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Re: SR examination question "survey" (Q4)



John Mallinckrodt wrote:

Q4: Consider the following descriptions of two events:

The place and time where/when Peter *observes* the second hand on
his watch to have made one complete revolution.

The place and time where/when the second hand on Peter's watch
*has* made one complete revolution.

Are these descriptions of the same event (within a trivial
uncertainty of about 3 feet and/or 3 microseconds) or not?

I think the *observes* is, in a practical sense, irrelevant. (Would 3
nanoseconds be a better estimate of the trivial time uncertainty,
ignoring, of course, the much greater uncertainty associated with the
travel time of the nerve impulses from retina to brain)?


Rick Tarara wrote:

The only real problem with the original question, IMO, is that it is not
100% clear that Peter is moving and Jane is at rest although that (again
to me) is clearly the intent.

It is not clear, but even if it were, it would not be relevant, as
Jane (Peter) would be at rest in her (his) own reference frame, but
not in any other valid frame.

Of course this is not the first (or last) time a topic that started out
as a discussion for a H.S. or College intro course has turned into a
free-for-all at the theoretical dissertation level. ;-)

I respectfully disagree. ;-)

The entire discussion I have seen is at a level that would be entirely
appropriate for the second-year college students to whom I attempt to
teach this fascinating topic (assuming, of course, that they had the
patience and insight necessary to follow it). Michael Edmiston (using
words) and John Denker (using a picture) have presented complementary
explanations that clearly indicate why the original question is
essentially meaningless. IMHO, the two taken together could easily
constitute a good textbook-quality example at this level.

In the "real world", students will naturally encounter poorly or
incompletely stated problems; this usually reflects a lack of full
understanding (or of due caution) on the part of the person or other
entity posing the problem. We should train our students to deal with
these questions by seeking clarification, not by making assumptions
about the original intent (taken to extremes, the latter approach
results in debacles such as occured in Florida's recent election).
However, examinations are not the appropriate venue for this training
to occur, and this question should not have appeared on one, at least
not in its present form. SR is difficult enough for beginners to
understand without piling on the additional burden of having to
interpret carelessly worded questions that don't exhibit correct use
of the technical jargon (e.g., "events", "spacetime intervals",
"proper time", "relativistic invariance") that is necessarily
associated with this topic.

Dissertation level? Not until someone pulls John D.'s string hard
enough to get him to say "gee mu nu" in public. ;-)