OT: Photon sailing

I guess most radiometers, and the ones in school, too, are of this type:

Measuring the radiation pressure is more difficult, but you can do it with this radiometer type:

Right, but the Crookes radiometer works by differential gas pressure. It rotates the wrong way for it to be radiation. The white side leads because the higher temperature on the black side pushes the gas away harder. If it were radiation, the black side would lead--the momentum change would be ~ -E/c per photon on the black side (photons are absorbed) and more like +1.5*E/c on the white side (photons hit and bounce off into a hemisphere). Exact numerical factors depend on where the light is coming from, the shape of the vanes, etc.

Solar sails have much the same issue due to solar wind, which iirc carries a lot more momentum than the light. Since it tends to stick to the sails instead of bouncing off, it's quite hard to tack against.

Cheers

Phil Hobbs

I would also imagine that higher energy impacts would fragment molecular structures, kicking out ions of various kinds, which would drift away for a bit leaving a charged 'system' of some kind, but gradually that charge itself would attract back some of them or other parts of the solar wind, and the eventual developing brown crud itself would be a pain in the ass, long term.

How does one actually design a system to survive effectively for long periods of time in near-sun space? Say on the order of 50 years and more?

Jon

Thin layer of aluminium or gold on the surface of mylar film and a mechanism for replacing it periodically.

You sometimes get comets with two obvious tails. The blue ionised gas tail dominated by solar wind pointing away from the sun and the white dust tail dominated by solar wind.

Hale Bopp showed it clearly see for example:

Regards, Martin Brown

Have you ever read the (detailed) theory on how these (Crookes radiometer) are supposed to work? When I read the explainations it seems like they should spin much faster if you put a bunch of holes in the vanes. And yet I can find anyone who has done the experiment. (That is make two types of vanes and see which has more force.)

(Go to Arxiv.org search for Crookes radiometer for a recent article.)

George H.

If it were radiation, the black side would lead--the momentum

Thanks, George. I read the text but haven't had a chance to read the earlier papers or to study and try and understand them. So the context of the 2004 paper there is currently lost on me, though I _do_ believe I may gather where the words point.

Something bothers me, just a little. In their summary, they suggest that to a rough approximation the additional force should be proportional to the "active area," as defined and they compute this active area for two cases which are more than a million different from each other.

However, the two cases differ in yet another fashion, in practice. The solid vane case is typically used in an evacuated bulb which increases the mean free path to a length consistent with a vane that may be practically constructed from mica sheets. At the same time, gas pressure itself needs to be lowered to achieve that mean free path and in doing so there are fewer molecules operating and this fact due to practicality also lowers the force. (Obviously, with no air... no force... with more air, shorter mean free path vs physical vane design also reduces the force eventually to near zero... so there is a peak here to be found, clearly.) In the proposed case of atmospheric pressure, the density is higher and this means more particles involved. Which I would imagine would be yet another factor in enhancing the force that goes beyond a simple comparison of relative active areas. This isn't mentioned in the summary.

Also, on page 2 and 3, the equation (1) is presented and some of the terms, m and n for example, are discussed on page 2 but not found in equation (1) on top of page 3. Did you happen to pick up where m and n are involved? I fully expect that the mass and density _do_ count. I just can't see the term. Sigma is described as the diameter, so that can't be it.

Anyway, thanks for the article.

Jon

The paper resides at:

The paper is a long winded way of saying that putting holes in a flat plate decreases drag (and in this special case also increases the amount of plate near to an edge that can contribute to the force).

What they are discussing in that last part is the dimensions of a vane with holes that would work at atmospheric pressure. It would be a challenge for silicon microfabricators to make but it might not be impossible.

Really fancy physics lecture Crooke's radiometers with powerful vacuum pumps attached can be pumped down through the peak of maximum force and then on to the point where they stop briefly and then start spinning slowly in the opposite sense when radiation pressure dominates.

They didn't discuss changing the gas, but kinetic energy scales with temperature so you would expect momentum for a monatomic gas

KE = p^2/2m = v^2/2m = 3kT/2

p ~ sqrt(3mkT)

So all things being equal you want the slowest moving heaviest noble gas to have the longest mean free path and the biggest momentum change per collision - probably xenon. Or radon if you like living dangerously - but that will quickly pollute itself with helium...

Using Xenon (mass ~131) instead of nitrogen (mass 28) should give a roughly twofold improvement in the force delivered based on this quick back of the envelope analysis.

A few nice pictures of old ones online at

Regards, Martin Brown

I'm guessing he just has a link to Amazon and gets a commission on sales linked from his site, but all three are "not currently available" MikeK

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Hmm, I think there is a bit more than that. I can imagine two plates with the same drag but different number/ size of holes. And the theory says that one should have a greater force. (The spinning buisness complicates things.) One could also do a static measurment, with some sort of torsional fiber.

I've never seen such things do you have a link? I thought that observing radiation pressure was really hard.... and measuring it even harder.

George H.

Crooke_s_Radiometer.html

AFAIK the video is not online but a description of one such demonstration on a filmloop "The Pressure of Light" is online at:

ISTR it is the version that MIT did.

It is a fun demo if you have access to hard vacuum pumps.

Regards, Martin Brown