Editing Crookes radiometer (section)

==Explanations for the force on the vanes ==<!-- This section is linked from [[Crookes radiometer]] -->
Over the years, there have been many attempts to explain how a Crookes radiometer works:

===Incorrect theories===
Crookes incorrectly suggested that the force was due to the [[radiation pressure|pressure of light]].<ref>{{cite journal |first=William |last=Crookes |author-link=William Crookes |date=1 January 1874 |doi=10.1098/rstl.1874.0015 |title=On Attraction and Repulsion Resulting from Radiation |journal=Philosophical Transactions of the Royal Society of London |volume=164 |pages=501–527|s2cid=110306977 |url=https://zenodo.org/record/1432450 |doi-access=free }}.</ref> This theory was originally supported by [[James Clerk Maxwell]], who had predicted this force. This explanation is still often seen in leaflets packaged with the device. The first experiment to test this theory was done by [[Arthur Schuster]] in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. This showed that the force turning the vanes was generated inside the radiometer. If light pressure were the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin. In 1901, with a better vacuum pump, [[Pyotr Nikolaevich Lebedev|Pyotr Lebedev]] showed that in fact, the radiometer only works when there is low-pressure gas in the bulb, and the vanes stay motionless in a hard vacuum.<ref name=":0">{{Cite journal|last=Lebedew|first=Peter|year=1901|title=Untersuchungen über die Druckkräfte des Lichtes|journal=Annalen der Physik|volume=311|issue=11|pages=433–458| doi=10.1002/andp.19013111102|bibcode=1901AnP...311..433L|url=https://zenodo.org/record/1424005}}</ref> Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction, as the [[photon]]s on the shiny side being reflected would deposit more momentum than on the black side, where the photons are absorbed.  This results from [[conservation of momentum]] – the momentum of the reflected photon exiting on the light side must be matched by a [[reaction (physics)|reaction]] on the vane that reflected it.  The actual pressure exerted by light is far too small to move these vanes, but can be measured with devices such as the [[Nichols radiometer]]. It is in fact possible to make the radiometer spin in the opposite direction by either heating it or putting it in a cold environment (like a freezer) in absence of light, when black sides become cooler than the white ones due to the thermal radiation.

Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was later effectively disproved by both Schuster's experiments<ref>{{Cite journal |jstor = 27757296|last1 = Brush|first1 = S. G.|title = Maxwell, Osborne Reynolds, and the Radiometer|journal = Historical Studies in the Physical Sciences|volume = 1|pages = 105–125|last2 = Everitt|first2 = C. W. F.|year = 1969|doi = 10.2307/27757296}}</ref> (1876) and Lebedev's (1901)<ref name=":0" />

===Partially correct theory===
A partial explanation is that gas [[molecule]]s hitting the warmer side of the vane will pick up some of the heat, bouncing off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side and as a result the vanes spin round with the black side trailing. The problem with this idea is that while the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the net force on the vane should be the same. The greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, [[Albert Einstein]] showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough.<ref>{{cite book |last1=Calaprice |first1=Alice |title=An Einstein encyclopedia |date=27 October 2015 |publisher=Princeton University Press |isbn=978-0691141749 |page=190|display-authors=etal}}</ref>

===Currently accepted theory===
The currently accepted theory was formulated by [[Osborne Reynolds]], who theorized that [[thermal transpiration]] was the cause of the motion.<ref>{{cite journal |first=Osborne |last=Reynolds |author-link=Osborne Reynolds |date=1 January 1879 |doi=10.1098/rstl.1879.0078 |title=On certain dimensional properties of matter in the gaseous state … |journal=Philosophical Transactions of the Royal Society of London |volume=170 |pages=727–845}}; Part 2.</ref> Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behaves like the pores in Reynolds's plate. As gas moves from the cooler to the hotter side, the pressure on the hotter side increases. When the plate is fixed, the pressure on the hotter side increases until the ratio of pressures between the sides equals the square root of the ratio of absolute temperatures. Because the plates in a radiometer are not fixed, the pressure difference from cooler to hotter side causes the vane to move. The cooler (white) side moves forward, pushed by the higher pressure behind it. From a molecular point of view, the vane moves due to the tangential force of the rarefied gas colliding differently with the edges of the vane between the hot and cold sides.<ref name="Usenet Physics FAQ"/>

The Reynolds paper went unpublished for a while because it was refereed by Maxwell, who then published a paper of his own, which contained a critique of the mathematics in Reynolds's unpublished paper.<ref>{{cite journal |first=J. Clerk |last=Maxwell |author-link=James Clerk Maxwell |date=1 January 1879 |doi=10.1098/rstl.1879.0067 |title=On stresses in rarefied gases arising from inequalities of temperature |journal=Philosophical Transactions of the Royal Society of London |volume=170 |pages=231–256|url=https://zenodo.org/record/1432458 |doi-access= }}</ref> Maxwell died that year and the [[Royal Society]] refused to publish Reynolds's critique of Maxwell's rebuttal to Reynolds's unpublished paper, as it was felt that this would be an inappropriate argument when one of the people involved had already died.<ref name="Usenet Physics FAQ"/>