Re: radiometer paradox

[Doug Craigen <dcc@cyberspc.mb.ca>]

On Sat, 14 Jun 1997, Bob Sciamanda wrote:
> Brad Shue wrote:
> > On Sat, 14 Jun 1997, Bob Sciamanda wrote:
> > > Addendum:  If you refrigerate the Crookes radiometer sufficiently, the
> > > thermal effect can be reduced so that the radiation pressure efect
> > > dominates and the thing turns "backwards" (ie., in accordance with
> > > Joseph's expectations) under a strong light.
> >
> > At what temperature is this effect seen?  How do you illuminate it?
>
> Nothing that I was able to do could make the radiation pressure effect
> dominant; however, the thermal effect can make it run "backwards".  When

If I was to guess, I would have thought this advice was backwards itself.
The thermal effect relies on the light generating large relative
temperature shift from one side to the other.  Wouldn't one make thermal
effects less significant by raising rather than lowering the temperature.

Somebody could try the following:  get a collimated light beam (laser or
whatever) and shine it such that the light only hits the white side.  How
much rotation can you get as compared with shining the same light only on
the black side.  Somne of the "white side" rotation may be due to lower
thermal effects, but this could at least give an upper bound on
the relative amount of photon pressure available.  (Mind you we could do
some ballpark calculations too ...)

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| Doug Craigen                                                 |
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| If you think Physics is no laughing matter, think again .... |
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