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

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. While this is not a unique
frame in the sense of some pervasive ether that sets it, it is only a
half a step behind, implying that there is something special about the
rest frame of an object, and that the true "physical" property of an
object is what we measure when it is at rest. 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. The
whole point of the principle of relativity is to convey that there is
nothing *physically* different or unique about measurements made in
one frame as opposed to any other.

I agree with Uncle Ben that "length" is physical insofar as it is the
result of a measurement and is in fact *defined* by a measurement
procedure, which happens to rely on simultaneity. If you insist that
length becomes "physical length" only in one frame, then this also
suggests that the simultaneity invoked in the measurement made in that
frame is somehow more "real" than what is simultaneous in other
frames, and again I think most would think this off the mark.

Now, between any two events in spacetime, there is an invariant
interval, and if that interval is spacelike, then there is a reference
frame in which that 4D interval numerically coincides with the 3D
length. But again, there is nothing *physically* special about that
frame, anymore than there is something physically special about the
choice of coordinate axes that makes the 2D length of a line segment
numerically coincide with its projection on the x-axis.

So I agree that the point is to use the word "measured", but if you
do, then you should abstain from using the term "physical length" at
all, because it is just as ambiguous as using the adjective "real".


But more generally, don't try to make sound bites, as they are
invariably mis-interpreted by people who don't already understand SR,
and they are useless to people who do understand SR.

Uncle Ben said:

If you measure it, it is real.

Hmmm. But the "real length" of a ruler is quite clearly its proper
length. So you have a length that is "real" but that is not its "real
length". Yes, there are PUNS on "real" throughout that, and THAT IS THE
PROBLEM -- "real" is not precise enough to capture the nuances of what
is going on, and such sound bites are invariably misinterpreted (except
by people who already understand all this).

Go back to my analogy of the ladder and doorway. The "real length" of
the ladder quite clearly does not change with orientation. But you claim
the "length" of a moving ruler is "real" and does change with
orientation. In this case, it is the PROJECTED length of the ladder that
matters, and that also applies to what is measured for the moving ruler.
All you need to do is be more careful to avoid PUNs.

It is a cop-out to qualify cetain results in relativity as "apparent"
or "measured" as if measurement was a lesser way to truth.

No. It is no "cop-out" to say "measured length" when THAT IS WHAT IT IS.
Or to say "projected length" when that is what it is. But "real length"
would CLEARLY be wrong for the measured length of a moving ruler, or for
the length relevant to the ladder fitting through the doorway.

It simply does not make sense to claim the length of a moving ruler is
"real", but is not its "real length" -- yet that's what your ambiguous
usages do.

It is
because the writer is still not quite accepting the results as real
in the back of his mind, or he thinks the reader wont accept it and he
is hedging his bets.

No. It is (at least in my case) because the writer does not want to use
an inappropriate PUN, and wants to specify what he/she is talking about.

Yes, the measurement of a projected length can have real, physical
consequences. But it is not the "real length" of a moving ruler, just
like the length of the ladder projected onto the doorway is not the
"real length" of the ladder.

There is no ambiguity at all in the "real length" of the ladder. But you
introduce ambiguity into "real length" of a moving rod. Those cases are
directly analogous, and it's silly to use the language differently for them.

That is why I always state it outright that, for example, the length
w.r.t. frame f "IS" so-and-so, not "appears to be".

That usage is OK, because you DID specify what you meant. The problem is
that you repeatedly use similar words and DON'T specify what you mean,
leaving your ambiguous words to be interpreted by the reader -- THAT is
the problem I'm trying to get you to understand. And it is a problem,
especially around here.

Length is defined
relative to a frame of reference.

To you, perhaps. But not necessarily to everybody else. When
communicating you must share definitions of words with your reader, or
it is hopeless. Do not count on "length" being defined that way to
everybody else, because it isn't: "length" and "real" are simple, basic
words, and you cannot possibly prevent people from applying their
simple, basic meanings. In the way you use those words, those basic
meanings are WRONG. You have no control over your readers, so you should
avoid such words, and use the correct terminology from the technical
vocabulary -- that's why it was invented. There are no shortcuts, and
sound bites are invariably misinterpreted.

Nobody will misinterpret "proper length", "measured length", or
"projected length", and there's no excuse not to use those phrases WHEN
THAT IS WHAT YOU MEAN.

Tom Roberts

.

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  • Re: Are SR effects real or not? Simplified case.
    ... The "real length" of the ladder quite clearly does not change with orientation. ... In this case, it is the PROJECTED length of the ladder that matters, and that also applies to what is measured for the moving ruler. ... Yes, the measurement of a projected length can have real, physical consequences. ... The problem is that you repeatedly use similar words and DON'T specify what you mean, leaving your ambiguous words to be interpreted by the reader -- THAT is the problem I'm trying to get you to understand. ...
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