Re: Einstein's Train Gedanken Re-visited

On May 27, 11:29 am, G <gehan.ameresek...@xxxxxxxxx> wrote:
On May 27, 3:50 pm, Cephalobus_alie...@xxxxxxxxxxx wrote:

On May 25, 11:03 am, G <gehan.ameresek...@xxxxxxxxx> wrote:

I think the problem is that one could say that the source of the
lightning srikes move with respect to both observers.

In Einstein's gedanken, a lightning strike stands for an event.

An event is a single point in spacetime defined by its x,y,z,t
coordinates.

Each and every observer's  x,y,z and t

Events do not "move".

When Manny, the track embankment observer, stretches a measuring
tape to the scorch marks on the ground, he is NOT measuring the
"distance" (or rather, the interval) between himself and the
lightning strikes. It is impossible for him to measure these
intervals with a measuring tape. The lightning strikes happened
in the past, and however hard he tries, Manny cannot twist and
turn the measuring tape to stretch from his current position to
the events which occurred in the past.

Manny can only stretch his measuring tape to the PROJECTIONS of
these events onto his frame.

Say Manny observered that the lightning struck points on the ground
that he had uniquely marked with a lightning rod. He previosly
measured the distance to each as x metres. When lightning strikes, and
the image on his retina is recognized as his beloved lightning rods,
instantly ( but not faster than light) he knows the distance to the
strikes. Why is this thinking not admissible?



Likewise, in measuring the distance between herself and the
crackedtrainwindows, Mary' is only measuring the distance
between the PROJECTIONS of the lightning strike events and her
position in thetrainframe.

The above statements apply. Mary knows the train is 50 metres long



The scorch marks on the ground, the cracks on thetrainwindows,
these are shadows of the actual events, and yes, these shadows
move along with the observers. But the actual events do not.

Where are the actual eventts located?

In the past.

If we draw an xt diagram

| |
t=1----o--------M-----
| |
| |
| |
t=0 ---*--------M-----
| |
| |

The strike event happen at * when M was at t=0 while o is where the
scorch marks are on the tracks when M is at t=1. The event is at *
not o, so if someone wants to split hairs M cannot measure the
distance to the event, he measures the distance to the space
coordinate where the event took place in his coordinate system.

It is a subtle distinction. I assume this was brought up to make the
distiction between an event and its assigned location. Put A standing
by the tracks where the strike hits, and A' on the train where the
strike hits. After the strike track observers say the strike happen
at A while train observers say it happen at A'. The two are not at
the same place after the strike.

IMHO it is ok to say Manny measured the distance to the point where
the strike hit.



However
the movement of the source does not make any difference in SRT.

Again, the movement of the source does not make any difference to
observers measuring the speed of light.

Your problem is that you do not WANT to believe the results of
simple logic.

Try me



1) Manny can see Mary's face.

Yes

2) Manny sees first one side, then the other side of Mary's face
   illuminated by the lightning strikes.

This is a stipulation then

No, it is a result of relative positions. Look at the setup after M
and M' meet.

A A' > M M' < B B'

There is one wave front headed from the strike at B/B' toward M' and
M. It doesn't matter if you consider that wave to have come from B or
B', it is going to hit M' before it hits M. It was given that both
waves hit M at the same instant. There is no way the wave from A/A'
can reach M' until after it hits M, so it could not hit M' before M
like the wave from B/B' did. The waves had to reach M' at different
times, one before they both hit M and the other after.

3) This observation shows that Mary' saw the strikes at different
   times.

OK

4) Mary' can measure the distances between herself and the cracks
   on thetrainwindows.

OK

5) Her measurements show that she was equidistant between the two
   strikes.

OK

6) Experimentally, light has been shown to travel at c regardless
   of the motion of source and observer.

Assume that to be true - I will give my reasons

7) The only way to reconcile (3), (5) and (6) is for the lighting
   strikes to have occurred at different times as measured in
   Mary's frame.

Logically there is no reason to assume (3) , (5) and (6) are
reconcilable

But it has been shown that they are, so long as you are willing to let
go of absolute time.

Logically, you stipulated (2)


The stipulation that the strikes were simultaneous in the track fame
dictates it. It is not a seperate stipulation.


However much your inner instinct revolts at the conclusion (7),
it is the only logical outcome.

Jerry

How about this:

Mary sees the flashes as simultaneous
Manny sees the flashes as simultaneous

Can't happen. Both flashes arrive at M at the same instant. The
flash from B/B' has already passed M' while the flash from A/A' has
het to reach M'.

Manny sees Mary
Since we worked from (1) where Mary sees the flashes as simultaneous
Manny sees the flashes reach Mary simultaneously

There is no way to reconcile these logically and hang on to
the 2nd postulate

There is if you give up absolute time. In the frame of M' the flashes
arrive at M' at different times because the strikes happen at
different times. Both frames still measure the speed of light to be c
in all directions.

.

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