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Re: The overpass

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Tom Roberts wrote:

> On 8/13/17 8/13/17   12:40 PM, Thomas 'PointedEars' Lahn wrote:
>> 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.
> 
> You are confused,

No, you are.  The relativity of simultaneity applies to this situation.

> and your discussion considers different events than the
> relevant ones. There are just two relevant events (given in the original
> description):
>   #1: N and A reach C simultaneously.
>   #2: M and B reach C simultaneously.

No, the description implies that *an observer at rest in C* sees N and A 
*and* M and B reach C simultaneously, respectively.

That other observers (not comoving with C) do _not_ observe *complete* 
simultaneity – which is correct – has been explicitly stated by the OP (but 
they did not consider all implications).

> Since #1m,occurs at a SINGLE EVENT in spacetime, all frames will see it as
> a single event, so all frames see N and A reach C simultaneously.

They may, if that is being agreed on, but:

> Ditto for #2.

AISB, *all* observers can only agree on either #1 or #2, but _not_ on both.

> (They all differ on the value of the time coordinate when each
> happens, and on the elapsed time between #1 and #2; all see #1 happen
> before #2 because they have timelike separation.)

No.
 
>> 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.
> 
> You have intermixed frames.

No.

> The trains move at the same speed ONLY in the rest frame of C,

Correct.

> but you applied that in the frame of each train.

No, I do not think so.  

Of course the speed of the train in its own frame is zero, but then the 
speed of C in that frame is not.
 
>>> 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,
> 
> Yes, BUT IRRELEVANT.

No, very relevant.  The relativity of simultaneity applies to those events.
 
> Yes, an observer on train AB will measure train MN as shorter than her
> own.

Yes.

> She will also measure it as moving at a slower speed relative to C
> than her own train.

This statement does not make sense.  If we measure the speed of MN relative 
to C, then we are measuring it *in the frame of C*.  If we measure AB’s 
speed relative to C, then we are measuring it *in the frame of C*, not of 
AB.

> The result is she will see N reach C right as A reaches C, and she will
> see M reach C just as B reaches C. Similarly for an observer on train MN.

No, I do not think so.

-- 
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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