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Re: The overpass

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Tom Roberts wrote:

> On 8/13/17 8/13/17   2:51 AM, LuigiFortunati wrote:
>> On the overpass C the head A of the train AB encounters the head N of the
>> train MN traveling in the opposite direction:
>> -------------M------->N------------------------
>> ----------------------C---------------------->x
>> ----------------------A<-------B---------------
>> 
>> Since the two trains have the same length and speed, in the land
>> reference also the tail B of the train AB encounters the queue M of the
>> train MN on the same overpass C:
>> ----------------------M------->N---------------
>> ----------------------C---------------------->x
>> -------------A<-------B------------------------
>> 
>> At that point, from the tail of both trains a bomb destroys the flyover
>> but the trains are now gone and so they are safe.
> 
> OK. That describes the physical situation. ALL reference frames will agree
> that A and N reach C simultaneously, and all frames agree that B and N
> reach C simultaneously, and the overpass is destroyed after both train are
> gone.

How did you get that idea?  Maybe they agree on the two events separately, 
but they cannot agree on both of them.

As I have shown in my picture, if the observers agree on the distance that 
the rear ends of the trains have from C, they must disagree on the distance 
that the front ends of the trains have from C, and so if both trains move at 
the same speed towards C, they must disagree about the simultaneity of the 
arrival of the front ends of the trains at C.  For C they arrive 
simultaneously, for AB and MN they do not as *only* the respective *other* 
train is shorter due to length contraction.

Likewise, if they instead agree that both trains’ front ends arrive 
simultaneously at C, they must disagree about the simultaneity of the 
trains’ rear ends passing C, which sets off the bomb.
 
> Simultaneous happenings that occur at a single event in spacetime are
> simultaneous in all frames. Here there are two separate events: A and N
> reach C, and M and B reach C -- these two events have timelike separation
> and therefore their order is independent of frame.

Their order yes, but not their respective time.
 
> It is only for spacelike-separated events that their order is frame
> dependent. There are no such events here.

The moving ends of the trains are space-like separated events, so is the 
distance between the front end of a train and C and the distance between
the rear end of a train and C.

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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Thread

The overpass LuigiFortunati <fortunati.luigi@gmail.com> - 2017-08-13 09:51 +0200
  Re: The overpass Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-13 15:53 +0200
    Re: The overpass Luigi Fortunati <fortunati.luigi@gmail.com> - 2017-08-13 19:13 +0200
      Re: The overpass The Starmaker <starmaker@ix.netcom.com> - 2017-08-13 11:02 -0700
  Re: The overpass Tom Roberts <tjroberts137@sbcglobal.net> - 2017-08-13 09:06 -0500
    Re: The overpass Luigi Fortunati <fortunati.luigi@gmail.com> - 2017-08-13 19:07 +0200
    Re: The overpass Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-13 19:40 +0200
      Re: The overpass Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-13 21:32 +0200
        Re: The overpass Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-13 21:45 +0200
          Re: The overpass Prudence Oppenheimer <phow@innfldo.sp> - 2017-08-13 21:32 +0000
            Re: The overpass Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-14 00:34 +0200
      Re: The overpass Tom Roberts <tjroberts137@sbcglobal.net> - 2017-08-13 23:03 -0500
        Re: The overpass Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-14 15:52 +0200
          Re: The overpass Tom Roberts <tjroberts137@sbcglobal.net> - 2017-08-14 15:21 -0500
            Re: The overpass Tom Roberts <tjroberts137@sbcglobal.net> - 2017-08-14 16:43 -0500
    Re: The overpass "Ross A. Finlayson" <ross.finlayson@gmail.com> - 2017-08-13 17:14 -0700
      Re: The overpass Tom Roberts <tjroberts137@sbcglobal.net> - 2017-08-13 23:04 -0500
        Re: The overpass "Ross A. Finlayson" <ross.finlayson@gmail.com> - 2017-08-14 19:55 -0700

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