Wikipedia:Reference desk/Archives/Science/2008 October 18

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.

October 18

How large can a Trojan planet be?

I'm trying to get a rough, seat-of-the-pants idea of how large a Trojan Planet or Trojan Moon could be relative the closer of the two bodies creating its L4 or L5 Lagrangian points. The Lagrangian point article only says "it needs not be negligible mass." How large of a Trojan Moon could Jupiter-Sun L4/L5 support, for instance? How massive a space station could Earth-Sun L4/L5 support? I anticipate that the main problem has to do with planet formation, and that the fuzzy nature of a Lagrange point causes formation-by-accretion to be unlikely. What I'm wondering is how large a planet could be held-onto in the unlikely event of capture. Thanks in advance for anything anyone has to say. DeepSkyFrontier (talk) 03:49, 18 October 2008 (UTC)

Theia (planet) mentions this issue, although it doesn't give any exact numbers. From that article, it would seem that a planet larger than Mars would not be stable at the Sun-Earth L4 or L5 points, but it doesn't say how much less than that it needs to be to be stable (it was about Mars sized when it is presumed to have hit the Earth, but that could well have been some time after it lost stability so it could have kept growing for a time). --Tango (talk) 12:27, 18 October 2008 (UTC)

Giant impact hypothesis says it was about Mars sized when it lost stability. --Tango (talk) 12:30, 18 October 2008 (UTC)

I remember discussing this on Usenet once... ah, here it is. The conclusion is that the restricted three-body system is stable regardless of the mass ratio of the smaller bodies, as long as the largest body's mass is at least 25.23 times the total mass of the other two bodies. Conversely, if the largest mass is less than 24.96 times the sum of the other two, the L4/L5 points will be unstable — only in that rather narrow range does the mass ratio of the smaller bodies matter. The Sun is about 1000 times as massive as Jupiter, so it seems that Jupiter's L4/L5 points ought to be stable regardless of what you put there (at least ignoring perturbations from other bodies in the solar system), as long as it's smaller than about 40 Jupiter masses. —Ilmari Karonen (talk) 00:36, 21 October 2008 (UTC)

orchic

Is orchic supplement bennificial ? —Preceding unsigned comment added by 68.221.224.43 (talk) 05:05, 18 October 2008 (UTC)

I don't see how. "Orchic supplements" (bovine testicular extracts) are broken down by the digestive tract, meaning they cannot be pharmacologically effective. They may exert a placebo effect, but other than that....they're pure bollocks ;-) Fribbler (talk) 11:58, 18 October 2008 (UTC)

I couldn't find any reliable sources about this. However Fribbler's second comment: "Orchic supplements are broken down by the digestive tract, meaning they cannot be pharmacologically effective" is not necessarily correct. Prescribed oral medications do have pharmacological activity, despite digestion. There are many plant extracts (medicinal herbs) that do have pharmacological effect: opium, belladonna, digitalis, etc.. Axl ¤ [Talk] 13:24, 18 October 2008 (UTC)

Of course oral medications are pharmacologically active. I should have been more clear. Testosterone, the "active ingredient" of Orchic Supplements, is heavily affected by digestion. Here is a link: [1]. This is why testosterone used in the treatment of hypogonadism is given either intramuscularly or transdermally: [2] Fribbler (talk) 16:12, 18 October 2008 (UTC)

Sure, testosterone would not be absorbed when eaten. However this might not be the only "active" component of orchic. I couldn't find any reliable evidence of appropriate testing of orchic. Axl ¤ [Talk] 16:39, 18 October 2008 (UTC)

Me neither. A red flag in my book. The medical community appear to be uninterested in this supplement. Fribbler (talk) 15:35, 19 October 2008 (UTC)

sex without condom

What are the most probable infections for a person who practices sex without condom?Mr.K. (talk) 09:32, 18 October 2008 (UTC)

Try reading the article Sexually transmitted disease. Jdrewitt (talk) 09:40, 18 October 2008 (UTC)

Note that I think you are implicitly meaning "with multiple partners" here. --98.217.8.46 (talk) 09:44, 18 October 2008 (UTC)

There are, in fact, two separate questions. One is how likely it is to become infected from an infected partner, the second is closely related to the statistical distribution of STDs in the population. Actually, a third interpretation would lead to an answer like Herpes simplex (infection rates of close to 90%), athletes foot or common cold ;-). --Stephan Schulz (talk) 11:33, 18 October 2008 (UTC)

Genital Herpies is pretty easily the most common STD, and it can infect you ( its a virus ) with a comdom, so condom or hot, the most probible infection is going to be the most common STD. $--~~~~$ —Preceding unsigned comment added by 99.185.0.29 (talk) 13:56, 18 October 2008 (UTC)

While HSV is exceedingly common, it is cumulative and most people don't have constant heavy shedding. This illustrates why you should not use prevalence to estimate incidence. Chlamydia may have higher incidence in some populations. Stephen Schulz was right in saying that it depends on specific factors (including locale, ages of involved persons, and what sexual practices are involved). --Scray (talk) 14:25, 18 October 2008 (UTC)

I was actually surprised a while back reading our article on HIV that vaginal unprotected sexual intercourse only gets you a 5 to 10 in 10,000 chance of receiving the virus, I had thought it was much much higher. (Not that it's worth even that risk!) -- MacAddct1984 ^{(talk • contribs)} 19:56, 18 October 2008 (UTC)

Those estimates are averages from multi-year follow-up of couples in whom coital frequencies have been recorded. This type of research involves finding discordant couples, i.e. couples with one person who has HIV and another who does not, then following them for years to determine risk. If you ask them on a regular basis how often they are having sex, and what they do when they have sex, then you can get an estimate per act. The problem with this approach is that these couples may not represent the setting in which HIV is usually spread.

It's known that there is an early phase of HIV infection when the individual tends to have very high levels of HIV in blood and body fluids. People are more susceptible to getting infected if they have ulcerating STDs, and they may be promiscuous. Thus, susceptibility to infection might occur in peaks and valleys, and when someone is at their peak of susceptibility they may also then be most infectious - with a high level of virus in body fluids, ulcers, and promiscuity - sort of super-transmitters. This would explain observations of rapid spread within mini-epidemics, and is supported by what's known about transmission mechanisms. It's likely to be very hard to find these people, in part because it's a transient state.

Adding to the confusion is that the same person could be highly infectious for a few weeks or months, then a year or two later might be different in many ways, with lower HIV level (their viral load "set point"), ulcers healed, and in a stable relationship. --Scray (talk) 00:42, 19 October 2008 (UTC)

STDs have different prevalences in different populations, so it depends on where you are, but really it only depends on who you are sleeping with. If your partner is clean, then the only STD you really have to worry about is pregnancy. --71.178.135.144 (talk) 06:41, 20 October 2008 (UTC)

size of a lysosome?

Hi could anyone give me an estimate of the length of a lysosome? The article on lysosomes does not give a value. Presumably we're talking nm here. I realise sizes will vary from cell to cell but a rough estimate would be good. Thanks in advance. —Preceding unsigned comment added by 139.222.240.110 (talk) 15:39, 18 October 2008 (UTC)

On a similar topic, does anyone have some kind of semi-visual (the parts that are possible) to compare sizes between things such as cells, mitochondria, chromosomes, genes, neurons... and then atoms, certain proteins/amino acids, you get the idea. I'm well aware that a cell has an insane amount of atoms, which is why I said semi-visual. :) -- Aeluwas (talk) 15:54, 18 October 2008 (UTC)

From "Cytology & Histology" by Wolfgang Kuehnel, p. 34: "Their sizes are 0.1 – 1.2 μm." Axl ¤ [Talk] 17:31, 18 October 2008 (UTC)

...With the caveat that they can get much larger in the lysosomal storage disorders. TenOfAllTrades(talk) 18:49, 18 October 2008 (UTC)

Thanks —Preceding unsigned comment added by 139.222.241.15 (talk) 11:36, 20 October 2008 (UTC)

where does the air in hypercaviation come from?

the supercavitation article says "drag is normally about 1,000 times greater in water than in air."

However, isn't the gas water vapor (h2o in gas form) NOT air (homogenous mixture, largely nitrogen, o2, etc)...? If it is indeed "air", where would it come from? —Preceding unsigned comment added by 79.122.55.76 (talk) 17:35, 18 October 2008 (UTC)

Yes, it's water vapour. The supercavitation article says "The pressure of the fluid can drop due to its high speed (Bernoulli's principle) and when the pressure drops below the vapor pressure of the water or the temperature increases thus vapor pressure increases reaching water pressure, it vaporizes — typically forming small bubbles of water vapour (water in its gas phase)." -- Finlay McWalter | Talk 17:42, 18 October 2008 (UTC)

Defrosting beef

Is it possible to defrost minced beef by putting it in the oven at a low temperature (about 50 degrees centigrade)? —Preceding unsigned comment added by The Defroster (talk • contribs) 18:21, 18 October 2008 (UTC)

Of course it is possible, but the outer part will tend to cook before the center is melted. Externally-applied heat will maximize the temperature gradient between center and exterior meat. If you were to slice the minced beef thinly, this effect won't be much of a problem. I don't see a WP article right off, but here's a link: [[3]] --Scray (talk) 18:36, 18 October 2008 (UTC)

It's worth noting that prolonged incubation at 50C could encourage bacterial growth in the interior of the meat. This is another reason to thaw at refrigeration temperature, or warm using more rapid or uniform heating methods (see the link previously provided). —Preceding unsigned comment added by Scray (talk • contribs) 18:46, 18 October 2008 (UTC)

In the case of minced beef, I generally find that it works fine to just start cooking it as you normally would - if you're frying it it defrosts pretty quickly as you do it as it has such a high surface area. ~ mazca ^t|c 19:03, 18 October 2008 (UTC)

I agree. If frying it, I usually (more or less) take it out of the freezer. Microwaving it for a while helps, and that's what I would do if I needed it thawed (to make meatballs, for instance). -- Aeluwas (talk) 19:11, 18 October 2008 (UTC)

Are you referring to Ground beef? Edison (talk) 19:19, 18 October 2008 (UTC)

I assumed he was... what else does the phrase "minced beef" refer to that could be confused? ~ mazca ^t|c 23:58, 18 October 2008 (UTC)

Mince means "cut into small pieces," which sort of fits ground beef, but it also means "to pronounce in an affected way," like saying "boeuf" for "beef" in English speaking countries, or it means to "walk with short steps or exaggerated primness," which would be atypical behavior for cattle. Edison (talk) 01:14, 19 October 2008 (UTC)

It depends on the person you ask, but to me ground meat is produced using a meat grinder and usually passed through the machine one ore twice. Mince beef, on the other hand is cut finely by hand and usually has a meatier and less "floury" texture when compare dot ground beef.Sjschen (talk) 02:47, 19 October 2008 (UTC)

If a recipe told me to prepare a quantity of "minced beef" I would slice and dice it as finely as possible with a sharp knife on a cutting board. As Sjschen said, to make "ground beef" I would run it twice through a grinder. "Two great nations divided by a common language." Edison (talk) 04:13, 19 October 2008 (UTC)

...and don't confuse your minced meat with your mincemeat. Gandalf61 (talk) 08:49, 19 October 2008 (UTC)

I have no problem understanding what someone means when they say mince beef or ground beef (even if the second is not something I'd use in conversation/writing) as I expect many kiwis and I expect Brits and Australians. It's only other people who have funny ideas about what mince beef is... Perhaps it's only one great nation that is at fault? :-P P.S. Before you argue it's my fault for not understanding your meaning for mince beef, I should ask, how often do you use or encounter 'mince beef'? Surely the more common use of the term should be given preference of one used rarely... ;-) Nil Einne (talk) 09:41, 19 October 2008 (UTC)

Heh, I was unaware of the extent of the American use of "grind" for where I (as a Brit) would use "mince". An American apparently uses a "meat grinder" to produce "ground beef" while I would use a "mincer" to produce "minced beef", with both the equipment and the final product being exactly the same. While to an American "mince" may instead mean "chop finely", to me "grind" implies reducing the meat to a horrible homogenous paste. ~ mazca ^t|c 11:54, 19 October 2008 (UTC)

Same here in Oz but being so far away from birth of the terms, we can mix them too, getting mince from a grinder. The current answer to an otherwise homogenous paste is to pack it with wavy grooves resembling nothing I can think of. ~:\ Julia Rossi (talk) 13:27, 19 October 2008 (UTC)

Don't forget spraying red dye on the outside of the lump to make it look fresh-killed. At least, that's how they do it here in Canadian supermarkets (another country where we (or at least I) can assimilate the concept of ground and minced being the same). Franamax (talk) 20:45, 19 October 2008 (UTC)

I don't think that's red dye, at least not in the U.S.A. It's a tiny amount of carbon monoxide gas placed in the package just before sealing, considered safe and simply cosmetic by the FDA and meat-packing industry. Seems people won't buy brown meat generally, and I would guess that's true. --Scray (talk) 01:30, 20 October 2008 (UTC)

To defrost something quickly and safely, place it in your sink with a small amount of water. Turn your tap on so that barely a trickle of cold water comes out. You want a stream, not drops. The purpose is to keep the piece immersed in moving cold water; the coldness of the water delays the proliferation of bacteria, the movement facilitates the contact of fresh water with the frozen product. Before long, the piece will assume the temperature of the surrounding water (i.e. defrost). Because of water's thermal properties, the meat will defrost faster than it would on the counter. And much more safely. Matt Deres (talk) 14:24, 19 October 2008 (UTC)

Yes, this is what was recommended in the link at the end of my original answer to the post. --Scray (talk) 15:42, 19 October 2008 (UTC)

Toaster in the ocean.

What would happend if you threw a toaster with a very long extension into the ocean?Bastard Soap (talk) 19:50, 18 October 2008 (UTC)

Lots of sparks around the toaster, then shortly a wire will burn through, breaking the circuit. Charge density will tend to disipate with the square of distance from the charge source, so anyone or anything near the toaster is likely to get a nasty shock; anyone a considerable distance away will notice nothing. --Jayron32.talk.contribs 20:18, 18 October 2008 (UTC)

If you threw a whole toaster in the ocean there is no obvious reason that melting would kill the circuit. A) the water will cool the wires, and B) conductive salt water will continue to carry current around the break even if some wires melt. Obviously a circuit breaker/GFI could stop the thing, and I suppose one might also melt part of the extension cord above the water, but if neither of those things happened I don't see any obvious reason for the circuit to ever shut down on its own. Dragons flight (talk) 00:15, 19 October 2008 (UTC)

It would get wet. Hopefully you unplugged the cord first for safety. If still plugged in the element would heat up the water and put some hazardous currents in the sea water. Hopefully a fuse would blow before the wires melt or ELP would disconnect it. Graeme Bartlett (talk) 20:38, 18 October 2008 (UTC)

Who is ELP? —Preceding unsigned comment added by Seans Potato Business (talk • contribs) 22:22, 18 October 2008

Most likely Earth Leakage Protection. -hydnjo talk 22:50, 18 October 2008 (UTC)

If you were smart/lucky enough to have plugged the extension cord into a GFI protected outlet then the stupid act of tossing a plugged-in toaster into the ocean would cause little harm except to the toaster. I suppose you could pull it back by the extension cord and try and try again but the tripped safety device would remain tripped and you would have no clue as to what was going on. How about making some toast instead? -hydnjo talk 22:34, 18 October 2008 (UTC)

For the sake of clarity: ELP and GFI (or GFCI) are the same thing. This form of protection has a large number of different names and the best-known ones vary from one country to another. Another one I remember seeing is RCCB. --Anonymous, 23:56 UTC, October 18, 2008.

Oh, and besides the usual "don't try this at home" admonishment regarding this dangerous stunt you'll probably not know how to properly secure the toaster cord to the extension cord so forget about pulling it back, it'll be sleeping with the the fishes. -hydnjo talk 00:18, 19 October 2008 (UTC)

I would not expect sparks, despite the drama shown in movies when electrical appliances encounter water. I have seen flooded electrical vaults where the water simply boils without great drama due to submerged 120/208 volt bus bars. Saltwater is not as conductive as some think. Current would flow through the salt water from the phase to the neutral connected wires near the entry point of the power cord, where the switch is, but only a small area of conductor is exposed with full phase to neutral voltage between the conductors. The heating element would continue to carry the current it carried in normal use (neglecting the slightly lower voltage due to the additional drop in the extension cord). More current would likely flow through the heating element than directly through the water. The heating element would not get red hot due to the cooling effect of the water. Current would also flow from the heating element directly to the ground connection afforded by the ocean, so that if it were connected to a Ground fault interrupter it would trip offline in milliseconds. Any bread in the toaster might cook, but it would never toast satisfactorily.There would be bubbles of hydrogen and oxygen liberated due to Electrolysis. Anyone attempting the experiment runs a serious risk of death from electrocution, and anyone in the water near the toaster might be electrocuted. Edison (talk) 00:51, 19 October 2008 (UTC)

Thanks Edison for your serious and comprehensive response to an inquiry that some of took as frivolous. -hydnjo talk 01:37, 19 October 2008 (UTC)

Bah! Cut the praise. I don't like Edison's answer. Specifically, the claims that "The heating element would continue to carry the current it carried in normal use" - and that "Saltwater is not as conductive as some think" are entirely misleading. We need to look at the numbers here...(So many of my answers start that way!)

The current has to follow the lowest resistance path - so we need to look at the resistances involved here. Since this is the science desk rather than the guessing desk - I just performed the simple experiment of measuring the resistance of the heating element in my toaster with a meter. I used the cold resistance on the assumption that the ocean water will prevent it from heating up so much (and because I didn't want to plug my toaster in while it was in pieces on my bench!)...and it measures 6,000 ohms. The hot resistance is going to be different - but (as we'll see) - it's not going to matter a damn. The resistance of sea water is just 0.2 ohms per meter. Is this "not as conductive as some think"? It's hard to tell.

So will the current flow through the sea-water between the two exposed metal contacts inside the switch of the toaster (less than 1cm apart in my toaster) or will it flow through the 6kohm heating element? Well, let's approximate this as a simple parallel circuit comprising one 6kohm resistor (the heating element) and one 0.002 ohm resistor (1cm of salt water). In a parallel circuit, the current splits by the reciprocal of the resistance - so the current that flows through the heating element is 0.002/6000 times less than flows through the water. Yes, TECHNICALLY, some electricity will still flow through the element - but unless the extension cord can carry 6 megawatts (the output of a small powerstation) - the heating element WON'T carry more than an utterly negligable much of the available current and it certainly won't get even slightly warm. In fact, it doesn't matter a damn whether it's a toaster or just the exposed end of an extension cord that we toss into the ocean - the result will be the exactly same.

If we assume that any protection device inside the toaster gets wet also - then it will get shorted out by the water on the 'hot' side of the circuit and since almost zero current will flow through it, it will utterly fail to operate. So a lot of current will flow through the water - at 110volts, ohms law says: I = V/R = 110/0.002 = 55,000amps. At 110 volts - that's 6,000,000 watts. That's a LOT of current. If the other end of the extension cord is plugged into any kind of protection device (a simple fuze, an ELCB, GFI or whatever) - it's obviously going to trip with that much current flowing. But if we hypothesise that there is no protection of any kind - then the next question is whether the copper in the cord will melt before the water boils. Certainly there is no extension cord in existence that can carry six megawatts - so clearly it'll break somewhere if it has to carry anything like that much current.

But it's a bit more subtle than that. If the water does boil - then you're concerned with the electrical resistance of steam - which is in the many-mega-ohms per meter range (a lot depends on temperature and pressure). But once the water boils, the circuit is broken and hardly any current flows until the bubble rises and is replaced by more sea water - so the average current flow will be vastly less than 6 megawatts - and perhaps the cable can survive. If so, then the system can keep that up until the ocean boils dry. Over one second, 6 million watts will produce 6 million Joules of heat energy. The Latent heat of water is 2.5MJ/kg - so one second of 6 million watts is enough to flash-boil more than two liters of water and the little bit of water between the the two wires might only represent a couple of cc's - so it's going to boil in a fraction of a millisecond. Since this is actually AC current - the current flowing as the voltage reaches some small fraction of a volt will be enough to create a bubble of steam and cut the current for a while. Since there is some capacitance in the system, when the connection re-establishes as the bubble moves out of the way, the voltage will build up slowly until enough current flows to boil another bubble. The voltage will never reach 110v and the current will be limited by the amount required to boil water at whatever rate it flows in as bubbles move out of the way. As soon as a continuous wet-path between the contacts forms, it'll immediately boil out of the way and cut the circuit again.

It's harder to figure the melting of the copper because too much depends on how well cooled it is by the sea water and on whether there are weak points in the cable (eg at the connections at either end. However, because the mechanism of bubble formation moderates the current flow to that required to boil fairly small amounts of water - I strongly suspect that the formation of bubbles at the end of the cable in the water will moderate the average current flow to the extent that the copper would not melt.

Conclusion. If there is any kind of protection device present in the circuit - it'll trip and end the experiment. If there is no protection whatever - then almost certainly the water near the end of the cable will boil and not much else will happen.

SteveBaker (talk) 15:30, 19 October 2008 (UTC)

This question should be graced by the answer "A wet salty toaster" ;) Sjschen (talk) 02:44, 19 October 2008 (UTC)

My answer would have been "wet toast", but I think that Steve's (usually great) answers have gone a little off track on this one. First of all, the units of resistivity are ohm-metres, not ohms per metre. The resistance of a 1 cm cube is not the resistance of a 1 metre cube divided by 100. The 1 cm cube is x100x less because of the decrease in length but x100² more because of the decrease in csa, making a net x100 increase in resistance. If a metre cube of material has a resistance of 0.2Ω then a 1 cm cube of the same material will have a resistance of 20Ω. Secondly, you will only get that resistance if the field is linearly distributed throughout the cube. This can be achieved by metallising the ends of the sample cube with a near perfect conductor (copper is good enough for most materials). The field from a point contact, eg your meter probes or the ends of the cable chucked into the sea will give you a completely different (much higher) result which needs some more than simple maths to calculate. So being a simple person, I have just put the probes of my meter in a half-pint glass of warm water saturated with salt. At a distance apart of 1 cm my meter measures around 100kΩ betwen the points of the probes and 55-60kΩ if I immerse them in the liquid. more than most peaple think . . it is cetainly more than I was expecting. I don't think you are going to get 55,000 amps going through that very easily. By the way, I have come across the use of a jar of water as a makeshift fuse while testing high current machinery. Too much current causes the water to boil away and break the circuit. SpinningSpark 18:08, 19 October 2008 (UTC)

How would we get wet toast? I thought we've established the toaster isn't going to toast in sea water? :-P Unless you put toast in to re-heat it you're going to end up with soggy, mushy bread Nil Einne (talk) 04:34, 22 October 2008 (UTC)

Steve's answer , excellent if off by a few orders of magnitude, assumes that the resistance of the heating element is higher than I expect it would be found in practice, and that the area of the exposed-to-seawater conductors is far greater, and possible at a closer spacing that I expect. I have used saltwater as a resistive element and a large area of electrode had to be immersed in the water to get appreciable current. I hate to ses the incorrect notion "electricity follows the lowest resistance path" repeated when, in fact, electricity follows ALL paths, in inverse proportion to the resistance. A resistance heating element immersed in water gets quite hot, if not red hot, changing the resistance from the value measured cold. If Steve's toaster had 6000 ohms resistance with 120 volts applied, it would only have 2.4 watts of energy dissipated. My toaster is rated 120 volts, 780 watts, which would imply a hot resistance of 18.5 ohms, which was about what I found the cold resistance to be, although the measurement was with a battered analog volt-ohm meter of dubious accuracy. The heating element would draw 6.5 amps in normal operation. I do not see evidence that it would draw much less immersed in seawater, or that the area of the exposed wires is sufficient to allow a large current to flow through the water from conductor to conductor. The inevitable buildup of bubbles of hydrogen and oxygen on the wires would further decrease the current flow through the water. I have not found a protective device inside toasters I have disassembled which would cause them to trip when immersed, unless the unit is plugged into a residual current device/ground fault interrupter. Edison (talk) 00:25, 21 October 2008 (UTC)

MRI history

When, where and which manufacturer installed the first commercially-available clinical MRI? —Preceding unsigned comment added by Henpecked (talk • contribs) 20:12, 18 October 2008 (UTC)

Not sure on commercial availibilty, but there is considerable controversy over the first workable NMR machine. There are dueling claims from both Raymond Vahan Damadian, who holds the first patent on what would later be called MRI, while Paul Lauterbur and Peter Mansfield won the Nobel Prize for producing theraputically viable MRI. No one disputes Damadian's contributions as coming first; there is however considerable debate over how much Lauterbur and Mansfield contributed to the field. Some say that Damadian's initial discoveries were the most important discovery; others claim that it was Lauterbur and Mansfield's work that made it a viable technology in the medical field. Click the links to get more info on all of these topics. I hope that helps some! --Jayron32.talk.contribs 20:22, 18 October 2008 (UTC)

As for where the first clinical MRI scanner was installed, I believe it was in my home town of Nottingham in the Queen's Medical Centre. —Cyclonenim (talk · contribs · email) 22:42, 18 October 2008 (UTC)

Quick eye shift breaking white into RGB

I was watching a movie which was being projected via one of those projectors which I suppose was fairly cheap and marketed to domestic users (not sure if the cost is relevant). When I looked at the screen and then shifted my eyes quickly, I noticed that the white areas of the screen appeared to break down into red, green and blue components. What's going on there? I've noticed it before, very occasionally, but it doesn't happen for example, with my PC monitor.

Cool! I wonder if it was a technology with sequential presentation of the three images and a fairly slow scan rate? Edison (talk) 01:00, 19 October 2008 (UTC)

Assuming you have a DLP projection system, see the DLP "rainbow effect" in single-chip systems. -- Tcncv (talk) 01:15, 19 October 2008 (UTC)

Yep. Normal TV's and computer monitors show the red, green and blue images simultaneously. In a single-chip projector, there is a monochrome image and a rapidly spinning three-color wheel in front of it that allows the system to display first all of the red, then all of the green and finally, all of the blue. There are three images displayed over every 1/60th of a second - so each image is there for 1/180'th second - but persistance of vision merges all three together visually into a single image. If you blink, you miss one or two of the colors so you get a brief flash of a magenta, yellow, cyan, red, green or blue image. (I find this effect particularly disturbing) But if some small object moves quickly across the screen there is something more complex going on. Because the three images are displayed separately - they appear at different times. Yet all three images are of the object have it in the same place on the screen - we get a rather peculiar effect. Imagine you plotted a graph of position on the screen versus time. The graph would look like this:

  ^   |                           . . .
  |   |                    . . . 
Posi- |              . . .
tion  |       . . .
      | . . .
      +------------------------------------
            Time ==>

(each dot being a red, green or blue 'rendering' of the object)

...instead of a nice straight line, we have a staircase. What our brains do with this ought to be something like seeing a jerky motion instead of smooth motion - but at these kinds of frame rates, what we actually "see" is three objects moving on a set of parallel paths across the screen. Since each one is a different color, we see a red object, a green object and a blue object moving close to each other - separated by one third of the distance the object moves in a single 1/60th second frame. For slow objects, this isn't noticable - for faster objects, you see a rainbow fringing effect and for very fast objects, you see three separate things tracking across the screen.

When you move your eye rapidly across the screen - it's just like the entire scene is moving rapidly in the opposite direction - and the result is the same as if the entire picture were moving quickly - so the fringing happens all over the screen instead of in just one place. That's why the effect is most noticable when you do that.

More modern projectors (or at least, more expensive ones) have three separate projectors inside - so it can display red, green and blue at the same time - which completely eliminates all three problems (fast eye motion, fast object motion AND blinking). Some intermediate price systems display at even higher than 180 frames per second (eg 360 frames per second) and they show red,green,blue,green,red,blue in every 1/60th second frame. This helps by confusing the eye and making it harder for the eye to misinterpret what you're seeing. That helps the blink problem a lot (because you can't blink and miss (for example) both green presentations and end up seeing a magenta image. The color fringing is also somewhat disguised.

SteveBaker (talk) 13:29, 20 October 2008 (UTC)

Weight of bones

A bird skeleton equals only about 5% of its mass. How does this compare with mammals? —Preceding unsigned comment added by 66.121.22.163 (talk) 22:27, 18 October 2008 (UTC)

According to our article on the Human skeleton, it accounts for about 20% of a human's body mass. I am not sure how representative we are for all mammals, but there you go... --Jayron32.talk.contribs 22:32, 18 October 2008 (UTC)

Heavier animals devote a larger percentage of their body mass to their skeleton than do lighter animals. Small mammals (e.g. mice and things) are generally also in the several percent range. [4] Dragons flight (talk) 23:07, 18 October 2008 (UTC)

• For the reason this is true, see square-cube law and particularly the "Biomechanics" section. --Anonymous, 23:59 UTC, October 18, 2008.

According to the Natural History Museum of Los Angeles Co., the weights of a bird's skeleton and that of a mammal of equal size are comparable, noting Dragons flight's observation that the larger the animal the heavier the bones. - Nunh-huh 23:11, 18 October 2008 (UTC)