Re: Are SR effects real or not? Simplified case.

On Jul 17, 8:44 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
PD wrote:
On Jul 16, 8:24 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
PD wrote:
To put Tom's point more colloquially, an given ruler at any given
moment has got thousands of physical lengths, [...]
I wouldn't say that, either. To virtually everybody, "physical length"
means "proper length", and that's not what you mean.

Say "measured length", because that's what it is. And don't worry about
whether or not it's "real" -- that's hopelessly ambiguous, and is never
needed.

Well, with all respect, I disagree. The problem philosophically and
pedagogically with assigning "physical length" to "proper length" is
that it singles out a reference frame.

No. It "singles out" the PHYSICAL OBJECT.

But you missed my point: don't use the phrases "physical length" or
"real length", because they will be confusing to many readers. Say,
rather, "projected length" or "measured length", as those phrases don't
have the inherent ambiguity and PUNs of the first two.

I find it distasteful,
and I believe Einstein would too, to think that "physical" length can
only be accessed directly by measurement in one reference frame.

I'm merely pointing out how the words are actually used by many people.

Go back to my ladder and doorway analogy and imagine using "physical
length" of the ladder as the length that is important for whether or not
it fits through the doorway. Ditto for "real length". Obviously both are
ludicrous -- so I feel it's silly apply the same words differently in
the analogous situation of a moving ruler.

A PROJECTED length is what this is. Trying to call it "physical length"
or "real length" is guaranteed to be confusing to at least some readers.
The underlying problem is that both "physical" and "real" have several
different meanings, and only some of them apply to the way you want to
use those words. As I keep saying: such unintended and unacknowledged
PUNs can destroy an argument and confuse the readers. So don't do that
-- avoid using the ambiguous words altogether and replace them with
PRECISE words from the technical vocabulary.

        If you never discussed "physical length", or "real
        length", but always said "projected length" or
        "measured length", I doubt very much that Einstein
        (or anybody else) would object to the result.

What's wrong with being precise in terminology?

Tom Roberts

IMHO, it's not being precise. It's being wishy-washy. Length of a
moving object is perfectly well defined. You mark the coordinates of
the two ends at the same time.

What you are trying to do is to preserve a pre-relativistic idea.
That's attractive and comforting. The thing is, relativity is
uncomfortable to common sense. But we need to get used to it, not to
dodge it.

People, even physicists, may have a hard time with it, but as far as
we can tell, moving objects do shrink; moving clocks run slow, but
only w.r.t. the frame of reference in which they are moving. The rate
of a clock is defined relative to a frame of reference, no matter what
your intuition tells you. Try to get used to it.

It's no more strange than the fact that if you run along beside a
moving charge, the magnetic field it used to produce disappears w.r.t.
you. It doesn't disappear w.r.t. someone standing still.

Or not. It's a free country.

Ben
.

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