Re: does circumference contract with velocity?

Subject: Re: does circumference contract with
velocity?

Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
David wrote:
If I have a disk that has a rotational
velocity V at its outer edge, does someone who is
at rest with respect to the center of the
disk conclude that some sort of length contraction
occurs because of V and therefore the rotating
disk has a smaller circumference than the
same disk when it is not rotating?

Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
Normally when one says "the circumference of this
disk", they mean a measurement made
_simultaneously_ around the entire edge of the
disk. For an inertial observer (e.g. of the
inertial frame in which the center is at rest) this
is easy; for a rotating observer on the disk itself
this is impossible -- there is no single self-
consistent definition of simultaneity for a
rotating system.

Gerald L. O'Barr <globarr@xxxxxxxxx> comments:
Sure sounds funny. Are you sure that such a
simple thing as a rotation is beyond a consistent
definition of simultaneity? LET has no problem with
doing any of these things. Maybe LET is superior to
SR?

Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
So you _must_ change your notion of what
"circumference" means for a rotating disk.

O'Barr comments:
Again, this sure sounds funny! Does the
circumference disappears? Or does it change its
nature? And at what velocity does all this occur?
What I think is that you are embarrassed at what SR
says, so rather than give up on your favorite
approach, you would rather change the meaning of what
is is.


Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
If instead of a solid disk you imagine a "disk"
made up of thin radial fibers with increasing
widths such that together they make up the disk ]
when not rotating, then as the set of fibers starts
rotating, small gaps will appear between the
fibers, getting larger as the tangential velocity
increases. Of course in practice this is
immeasurably small, and for practical materials the
fibers will be torn apart long before either an
appreciable fraction of c is achieved or the gaps
are observable.

O'Barr comments:
The only gaps I know about are the gaps in your
head. There are no gaps in materials that have
rotational motion, never more than what are always
there, just as in linear motions, except what can be
attributed to internal stresses.

What Tom says about any inertial frame is correct.
In any inertial frame, every clock can be perfectly
synced, and every measurement will be SR perfect.
What Tom is slow in saying that in such a perfect
measurement system, the length of all moving rulers
in that frame will be measured by these perfect tools
to change their lengths in the direction of their
motions, and all clocks will be measured to slow
down. These things are exactly what are measured.
But Tom does not like to say that any of these
measurements are what is actually happening. He
really doesn't think that there are any real changes
at all.
But when you go into a rotation mode, then these
changes are harder to ignore. Their reality become
more clear. And thus Tom needs to say that other
things become a problem, so he can ignore these
impossible possibilities that real changes really do
occur.
Sorry, Tom, you simply are wrong to support SR,
****as you want to understand it!****
and you know are wrong! I am waiting for the day you
retire, and will be free to say what you ought to be
saying. Yes, some changes in SR are due to other
things changing, but not all. Some changes, under
some conditions, have to be real in order for it to
all happen the way it happens.

Thanks for reading.
Gerald L. O'Barr <globarr...@xxxxxxxxx>
Remove ... for e-mail.

.

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