Wikipedia:Reference desk/Archives/Science/2011 April 27

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

April 27

quarks

how do they know that the quarks in a proton maintain their color? in other words do we know that the "red" up quark stays "red"? since we can only take still shots could the red up change to a blue up quark in different still shots without detection? —Preceding unsigned comment added by 98.221.254.154 (talk) 03:58, 27 April 2011 (UTC)

Quarks are constantly exchanging colors with their neighbors. That's part of how the strong interaction works. Dragons flight (talk) 04:04, 27 April 2011 (UTC)

Couldn't that be explained more simply as at the time of the observation one quark was moving toward the observer and another was moving away, or more complexly spinning right and moving toward or spinning left and away? Color seems awfully similar to length width and height, why not use those instead? —Preceding unsigned comment added by 98.221.254.154 (talk) 04:30, 27 April 2011 (UTC)

Because calling it a form of motion would be wrong. The Color charge of quarks was specifically called a color because it is a fundemental property which is quite unlike other properties such as "spin" and "electric charge" and stuff like that. Since the tripartite existance of quarks within nucleons required 6 values to capture all possible combinations, the 6 color charges (red, antired, blue, antiblue, green, antigreen) allow that to work. Electric charge only requires 2 values (+ and -) to work out. The goal is to pick an analogy which won't be confused with other properties, like length, volume, or spin. --Jayron32 04:41, 27 April 2011 (UTC)

(edit conflict) Color charge has nothing at all to do with visible color. Quarks don't "look" red or blue, and their color charge has nothing to do with how they are moving. The "red", "blue", and "green" quark charges are simply labels used to refer to different kinds of charge. This is analogous to how objects can have an "electrical" charge, except that in this case "red", "blue", and "green" refer to other kinds charge that only encountered in the hearts of nucleons. These are simply labels that physicists adopted. Admittedly, using the familiar terms and calling it "color" can be confusing to outsiders since the whole process has nothing at all to do with the normal experience of color. Dragons flight (talk) 04:46, 27 April 2011 (UTC)

No, I mean like the "height" position of the quark in the nutron at the time of the photo or single observation, as in one of the three dimensions of space that that particular quark is occupying at that particular time. —Preceding unsigned comment added by 98.221.254.154 (talk) 04:48, 27 April 2011 (UTC)

Except you can't take a photo of a quark in an observation. Their position exists only as a statistical average of their location taken over an arbitrarily long period of time, exactly as electrons do. The three quarks in a nucleon exist in a small sphere whose volume is defined by the distance over which the strong interaction operates. They don't actually exist in any specific place within that sphere, and it is completely meaningless to speak of a quark as a little ball which can be "frozen in time" by a photograph. Fundemental particles don't work that way. Your presumption that you could define it as such is flawed at the most basic level. --Jayron32 04:56, 27 April 2011 (UTC)

Ok, the height "quality" of the nutron the length quality of the nutron and the width quality of the nutron gives the nutron the ability to be in all three dimensions, but each quark takes turns as each of those qualities equally. —Preceding unsigned comment added by 98.221.254.154 (talk) 05:02, 27 April 2011 (UTC)

(edit conflict) with below. This response is to the above.I guess so. We can "call" the three quark charges anything we like. You could say that the three quark charges are called "Tom, Dick and Harry" if you wanted to, then describe gluon interactions as "A tom quark emits an tom-antidick gluon, which converts the neighboring dick quark to a tom quark, which itself then emits a dick-antiharry gluon, converting the harry quark to a dick quark" The choice of the three names for the three quark charges is completely arbitrary, and any set of three names would work. Height-Width-Length is a nice three-part set, the problem with using it is that those three words already apply to a part of physical reality which is already "coded for" in the x-y-z axis system inherent in the particle spin quantum number. Since the particle spin axis is defined as the z (height) axis, it doesn't make any sense to define both spin and quark charge using the same set of terms. Since the height-width-length (z-y-x if you prefer) is already fundemental to how spin is defined, it would be beyond confusing to use the same terms to define quark charge. Even the Tom-Dick-Harry system would be better. The advantage of the color system is that quark-quark interactions through gluon exchange can be directly modeled by how colors interact. Just as combining red with its complementary color (cyan or "antired") results in colorless (white) light, combining a red quark with an antired gluon will "cancel" the red color. --Jayron32 05:20, 27 April 2011 (UTC)

Quarks already have positions. Color charge refers to an entirely different set of properties that are in addition to the position characteristics. Dragons flight (talk) 05:12, 27 April 2011 (UTC)

That's not what Jayron just said. —Preceding unsigned comment added by 98.221.254.154 (talk) 05:13, 27 April 2011 (UTC)

(post EC response). A quarks spin defines the three dimensions (the axis of the spin fixes the "z" axis of the coordinate system). However, quarks, like other fundemental particles such as electrons, can't be localized to a specific point within their defined probability distribution. As far as I know, there's no way to specifically define a position within the nucleon for each quark to occupy. We draw pictures of quarks as three little circles within a bigger circle because this allows us to visualize them, but this is no more an accurate model of the quark than the lewis structure is an accurate model of the electron. --Jayron32 05:20, 27 April 2011 (UTC)

I get that the "three little colored circles" are not representative of the dynamics of a nutron. I don't believe that simply identifying the xyz coordinates is equivalent to explaining how a nutron exists in those dimensions. Would you agree that just as two electrons cannot occupy the same state two quarks in a nutron cannot occupy the same dimension. that in order to have a nutron it must occupy three dimensions. To have a nutron with two height qualities and a length quality would leave out the width quality —Preceding unsigned comment added by 98.221.254.154 (talk) 05:35, 27 April 2011 (UTC)

Then we're just debating semantics here. You can literally use any three words to define quark charge that you want (see my Tom-Dick-Harry explanation above), so long as you don't mistake the three words you choose as representing any real property except for the quark charge. The disadvantage of using the positional terms is that it implies a connection to reality which does not exist and may confuse. The advantage of the color system is twofold. First, it is less likely to confuse since it uses terms and concepts which are not encountered elsewhere in the model. Secondly, the way in which colors interact (see my red-cyan colormixing example above) makes a nice analog for how color-charge works. So, yes, you are technically correct that you could choose any set of three words to represent the quark-charge concept. However, don't overextend the words you use to take on meanings that do not correlate to behavior. Using words like "height, width, and length" to describe the three aspects of quark charge has no connection to any other definition of those terms. It would be just as arbitrary of a system as the color system would, so why upset the applecart. Instead, just stick with the system that exists which, while equally arbitrary as your proposed system would be, has the distinct advantage that everyone is already using it. --Jayron32 05:48, 27 April 2011 (UTC)

The animation of gluons in color charge shows a neutron transiently having two red down quarks. But the article says "...introduced the notion of color charge to explain how quarks could coexist inside some hadrons in otherwise identical quantum states without violating the Pauli exclusion principle." Is either of these things wrong or misleading? Wnt (talk) 05:52, 27 April 2011 (UTC)

The picture is attempting to show the gluon exchanges happening in isolation. It is my understanding that, in reality, all three gluon exchanges would happen simultaneously, so all three gluons would change color at the same time. If the gluon exchanges happened sequentially (as shown in the animations) rather than simultaneously, it would lead to some rather impossible situations. The animations are showing isolated gluon exchanges probably because it makes it easier to show how the two-color gluons (say red-antigreen) can change the color of the target proton. In reality, these exchanges are happening simultaneously, and at the speed of light, over a distance of a few femtometers, which is about as instantaneous as the universe will allow. --Jayron32 06:02, 27 April 2011 (UTC)

The animation is accurate and consistent with the exclusion principle. Dauto (talk) 13:40, 27 April 2011 (UTC)

So noone knows what physical properties the colors represent? I figured that the colors were to make it easier to understand, not that no one knows how else to interpret it. wow. —Preceding unsigned comment added by 165.212.189.187 (talk) 18:44, 27 April 2011 (UTC)

Sure, we know what physical property the colors represent. They represent the 6-fold "quark charge" which is fundemental to how the strong nuclear force works. This is exactly analogous to the 2-fold electric charge which is fundemental to how the electromagnetic force works. If you understand what physical property + and - mean with regards to the electromagnetic force, then you also exactly understand what the 6 colors of color charge mean with regard to the strong nuclear force. Its the exact same sort of thing. --Jayron32 18:48, 27 April 2011 (UTC)

Think carefully about it. What do you mean by a physical property? For instance, what physical property does the mass of a particle represent? Dauto (talk) 18:57, 27 April 2011 (UTC)

Why couldn't it be the H-W-L qualities? —Preceding unsigned comment added by 165.212.189.187 (talk) 18:56, 27 April 2011 (UTC)

H-W-L are spacial properties. color charge are not spacial properties the same way that electric charges are not spacial properties. Dauto (talk) 18:59, 27 April 2011 (UTC)

How do you know: Photon 1 dimension = no charge; Electron 2 dimensions +,-; Quark 3 dimensions HWL? —Preceding unsigned comment added by 165.212.189.187 (talk) 19:02, 27 April 2011 (UTC)

Do you mean 0, 1 and 2 dimensions respectively? – b_jonas 19:34, 27 April 2011 (UTC)

I don't think so? How do you mean? —Preceding unsigned comment added by 165.212.189.187 (talk) 19:46, 27 April 2011 (UTC)

Again, I need to ask the question: Why are you so tied to the words used to describe the property? You seem to be having a problem conceptualizing that the word is not the thing itself. This is purely a linguistic problem. We've already conceded that the words you use to describe the quark charge thing are completely arbitrary, there's nothing inherently wrong with your system, excepting that it isn't any improvement on the current system, which has the advantage that its already used. Why is height-width-length any better to describe the system than red-green-blue is? The fact that HWL is coincidentally a 3-dimensional system? So isn't red-green-blue (see color space). What makes your three-dimensional system better than the existing three dimensional system already in use? --Jayron32 21:01, 27 April 2011 (UTC)

I guess its better to me only because it helps me understand.

That's cool. If you've got a model that works for you, then stick with it I guess; but you also need to be able to work with the existing model and understand it as well. --Jayron32 01:01, 28 April 2011 (UTC)

Why does coldness hasten the browning of a banana peel?

I took two nearly identical bananas and put one inside the cold refrigerator and one on the room temperature counter and after a couple of hours the one in the fridge was significantly more browned. I assume the chemical content of the air (%O2, %N2, etc...) is the same since the fridge door opens and closes frequently enough and there's nothing else in the fridge I think is giving off or absorbing gases much. The physical difference between the different temperature airs is then pressure. I don't have a bell jar with vacuum pump, though. Will a banana peel brown at an accelerated rate in a vacuum? Thanks. 20.137.18.50 (talk) 14:43, 27 April 2011 (UTC)

Surveying the Web, it's apparent that cold injury sets in below about 10 C. What happens is that the plant senses damage, and (perhaps via ethylene) activates enzymes such as phenylalanine ammonia lyase and polyphenol oxidase. It is possible to slow this down with a modified atmosphere with less oxygen and more carbon dioxide.[1] I saw claims on non-reliable sites that you could slow down the browning by keeping the bananas in a bag, but I don't know if the banana actually respires enough to build up CO2 in a bag. (the truth is out there, but I might go bananas trying to track it down) This is essentially a pigmentation reaction, producing melanin - though the details are not quite the same as the reaction in insects, the basic function of responding to injury or infection using a pigmentation reaction is the same. Wnt (talk) 15:49, 27 April 2011 (UTC)

Thanks for that information. You've well answered how the browning happens. "What happens is that the plant senses damage,..." How it does that (how it figures out that it's cold outside and to start doing what you described, which has a side effect of making them appear brown) would be a mechanism of interest. 20.137.18.50 (talk) 17:21, 27 April 2011 (UTC)

Blood supply to the dartos muscle

Does the dartos muscle in the scrotum have a named blood supply?

This is mostly for personal interest, but should possibly be referenced in the article. Kind Regards, Captain n00dle\^Talk 14:47, 27 April 2011 (UTC)

I did a quick search:

"The blood supply to the skin of the penis and the anterior scrotal wall are from the external pudendal arteries. The blood supply to the posterior aspects of the scrotum is from the posterior scrotal arteries, which is a branch of the perineal artery, which is a further branch of the internal pudendal arteries (5) (Fig. 2.1)

Branching off the medial aspect of the femoral artery are the superficial/ superior branches and the deep/inferior branches of the external pudendal artery. These superficial external pudendal branches pass from lateral to medial, in a variable pattern, across the femoral triangle, and within Scarpa's fascia (a loose membrane of superficial fascia; Fig. 2.2).

After giving off scrotal branches to the anterior scrotum, the superficial external pudendal artery cross the spermatic cord and enter the base of the penis as posterolateral and anterolateral axial branches. Together with interconnecting, perforating branches, they form an arterial network within the Dartos fascia. The Dartos fascia is not really the blood supply; it is more accurate to visualize the fascia as a trellis and the blood supply as the vine entwined on the trellis. At the base of the penis, branches from the axial penile arteries form a subdermal plexus which supplies the distal penile skin and prepuce (Fig. 2.3). There are perforating connections between the subcutaneous and subdermal arterial plexuses. These connections typically are minimal and very fine and, thus, a relatively avascular plane can be developed between the Dartos and Buck's fascia. Because the fascial plexus is the true blood supply to the penile skin flaps that we use in urethral reconstruction, the flaps are considered axial, penile skin island flaps that can therefore be mobilized widely and transposed aggressively.^[2]

I would like to copy the images from the book here, as not everyone will find themselves able to access this page from Google each time they check the link; but unfortunately, I don't think that Wikipedia's Fair Use image policy has thus far extended to uploading local Fair Use images for the Ref Desk archives. If people think we have a chance, this might be time to press the issue on behalf of one or more images. Alternatively, they might be redrawn from the source at some low level of quality (it really is pretty schematic as it is, presumably due to some anatomic variation that they discuss after the section I quoted). Wnt (talk) 22:14, 27 April 2011 (UTC)

Thank you very much for your answer, that was more than helpful! Regards, Captain n00dle\^Talk 11:47, 28 April 2011 (UTC)

Selling spent nuclear fuel

Why don't states that practice nuclear reprocessing buy the nuclear waste from states that don't? Is it uneconomical? (If so, by how much?) Or is it just political? I can see why the US would not sell to China, for example, because there is always a chance that any plutonium reprocessed in China could be alleged to enter into their nuclear stockpile, which would be political poison to whomever proposed it in the US. But there are other, safeguards states, like the Netherlands, or Japan, where this wouldn't presumably be a problem. Has this ever been seriously proposed? --Mr.98 (talk) 15:43, 27 April 2011 (UTC)

For perspective, clothing worn by technicians who work in laboratories near hallways connected to the reactor core is considered "low level nuclear waste" and can't be driven on non-Federal roadways without police escort. (A bit of hyperbole, perhaps, but not much - that's the regulatory environment that years of paranoia and FUD have created). The prospect of actually exporting actual nuclear material is so far off the political table right now, I can't imagine it ever being discussed by a serious high-level legislator. See Radioactive Waste from the US Nuclear Regulatory Commission for more information about relevant policy and procedure; particularly, transportation guidelines for spent fuel. Nimur (talk) 15:53, 27 April 2011 (UTC)

(In actual truth, there are only three locations that are legally permitted to accept incoming shipments of low level nuclear waste, including dirty laundry from nuclear facilities). My exaggeration in the previous paragraph was not that far off the mark. Nimur (talk) 15:57, 27 April 2011 (UTC)

Sellafield's Thermal Oxide Reprocessing Plant provided reprocessing for material from several nations: Japan, Germany, Switzerland, Spain, Sweden, Italy, Netherlands and Canada. [3] Waste was (and may still be) processed for a fee and returned to the origin. Polititally it seems undesirable for any nation to accept radioactive waste from another nation without a promise to return it. There are also numerous problems with transporting it, not least of which is the reluctance of intervening countries to let nuclear waste pass through, and while some of the public grudgingly admit the need for nuclear power, the need for reprocessing is less obvious. --Colapeninsula (talk) 16:01, 27 April 2011 (UTC)

According to Peak_uranium#Reprocessing_and_Recycling, mining uranium is far cheaper than reprocessing nuclear waste, so it does not make economic sense to buy waste in order to reprocess it. --Colapeninsula (talk) 16:06, 27 April 2011 (UTC)

Though you could imagine states like the US, who have literally zero long term waste policy at the moment (other than "store it on site at every individual reactor"), might find the service itself to be worth paying a premium for, well beyond what it would get them in terms of fuel. --Mr.98 (talk) 21:15, 27 April 2011 (UTC)

Part of the economic theory which justified the design of THORP in the 1970s was that there would be a market (subject to NNPT restrictions) for plutonium for fast breeder reactors.(ref:NewScientist 4 Aug 1977). But the UK didn't build a generation of commercial fast breeder reactors (ref: Bulletin of the Atomic Scientists March 1993), and they didn't really catch on elsewhere either. And given that nuclear weapons delivery systems have become so accurate, and the mania for competing in megatonnage has gone with SALT, START etc., the declared nuclear powers have a surplus of Pu anyway (and so have come to realise that it's a liability rather than a boon). If the proposed new generation FBRs catches on, there might after all be a decent market for Pu, but there seems to be quite a lot of it sitting around anyway (from military applications and existing reprocessing). This paper(from 2001) gives numbers then for surplus Pu and describes the plutonium fuel business as uneconomic and over-subsidised. -- Finlay McWalter ☻ Talk 21:50, 27 April 2011 (UTC)

ISS lights

I have been able to spot the ISS in the Earth's shadow with binos quite often. In fact, it is always observable when it is higher than about 20° above the horizon. I would say that it is roughly magnitude +8 when it is at 45° altitude, which by my calculations corresponds to a lightsource of about 30 Watt. Because over the last three years, the brightness is similar, I think that there is an outside lightsource on the ISS and it isn't light escaping through the windows. Does anyone know more about the lights of the ISS? Count Iblis (talk) 17:40, 27 April 2011 (UTC)

What makes you think it isn't just reflecting some combination of the ambient light reaching it from the Earth/Moon/Sun. It has giant solar panels and a shiny metal skin. I think it'd be a pretty decent reflector, and wouldn't need a porch light to be visible at night with some binoculars. --Jayron32 18:09, 27 April 2011 (UTC)

I doubt any external lights are normally on. They would serve no purpose except during an EVA, robotic operations, or docking/undocking with a visiting vehicle. You're probably seeing reflected earthshine. anonymous6494 18:25, 27 April 2011 (UTC)

Also, solar panels provide a limited amount of power. And every extra light fixture implies extra wiring, extra fixtures, etc, each one of them having a small chance of breaking and/or becoming a hazard and/or becoming a nuisance to do something else. Also, extra control circuits and extra items in your maintenance checklist. --Enric Naval (talk) 13:51, 28 April 2011 (UTC)

It is usually visible for a considerable time after the sun has set at the ground observer's location. Just because it's dark where you are does not mean the ISS is not in sunlight when you see it. Roger (talk) 18:29, 27 April 2011 (UTC)

I know, but if the ISS is not in the Earth's shadow it is magnitude -3.7 or so, easly visible to the naked eye (as bright as Venus). E.g. a few days ago, I could see it entering the Earth's shadow already above the Western horizon, and then it became invisible to the naked eye (brightness dropping rapidly from -3.7 to about +8). In binoculars it is then still visible. Then, when it rises toward the Zenith it actually brightens a bit to magnitude +7 or so and then, when it moves toward the East and starts to set, it becomes less bright. Count Iblis (talk) 18:50, 27 April 2011 (UTC)

(I'm going to assume that you meant -7 in the post above). The apparent brightness of the ISS viewed with the naked eye or through binoculars will depend on a number of factors. Less atmospheric haze, skyglow, or residual twilight on a given evening and time will make the station appear proportionately brighter because it will be seen against a darker background sky. Less haze and good seeing will also scatter less of the station's light on its way to you, meaning that it will be genuinely brighter as viewed, even if the amount of light it reflects is unchanged. Of course, it's not a good idea to assume that the amount of reflected light is unchanged—in fact it will vary quite a bit. The amount of light reflected will depend quite heavily on the relative position and orientation of station, Sun, and Earth-based observer. The moderately-experienced stargazer will be familiar with the swift (and often periodic) variations in apparent brightness associated with inactive satellites and other tumbling space debris that rapidly change their orientation relative to the Sun and Earth. While the ISS isn't tumbling, it's apparent orientation relative to the observer changes as it arcs across the sky. Usually this effect is subtle, but if one is in the right place at the right time one can get very bright specular reflections off the broad, flat solar panels. These so-called satellite flares are most often associated with the Iridium communication satellites, but observers have also recorded ISS flares—this short article has some incredible photographs. TenOfAllTrades(talk) 14:12, 28 April 2011 (UTC)

diminished brightness when closer to the horizon could be caused by the greater distance that the reflected light has to pass through the air to reach you.190.148.133.64 (talk) 21:28, 27 April 2011 (UTC)

There is a reason why distant objects appear brighter: Viewed from the moon it self the moon dust is quite dark, but from the earth the moon looks so bright it shines, I think that's probably part of the effect you're noticing. have a look at the pics on our ISS article, and if you're still not convinced, do a google image search for telescope iss photo, there are no obvious external lights to be seen. Vespine (talk) 23:27, 27 April 2011 (UTC)

Virtualy all the light you see reflected from the ISS is from the sun. This can be demonstrated by the fact that the brightness (and thus visibility) of the passes over a particular spot on the earth's surface are related to the passage of the station through the still present sunlight high above the observer, although the ground is in darkness. The station is never visible from the ground by naked eye more than about three hours after sunset because it passes throught the earth's full shadow when observed by a ground observer more than (say) three hours after sunset. The same principle applies before dawn. The station is visible from earth during evenings or pre-dawn periods even when there is no visible moon. I get my info from here Richard Avery (talk) 07:58, 28 April 2011 (UTC)

I think this thread is going off the rails a bit, perhaps I was not so clear in my postings here :) I'm talking about the ISS being in the complete shadow of the Earth, and then it is "invisible", but actually not quite invisible, you can still see it in binoculars. It seems consistently magnitude +8 to me when it is 45° above the horizon (and obviously a bit brighter when it is higher in the sky, but I have fewer observations of such cases and because it is then moving fast, it more difficult to estimate brightness). If you attribute that to a lightsource on board the ISS, you can compute that you need that lightsource to be about 30 Watt. You can easily imagine that 30 Watt can escape through the windows, or you could postulate that there is an outboard light that is always burning.

Now, Jayron32 says that it could be due to reflection of ambient light from the Moon, the Earth etc. But we can discount the Moon, because you can see the ISS in the shadow just as well during New Moon. Also, if the ISS is in the Earth's shadow, you won't have reflected sunlight from the Earth shining on the ISS. Then what remains are city lights. Now, where I live, there is a fair degree of light pollution, so you could imagine that this is possible.

To make the ISS appear to be magnitude +8, you need a total of 15 Watts reflecting off it (half of the 30 Watt because that 30 Watts was assumed to radiate isotropically). But to get 15 Watts of city-lights reflecting off the ISS, you need quite a bit more than of 10 GigaWatt of lights on the ground near a radius of a few hundred kilometers of where I am (I get 14 Gigagawatts when I compute flux by taking the reflecting area of the ISS its width times its length, which is obviously a big overestimate, and then assuming that most light sources are where the ISS apears 45° above the horizon, which is also not realistic). If I assume that most city lights have a luminous efficacy of 60 lumens/Watt instead of the 15 lumens/watt I've been assuming all along, you still are left with at least about 3 GigaWatts of street lighting which still doesn't sound realistic to me.

Instead, assuming that you simply have 30 Watts of lights (or perhaps just 10 Watts of higher luminous efficacy light source than an incandescent light bulb), escaping from the ISS, sounds more realistic. The only thing then is that the ISS seems to have the same brightness consistently over the last few years when I have seen it in the Earth's shadow. So, that's why I was wondering whether there is a (small) light source on the outside of the ISS that is always switched on. Count Iblis (talk) 15:12, 28 April 2011 (UTC)

You may simply be seeing cabin light escaping through the laboratory science window, a 510mm diameter circular glass window on the ISS' Destiny module. Subsequent to STS-131, the WORF is installed at that location, and when it is in use it can be configured to entirely block cabin light entering the WORF (and thus escaping to be seen by you). If that's the case, if you can find a mission schedule that shows when the WORF is and isn't swung over the window, if that correlates with changes in the apparent brightness of the unilluminated ISS, then the window is implicated. But you'd probably need a better calibrated system than binoculars and human eyes, and quite a lot of observations, to be sure. 87.115.52.162 (talk) 15:40, 28 April 2011 (UTC)

...although they probably keep the external shutter on that window closed most of the time, to avoid it getting scuffed up unnecessarily (by micrometeorites). 87.115.52.162 (talk) 15:47, 28 April 2011 (UTC)

How about heating from atmospheric drag? Probably very little of it, but maybe 15W worth. Googlemeister (talk) 13:02, 29 April 2011 (UTC)

It is not clear to me that the dissipated energy would be in the form of visible light. From the drop rate of about 1 km per ten days, I find that about 2100 Watt is being dissipitated due to friction. Count Iblis (talk) 16:09, 29 April 2011 (UTC)

Perhaps Googlemeister's suggestion does work. We only need to get a few Watt of light energy out of the 2100 Watt, the 30 Watt is the power of a lightbulb which only has a luminous efficacy of 15 lumens/Watt. If all the light energy were at 555 nm, then we would only need 0.7 Watt. So, the question is if the frictional processes the ISS is subject to, can lead to something of the order of, say, 0.2% of the energy being emitted in the form of visible light. Of course, the space station being heated by 2100 Watt won't make it radiate thermally into the visible spectrum. However, the collisions of the atoms with the space station can generate light non-thermally. In a collision of a Nitrogen atom with an object at a relative speed of 8 km/s, 4.6 eV of energy is available for causing atomic transitions, enough for the atom to emit light. Count Iblis (talk) 17:15, 29 April 2011 (UTC)

Do fluorescent lights have a preferred frequency when they flicker?

Resolved

As the title says, I'm wondering about the flicker in fluorescent lights. Not a flicker as they power up or down, but a sustained flicker that indicates something is not functioning properly. I observed this today, and while the pulses were not of even intensity (very bright pulses ~1/sec), I began to suspect the underlying frequency (almost too fast to notice) may be related to the 60 Hz of AC power. Is there any merit to this idea? Does the frequency of AC current have any effect on the frequency of flicker in the light? Thanks, SemanticMantis (talk) 21:47, 27 April 2011 (UTC)

See Fluorescent_light#Flicker_problems. Wnt (talk) 21:57, 27 April 2011 (UTC)

Oops, I should have checked there more thoroughly. Thanks! SemanticMantis (talk) 23:29, 27 April 2011 (UTC)