Re: Sensing light at rt angles to it direction?
From: Old Man (nomail_at_nomail.net)
Date: 09/23/04
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Date: Thu, 23 Sep 2004 14:31:44 -0500
"Ken S. Tucker" <dynamics@vianet.on.ca> wrote in message
news:2202379a.0409222042.870d063@posting.google.com...
> "Old Man" <nomail@nomail.net> wrote in message
news:<i8udnXZ5drh-QszcRVn-vA@prairiewave.com>...
> > "Bill" <bill.thomson@tka.co.za> wrote in message
> > news:4b1e845e.0409212310.7e282c40@posting.google.com...
> > > Is it possible to detect/sense/observe 'a beam of light' at right
> > > angles to its direction of propagation without interference? Some
> > > pointers would be appreciated.
> > >
> > > Bill
> >
> > For a parallel beam, no, and measurements always involve
> > a physical interaction wherein action yields reaction.
> >
> > A diverging light beam carries inertial mass and inertial
> > mass gravitates. In principle, one could measure the
> > gravitational force on an adjacent test mass to detect the
> > proximity of a diverging light beam, but the beam will
> > respond by being deflected towards the test mass.
> >
> > This appears to provide a method of detecting light
> > without absorbing or absorbing and re-emitting any of
> > the constituent photons. It doesn't seem right. There
> > ought to be a Law against such transgressions.
> >
> > [Old Man]
>
> Ha, the photon is "red shifted".
> ken
Yes, there's a red shift as the beam recedes from the
test mass, but that's subtracted from the blue shift
on the beam's approach. The gravitational field is
conservative.
However, in the lab frame, the test mass experiences
a net integrated impulse that's directed towards the
beam, and, energy being conserved, that kinetic energy
has to come from the beam.
So, Tucker is correct: If the test mass is free to move
WRT the beam, the beam necessarily undergoes a shift
to the red.
[Old Man]
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