Re: On the observation of a moving beam of light.
From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 11/20/04
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Date: Sat, 20 Nov 2004 22:55:22 GMT
"kenneth couesbouc" <kencouesbouc@yahoo.fr> wrote in message
news:1c9636fc.0411200729.7d999dd0@posting.google.com...
> I apologise for the poor quality of the drawings.
>
>
> 1. Let's suppose that a coherent light projector, A, and a target, B,
> are fixed at either end of a rod. The rod and its fixtures are moving
> lengthwise from left to right, as is the observer, at a constant speed
> s.
>
>
>
> A' A B' B
> --------------_____________________
> ---> s
>
>
>
>
> . O
>
> ---> s
>
>
> The perceived distance A'B' varies according to the observer's
> position, as for a simple rod. What of the light beam ?
>
>
>
>
> A" A B" B
> ........._____________________
>
> ---> s
>
>
> By the time the light from the projector at A" reaches the target at
> B, A" has moved to A and B" to B. (A"A = B"B). This means that the
> light shining on B (the target's present position) comes from A" (the
> projector's past position). Meanwhile, a light beam is being emitted
> by A (the projector's present position). This can only mean that the
> beam of light from A joins up with the beam of light from A",
> somewhere between A and B.
That does not follow. Since the speed of light is always that same a light
beam emitted form a moving source will always lag behind one emitted from
the same source some time ago - forward is always forward.
Bill
> However, the projector does not move instantaneously from A" to A.
> To do so, it passes through a vast number of intermediary stages (An,
> ..., A3, A2, A1) and emits light from each one of them. The beam of
> light between A and B consists of all these brief emissions from past
> positions of A. The oldest comes from A" and lights up the target B.
> The most recent comes from A.
>
>
>
> A" An ... A3A2A1A A A1 A2 A3 ... An A"
> . . ... . . . . - - - - - - - -
>
>
>
>
> We've said that the observer perceives A'B' (the past images of AB)
> as longer, shorter, or equal to AB. He therefore perceives the beam of
> light in the same way. This does not mean, however, that the light
> emissions go faster or slower, nor does it mean that there is a time
> warp. It merely means that he perceives wave-lengths that are longer,
> shorter, or equal to those of the actual light beam, depending on
> where he places himself.
> If the beam of light AB and the observer are not moving at the same
> speed, either the beam is going faster than the observer, or it is
> going slower. If the beam is going faster, it overtakes the observer
> from left to right and appears to get shorter. If the beam is going
> slower than the observer, its relative movement is from right to left
> and it appears to get longer. In neither case is the perceived beam of
> light A'B' equal in length to the real beam of light AB when A'O =
> B'O. For A'B' = AB, A'O < B'O, as for the simple rod.
>
>
>
>
>
> 2. The lengthwise motion of a beam of light divides it up into a
> series of short successive impulses. But this longitudinal movement
> does not modify the actual length of the beam, nor its direction.
> Let's now suppose that the rod and the beam of light are moving
> sideways.
>
>
> B" B
> : !
> : ; !
> : ; !
> : ; !
> : ; ! ----> s
> : ; !
> : ; !
> : ; !
> : ;!
> : !
> A" A
>
>
>
>
> In this case, by the time the light from the projector at A" reaches
> the target at B", A" has moved to A and B" to B. (A"A = B"B). This
> means that all the segments of light, emitted by the intermediary
> positions of the projector, join A to B". Instead of A to B as was the
> previous case.
>
> B" B
> ;A" !
> : ;An !
> : . !
> : . !
> : . ! ---> s
> : ;A3 !
> : ;A2 !
> : ;A1!
> : ;A
> A" An ... A3 A2 A1 A
>
>
>
> When the beam of light moves laterally, it is bent backward and,
> instead of shining on B, it shines on B", a past position of B.
> Moreover, this bending back of the beam lengthens it (AB" > AB). Here
> again, the speed of light is unmodified, nor is there a time warp. The
> light shining on B is simply of a longer wave-length than the light
> that shone on B, when the beam was moving lengthwise.
> Can this be tested experimentally ?
> By using the speed at which we are moving round the galaxy (200
> Km/s) the respective lengths of B"B and AB would be of the order of 1
> : 1500. It seems to me that these proportions make the experiment
> feasible. At least for the measuring of B"B. The spectrography of the
> light shining on B" is something else. It needs a spectrograph.
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