# How fast will a radiometer spin in a frictionless enviroment?

#### carl fischbach

If you place a radiometer in a frictionless
enviroment and eliminate mechanical failure how
fast will it spin?
Since the frequency of c is very high, the
doppler shift of light reaching the white and
black side is virtually negilable, even if the
radiometer is spinning at high speeds,this also
applies to infrared radiation leaving the black
side.Therefore torque acting on the radiometer
doesn't decrease greatly with increasing speed
high speeds? Which begs another question is the
the kenetic energy contained in the rotating
greater than the light energy radiated on to
the radiometer in the first place?

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#### pmb

Originally posted by carl fischbach
If you place a radiometer in a frictionless
enviroment and eliminate mechanical failure how
fast will it spin?
Since the frequency of c is very high, the
doppler shift of light reaching the white and
black side is virtually negilable, even if the
radiometer is spinning at high speeds,this also
applies to infrared radiation leaving the black
side.Therefore torque acting on the radiometer
doesn't decrease greatly with increasing speed
high speeds? Which begs another question is the
the kenetic energy contained in the rotating
greater than the light energy radiated on to
the radiometer in the first place?
Since you're assuming that there are no dissipative forces then the constant torque will keep increasing the angular momentum and thus make it spin faster and faster until the material can't handle the mechanical stresses and it will then fall apart. So it really depends of the material. Otherwise it will spin at a rate at which the speed of all particles is just less than the speed of light. But it would fall apart way before then.

Pete

#### Jonathan

Radiometers don't spin by the momentum of photons. At some point someone proved that it is a result of the temperature differential in the rarified gas caused by the different colors of the paddles. The black side gets a tiny bit hotter than the white/silver side and the temperature differential causes the molecules to hit at a slightly more oblique angle on one of the sides. However, I do think that if we ignore mechanical failures, and after an infinite amount of time, for all intents and purposes its instantaneous tangential velocity will be the speed of light. That'd be one hell of a radiometer. Btw, carl fischbach, I've PM'd you and you haven't responded to that or to the one (and now two) thread(s) of yours that I've posted in, what's that about? I'm interested in what you have to say about angular momentum and vorticies! I notice a recurring theme here with all your threads, what's with it? You post several threads with topics relating to rotational anything and you never really say anything in them.

#### Integral

Staff Emeritus
Gold Member
The black side gets a tiny bit hotter than the white/silver side and the temperature differential causes the molecules to hit at a slightly more oblique angle on one of the sides.
What does this mean? What molecules and how does the temperature of the surface affect the angle they hit at? I thought this was in a vacuum, so what molecules are you talking about?

If the cause of motion is a temperature difference then some form of gas must be present to make anything move.

Edit: Rereading the original post I see that he does not specify a vacuum but says frictionless. Wouldn't that imply a vacuum? I would expect air resistance to be the main friction acting.

Last edited:

#### Jonathan

Yes, if one could make a perfect vacuum in a radiometer, it would quit working. I was assuming that these air molecules where perfectly frictionless. The thing about the angle that I said: I was just repeating from what I remember. I have no idea what that was refering to when I heard it, it didn't make any sense to me now or then. It's too bad I don't know the name of the paper or the guy who wrote it or anything. I am pretty sure this is right, it has to be rarefied or there'd be too much friction, and one can't easily get such a good vacuum that you couldn't get it to work, so maybe this is bunk? I really was sure of it a minute ago.

#### Jonathan

IIRC, part of the proof was that a radiometer spins in the opposite direction than you'd think. You'd think that there'd be two impulses (hit, rebound) on the white/silver side and one (hit, absorb {no rebound}) on the other. But that would make it spin black side forward, which I remember them (I don't know who) saying that they(radiometers) didn't. I've never gotten around to checking though.

#### Jonathan

Oh good, I'm right!

#### pmb

Originally posted by Jonathan
Radiometers don't spin by the momentum of photons.
Correct

One has to completely evacuate the tube instead of leaving a poor vaccum for it to work on radiation pressure alone

Pmb

#### Jonathan

In which case it would spin the other way.

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