by huge factors
WITHOUT any destruction of the lattice structure.

The thing to ask yourself is why do you think there NEEDS to be stress
to make the lattice dimension change? If you say, "well, while it's at
rest I have to do that," that is certainly not extrapolable to be the
only explanation for what happens under a translation.

I go back to the kinetic energy example. If we have an object just
sitting at rest and I ask you how you can change the kinetic energy,
you'd like say, "Well, I have to do mechanical work on the object,"
and that would be right. But now I say, "OK, the same object that has
0 J kinetic energy now has 25 J kinetic energy in another reference
frame. Did the observer at rest in that frame have to do mechanical
work to the object to give it 25 J?"


It just isn't. And in fact, collisions of nuclei on nuclei
demonstrate just this fact, that the physical density of that nuclear matter is *higher* when the nucleus is in relative motion.

Are these experiments related to rigid lattices or to free atoms?

Yes, in the sense that those nuclei are lattices of quarks and gluons,
and that the introduction of additional stress would have experimental
implications that are not seen.


This is precisely the point. For centuries, we knew that there were
frame-dependent, extrinsic quantities and frame-independent, intrinsic
quantities. We knew that velocity and kinetic energy and 2D projected
distance were not intrinsic, but we felt sure that time interval and
length and a few other things were intrinsic. It turns out that some
of the quantities we thought were intrinsic are in fact extrinsic, and
that there are other intrinsic properties (like interval) that we
didn't know about.

No argument here, I just think that the INTERPRETATION of the above
scientific THRUTHS , when cast in the "pole in the barn paradox" is
wrong.

There is a way of settling it. While this experiment may not be
realizable today, it might be in the near future.
Imagine a very rigid 10m rod that is being nearly Born-rigid
accelerated by a maglev to 360km/h in such a way that the rod is not
distorted.
There is a laser beam perpendicular on the path of the rod and a
counter attached to a sensor. When the rod cuts off the light path of
the laser, the counter starts counting. The counting stops when the
light path is restablished.

If relative uniform motion truly contracts moving objects, the counter
shoud count:

Delta_t=L_0/v(1-sqrt(1-(v/c)^2))

where L_0=10m (proper length of the rod)
v=100m/s

The effect should be of the order of:

.5*L_0/v*(v/c)^2=5*10^-15 sec

I agree it would be nice to do the direct measurement someday.
However, it has already been verified another way already.

PD

.

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