Re: [Phys-l] Re. Simultaneity

[John Denker <jsd@av8n.com>]

On 05/20/2009 12:34 PM, Eric Lane, Lane wrote:
> Re. Simultaneity: With reference to the train/platform thought
> problem, suppose a mid-point observer in the train frame S-prime
> confirms simultaneity when two light sources flash at the ends of his
> train of length L. 

OK, mostly.

> Then an equivalent observer (equivalent in the
> sense that he is situated at the mid point of the platform) in frame
> S, also of length L, will not observe simultaneity. However, a
> non-equivalent observer in the platform frame S located (beta)L/2 to
> the right of the mid-point observer, will observe simultaneity. We
> derived this result from our Equation of Light, and the same result
> can be obtained by the usual Lorentz transformation procedure. Why
> restrict considerations of simultaneity to equivalent observers?


As John M. pointed out, the notion of "equivalent" observers is
nonstandard and unhelpful.

There is a conventional, sensible way of analyzing all problems of this 
ilk:

  1) Draw the spacetime diagram.

  2) Analyze the situation in terms of _events_, i.e. points in spacetime.

  3) Draw the spacetime diagram already.

  4a) When we directly _observe_ an event, we observe it _at_ the event, 
   i.e. at the place and time that the event occurred.  This eliminates
   all questions about the meaning of the observation.

  4b) Sometimes it is possible to observe an event indirectly, perhaps by
   observing some other event and applying corrections for distance=rate*time
   and/or other corrections.  It is up to you to prove that your indirect
   observation is equivalent to the direct observation.

   5) See item 1.

   6) Let's not play word games.  The question is supposed to concern the
    simultaneous (or not) emission of the light pulses.  Those are the events
    we are supposed to be observing.  Arrival of the pulses at this-or-that
    observer is *not* directly what we are supposed to be observing.  If the
    non-midpoint observer thinks the emission events are simultaneous, he is
    observing the wrong events and/or is applying the wrong correction terms.  
    The emission events are not simultaneous w.r.t the platform frame, and 
    fiddling with the observers cannot change this.

On 05/20/2009 01:51 PM, Mark Sylvester wrote:
> > Doesn't this "non-equivalent observer" having observed the flashes 
> > arrive simultaneously, go on to deduce that the events that emitted the 
> > light flashes were not simultaneous since they were not equidistant from 
> > him/her?

Exactly so.