Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > sci.physics.relativity > #428488 > unrolled thread

The overpass

Back to article view | Back to sci.physics.relativity

Contents

  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

#428488 — The overpass

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.

Instead, in reference to both trains, the passage of trains on the 
overpass is no longer simultaneous and the train that passes first 
destroys the overpass while the other one has not yet passed.

So a train perishes.

In the AB train reference, the MN train is contracted and therefore the 
M tail passes on the overpass (and destroys it) while the tail B of the 
train AB has not yet passed.

In the AB reference you save the MN train (which passes first) and 
destroy the AB train:
----------------------M--->N-------------------
----------------------C---------------------->x
-----------------A<-------B--------------------

In the MN reference you save the AB train (which passes first) and 
destroy the MN train:
-----------------M------->N-------------------
----------------------C---------------------->x
-----------------A<---B--------------------

Which of these scenarios is the right one?

-- 
Luigi Fortunati

Credere e' piu' facile che pensare
Believing is easier than thinking

[toc] | [next] | [standalone]

#428509

LuigiFortunati wrote:
^^^^^^^^^^^^^^
If “Luigi Fortunati” is your real name, you would be well-advised to insert 
a space. character

> 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.
> 
> Instead, in reference to both trains, the passage of trains on the
> overpass is no longer simultaneous and the train that passes first
> destroys the overpass while the other one has not yet passed.
> 
> So a train perishes.
> 
> In the AB train reference, the MN train is contracted and therefore the
> M tail passes on the overpass (and destroys it) while the tail B of the
> train AB has not yet passed.
> 
> In the AB reference you save the MN train (which passes first) and
> destroy the AB train:
> ----------------------M--->N-------------------
> ----------------------C---------------------->x
> -----------------A<-------B--------------------
> 
> In the MN reference you save the AB train (which passes first) and
> destroy the MN train:
> -----------------M------->N-------------------
> ----------------------C---------------------->x
> -----------------A<---B--------------------
> 
> Which of these scenarios is the right one?

[The proper term is “reference _frame_” or “frame of reference”; and one 
does not say “in reference to”, but “in the (reference) frame of” or “from 
the perspective of” or simply “for”.]

The first scenario is *basically* the right one (but see below), because you 
have not considered length contraction *properly* in the second and third 
scenario.

For AB, MN is shorter *all the way*, so if both trains start with their rear 
ends the same distance away from C (as measured by C), then MN’s front has 
not yet reached C when AB’s front already has.  Thus, AB has enough time to 
pass C:

1.
  -------------M--->N----------------------------
  ----------------------C---------------------->x
  ----------------------A<-------B---------------

2.
  ---------------M--->N--------------------------
  ----------------------C---------------------->x
  --------------------A<-------B-----------------

3.
  -----------------M--->N------------------------
  ----------------------C---------------------->x
  ------------------A<-------B-------------------

4.
  --------------------M--->N---------------------
  ----------------------C---------------------->x
  ---------------A<-------B----------------------

5.
  ----------------------M--->N-------------------
  ----------------------C---------------------->x
  -------------A<-------B------------------------

For MN, AB is shorter *all the way*, so AB’s front has not yet reached C 
when MN’s front has.  Thus, MN has enough time to pass C:

1.
  -------------M------->N------------------------
  ----------------------C---------------------->x
  --------------------------A<---B---------------

2.
  ---------------M------->N----------------------
  ----------------------C---------------------->x
  ------------------------A<---B-----------------

3.
  -----------------M------->N--------------------
  ----------------------C---------------------->x
  ----------------------A<---B-------------------

4.
  -------------------M------->N------------------
  ----------------------C---------------------->x
  --------------------A<---B---------------------

5.
  ----------------------M------->N---------------
  ----------------------C---------------------->x
  -----------------A<---B------------------------

However, you have also neglected length contraction from the perspective of 
an observer in C:

1.
  -------------M----->N--------------------------
  ----------------------C---------------------->x
  ------------------------A<-----B---------------

2.
  ---------------M----->N------------------------
  ----------------------C---------------------->x
  ----------------------A<-----B-----------------

3.
  -----------------M----->N----------------------
  ----------------------C---------------------->x
  --------------------A<-----B-------------------

4.
  -------------------M----->N--------------------
  ----------------------C---------------------->x
  ------------------A<-----B---------------------

5.
  ----------------------M----->N-----------------
  ----------------------C---------------------->x
  ---------------A<-----B------------------------

[You assumed the rest length of a train to be 10 characters, and the moving 
length in the other train’s reference frame to be 6 characters. So the 
moving length in C’s frame should be less then the rest length, but more 
than 6 characters; here, e.g., 8 characters.]

HTH

-- 
PointedEars

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

[toc] | [prev] | [next] | [standalone]

#428527

Thomas 'PointedEars' Lahn alle ore 15:53:17 del 13/08/2017 ha scritto:
> LuigiFortunati wrote:
> ^^^^^^^^^^^^^^
> If “Luigi Fortunati” is your real name, you would be well-advised to insert 
> a space. character

  Done.

>
> [The proper term is “reference _frame_” or “frame of reference”; and one 
> does not say “in reference to”, but “in the (reference) frame of” or “from 
> the perspective of” or simply “for”.]

Ok thanks to my english is not good and these corrections help me.

Tom Roberts's response was enough to me.

Bye.

-- 
Luigi Fortunati

Credere e' piu' facile che pensare
Believing is easier than thinking

[toc] | [prev] | [next] | [standalone]

#428539

Luigi Fortunati wrote:
> 
> Thomas 'PointedEars' Lahn alle ore 15:53:17 del 13/08/2017 ha scritto:
> > LuigiFortunati wrote:
> > ^^^^^^^^^^^^^^
> > If “Luigi Fortunati” is your real name, you would be well-advised to insert
> > a space. character
> 
>   Done.
> 
> >
> > [The proper term is “reference _frame_” or “frame of reference”; and one
> > does not say “in reference to”, but “in the (reference) frame of” or “from
> > the perspective of” or simply “for”.]
> 
> Ok thanks to my english is not good and these corrections help me.
> 
> Tom Roberts's response was enough to me.
> 
> Bye.
> 
> --
> Luigi Fortunati
> 
> Credere e' piu' facile che pensare
> Believing is easier than thinking



Strange....that Luigi and Tom Roberts seem to be one and the same person.


Did Tom Roberts posted a question (in a different handle), and then answered it with his handle?


Why would someone do something like that????

[toc] | [prev] | [next] | [standalone]

#428511

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.

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.

It is only for spacelike-separated events that their order is frame dependent. 
There are no such events here.

Tom Roberts

[toc] | [prev] | [next] | [standalone]

#428526

Tom Roberts alle ore 16:06:41 del 13/08/2017 ha scritto:
>> 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.
>
> 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.
>
> It is only for spacelike-separated events that their order is frame 
> dependent. There are no such events here.
>
> Tom Roberts

Ok, grazie, bye

Luigi Fortunati

-- 
Luigi Fortunati

Credere e' piu' facile che pensare
Believing is easier than thinking

[toc] | [prev] | [next] | [standalone]

#428531

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.

[toc] | [prev] | [next] | [standalone]

#428551

Thomas 'PointedEars' Lahn wrote:

> Tom Roberts wrote:
>> 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.

This was badly worded.  I meant to say: The moving ends of the trains – A 
and B, and M and N – and C are *all* spacelike-separated events.
 
-- 
PointedEars

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

[toc] | [prev] | [next] | [standalone]

#428554

Thomas 'Ingrid' Lahn wrote:

> Thomas 'PointedEars' Lahn wrote:
>> Tom Roberts wrote:
>>> 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.
> 
> This was badly worded.  I meant to say: The moving ends of the trains – A
> and B, and M and N – and C are *all* spacelike-separated events.

Finally,

“The ends of the moving trains – A and B, and M and N, respectively – and C 
are *all* spacelike-separated events.”

should remove all ambiguity :)
  
-- 
PointedEars

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

[toc] | [prev] | [next] | [standalone]

#428566

Thomas 'PointedEars' Lahn wrote:

>> This was badly worded.  I meant to say: The moving ends of the trains –
>> A and B, and M and N – and C are *all* spacelike-separated events.
> 
> Finally, “The ends of the moving trains – A and B, and M and N,
> respectively – and C are *all* spacelike-separated events.”
> should remove all ambiguity

Still wrong, doesn't remove anything. The observer is also spacelike. And 
the most important. You keep making a fool of yourself.

[toc] | [prev] | [next] | [standalone]

#428569

The ’nym-shifting troll trolled as "Prudence Oppenheimer" <phow@innfldo.sp>:

> Thomas 'PointedEars' Lahn wrote:
>>> This was badly worded.  I meant to say: The moving ends of the trains –
>>> A and B, and M and N – and C are *all* spacelike-separated events.
>> Finally, “The ends of the moving trains – A and B, and M and N,
>> respectively – and C are *all* spacelike-separated events.”
>> should remove all ambiguity
> 
> Still wrong, doesn't remove anything. The observer is also spacelike.
> And the most important. You keep making a fool of yourself.

That is just your usual pseudo-scientific word salad.

Instead, two _events_ can be spacelike-_separated_, namely when they are

  e₁ = (x₁, t₁)

  e₂ = (x₂, t₂)

and

  c (t₂ − t₁)² − (x₂ − x₁)² < 0,

where c is the speed of light.

*PLONK*

F’up2 trash can

-- 
PointedEars

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

[toc] | [prev] | [next] | [standalone]

#428605

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, 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.
Since #1 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. Ditto for #2. 
(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.)

> 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. The trains move at the same speed ONLY in the rest 
frame of C, but you applied that in the frame of each train.

>> 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.

Yes, an observer on train AB will measure train MN as shorter than her own. She 
will also measure it as moving at a slower speed relative to C than her own 
train. 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.

Tom Roberts

[toc] | [prev] | [next] | [standalone]

#428654

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.

[toc] | [prev] | [next] | [standalone]

#428738

On 8/14/17 8:52 AM, Thomas 'PointedEars' Lahn wrote:
> 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.

No, relativity of simultaneity does NOT apply to happenings that occur at a 
single event in spacetime. You merely repeated your earlier error.

>> 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.

You mean "Yes, the description implies...", but your "*and*" joins two separate 
events in a way that can be confusing (which is why I explicitly separated them 
and named them #1 and #2).

	Part of the problem could be English -- you use C as if it
	were a frame, but it is really a specific place. There
	is a frame in which C is at rest, and that frame is commonly
	also called "C".

The problem stated that to an observer at C, N and A reach C simultaneously (my 
event #1). As C is a single place, this means they happened AT A SINGLE EVENT IN 
SPACETIME. That means that ALL observers see N reaching C and A reaching C as 
happening at a single event in spacetime, so they ALL agree that A reaching C is 
simultaneous with N reaching C (they also agree that this happened at a single 
location).

The problem stated that to an observer at C, M and B reach C simultaneously (my 
event #2). As C is a single place, this means they happened AT A SINGLE EVENT IN 
SPACETIME. That means that ALL observers see M reaching C and B reaching C as 
happening at a single event in spacetime, so they ALL agree that B reaching C is 
simultaneous with M reaching C (they also agree that this happened at a single 
location).

Yes, different observers measure different time intervals between event #1 and 
event #2. And except for observer C they see #1 and #2 happening at different 
locations. But they all agree that event #1 is a single event, and that event #2 
is a single event. And they all agree that event #1 includes BOTH N reaching C 
and A reaching C, and they all agree that event #2 includes BOTH M reaching C 
and B reaching C.

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

That is wrong. All observers agree on both #1 and #2. Remember that each of 
those events includes two happenings that occur AT THE SAME EVENT IN SPACETIME. 
Relativity of simultaneity does not separate happenings that occur at the same 
event in spacetime.

	Just THINK a bit: A reaching C MANIFESTLY happens at the exact
	same place and time as C reaching A (this is just part of #1).
	How could relativity of simultaneity possibly "separate" these
	two happenings that occur at a single event in spacetime? --
	after all they are MANIFESTLY two descriptions of A SINGLE
	happening (the meeting of A and C). But N reaching C occurs at
	that same event, so it cannot be separated, either. Ditto for #2.

I repeat: relativity of simultaneity does not apply to happenings that occur at 
a single event in spacetime. This applies to #1, and it applies (separately) to 
#2. All observers agree that the happenings of #1 occur together (both 
co-located and simultaneous), and also that the happenings of #2 occur together 
(both co-located and simultaneous). Nobody thinks #1 and #2 are simultaneous, 
and everybody agrees #1 happened before #2.

Tom Roberts

[toc] | [prev] | [next] | [standalone]

#428754

On 8/14/17 3:21 PM, Tom Roberts wrote:
> [...]
>      Part of the problem could be English -- you use C as if it
>      were a frame, but it is really a specific place. 

Misstatement. I meant that C is a specific OBJECT (an overpass), which is 
assumed to be pointlike. This object is, of course, at a specific place in its 
rest frame.

Tom Roberts

[toc] | [prev] | [next] | [standalone]

#428583

On Sunday, August 13, 2017 at 7:06:49 AM UTC-7, tjrob137 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.
> 
> 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.
> 
> It is only for spacelike-separated events that their order is frame dependent. 
> There are no such events here.
> 
> Tom Roberts

It goes through the transfer of their reference frames.

This is for the objects in the geodesy, 
the trains passing in the relativistic 
are relativistic in the inertial content 
of their frames.

Then they're also relativistic in each 
others' moving frames.

This is that the trains pass each other 
in the middle.

This is that the relativistic arrives 
from the moving frames (in the relativistic, 
for example motion).

Interference or disruption of the moving 
stages of objects has basically considering 
them at rest together.

Abruptly.

[toc] | [prev] | [next] | [standalone]

#428606

On 8/13/17 8/13/17   7:14 PM, Ross A. Finlayson wrote:
> [... word salad and gobbledygook]

You REALLY need to learn what the words you use ACTUALLY mean. And how to apply 
them to physical situations and theories.

Tom Roberts

[toc] | [prev] | [next] | [standalone]

#428802

On Sunday, August 13, 2017 at 9:05:05 PM UTC-7, tjrob137 wrote:
> On 8/13/17 8/13/17   7:14 PM, Ross A. Finlayson wrote:
> > [... word salad and gobbledygook]
> 
> You REALLY need to learn what the words you use ACTUALLY mean. And how to apply 
> them to physical situations and theories.
> 
> Tom Roberts

Am I not making sense again?


Makes perfectly good sense to me.

Excuse me then if I am not making sense to you.

Here it's the written word, "if it doesn't make 
sense, don't go making nonsense out of it".

Here you might at least agree I have a hedged opinion 
that what as far as I can tell, largely, what there is 
there is the "dynamic behavior of relativistic systems 
of matter" or here the gravity.

So, it's usually the written, spoken word.

Words and what they mean.

Also "how to apply them to physical situations and theories".

No, if I'd efface you, you shouldn't criticize me like 
I would stare at your face.

Also then a usual reference to my agen-da.

If there's one thing that's very well established in my opinion, 
it's my opinion.

If it would help, I guess it's not so often people actually 
post "questions" on the usenet where it takes such firm believers 
in their own opinion to maintain an edifice of sage (or sagacity).

Learning physics is here all through the (usually) academic  
course really as it were where this is just fundamental 
theoretical physics, par for the course for usual directions 
in the applied, this just being Maxwell and here you go.

Maxwell and relativity, right.

Chemistry all that even power physics is way above all that there.

States of matter and particle physics and all that has here the 
states of matter and all that from the surface forces of the 
solids, liquids, gases, etcetera.  

Here this is indeed a very "theoretical" physics.

Theoretical and "fundamental".

[toc] | [prev] | [standalone]

Back to top | Article view | sci.physics.relativity

csiph-web