Re: Distance-dependent time contraction

From: Dirk Van de moortel (dirkvandemoortel_at_ThankS-NO-SperM.hotmail.com)
Date: 06/24/04 

Date: Thu, 24 Jun 2004 22:37:28 GMT

"mich" <mich@efni.com> wrote in message news:10dmka6o8pn4215@corp.supernews.com...
>
> "Dirk Van de moortel" <dirkvandemoortel@ThankS-NO-SperM.hotmail.com> wrote
> in message news:rvxCc.4639$9t2.1064@news.cpqcorp.net...
> >
> > "mich" <mich@efni.com> wrote in message
> news:10dkeib1unj4db4@corp.supernews.com...
> > >
> > > "Dirk Van de moortel" <dirkvandemoortel@ThankS-NO-SperM.hotmail.com>
> wrote
>
> > > in message news:u%gBc.161297$Yb2.8243253@phobos.telenet-ops.be...
> > > >
> > > > "mich" <mich@efni.com> wrote in message
> > > news:10db3ut6mqs0g97@corp.supernews.com...
> > > > >
> > > > > "Dirk Van de moortel" <dirkvandemoortel@ThankS-NO-SperM.hotmail.com>
> > > wrote
> > > > > in message news:0gYAc.160388$BL2.8269913@phobos.telenet-ops.be...
>
> I went through our posts and I found something that you wrote:
>
> ________________________________________________________
> You wrote
> Remark:
> In order to measure lengths you don't actually need rods.
> You can measure lengths with light and a clock only,
> using the assumed fact (postulate) that light speed is
> always the same. Send a signal, wait for the echo, find
> the time between sending and receiving, and you know
> how far away (and when) the signal hit the target.
> To use this system for measuring a moving rod requires
> two signals and echoes, in such a way that the front and
> the back of the moving rod are hit by your signals at the
> same time (time measured by yourself of course!).
>
> _______________________________________________
>
> The second explanation I wrote seems to match yours, although I may not have
> been as clear.
> _____________________________________________
>
> a quote from you:
>
> "To use this system for measuring a moving rod requires
> two signals and echoes, in such a way that the front and
> the back of the moving rod are hit by your signals at the
> same time (time measured by yourself of course!)."
> _________________________________________
>
> The reason why I elaborated a little bit is due to the difficulty of knowing
> whether the two signals hit the front and back of the moving rod at the same
> time.So here's my second explanation again, and I'll wait for your response.

Very good! You found exactly what you needed to solve it,
and there really is no need to repeat that explanation.
You just found it!

Look:
        "To use this system for measuring a moving rod requires
        two signals and echoes, in such a way that the front and
        the back of the moving rod are hit by your signals at the
        same time (time measured by yourself of course!)."
That's all there is to it.
You can measure the distances and the times of the reflection
events on both sides of the rod, so just make sure that both
times are the same, and then you take the difference of the
distances of the reflection events, and hey presto!

Recall the definition of "the time of an event" as
        t = (te+ts) / 2
and distance as
        x = c*(te-ts)/2.

So for the reflection event on the near side of the rod you
calculate
        t1 = (te1+ts1)/2 and x1 = c*(te1-ts1)/2
and likewise for the other reflection event on the far side
        t2 = (te2+ts2)/2 and x2 = c*(te2-ts2)/2 .
So, if you arrange for things such that
        t1 = t2 [ in other words delta(t) = t2-t1 = 0 ],
then the length of the rod is given by the difference of the
distances of the near and far side:
        delta(x) = x2 - x1

It works for a moving rod, and for a non-moving rod it works
just as well of course. It's *that* simple.
Nice, isn't it?

Dirk Vdm

Relevant Pages

  • Re: Distance-dependent time contraction
    ... To use this system for measuring a moving rod requires ... the back of the moving rod are hit by your signals at the ... The second explanation I wrote seems to match yours, ... >> At the instant I stop receiving the reflected signal, ...
    (sci.physics.relativity)
  • Re: Distance-dependent time contraction
    ... >> the back of the moving rod are hit by your signals at the ... > You can measure the distances and the times of the reflection ... > So for the reflection event on the near side of the rod you ...
    (sci.physics.relativity)