# Thread: Random Photon Question

1. ## Random Photon Question

Okay, if photons have mass, but GR/SR predicts an object's mass increases as it's speed increases, then wouldn't a photon of light have infinate mass?

2. Photons do not have rest mass, so most people would say they don't have mass. One can also sometimes mean "relativistic mass", which is equivalent to energy, so if that's what you mean, then a photon's relativistic mass is determined by its energy, not its speed.

3. I knerw I hadta be missing something because we're not being pelted by infinately large photons :-P forgot that rest mass is 0. anyway was just a random thought that crossed my mind. thanks.

4. Is that demonstrated by that glass ball with the black and white paddles inside that spin faster the brighter the light?

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Originally Posted by Pinemarten
Is that demonstrated by that glass ball with the black and white paddles inside that spin faster the brighter the light?

It looks like nobody's sure how they work. It appears that it's not photon pressure causing the rotation, and there must be a certain amount of air in the globe for them to work:

The effect begins to be seen at partial vacuum pressures of a few mm of mercury (torr) , reaches a peak at around 10−2 torr and has disappeared by the time the vacuum reaches 10−6 torr (see explanations note 1). At these very high vacuums the effect of photon radiation pressure on the vanes can be observed in very sensitive apparatus (see Nichols radiometer) but this is insufficient to cause rotation.

6. I remember one from physics class. We used it to prove the 'particle' properties of light.
I guess my teacher was a bold-faced liar or an incredibly gullible dupe.

7. I'll go with the latter. Light need not be made of particles to carry momentum, classical waves do that all the time.

As for understanding why you need some air in there to make it work, one might want to start with the recognition that it is wrong to think the white side feels double the force of the black side on the grounds that the white side "reflects" light while the black side "absorbs" it-- as energy is conserved, both paddles re-emit light energy equally. The momentum consideration is more complicated because you need to know the angular distribution of the scattered (white) light, versus the absorbed and re-emitted (infrared) light, but the article says that the momentum of light itself would never be enough to explain the effect-- and indeed the things turns even if immersed in an isotropic radiation field, like the middle of a bright white room. So you actually need the dynamics of the air in there, due to the fact that the black side gets hotter. Hot air expands, acting like a little jet engine. The article goes into more detail but I don't think it brings any greater insight than that.

8. I might just buy one and try the following:

1)Put it in a darker room and measure the RPM.
2)Turn on my Ebay flashlight with 23 white LEDs in the head. ('tis a bright sucker)
3)Measure the RPM change when I shine on it from the longest distance possible.

LED is 'cold' light.

What should any change be in RPM?

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Originally Posted by Ken G
As for understanding why you need some air in there to make it work, one might want to start with the recognition that it is wrong to think the white side feels double the force of the black side on the grounds that the white side "reflects" light while the black side "absorbs" it-- as energy is conserved, both paddles re-emit light energy equally.
Absorption and re-emission equality would be true in thermal equilibrium, but that's not guaranteed to hold here, so you could have the black surface absorbing more than it emits. And the black surface would have a higher emissivity, so saying that they both re-emit equally isn't accurate.

Further, if one side tends to absorb rather than reflect, the whole panel should heat up (there will be conduction in the panel), so that the thermal emission happens in both directions (though different emissivities means they would not be equal). But more absorption from one direction and symmetric emission will impart momentum to the paddles; it's exactly the concept at play in laser cooling.

Lebedev and Nichols & Hull did experiments demonstrating light pressure on metal vanes in a slightly different configuration (torsion was present, so that a measurement could be made). So while the gas heating is why the radiometer rotates, light pressure does exist here.

10. Originally Posted by Ken G
Hot air expands, acting like a little jet engine. The article goes into more detail but I don't think it brings any greater insight than that.
Actually it does, and this matches what I've learned about radiometers. Especially note points 3 and 4; it's not just as simple as the black side providing a bigger "kick" to the air molecules.

Originally Posted by Wikipedia
Explanations for the force on the vanes
Over the years, there have been many attempts to explain how a Crookes radiometer works:

1. Crookes incorrectly suggested that the force was due to the pressure of light. 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 disprove 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 was 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 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. Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed. 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.
2. Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was effectively disproved by both Schuster's and Lebedev's experiments.
3. A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat i.e. will bounce 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 the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the force on the vane should be exactly 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.
4. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds, but first published by James Clerk Maxwell in the last paper before his death in 1879. 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 behave like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move cold (white) side forward.
Both Einstein's and Reynolds's forces appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger.

11. To be honest, points (3) and (4) is just what I meant about not bringing much further insight. The strawman argument that the pressure is always equal everywhere so would be expected to "cancel out" is a steady-state assumption, and applies to the neither the radiometer nor a jet engine-- I interpret Einstein's analysis as quite similar to the jet principle. Point (4), on the other hand, might have something to say for a porous paddle, but to claim that air going around the sides of the paddle is analogous to it being porous once again just sounds to me like what happens when air expands in the vicinity of the temperature gradient. It short, it just keeps coming up jet engine to my ear, only it is made to sound much more incomprehensible. No doubt the concept of "jet engine" is itself a complex subfield with many variations, but the overall image seems quite appropriate in both points (3) and (4), as well as for additional factors that would seem to be needed physically that are not included in either of those points.

12. Originally Posted by swansont
Absorption and re-emission equality would be true in thermal equilibrium, but that's not guaranteed to hold here, so you could have the black surface absorbing more than it emits. And the black surface would have a higher emissivity, so saying that they both re-emit equally isn't accurate.
All you are saying is that there is some thermal inertia as the black paddle heats up. I was aware of that, but I suspect it is not the central issue here, as it is probably quite short compared to the timescale it takes to get the thing rotating. Once it is rotating faster and faster, it might become a factor requiring consideration. If it matters before that, it would certainly be a pretty shocking oversight by both Wikipedia and Einstein. And not that thermal equilibrium is not required here-- all we are talking about is radiative equilibrium, i.e., conservation of energy.
Further, if one side tends to absorb rather than reflect, the whole panel should heat up (there will be conduction in the panel), so that the thermal emission happens in both directions (though different emissivities means they would not be equal). But more absorption from one direction and symmetric emission will impart momentum to the paddles; it's exactly the concept at play in laser cooling.
That is one of the issues that comes under what I mentioned about the angular distribution of the ultimate re-emission. I was originally thinking along those lines, but then I read the Wikipedia article and it clearly establishes that simple photon momentum is never enough to explain the effect.

Lebedev and Nichols & Hull did experiments demonstrating light pressure on metal vanes in a slightly different configuration (torsion was present, so that a measurement could be made). So while the gas heating is why the radiometer rotates, light pressure does exist here.
Yes, it's just not the explanation for why it rotates-- I'll stick with the "jet engine" analogy there.

13. Originally Posted by Ken G
Photons do not have rest mass, so most people would say they don't have mass. One can also sometimes mean "relativistic mass", which is equivalent to energy, so if that's what you mean, then a photon's relativistic mass is determined by its energy, not its speed.
Okay so back to my OP question, photons do not have a rest mass (but are they ever at rest anyway?). So when photons are energized (or not at rest, whatever the term is), they have relativistic mass (the worlds most recognizable formula). Is this what allows light to be affected by gravity?

14. Yes. I think it is called 'gravitational lensing'.

15. Originally Posted by Ken G
To be honest, points (3) and (4) is just what I meant about not bringing much further insight. The strawman argument that the pressure is always equal everywhere so would be expected to "cancel out" is a steady-state assumption, and applies to the neither the radiometer nor a jet engine-- I interpret Einstein's analysis as quite similar to the jet principle. Point (4), on the other hand, might have something to say for a porous paddle, but to claim that air going around the sides of the paddle is analogous to it being porous once again just sounds to me like what happens when air expands in the vicinity of the temperature gradient. It short, it just keeps coming up jet engine to my ear, only it is made to sound much more incomprehensible. No doubt the concept of "jet engine" is itself a complex subfield with many variations, but the overall image seems quite appropriate in both points (3) and (4), as well as for additional factors that would seem to be needed physically that are not included in either of those points.
Except that you were talking about heated air expanding, with that being the reason for the "jet engine" effect, but neither Einstein's work nor Reynold's sounds like an effect directly due to expansion of air. Even the brief description in Wikipedia is more detailed than just the vague description you gave. If you want to simplify it yourself, that's fine, but it doesn't make sense to imply that there wasn't more insight available. It also provides a link (here) that gives more detail about Reynold's work. It's not an effect related to expansion of gas, but more akin to superfluidity.

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Originally Posted by Fazor
Okay so back to my OP question, photons do not have a rest mass (but are they ever at rest anyway?). So when photons are energized (or not at rest, whatever the term is), they have relativistic mass (the worlds most recognizable formula). Is this what allows light to be affected by gravity?
Photons are always moving

I think that that is a basic somewhat functional explanation (good enough for layman use, but not really correct), but that the real answer lies in having a good understanding of the geometry used in general relativity.

17. but that the real answer lies in having a good understanding of the geometry used in general relativity. lol I've been fighting to understand GR and SR ever since I discovered this board. My two biggest interests in science are light and gravity--therefore gaining and understanding of Einstein et al's explaination of the two is very important. Unfortunatly, for the layman such as myself, it's also like trying to sumo wrestle a greased-up humpback whale.

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Originally Posted by Fazor
but that the real answer lies in having a good understanding of the geometry used in general relativity. lol I've been fighting to understand GR and SR ever since I discovered this board. My two biggest interests in science are light and gravity--therefore gaining and understanding of Einstein et al's explaination of the two is very important. Unfortunatly, for the layman such as myself, it's also like trying to sumo wrestle a greased-up humpback whale.
I have a bachelors in physics and several years of graduate study, and I still dont really understand GR. What you see here is getting pretty close to my limit. Unless you have the math background to understand it, your analogy is pretty accurate

19. Originally Posted by Grey
Except that you were talking about heated air expanding, with that being the reason for the "jet engine" effect, but neither Einstein's work nor Reynold's sounds like an effect directly due to expansion of air.
I'm a bit mystified how you can say neither of those sound like the way heated air acts in a jet engine. To my way of looking at things, what the Wiki article says (and it basically requires an interpreter to extract this) is that a boneheaded argument says you can't get pushes on paddles by heating an ideal gas, because the ideal gas equilibrates its pressure even if some areas are hotter because other areas will be denser, but this is of course silly as even jets wouldn't work if this were true. Wiki doesn't make that last fairly obvious point, it instead claims it took Einstein to see how ridiculous that argument is, and Einstein found some peculiar edge effect involving temperature gradients. I think the truth of the situation is that everyone knows heating air will have dynamical consequences, and that is essentially the jet engine affect I alluded to because it does not engender confusion and obfuscation like the Wiki article (which really outdoes itself when it brings in a porous-medium effect for non-porous paddles).

I think the simple way to look at all this is that when you heat air, you can get pushes on the hot side of the paddle for a wide array of reasons, all centering on the resulting dynamics in the air. One type is simple pressure forces across temperature gradients where those gradients are strongest, i.e. the edges (Einstein's approach), and another type involves diffusion of faster molecules past slower ones (which they called "thermal transpiration" but is more generally known as "diffusive energy transport"). No doubt there are many other ways that momentum fluxes, which is what we are talking about here, can be generated by heated air. Said like that, it's not nearly so intractable as the Wiki article makes it seem, it's just as difficult as all problems that are poorly attacked by hydrodynamics.

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Originally Posted by Ken G
All you are saying is that there is some thermal inertia as the black paddle heats up. I was aware of that, but I suspect it is not the central issue here, as it is probably quite short compared to the timescale it takes to get the thing rotating. Once it is rotating faster and faster, it might become a factor requiring consideration. If it matters before that, it would certainly be a pretty shocking oversight by both Wikipedia and Einstein. And not that thermal equilibrium is not required here-- all we are talking about is radiative equilibrium, i.e., conservation of energy.
But, strictly speaking, no re-emission is necessary to conserve energy. An object can absorb a photon and create a phonon and energy will be conserved.

Originally Posted by Ken G
That is one of the issues that comes under what I mentioned about the angular distribution of the ultimate re-emission. I was originally thinking along those lines, but then I read the Wikipedia article and it clearly establishes that simple photon momentum is never enough to explain the effect.

Yes, it's just not the explanation for why it rotates-- I'll stick with the "jet engine" analogy there.
Certainly, but your post sounded like you were saying that radiation pressure had no effect at all.

21. Originally Posted by swansont
But, strictly speaking, no re-emission is necessary to conserve energy. An object can absorb a photon and create a phonon and energy will be conserved.
Certainly. But that doesn't happen here, instead we have radiative equilibrium being set up after a brief thermal inertia that could in principle start movement but in practice is negligible (according to the Wiki article, and it makes sense).

Certainly, but your post sounded like you were saying that radiation pressure had no effect at all.
What aspect of my post gave you that impression? I thought I was pretty clear on how to include radiation pressure-- analyze the angular distribution of the re-emission.

22. Round and round we go with the slightest hint of 'EUREKA' Thats it. I know what light is. Tis not a wave but, behaves like one. Can't be a particle because it can not have mass. Its energy pure and simple. OK, ok maybe not.
Then I know what gravity is. Its........ see above.

With all the postulating of a know it all. We do not.
When we look at these questions we can be very flippant with the. ' I know what its not.' All sorts of very good arguments have been used to attempt to make simple what is not.

Right now in this forum if you now what light is tell me. Please use English language that a mind of limited ability might comprehend. While you are at it a satisfactory explanation of gravitational force would be handy also.
Keep it simple because My education does not afford me the highest levels of Quantum physics understanding.... Thank you.

From the opposite end of not so bright thinking comes the next thought and or question; How do we see?
Please do not answer this last question as this one I do know. The small part of the spectrum that is defined as visible light is detectable to us by means of what we call vision. As I have studied and worked in opthalmology an explanation would not be required here. I still do not fully understand what light is. So Its enlightenment I am looking for.... sorry.

There is a small point to be made here,. I am not a creationist. Do not hold with ' inelegant designer ' beliefs and will run and hide at the very mention;...mark.

23. astromark,

Right now in this forum if you now what light is tell me. Please use English language that a mind of limited ability might comprehend. While you are at it a satisfactory explanation of gravitational force would be handy also.
Keep it simple because My education does not afford me the highest levels of Quantum physics understanding.... Thank you.

From the opposite end of not so bright thinking comes the next thought and or question; How do we see?
Please do not answer this last question as this one I do know. The small part of the spectrum that is defined as visible light is detectable to us by means of what we call vision. As I have studied and worked in opthalmology an explanation would not be required here. I still do not fully understand what light is. So Its enlightenment I am looking for.... sorry.

There is a small point to be made here,. I am not a creationist. Do not hold with ' inelegant designer ' beliefs and will run and hide at the very mention;...mark.
To speak in plan English "run".

Light is a wave that travels in the creation and expansion of the electron.

Gravitational Force is a higher expansion of electrons pushing you to a lower pressure of expansion.

Energy = 1 kg x (the speed of light in meters/second)2
Energy = 1 x 300,000,000^2
Energy = 90,000,000,000,000,000 joules (a metric energy unit)

This is equivalent to 25,000,000,000 kWh (kilowatt hours) of electricity, or 25 billion kWh. This is enough electricity to power 47,564,688 60-watt light bulbs for an entire year. Thus, it is quite easy to see that just one kilogram of matter is equivalent to an enormous amount of energy. This is proof that mass is a condensed form of energy. Thus, the faster expansion rate of energy pushes you to a lower expansion rate of condensed energy. That's in a nutshell without quantum physics.
Last edited by rebel; 2007-May-27 at 07:29 PM.

24. .... breathless from running...
Thanks 'rebel'. that is interesting.

25. Astromark,

There is more where that came from in a closed post called "My theory of the creation of the universe". There is even more but as I stated, it is closed and I'm not getting any answers back to see if I can reopen it.

26. Originally Posted by Ken G
I'm a bit mystified how you can say neither of those sound like the way heated air acts in a jet engine.
Well, I wouldn't call something considered analogous to the way a superfluid will move up the walls of a container particularly like the way air behaves in a jet engine, except perhaps in the most basic way that material comes in one end and goes out the other. It seems strange that you'd prefer this extremely simplistic description, and yet you complain that the Wikipedia article lacks detail.

Originally Posted by Ken G
To my way of looking at things, what the Wiki article says (and it basically requires an interpreter to extract this)...
I think this must just be you, because I thought it was reasonably clear, and provided links to more detailed discussions for anyone who wanted more out of it.

Originally Posted by Ken G
...is that a boneheaded argument says you can't get pushes on paddles by heating an ideal gas, because the ideal gas equilibrates its pressure even if some areas are hotter because other areas will be denser, but this is of course silly as even jets wouldn't work if this were true. Wiki doesn't make that last fairly obvious point, it instead claims it took Einstein to see how ridiculous that argument is, and Einstein found some peculiar edge effect involving temperature gradients.
Except that historically, it did take several years and the proposal of a number of ideas that turned out to be incorrect before physicists figured out how a radiometer worked. Sure, now it's pretty well understood, but it certainly makes sense to discuss the historical failed attempts before working that out.

Originally Posted by Ken G
I think the truth of the situation is that everyone knows heating air will have dynamical consequences, and that is essentially the jet engine affect I alluded to because it does not engender confusion and obfuscation like the Wiki article (which really outdoes itself when it brings in a porous-medium effect for non-porous paddles).
Except that this is the currently accepted explanation for how a radiometer actually works. So you're complaining that the article lacks sufficient insight, and yet you're suggesting that it be replaced with something vague and inaccurate.

27. Originally Posted by Grey
Well, I wouldn't call something considered analogous to the way a superfluid will move up the walls of a container particularly like the way air behaves in a jet engine, except perhaps in the most basic way that material comes in one end and goes out the other.
I'm missing the reference to superfluid. We have a superfluid here? And as for material coming in one end and going out the other, you bet I think an article that doesn't even allude to that in the least way is pretty darn incomplete!
It seems strange that you'd prefer this extremely simplistic description, and yet you complain that the Wikipedia article lacks detail.
My complaint about the Wiki is that it was obfuscating. "Detail" is only a good thing when it answers more questions than it raises. My point is that the Wiki article really only did two things-- raised a strawman argument and then said it took Einstein to see that it was wrong, and then focused on two extremely esoteric and poorly explored dynamical effects, while overlooking a simple broad-brush explanation: if you heat gas on one side of a paddle and not the other, you'll get a push from the hot side. How that actually happens is probably very difficult in detail, and I highly doubt that either explanation offered by the Wiki is much more than an idealization. So if you're going to idealize anyway, at least use an analogy that people can understand. It's like the explanations for how an airplane flies-- they're never really right, because it still has elements that are not well understood, but it conveys the essence.
I think this must just be you, because I thought it was reasonably clear, and provided links to more detailed discussions for anyone who wanted more out of it.
If I'm claiming it raises more basic questions than it answers, I suppose it falls to me to back that up. Here are the questions that occur to me:

1) Why would it take Einstein to realize that an ideal gas doesn't have the same pressure everywhere under dynamical conditions?
2) Why is it surprising that the dynamical forces that appear should require consideration of the places where the warmer air meets the cooler air, i.e., at the edges where the temperature gradient is largest?
3) Why does "thermal transpiration" have anything to do with forces on the paddle, when their explanation suggests that it can happen if there's a temperature difference even when there's not pressure difference-- but then they go on to say it's the pressure difference that produces the force! The unexplained question is, how did the thermal transpiration act to yield the pressure difference, because their explanation applies in the presence of a pressure difference, and prefers that it be small.
4) If you have a paddle that warms the air on one side, why should we be at all surprised that the paddle moves away from the warmed air?
5) If that is what they are trying to make us see, why would they not mention the most obvious example of all-- a jet?
6) Given the vast array of complex and poorly understood phenomena in hydrodynamics and more complicated treatments of the fluid, if one wants to do better than a simplified analogy, why would we think that the two idealized effects they did mention somehow give a "complete" explanation, even though they cannot even tell us which one dominates, and both explanations come from work done prior to the invention of the jet engine and the associated new physics?
Except that historically, it did take several years and the proposal of a number of ideas that turned out to be incorrect before physicists figured out how a radiometer worked. Sure, now it's pretty well understood, but it certainly makes sense to discuss the historical failed attempts before working that out.
If it's pretty well understood now, that certainly doesn't come out from the Wiki article's explanation-- see the above questions.
Except that this is the currently accepted explanation for how a radiometer actually works. So you're complaining that the article lacks sufficient insight, and yet you're suggesting that it be replaced with something vague and inaccurate.
What I'm complaining about is that I have only the vaguest idea of what the "current explanation" is, and I had to supply most of it myself, because the Wiki article reads like something from a science reporter that read enough to get the buzz words but really doesn't understand the physics being reported on. Again, that kind of reporting always raises more questions than it answers, because it simply isn't cogent.

28. Ken G,

Don't worry, I find it hard to keep up with this theory supporting that theory and these theories supporting them.

I don't understand why, much like your hot air on one side of a paddle, a higher expansion rate pushing to a lower expansion rate is fine for all theories except one. The theory of gravity, this one has to be a pull instead of a push. With all the other evidence of natural phenomena I don't understand why only gravity breaks the rules. Actually, I find that gravity makes much more sense as a push to a lower expansion. I mean, the cold air isn't said to pull the hot air over causing the paddle to move, it's the hot air that is said to push the paddle.

Now if more theories were made up to explain where the cold air came up with the extra energy to pull the paddle, would this make it right? That's why I don't like a theory on a theory, because with this anything is possible.

29. Originally Posted by rebel
Ken G,

Don't worry, I find it hard to keep up with this theory supporting that theory and these theories supporting them.

I don't understand why, much like your hot air on one side of a paddle, a higher expansion rate pushing to a lower expansion rate is fine for all theories except one. The theory of gravity, this one has to be a pull instead of a push. With all the other evidence of natural phenomena I don't understand why only gravity breaks the rules. Actually, I find that gravity makes much more sense as a push to a lower expansion. I mean, the cold air isn't said to pull the hot air over causing the paddle to move, it's the hot air that is said to push the paddle.

Now if more theories were made up to explain where the cold air came up with the extra energy to pull the paddle, would this make it right? That's why I don't like a theory on a theory, because with this anything is possible.
Gravity is irrelevant to the topic of how this radiometer works. In my opinion most of this post belongs in ATM.

30. Originally Posted by Ken G
I'm missing the reference to superfluid. We have a superfluid here?
Follow the link that the article provides for those who want more detail. It's not an example of superfluidity, but apparently has some similarities to it.

Originally Posted by Ken G
And as for material coming in one end and going out the other, you bet I think an article that doesn't even allude to that in the least way is pretty darn incomplete!
Originally Posted by Wikipedia
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 behave like the pores in Reynolds's plate.
As you can see, the article does more than "allude" to this. Of course, it's not very much like a "jet engine", but that's my point. A jet engine is a poor comparison for what is actually going on, though it has some vague similarity to the process.

Originally Posted by Ken G
If I'm claiming it raises more basic questions than it answers, I suppose it falls to me to back that up.
I won't argue that it reaises questions, but that's irrelevant. Any encyclopedia article about any topic is likely to raise more questions than it answers. That's because an article at this level can only give a fairly broad description of the subject, and provide references for those interested in more information. This article does that.

Originally Posted by Ken G
Here are the questions that occur to me:

1) Why would it take Einstein to realize that an ideal gas doesn't have the same pressure everywhere under dynamical conditions?
2) Why is it surprising that the dynamical forces that appear should require consideration of the places where the warmer air meets the cooler air, i.e., at the edges where the temperature gradient is largest?
3) Why does "thermal transpiration" have anything to do with forces on the paddle, when their explanation suggests that it can happen if there's a temperature difference even when there's not pressure difference-- but then they go on to say it's the pressure difference that produces the force! The unexplained question is, how did the thermal transpiration act to yield the pressure difference, because their explanation applies in the presence of a pressure difference, and prefers that it be small.
4) If you have a paddle that warms the air on one side, why should we be at all surprised that the paddle moves away from the warmed air?
5) If that is what they are trying to make us see, why would they not mention the most obvious example of all-- a jet?
6) Given the vast array of complex and poorly understood phenomena in hydrodynamics and more complicated treatments of the fluid, if one wants to do better than a simplified analogy, why would we think that the two idealized effects they did mention somehow give a "complete" explanation, even though they cannot even tell us which one dominates, and both explanations come from work done prior to the invention of the jet engine and the associated new physics?
The important point is that historically, it did take a few years for physicists to understand what was going on in a radiometer, and some of the "obvious" explanations turned out to be wrong. Maybe you're just smarter than them, but it makes sense in a discussion of the device to talk about the failed attempts to explain its motion, as well as the eventual explanation that is currently accepted. They don't compare it to a jet because it's just not very much like a jet.

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