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

On Feb 26, 2:03 pm, Dono <sa...@xxxxxxxxxxx> wrote:
On Feb 26, 11:27 am, PD <TheDraperFam...@xxxxxxxxx> wrote:





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.

Very nice, I have two questions:

1. are the particles in cause in uniform motion or accelerated?

Uniform.

2. can you send me the original paper, I would highly appreciate it.

I'll have to hunt for it. It's a paper comparing ISR (collider)
rapidity distributions with those at FNAL (fixed target), if I
remember right. I think I remember seeing some references in Don
Perkins' book on particle physics, too.


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.

Because there should be no difference between rest and uniform
relative motion, I thought that this should be self-evident.

And there isn't. If you have an object that is in motion in a
particular reference frame and you want to change its length without
changing its motion in that frame, then you have to compress the
lattice by introducing a stress. However, this does not mean that the
change that is observed in going from frame to frame has to be
attributed to the same process.


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.

Is there any acceleration involved in the above?

Not in the region being probed.


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.

The little  sketch I made shows the formidable obstacles before we get
such a thing going.
.

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