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


"PD" <TheDraperFamily@xxxxxxxxx> wrote in message
news:fb0917fd-dd35-4d2f-86f3-cc62e443abb1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Feb 26, 11:04 am, Dono <sa...@xxxxxxxxxxx> wrote:
On Feb 26, 8:31 am, PD <TheDraperFam...@xxxxxxxxx> wrote:





On Feb 26, 9:15 am, Dono <sa...@xxxxxxxxxxx> wrote:

On Feb 26, 7:09 am, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:

kenseto wrote:
"Tom Roberts" <tjroberts...@xxxxxxxxxxxxx> wrote in message
news:HJMwj.8029$xq2.1334@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Rotations can have direct experimental consequences -- try
carrying a
20-foot ladder through a door that is 3 feet wide and 7 feet
tall, and
you'll see how important rotations can be in the world we
inhabit. I
find it hard to see how one could claim the word "physical" does
not
apply....

But that is not length contraction.....the length of the ladder is
not
changed.

Right. I did nopt claim it was. This is merely an example of the
"physical" consequences of a rotation in space, and its effect on
the
length PROJECTED ONTO A PLANE (the length of the ladder projected
onto
the plane of the door). But "length contraction" is a similar
quantity
-- it is proper length PROJECTED ONTO A PLANE (the proper length of
the
pole projected onto the frame of the barn).

Tom Roberts

Your shadow fits inside a match box. Did I trap you inside the match
box?
This is the 10,000$ question.

The shadow *is* the analog of the length. Length IS the 3D shadow of a
4D interval between two spacetime events. The 4D quantity is interval,
not length.

PD

Precisely. So can you fit the 50 foot pole in the 30 foot barn? Aren't
you comparing "shadows"?

There IS no reality for length except for the shadow. That is
PRECISELY the point. Length is a physical quantity that is *defined*
by taking a projection of a 4D quantity that is NOT a length and
projecting it onto 3D space (like a shadow, if you like), with the
projection BEING the length. There IS NO deeper reality to length than
this. The ONLY absolute (frame-independent quantity) is a *4D*
quantity (spacetime interval), and that is not a length.

ROTFLOL....so physicist have to invent a new meaning for length to make the
theory of SR valid. Similarly physicists have to invent a new circular
definition for the meter length (1 meter =the distance light travel in
1/299.792,458 seconds) to insure that the speed of light is measured to be a
constant.

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

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.


PD


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