last? blow to SR dead horse

[Geoff Nunes <gnunes@MAILAPS.ORG>]

"Michael J. Moloney" wrote:
> ...Later, Peter passes Sam, another observer in
> Jane's frame. One minute has elapsed on Peter's clock. What is the elapsed time
> difference on the clocks of Sam and Jane?
Thank you Mike for finally bringing up the issue that continually
confuses begining students precisely because it is so obvious to us
professionals that we can't even see it.  Michael Edmiston gave a
wonderfully clear explanation of the importance of thinking in terms of
events and the intervals between them.  But even he says:

> ...see what Jane records for this event.  She
> records x=-6.19x10^8 meters and t=2.29 minutes.

No she doesn't.  She isn't at x=-6.19e-8 meters where the flash occurs
(using Michael's clarifying strobes).  It takes time for the flash to
reach her.  Bright students immediately see this and get (rightly!)
confused.

I always told my students to imagine each reference frame to be
absolutely packed with observers.  When each sees the flash, they mark
the time in their own notebook.  Later, at their leisure, they all get
together with Jane and compare notes.  When finding the interval between
flashes, the time to use is the time recorded by the observer under
whose nose the flash went off (Sam in this case).  It's imperfect, but
seems to be generally helpful.

 From a broader perspective I would say that the discussion over this
problem arises because it belongs to a peculiar class of examination
questions (of which I have written a few) that bright students fail at
and dull ones get right.  It's poorly worded, but if they don't think
too hard about it they do just what the examiner intended.  But if they
stop to think about it, all the ambiguity comes out of the box and
they're done for.

-Geoff Nunes