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Re: A Question of Simultaneity



The scenario I proposed (see below) provokes a question about what happens
in the innards of the coincidence apparatus as viewed by the train and
ground observers.
Keep it simple: Let the coincidence apparatus be two relays whose SPST
contacts are wired in series so as to blow the train whistle only when
both relay coils are energized (by currents from photocell detectors). In
the train frame both coils are simultaneously energized and the whistle
blows.
In the ground frame the relays are not simultaneously energized and yet
the whistle blows ? Explain!

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Bob Sciamanda" <trebor@velocity.net>
To: "phys-l@lists.nau.edu: Forum for Physics Educators"
<PHYS-L@lists.nau.edu>
Sent: Friday, January 25, 2002 2:57 PM
Subject: Re: A Question of Simultaneity


Set up two detectors on a railroad car (spaced along the direction of
travel). Wire the detector outputs into a coincidence counter whose
output blows the train whistle when coincident events occur. All of
this
apparatus is on the moving train.
Now add, on the train, an isotropic light source midway between the
detectors. When this light source fires a pulse, the train whistle will
blow.
A second observer standing on the ground sees that the whistle was
triggered by light arrivals which were not simultaneous in the ground
frame, but he realizes that the apparatus is rigged to fire only for
light
arrivals which are simultaneous in the TRAIN frame.
Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor