Crookes Radiometer: a Surprisingly Persistent Scientific Mystery

[@oliversmith9200]

He gets my sub for the best talk on Crookes Radiometer in my life.

[@MervynPartin]

I had not realised that there was still so much controversy surrounding this device, so thank you for this excellent presentation. Perhaps a Nobel Prize awaits the definitive answer?

[@stevecausey545]

This is a wonderful channel! I miss learning...lol I'm 64 and this channel has captured my mind and heart...thank you so much.

[@dominicestebanrice7460]

What a FANTASTIC exposition; thorough, organized, comprehensive and yet concise! I assumed radiometer mechanics was "sorted" once we understood the phenomenon of a comet's tail always pointing away from the sun on its orbit but I did not know that at ultra-low vacuum the vanes won't rotate - fascinating. It's a mark of physicists' hubris today that we claim to know what happened 10^-34 seconds after the big bang and how Beta decay really works (just two of the many examples of conjecture-not-science that reign today) but we still don't have consensus on the radiometer, the 'glass transition, or how Tylenol works!

[@Torby4096]

Fascinating! (No Spock emoji?) Grandpa always had one where the sunshine made it work during the day and a lamp in the evening. Loving spinning things, I watched it a lot. When I was 10, the teacher showed us one. He left it on the window sill while we opened our science books and read that it worked because photons bouncing off the light side produced more force than the ones being absorbed by the black. But it was spinning the other way. "Um, this can't be right," I said. This resulted in a big fight because, of course, Torby's observation can't be right, and who is he to question the book? I got sent to the principal, who gave me a stern talking to. I could not believe the first person to notice was a 10-year-old boy who never got his homework done. I keep one in a window where it spins the wrong way in the afternoon. Oh, and today I find out it does not work the way I thought either😅

[@miinyoo]

Neat. I always thought of it as excitations of the gas right next to the hotter side providing slight momentum which adds up overcoming the drag. Electrostatic imbalance is even better because it's so localized. The things have a top speed so a kind of equilibrium should be achieved between drag and exposure time to light on the hot side of any given vane.

[@Information_Seeker]

likely correct about gas temp, as opposite spin is achieved by spraying an upside down air duster can on it.

[@dragonbrains6045]

Im gonna be honest most of the science went over my head. But I still thoroughly enjoyed this video! Good stuff!

[@FranktheDachshund]

My neighbor had one in his garage window I was probably 7 years old. It was about 1969 and he explained that particles from the Sun made it move. It worked in the sunlight, so it seemed reasonable to me. I was fascinated by the device, it was as close to magic as i had seen.

[@williesnyder2899]

My grandparents has one next to a “drinking bird” on a kitchen window ledge. Always fascinating, always mysterious!!

[@williesnyder2899]

“had” not has…

[@iandavidson6604]

Another fascinating video, thank you. I used to keep one of these on my desk when I delivered lectures on spectroscopy and hazard detection. My students, who were often mesmerised by the motion, were invariably disappointed when I told them that, even after all this time, we still didn't really have a good explanation for the device. Hopefully Mr Liu's paper will change that .

[@speed8701]

Had one as a child, 66 years later still have it and it still works.

[@prestonburton8504]

me too - and the charge box (two pieces of foil that will deflect away from each other showing same charge will always move away from each other)

[@AdamosDad]

I have been fascinated by this device since I first saw one in my grandmother's house in about 1953.

[@williesnyder2899]

If I’m not mistaken there is a “drinking bird” device on the upper shelf over our presenter’s left shoulder. Sure brings back good memories!

[@Allan_aka_RocKITEman]

While sitting here at my dinner table watching this video, I grabbed my radiometer {currently sitting on the piano next to the table}, and an L.E.D. flashlight I had left on the table, and AWAY SHE GOES {i.e., the radiometer SPUN}. I thought radiometers only spun 'well' for incandescent bulb light sources, but I now know that is incorrect. I also carry another L.E.D. flashlight in a belt pouch. I then shined BOTH FLASHLIGHTS on the radiometer and *IT REALLY SPUN.* 😊 *EDIT→* I shined both flashlights on the lighter sides of the vanes -- they appear to be bare metal, not painted white -- and the vanes did _slowly_ move _towards_ the lights.

[@Schwertsan]

This was a very fun episode.

[@fredblonder7850]

My brain hurts after listening to that. I will now relate an anecdote: I once noticed that my radiometer had collected a lot of dust, so I took it into the bathroom and ran warm water over it. It spun as long as I ran warm water over it, the same as if I had held it in bright light. Note that this was an interior bathroom, so the only source of light was two CFL bulbs in globes, so the light was very diffuse. I assumed that the radiometer was responding to infrared being emitted from the warm glass bulb. As my primary objective of cleaning the radiometer had been accomplished, I put it back in the window, without any further experiments, but now I’m tempted to try other things.

[@leocurious9919]

It would have stopped spinning if you kept going with the warm water.

[@fredblonder7850]

@@leocurious9919 I know. I didn’t want to get into a long essay here.

[@mikep3226]

This is probably the inverse of the fridge effect that got mentioned. Perhaps moving between locations with different ambient temperatures would be an interesting experiment. In the winter my house often gets to a state where there are places over 80F (30C) and other places below 50F (10C), I wonder if that would be enough? Well, I guess I need to get a radiometer and run some experiments...

[@jermeyplunkett3744]

Electrostatics

[@Torby4096]

Oh, yes! Your warm water was emitting infrared, which will spin it. The warmth of your hand will make it go if you hold the bulb. I am going to go play with mine now😊

[@Muonium1]

Wow, a highly comprehensive tour de force as usual. Very excellent treatment of the topic. This is the first I'm hearing of that Liu theory at the end there. On closer inspection it appears that Jerry Z. Liu is somehow associated with the CS department, but not actually employed by, Stanford. Perhaps he is an alumnus. His other theories listed on his page are highly...how shall I put this diplomatically...eccentric. His paper does not seem to be published anywhere but on his own Stanford vanity page, and not even on the ArXiv (or for that matter even on viXra!), let alone in peer reviewed literature. I'm not saying that alone should discount his hypotheses, but it is, as the kids would say today, "sussy". Further, the postulated mechanism he offers makes little sense to me. He seems to be under the impression that an electron in an atom "jumping" to a higher orbital when excited is *actually* jumping or moving like a ball being shoved away from a thrower, and that this somehow imparts a motive force on the parent atom and onto any random molecule in its vicinity. But of course, this is not at all how things work in the quantum realm of the atomic scale. When an electron in an atom is excited by absorption of an external quantum of energy, say electromagnetic energy, it merely changes the conformational structure of the probability distribution of the electron orbitals or "clouds" around the atom. And importantly, if an electron is not ejected in the process, no electrostatic charge will be imparted to the atom which could repel external atoms or molecules. It will remain entirely neutral. I recommend the video Cody did on this curious item a few years ago where he actually built his own functioning Sprengel pump to pull high vacuum on his radiometer (nearly offing himself in the process by inadvertently giving himself a massive shock from the spontaneously developed electrostatic charge imparted by triboelectric charging of the flowing mercury!). It is to this date the only demonstration of that device that I have ever seen anywhere.

[@CanadianMacGyver]

Yes: I was hesitant to mention his theories for those reasons, but ultimately decided to include them to show that work on radiometer physics is still ongoing. My understanding of the repulsion force he describes is that it is similar to London Dispersion or Van der Waals Forces - as the electrons jump to a higher orbital, they get slightly closer to those of surrounding atoms and thus the repulsion force between said atoms slightly increases. This at least seems plausible.

[@Ming-vz1sh]

Great job. Actually, the latest version of this article provides more experimental evidence supporting the theory. Click the title of Liu's article to open the latest version.@@CanadianMacGyver

[@pauls5745]

Liu was saying the excited electrons caused a physical disturbance among all the surface molecules. Interesting, but there is more to why excited surface atoms are repelled.

[@TravisTellsTruths]

I need to see that video... link? The question is, is it the photoelectric effect in action? Does it spin with less energy light than UV? Does it work with other materials (i.e. one side not metal). Does it always use metal foil on one side or actually just white stuff works? Can we rule out a "mild" photoelectric effect that first pushes charges (electron clouds) before the electrons would be ejected? It seems it must be heat related or ionic in nature as the air is still required. Could this effect somehow involve both heat and tiny static charges? I'm a new sub!

[@ghlscitel6714]

​ @greatawakeningforall Here is Cody's lab. kzbin.info/www/bejne/pHKynmqJrpeYm7c "Applied Science" has also comments on this subject: kzbin.info/www/bejne/qGixdnyVeJ6MnpI

[@wbeaty]

Spinning vanes, if charged, should be easily detected outside, by an electrometer, even at incredibly low voltages. Use copper telescope mirror or germanium lens as an "un-spotlight" to optically focus a small cold (0K degrees) object at the radiometer, so it spins backwards constantly! Or, as your "anti-lamp," try a light-bulb filament that's turned off, and cooled with droplets of liquid nitrogen. Is the net force actually proportional to the area of the vanes, or to the edge-length of the vanes? Vane shapes: I've seen toy radiometers with circular vanes (probably circular cover-slips for microscope slides.) Employ sodium silicate glue, to assemble odd-shaped vanes from fragments of cover slips (then soot-coat one side black, the other side white using smoke from burning Mg or W. Hmm, if EVERYONE always uses carbon-black as the dark part (soot from kerosene flame, etc., ) maybe the sharp tips of the nanoforest, in near vacuum, are launching little atomic beams made of air molecules? What if they're functioning as light-powered ion engines?!! Just compare black paint to black soot coating. Oddity: if a Tesla Coil is used to cause the gas to glow, the vanes immediately spin. Do this in a microwave oven, and in a brief moment, the vanes are violently "kicked," and spin in a blur! (Both devices cause black side to retreat.) I have 15 of these, from a $5 post-xmas sale, plus momentary $12 price at Sciplus catalog. So I had no problem with nearly destroying one inside a one-kilowatt microwave oven, although I did put 2cu of water in the oven, then gradually reduce it and re-test, until the radiometer gas would finally be ionized. Didn't destroy the radiometer. Huh, I should go back and see how much it takes to actually fry the radiometer. Was it Actionlabs who discovered that black soot paint in near-vacuum, hit by ?100mW? IR laser, will glow brightly orange?

[@uploadJ]

Oh cool, William. I think I know your work from previous work and discussions on the internet. Cheers.

[@uploadJ]

Hmmm ... my comment just got eaten ...

[@wbeaty]

@@uploadJ don't include any links. Sometimes it will survive if it has one link, but not two. If you want to make an old comment vanish, edit it, and add an URL. (Even including youtube links will make comments vanish.)

[@weldonhudson5535]

Very interesting question!! Perhaps I will write an essay on possible theories! Thanks for fostering the idea! Sounds like fun! In the way mental gymnastics! I really enjoy your work!!

[@alokraj3128]

Given that the inner surface of the radiometer in the explanation given by Liu is involved in the effect, it should be easy to carry out experiments with radiometers of differenct diameter glass bulbs. Then the surface area of glass versus surface area of black plates will be different and they should effect rate of rotation.

[@daphneblake7889]

I have had a crooks radiometer since I was little. A peculiar effect I observed was that it would spin if I rubbed the surface of the glass bulb with my fingers in a brisk fashion as to possibly be creating a static electric charge upon the glass. As a result, the veins would rotate very briskly white side leading the direction of rotation. Reaction time was nearly instantaneous and would stop shortly after I stopped. This effect is greatly enhanced when I hold the radiometer by its base in one hand as opposed to leaving it stationary upon the table (formica covered wood table) This leads me to believe the explanation about electrons given in the video

[@iangriffiths9840]

Interesting experience. If you use an old camera flash gun and fire it at the radiometer, the vanes spin wildly as if hit by a major force. If you then fire the flash gun at your hand you can feel a force, or a rapid transient change in temperature. Feeling this force on your hand is noticeable even if you cannot see the flash and it is fired by another person.

[@Torby4096]

Yes! I used to play with my old camera flash this way. Also, if you take a little scrap of paper and fold it so it stands up, the flash will knock it over or scoot it across the table

[@melody3741]

Sometimes it hurts lmao

[@MrJMaq]

Thank you! It is somehow comforting to understand that nobody else knows this works either.

[@larrybell726]

What an absolutely intriguing and geeky review. Huzzah!!

[@hu5116]

VERY INTERESTING! Thanks so much for making this video! I have long been fascinated with the radiometer, and have read up on the transpiration theory in the past. I had not heard of this new potential explanation. I will have to read up on it some more before, being convinced that it is a real thing. But that said, there is nothing that would prevent both the transpiration, and the electrostatic phenomena from both working at the same time. I’m now going to have to schedule some time to look into this again :-). Thanks for a great video!

[@hagerty1952]

This was a really fun one! BTW, I don't know if it was deliberate or not, but your focus (i.e., the camera's focus) was on the radiometer, which put you slightly out of focus.

[@dnomyarnostaw]

Probably a good outcome. The device is the subject, and talking heads ( arms and shoulders) are a bit irrelevant.

[@finnmcool2]

I like knowing there are unknowns so readily to hand.

[@philipoakley5498]

Definitely some interesting stuff in there. I also thought I knew what was going on, but I like the photonic nature of Lui's idea - very Einstein. A subtle big question is how to determine the range of pressures over which the phenomena should work, and how to control / adjust the pressures (along with control of the coatings / 'black' and 'white' materials).

[@geoffreypiltz271]

Why seemingly has no one conducted a long series of experiments on this but just spent their time theorizing?

[@joutoob9]

because it's a desk toy and there's no money in it.

[@geoffreypiltz271]

@@joutoob9 Whatever happened to pure scientific research simply for knowledge?

[@agranero6]

At University you learn the Reynolds theory as the accepted one for the radiometer. I almost forgot it and was writing another comment that I deleted. The water effect is the main evidence. I thought that this question was settled 100 years ago. Nice to learn a new thing and see that it may not so simple. But I have heavy doubts: applying it to Brownian movement is too much IMHO, Einstein calculated the Brownian movement with a high degree of precision without the need for additional hypothesis Besides it is easy to test that using different substances with different elements modifying the behavior and orbital transitions are sensitive to frequency what also could be measured. It is an almost obvious test. Besides yet, how a charge can repel neutral atoms? Some dipole effect? Besides how this explains the cold water effect? I will need to read Liu's work to decide what I think, but I am very skeptical. A referee stealing an idea that he avoid publication is the epitome of ant-ethic work another thing I learned.

[@jonbold]

Some of it is smoke and mirrors but most of it is curtains and paint. Great video, great explanation!

[@JefeSpace]

I did this experiment I made black veins and silver veins by depositing aluminum on one side carbon black on the other side, made my own bearing, put it in a vacuum chamber, pumped the chamber down, plotted rotation, as a function of pressure at a certain pressure It turns away from the black then it stops, and it very low pressure. It turns away from the silver.

[@RGD2k]

Very interesting! I agree with Liu. In addition, the kinetic energy produced depends on the time each atom remains enlarged due to orbital jump: On the reflective side, this time is essentially near zero, because being unable to retain a new orbital equilibrium, an interacting electron can only immediately return to the orbital condition (honestly, more likely one of the sea of conduction electrons) that it *was* in, which is what results in reflection: a new photon, of exact energy as the incoming, just in a new direction. There's radition pressure there too, but it's too small to move anything unless you really brute-force it like with a pulse laser. (there's a method to actually measure pulse laser pulse energy, by physically measuring the recoil of a mirror deflecting the pulses! only possible because the pulse peak power is around 10^7 W or so) Therefore, absent atoms storing energy as excited electrons, and those excited electrons increasing the kinetic diameter of the atoms for a time (until they relax and reemit light), allowing this strong repulsion force time to push on other molecules caught 'too close', and thus transfer energy to them. There's no such thing as a 'soild surface' at this scale, it's just that such an overlap between atoms creates very strong electrostatic repulsion for as long as the overlap exists, pushing the atoms apart and resulting in acceleration and a change in velocity. This is likely to also transfer kinetic energy from the atom having the absorption, slowing/cooling it from the excess energy it picked up during the electron's transition. We'd expect this absorbed kinetic energy to generally be much less than the energy of the transition, and is effectively a phonon contributing to the total kinetic energy of the molecule or solid surface. So, in the end, it ends up being a heat engine after all: just one due to each atom's 'orbital jump' resulting in a change of kinetic diameter, causing it to act like a little spherical piston on all its (very local) surrounds. Thus it transfers some momentum both into the nearby gas, as well as into its solid neighbors, and this is ultimately integrated into the total force available to move the radiometer. It is interesting, is is not, that the mfp of the best pressures for a radiometer, just so happens to be (in nitrogen at least) about the same length as the scale of the radiometer's vanes. 1 Pa at room temp, in nitrogen, give a mfp of about 7 mm. Come to think of it, the atom acts like a little 'vacuum-backed' speaker: This is one where you put an electromagnet inside an evacuated container, such that it can emit sound without producing a backwave like a conventional speaker (becomes infinitely-baffled if the MFP inside is much longer than the radius of the speaker. Thus, it's able to emit a pulse of pressure on the surrounding environment, without also producing a movement wave in the opposite direction, such as a typical speaker does from the back of the moving surface. In this case, the same process would occur again (a negative pulse) when the electron relaxs and emits a characteristic light emission - but by then its surrounds are further away, and there's no corresponding 'suction' at this scale, just the recoil from the small momentum of the photon emission, probably nulled out by the corresponding jump-reduction in kinetic diameter.

[@pianplays3659]

@CanadianMacGyver found an old experiment proving the edge theory apparently. It’s read as “n 1925, H.E. Marsh, E. Condon and L.B. Loeb confirmed the edge hypothesis [16], and more or less provided the final, satisfactory explanation of the radiometer with an ingenious experiment. They designed a collection of radiometer vanes with edges of different length but similar surface areas, as illustrated below. If the radiometer force arose primarily from the surface of the vanes, then the different vanes should have turned at same same rotation speed. If the radiometer force instead arose primarily from the edges of the vanes, then each vane should turn at a different rotation speed for the resulting radiometers; this latter result was in fact found to be the case”

[@user-eh6th9wj5k]

This video was amazing! Bravo sir!

[@malectric]

Riveting, compelling and very interesting!. Subscribed. An obvious next step would be to use different coatings on each side, for example, different colours, a coating which emits electrons vs one which doesn't etc. I need a vacuum pump and a sealable bulb. It is obvious that the gas is an integral part of the (re)action.

[@petehiggins33]

As a non-physicist it seems obvious to me how it works. The light is absorbed by the black surface and heats the air adjacent to it raising the pressure compared to the white side. This causes the vanes to rotate. As they rotate a hot thin film of air 'sticks' to the surface of the vanes due to "fluid sticks to a surface effect" and does not mix with the air in the bulb. Of course the heated air leaks around the edges of the vanes as it tries to equalise the pressure but friction between the air and the surface of the vanes slows it down sufficiently that the replacement air can be heated by the vane.

[@MichaelKingsfordGray]

Good work. There could be yet another contributing phenomenon: Relativity. Just as electromagnetism has nothing to do with magnetism, but has been demonstrated to be solely due to moving electric charges "seeing" the other like charges "shrink" due to the relative Lawrencian contraction A similar thing might explain these phenomena.

[@OnTheRiver66]

Best information on this subject I have ever seen!

[@kdeuler]

I wonder if the paddles were replaced with balls, each colored half black and white like the paddles, would prove or disprove the edge theory. ( balls having no edges in their shape).

[@KenFullman]

Seems to me that someone should build an easy to assemble kit where paddles of varying kinds could be tested. Would it make any difference if the black and white was replaced by conductive materials? Shouldn't be too hard to just create a bulb that can simply be lifted off for easy repleacement of the paddles. Seems a lot more experimentation is needed.

[@jackx4311]

@@KenFullman - but the radiometer only works in a near vacuum. Making a lift-off bulb is easy - but how would you recreate the extremely low pressure required?

[@fantabuloussnuffaluffagus]

@@jackx4311 With a vacuum pump.

[@alexsis1778]

@@fantabuloussnuffaluffagus That makes a kit like that a lot more expensive though. Not too many people will just have a vacuum pump on hand but even a pretty cheap one is going to run you like $70. A radiometer kit that costs 100+ is a bit silly for a completed item that is closer to 20

[@fantabuloussnuffaluffagus]

@@alexsis1778 Do experiments have to be cheap?

[@Carstuff111]

I honestly think that this is a case of, everyone was both correct, and wrong, about how these work. It is not unheard of for discoveries to be a multilayered find. It very well could be a combination of a couple, or even all, of the theories on how these work. This is why I love science, trying to unravel the unknown.

[@r0bhumm]

My thoughts exactly, I see no reason why more than one of these theories should be correct, especially since several of them seem to give results that say, but this wouldn’t give enough faults.

[@GroovyVideo2]

Great Show - Thanks

[@PeterEdin]

Unless Ive missed it, why are the vanes set at an angle like a diamond shape as the surface area would be the same even if set horizontally. 🤔

[@hitchpost5822]

I agree with fredblonder, my brain hurts after listening to that. My Radiometer has American flags in place of the black side of the vanes I also once saw one with airplane shaped vanes in an antique store, now I wished I had purchased it.

[@bill6255]

I wish I could make an automatic like button for this channel - Great Video!!

[@dhy5342]

I wonder if or how variations on the usual design would work. Such as one side mirrored instead of white; half painted spheres instead of flat plates; red, blue, green, white, black, or mirrored sides in various combinations; unpainted vanes made of sandwiched materials of different reflective qualities such as lead on one side and polished gold on the other; different underlying plate materials such as aluminum, gold, or tungsten; larger bulb sizes; laser light at various wave lengths; and finally, various density gases such as helium or sulpher hexaflouride residue the evacuated bulb.

[@ruperterskin2117]

Cool. Thanks for sharing.

[@DrewNorthup]

(1) Interesting new explanation. I'll have to give that a look. I'd love to know how they deal with Plank radiation and band-gapping. (2) FWIW, Brownian Motion exists in any "fluid", including air and the plasma of our Sun-not just water. (3) Good thing you didn't leave your bow tie unsatisfied. It would have been hard to end the video. Cheers.

[@wolfgangkranek376]

Such a simple object that leads into the depths ob physics, with many plot twists

[@roberttriner6242]

What about using laser light emission? Perhaps one could strike the white side of the radiometric blade with 405nm laser, whilst striking the dark side with a laser in the 600nm+ wavelengths in opposition to eachother (of course with equal power emission from both sources). What happens then? Is this type of light source too narrow in bandwidth for noticeable change in reaction? Though much wider in bandwidth, LEDs were usable for experiments. what wavelengths were used in those cases? What properties of light give rise to more noticeable effects; does wavelength play a significant role to affect change? What about bandwidth density? Or perhaps it's just bulk emission photons regardless of the range of applied wavelength s or bandwidth(s)??? This would be awesome for a science fair project! But not for someone in their 40s 😒

[@JohnSmall314]

Very informative explanations

[@daskritterhaus5491]

neat. after seeing one of these in hgh school over 55 yrs ago thanks to YT and poster l find out more about the workngs !!!

[@slc123ss]

How it's made. Is a freaking joke by these standards. Independent Video entertainment has surpassed television.

[@beatbasher]

This is why I now watch streaming video far more than television - the quality of content now is getting higher and higher.

[@dnomyarnostaw]

@@beatbasher Its really hard to go back to the inane TV content these days. Also, important news gets more detailed coverage, quicker , than commercial sources.

[@sandysand3097]

Yea, i haven’t bothered with cable tv in over 15 years

[@tubereactor7393]

Amen to that!

[@enginerdy]

This is what educational TV used to be like many years ago.. but the scale is 100x now that all the well-spoken nerds have their _own_ PBS channels

[@Blink_____]

Could you build this at scale, focusing light on the veins through mirrors and then feed the motion through a series of gear reductions (like a water wheel powering mill equipment) to drive an electrical generator ?

[@richtomlinson7090]

I was thinking at least try one that's two or three feet in diameter, maybe it could work on a magnetic clutch on the top or bottom, that way it be totally sealed.

[@Information_Seeker]

crooke's radiometer is likely working based on air flow caused by temp, as you can cause the opposite spin by spraying an air duster can upside down on it to cool it.

[@JulianMakes]

I’m going to do some experiments, this is fascinating thank you for another great video

[@catman492000]

Has this been tried with different gases, or always air?

[@renedekker9806]

I fail to see why the normal explanation would be an edge effect. We assume the black side of the vane is hotter than the white side. If the air is cooler than the black side, that means that air molecules hitting the black side will heat up: they will gain momentum. That extra momentum is directed away from the vane, and therefore the vane will gain momentum in the opposite direction. That process happens over the whole surface of the vane, because the whole surface of the vane is hotter than the air. Similarly on the white side. The air is warmer than the white side, therefore air molecules will loose momentum, momentum that is transferred to the vane, again over the whole surface area of the vane. The Wikipedia article claims that "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". But that is only the case assuming the air next to the black side is already warmed up and the pressure has evened out (it is basically the ideal gas law). It is not the case _while the warming process is ongoing._ And because in a radiometer, the warming process is continuously ongoing, and the temperature of the air quickly evens out, that caveat does not seem applicable. Somebody has any ideas on this?

[@rossr100]

Fascinating. I have one of these on the kitchen window sill. I'll give it a clean heh

[@slashmais5261]

Just came across your channel, liked, sub'ed & commented. thx

[@springleaf1035]

Good work. The Liu theory discussion didn't address the need for air to be present. The electron orbital change in the fins somehow agitates the air nearby?

[@CanadianMacGyver]

Yes, that's the idea. The electron orbital jump in the vanes creates a repulsive force that pushes away adjacent gas molecules, creating a reaction force that pushes the vanes.

[@TravisTellsTruths]

I subscribed for this video. It was excellent.

[@jackx4311]

I've no concept of what a Pascal is (nor a Newton), so I did a rough calculation, and worked out that a pressure of 1 Pasacl is about 0.000145 pounds / sq. inch - or 0.0023 ounces / sq. in. If I'm wrong, please correct it.

[@kotahurt]

Awesome video

[@ibrahimkocaalioglu]

Your idea makes sense 👍

[@JanSalzman-m1i]

Since it doesn't work in a vacuum, the gas must play a role. The action must involve thermal/fluid mechanics, not just radiation alone. How does changing the pressure, bulk temperature, and composition of the gas inside the bulb effect relation between rotation rate and radiation? What about the size and geometry of the plates? so many questions!

[@davidvavra9113]

Thank you!

[@ulyssesfewl1059]

There is so much more experimentation that could be done. Change the gas inside the bulb, change the colour of the vanes. Use different colours on each side of each vane (not only black and white), use lasers of various wavelengths, inspect the characteristics of the reflected radiation, and so on and so forth. This simple device may hold the key to the universe!

[@DK-jt6be]

I love this man! Cheers from Denmark

[@davidduffy9806]

Brilliant, excellent vid

[@chonpincher]

If it depended on a temperature difference between the sides of a vane, then one would expect the effect to be enhanced by an insulating layer (e.g. polystyrene foam) between the coatings. Such a device should be compared with one where the coatings were deposited on a highly conductive (e.g. copper) layer of the same mass.

[@MarkVrankovich]

Likely more than one of the theories combine to produce the resulting movement.

[@michaelisaacson9735]

I'm at 9:37 in this video and almost don't want to hear the explanation. Growing up, I had plenty of these from Edmund Scientific in Barrington, NJ. I was always satisfied with the explanation that came with them, which one that was I have no idea. But knowing ahead of time that this is a puzzle, I had to stop the video after 3:00 to ponder, for quite a while, what the actual explanation might be. I suspected that the black side trailed in motion and had to go back to the beginning and check, Yes, it spins opposite from the direction it would if photos were bouncing off of the white side. I settled on the absorption of light by the black side and then that energy being re-emitted being greater than the same amount of light being reflected by the white panels. Why this would cause torque I couldn't imagine and what would happen at thermal equilibrium? Know well that these things will spin very quickly with intense light and do so happily forever killed that idea. When the section explaining all of the different theories from mighty physicists was read, none of which were correct, I calmed down. I wasn't the only one baffled by this. Now, at 9:39, let's finish the video... Wow. Not what I was expecting and, as noted, I feel an awful lot better about not being able to figure this out. Count me as SUBSCRIBED!

[@TheGreatTimSheridan]

The plates should be little pieces of screen. This would optimize the effect.

[@pgm3]

Can a radiometer with near-absolute black surfaces and first-surface silvered (or aluminized) reflective surface be made? Can one be made where the vane surfaces are half-silvered? Can particular radiometers be made which respond to EM wavelengths such as IR, radio, x-ray, UV? Is the spin rate a function of the molecular weight of the trace-pressure gas or mixture?

[@iainmackenzieUK]

Regarding phase changes: My students and I were wondering why it is that a material (such as water-ice) being heated, will (apparently only change its potential energy during phase change and (only) kinetic during temperature change. Could this theory have something to do with this observation?

[@jonahansen]

I don't think the Liu proposal/hypothesis is anywhere near tied down. One would have to perform a number of experiments to see if it could be disproved and only occurs in situations consistent with his hypothesis. For example, the explanation that the vanes chase the white side when it is illuminated because of internal reflection onto the black side could be tested by coating the inside of the radiometer with a black absorbing coating...

[@bohenriksson2330]

It was a mystery to me 60 years ago. It’s still sitting by my window. And it’s still a mystery 😊!

[@dennisk5818]

I question the IR existence. If an LED light is used, but the LED does not emit energy in the red to IR spectrum, then how can 'heat' be the reason for the movement of the vanes? What happens if only a UV emitting source is directed toward the vanes? Also, If a material that fluoresces and emits a long wave (red) radiation when exposed to UV, is there sufficient near infrared to generate the heat energy?

[@iainmackenzieUK]

is the force dependent on the frequency of incident light?

[@iainmackenzieUK]

10:20 I didn't get how we moved from Reynolds porous material and temperature difference to particles striking the white and dark sides (and then the relative speeds of said particles) - I mean, what are the particles and why do they differ in KE?

[@johnwax9759]

I'm wondering if an experiment has been done with different wavelengths of light, to see if there is a difference in response vs wavelength. My radiometer spins way faster with a tungsten lamp than it does from an LED lamp with same intensities.

[@marcmarc172]

So good, thank you!

[@Cornz38]

Astonishing to think that devices invented 200+ years ago A: had such a profound effect of everyday life and B: were invented 200+ years ago!!!!!!

[@jpt3640]

There would be an easy way to test if this is depending on the temperature difference between black and white: simply add to the thickness. Try different materials with low and high thermal resistance. Does this change the effect?

[@davidhuber6251]

Just when I think most of the basic stuff has been figured out, something like this comes along. I guess we have a long way to go before we really understand the universe, assuming we ever can.

[@andys9678]

[@thinker5280]

Could the shape of the vanes and/or the glass bulb have any effect?

[@gyrateful]

As soon as you said it has a partial vacuum, I thought it must be electrostatic repulsion. I wonder if one made the vains out of a photovoltaic semiconductor that would cause a cathode side and an anode side to the vains, would that also cause it to spin?

[@janverberne5255]

I wonder why every radiometer has diamond shaped vanes. And why indeed not trying vanes with holes, as you suggested, or cogwheel shaped vanes to increase the amount of edges...

[@berndmayer3984]

It's not the momentum of the light, that is far too weak. the thing also turns the wrong way around. The decisive factor is the very, very low gas pressure so that the free path lengths of the gas molekules are in the inch range

[@davepowder4020]

"Since thermal transpiration is an edge effect, it stands to reason that if you could greatly increase the number of edges on a vane, for example, by drilling it full of tiny holes, you could increase the force and thus, increase the speed of the vanes. Now so far as I know, nobody's ever constructed a radiometer like this." --- Well, Mr. Messier? How much would it cost to get the 2 initial radiometers, and then have someone drill those holes in one and then re-vacuum the container? **Throws down the science challenge gauntlet (and then runs and hides!)** 😆

[@Ben-g4w5j]

Read Ken Wheeler's "Uncovering The Missing Secrets of Magnetism" for a better comprehension of light as well. Quoting Nikola Tesla, "Light is a soundwave in the ether". The Standard Model tries to make sense of a reality of waves as particles. There are no photons, electrons, or quantum thingeys.

[@tedarcher9120]

Radiometer would spin the other way if it was just light pressure. Weird that everyone didn't notice

[@SubTroppo]

I have a Crookes Radiometer working in front o as I write. It rotates without direct sunlight with radiation reflected from the house next-door which IS getting direct sunlight here in sunny Brisbane Qld (temp. 30C). So is it a "quantum" device? Say it is - pretty please.

[@user255]

IR does not cause charge separation, you need higher energy photons. Thus the last explanation cannot be correct. The Osborne Reynolds theory doesn't seem to make sense either. If molecules on the black side have more energy, then why it needs to be edge effect? Don't the higher energy molecules cause net force just by hitting anywhere on the black side?

[@maxjakobsen5526]

It's looks like an HP frequency counter that is on the shelf behind you.

[@DeactivatedCharcoal]

How about Helium as the gas and the paddles have Gold on one side and Lithium on the other?

[@DeactivatedCharcoal]

They should experiment on a version of this device on the ISS. Microgravity would require it pinned between two bearings. Has anyone changed the gas that is at partial vacuum? Hydrogen only verses heavier gasses Radon is more than 200 times heavier than Hydrogen.

[@TheAnimammal]

Excellent video. Thank you. I have another theory: When the vacuum is made stronger, the bearings take pressure which stops the vanes turning. It is a misinterpretation that high vacuum stops the effect. The reason that the black vanes are driven is because the black absorbs the momentum of the light. The white side reflects the light and it escapes with its momentum. Reflecting is much less momentum transfer than absorbing. That is why the radiometer works like it does. The other explanations are literal grasping at straws.

[@pianplays3659]

I’ve also been thinking about the effects of friction with the bearings. What would this experiment be like in a 0g environment

[@TheAnimammal]

@@pianplays3659 aren’t these very low friction bearings?

[@pianplays3659]

@@TheAnimammal yes but I also think the amount of force light could potentially generate is significantly less than it still

[@pianplays3659]

I don’t actually know I still have an entire rabbit whole to dive down

[@TheAnimammal]

@@pianplays3659 light pressure is well known fact and I think we may have got it wrong by using a reflective target. If we measure it properly by absorbing the mass, then the force is much larger. That is what I am saying.