[ExI] QT and SR

Jeff Davis jrd1415 at gmail.com
Sun Aug 10 07:46:39 UTC 2008

John, I'm trying to visualize this.  Help me out with one
clarification, if you will.

Two  ships connected by a ten meter string. (Not thirty meters.) Ships
and string set to travel parallel to the line of the ships and string.
 Two fellows at either end of the string with laser lights and clocks
prepared to make measurements.

To help me visualize this, when you say below "Look at it from the
point of view of an observer at one end of the string", which end of
the string are you thinking of, the trailing or the leading end?  And
I take it the string has some stretch to it, so that a variation in
length in either the shrinking or stretching sense, will be
measurable, with neither sagging nor instantaneous breakage upon
stretching, yes?


On Sat, Aug 9, 2008 at 10:43 PM, John K Clark <jonkc at bellsouth.net> wrote:
> "Jeff Davis" <jrd1415 at gmail.com>
>> Such an observer would see the entire assembly shrink
>> proportionately along its length.  No breakage.
> I am absolutely positively 100% certain the string will
> break, I might even be correct; at least that's what my
> intuition insists is true.
> Look at it from the point of view of an observer at one end
> of the string. Suppose a fellow at the far end of the string
> has a clock and sends a pulse of LASER light to you every 10 seconds and
> suppose you also have a clock and it's synchronized with his, so you know
> how long it took the light to reach you, hence you know how far away the
> other end of the string is. The other fellow also reports from time to time
> on how fast he is moving relative to some fixed point that both of you can
> both see.
> An instant after you start moving you receive a report from
> the fellow at the other end of the string saying he hasn't started moving
> yet. Because of this you predict that when you make the next distance
> measurement you will find that
> the distance has decreased, but when the next LASER pulse arrives you find
> that the distance is just the same. You can only conclude that sometime
> after the last report the other fellow started to accelerate and did so
> faster
> than you did.
> When he sends his next report you find he has indeed started to move but he
> still isn't moving as fast as you are, and yet the distance is the same as
> before.  The fellow at the other end of the string must still be
> accelerating faster than you and in fact he always will be.
> Of course an observer at the other end of the string could
> make similar observations and conclude that you are accelerating faster than
> he is. The two observers disagree
> on who started moving first, but both agree that the other
> end of the string is accelerating faster than their end; and that pulls the
> string apart.
> John K Clark
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