Wikipedia:Reference desk/Archives/Science/2009 August 8

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.

August 8

Ringworm

What causes ringworm to be circular shaped? --jpgordon^{::==( o )} 00:06, 8 August 2009 (UTC)

It's a fungal infection. Probably it starts as a tiny dot at the center - as it spreads outwards, the damage it does is being repaired by the victims' body - so you wind up with a circular infected area with a smaller circular healed area in the middle. But that's just a guess...I don't know for sure. SteveBaker (talk) 02:38, 8 August 2009 (UTC)

That seems to be vaguely confirmed by the literature (it seems the fungus feeds on keratin) - though I can't find an explicit statement of the reason.83.100.250.79 (talk) 15:38, 8 August 2009 (UTC)

The same principle applies, but not the same effect results, in Fairy Rings. 86.4.181.14 (talk) 07:17, 8 August 2009 (UTC)

<curmudgeon> Does circular shaped mean something other than circular ? </curmudgeon> —Tamfang (talk) 04:28, 11 August 2009 (UTC)

In some contexts, yes. A circular argument or a circular definition is quite different from a circular-shaped argument or a circular-shaped definition. In British English, a circular might also be a piece of junk-mail - which might or might not be circular-shaped. SteveBaker (talk) 13:10, 11 August 2009 (UTC)

Surely circular-shaped means shaped like a circular. —Tamfang (talk) 03:48, 13 November 2023 (UTC)

Negative Matter Superluminal Travel???

Suppose there is Negative Matter. Assume we have a negative positron at position p. When it moves to another position, say k, subluminally external observers may interpret this as an electron moving back in time. Is this counted as superluminal travel? Also, this travel into the past does not violate casualty because the information is sent from future to the present, which affects the future.
The Successor of Physics 04:18, 8 August 2009 (UTC)

What is "Negative Matter"? --Tango (talk) 04:49, 8 August 2009 (UTC)

Do you mean Antimatter? Mitch Ames (talk) 04:56, 8 August 2009 (UTC)

Antimatter moves through normal space in the normal way. It does not travel back in time.

There is no such thing as a "negative positron".

I don't understand how moving a "negative positron" from one place to the other would be the same as moving an electron back in time.

If information was sent from the future to the present it would absolutely violate causality as we understand it.

You might be trying to describe Tachyons. Keep in mind that Tachyons are a hypothetical construct. There is no theoretical or experimental evidence of their existence. APL (talk) 05:29, 8 August 2009 (UTC)

I think the OP might be talking about Feynman's idea of anti-particles being thought of as their regular counterparts traveling backward through time. It's important to stress that this is just an interpretation used to help intuitively understand what's happening. A given particle exists at various point in time-space, but it's not really meaningful to say that a particle is traveling in one direction along that path or the other. There's also no violation to causality. If we assume determinism in both directions, then the state of the universe at one time determines both the state immediately before and immediately after. The direction of causation is which ever direction we choose to look at things from, so it's not really meaningful to say that in this case events in the future are causing events in the past. It's just a matter of perspective. Rckrone (talk) 05:53, 8 August 2009 (UTC)

Guys, I think you should take a read on the (redirected, but still) page on Negative matter. It's not the same as antimatter. (One may not exist, while the other certainly does!) -- Aeluwas (talk) 07:36, 8 August 2009 (UTC)

Thanks, Aeluwas, thats the negative matter I was discussing! The Successor of Physics 13:22, 8 August 2009 (UTC)

"Negative matter" doesn't mean anything to a physicist, the existence of a Wikipedia redirect notwithstanding. It could just as well redirect to antimatter. The anti-positron is the electron, but exotic matter is supposed to be an entirely different kind of matter that has negative mass (and is presumably also different in other ways), so there's no such thing as an exotic positron. I don't think it's true that negative mass going backwards in time would be like positive mass. Mass isn't like charge—there's no symmetry of nature that exchanges positive and negative mass (in the presence of gravity). CPT symmetry swaps positive and negative charge as it reverses the direction of time, but it doesn't swap positive and negative mass. -- BenRG (talk) 16:19, 8 August 2009 (UTC)

Negative matter has been hypothesised, the fact that it remains unobserved doesn't detract from it status as a hypothesis. Having said that, I would otherwise agree with BenRG that inversion of T is equivalent to inversion of C and P, but no such known symmetry exists for mass. On an ending note, a symmetry that involves the inversion of T does not constitute superluminal travel. The particle is still going forward in time. —Preceding unsigned comment added by 92.8.8.69 (talk) 22:28, 9 August 2009 (UTC)

Purple dancing interstellar hippo's have also been hypothesised (I just hypothesised them - so I can say this with confidence!) - that doesn't give them any more scientific status than any other hypothesis...including negative matter. Being unobserved certainly doesn't detract from the hippo's status as a hypothesis either. But being a "hypothesis" doesn't confer any kind of meaning onto some vague concept - it's just a fancy term for "a wild-assed idea that someone thought up when they had nothing better to do". The hypothesis of negative matter doesn't appear to solve any problems that physics currently has - there is no proposed experiment that would reveal whether the stuff exists - until or unless one or other of those things becomes true, we can consign this to Occam's razor. Without anything more than a hypothesis - there is no possible way to answer the OP's question...no more than if (s)he had asked whether those hippo's do the tango or the waltz. (Actually, they can-can - but that's another matter). SteveBaker (talk) 14:08, 10 August 2009 (UTC)

A big difference between hypothetical negative mass and Baker's Hippo Burlesque is that negative mass is actually an interesting mathematical abstraction. Negative mass is something that, even if it doesn't actually exist, can still be examined in theoretical physics, asking "what would happen if this existed?" Abstract mathematical curiosity is in itself interesting and occasionally produces side results that are also practical. Another difference is that negative mass is something that has actually been explored in reliable scientific sources, whereas (far as I know) purple dancing hippos haven't. So the moral is that just because something probably doesn't exist doesn't mean it's not worth exploring the concept hypothetically. 63.95.36.13 (talk) 14:33, 10 August 2009 (UTC)

slow cooking of pork -- where does all the fat go?

A lot of recipes say the fat "melts" after a long slow roast (or a braising) ... and sure enough, the huge chunk of fat on the pork slowly seems to dissolve; if I'm braising the liquid becomes quite oily. What happens to the fat? Is it broken down into smaller fatty acids (and maybe some of it gets oxidised into CO2 and water?). I also note that we generally use acidic liquids like tomato sauce, fruit juice, beer, or some liquid with a bit of vinegar tossed into it ... does some sort of acid-catalysed elimination reaction occur? John Riemann Soong (talk) 10:35, 8 August 2009 (UTC)

If you're heating triglycerides (i.e. fat) in an acidic aqueous solution, the fat is hydrolysed to glycerin and free fatty acids (the glycerin is fully water-soluble, the fatty acids only partly water-soluble); however, oxidation to CO2 and H2O does not take place. 98.234.126.251 (talk) 10:52, 8 August 2009 (UTC)

Animal fats are just solid oils. (Or rather, edible oils are simply liquid fats). You said it yourself - the braising liquid becomes oily. No quotation marks needed, the fat literally is melting, and is being added to the drippings. If you poured off the liquid and separated out the oil, not only would it solidify when it cooled, but it would probably be equivalent in volume/weight to the fat lost from the roast. One caveat to this would be that some of the fat may be emulsified into the water portion. I seriously doubt that a significant portion of the fat has been converted to fatty acids, as free fatty acids are, well, soap. (Your bath bar possibly lists sodium tallowate as an ingredient - that's saponified (hydrolyzed) beef fat.) A small amount of free fatty acids as well as mono- and diglycerides may form, which would help to emulsify the fats, but if a significant amount of free fatty acids were formed, your roasts would taste "soapy". -- 76.201.158.47 (talk) 14:36, 8 August 2009 (UTC)

Soap gets part of its taste from the sodium in it, hydrolysed fats would be the free acid, and have a different taste. Mostly though the fat just melts, rather than reacting as mentioned above.83.100.250.79 (talk) 15:31, 8 August 2009 (UTC)

Light as a feather, stiff as a board

There is a fairly common "party trick" where somebody sits in a chair and a small group of people lift the chair and the person, using only one finger each (alternatively the lifters put their fingers under the subjects armpits and knees). I have found some discussions about how it works (it seems that it is called "light as a feather, stiff as a board" by some) but no definitive answers as to how it works. It seems to me that it is likely the case that the combined lifting force of the lifters is simply sufficient when the weight is shared between them (even though that seems counter-intuitive). Is it that, or is something else involved, or maybe is it really a "trick" with the lifted individual or one or more of the lifters in on the trick and somehow "cheating"? There is a Wikipedia article on the game, but it seems pretty vague, I am hoping that there is, in fact, a recognised scientific explanation for it. Bury me in a Y shaped coffin (talk) 10:57, 8 August 2009 (UTC)

We have an article Light as a feather, stiff as a board on that, it doesn't seem to suggest it is anything special - just that a number of people can easily lift a person and so the 'effort' is a trick of the mind (though it has a 'dubious' statement around it). ny156uk (talk)

Thanks, yes that is the article that I mentioned. However, it does not say conclusively how it works. Because this is so common, and yet at first sight so counter-intuitive, I imagine that there must be a definitive answer somewhere. Bury me in a Y shaped coffin (talk) 15:59, 8 August 2009 (UTC)

I think that's more a function of journaled researchers not spending a lot of time playing a 12-year old girls' sleepover game than anything else. The article offers a number of suggestions, however, all of which probably work in concert. Essentially, if you're trying to lift a 125 pound individual with 5 people, no one needs to lift more than 25 pounds. ~ Amory (user • talk • contribs) 16:17, 8 August 2009 (UTC)

I think that you are forgetting that many of the reseachers did spend some time as 12 year old girls at some point in their lives, and the rest spent some time as 12 year old boys. My point being that this "trick" seems to have been very well known for centuries and, if it didn't have an obvious explanation, it would have been investigated by somebody. I am surprised that there is no definitive published answer on this, although I appreciate your input and that of the other people who have responded. Bury me in a Y shaped coffin (talk) 18:40, 8 August 2009 (UTC)

I suspect many people, even when they were 12 years old, didn't play games like this. Even if they did, many probably didn't or don't now think there's anything extraordinary going on here. (The 'don't now' is perhaps a key point. Even if they really thought there was something extraordinary then, many would probably regard it now as such one of the silly things they believed while young.) Most scientists only really study things that are of interest to someone, probably them. There are some who do try to discredit pseudoscience or and few who perhaps are more open and willing to study pseudoscientific claims in their own right, but I don't think many people actually believe or claim there's anything unusual going on with the trick. (There are a lot of other areas where there are far more people who believe in some sort of pseudoscientific explaination which many would regard as more important to study or discredit.) In other words, the vast majority of people are unlikely to have any interest in studying the trick. In any case, it's not entirely clear what there is to study since as has been explained, there are many rather obvious things that could be going on here which most would consider entirely explain why some people think there's something unusual. Edit: It seems there's so brief discussion here [1] Nil Einne (talk) 10:00, 9 August 2009 (UTC)

I don't think that there is anything pseudoscientific here. You have a phenomenom were people seem to be able to lift a large amount of weight, yet they feel that it is very light. This has been experienced by large number of people, it is not an unsubstantiated claim. Now, someone of a scientific mindset is likely to wonder what is going on here and want to be able to explain it to themselves. Now, there is either a very obvious explanation (obvious to somebody with the required knowledge) which you would think that somebody would state as the definitive explanation or somebody would be interested enough to investigate (not to the extent of multi-billion dollar research, but some kind of investigation). Surely this situation of spotting a phenomena, yet not being able to explain it, is the basis of a lot of good science (not pseudoscience). Remember also that I didn't suggest that this would be heavily researched, I just asked if there was a recognised scientific explanation. Bury me in a Y shaped coffin (talk) 14:21, 9 August 2009 (UTC)

I think the key thing is that a finger locked out straight is pretty much as strong as the arm it is attached to. Since the finger is locked the muscles controlling it don't have to do any work, they just have to maintain the tension. People think that the finger is doing the lifting when it really isn't, the arm is doing the lifting and of course an arm can lift a fifth of a person. --Tango (talk) 17:34, 8 August 2009 (UTC)

Pushing things out of earth orbit

Would it be possible for an astronaut on the International Space Station to easily shove some garbage in the optimal direction so that it fell to earth and got burnt up? Or in reality is this not possible? If something was pushed towards earth, then as there is virtually no friction in space it should keep on going to earth, at least it would seem that way when viewed from the Space Station. 84.13.197.233 (talk) 14:55, 8 August 2009 (UTC)

No. Unless the astronaut pushes really, really fast (as in much faster than any astronaut could conceivably push any significant amount of garbage), the garbage will keep orbiting, just in a slightly different orbit. —Preceding unsigned comment added by 81.11.170.162 (talk) 16:59, 8 August 2009 (UTC)

No, it's quite impossible. Pushing something like this changes its speed (called giving it a delta-v, a change of velocity). Changing the speed of an orbiting body causes it to move into a different orbit (slow it down and it goes into a tighter orbit, speed it up and it goes into a wider orbit). To get from a low-Earth-orbit (LEO) to land on the earth takes a delta-v of more than 8km/s or more (per the diagram on the delta-v article), because that's how fast the astronaut and the ISS are orbiting. So the change to that 8km/s that the astronauts throw would induce would be so trivial that it would make a trivial difference to the orbit of the garbage. In fact the garbage will reenter in time (but so will the astronaut and the ISS, unless they do something about it), as they're all orbiting in the Thermosphere, one of the upper layers of the Earth's atmosphere. Although the drag from the very thin atmosphere is very little, it all adds up, and eventually it burns off enough velocity from things orbiting at the altitudes we're talking about for them to reenter. -- Finlay McWalter • Talk 17:34, 8 August 2009 (UTC)

Yes, they can, because there is still a considerable amount of atmospheric drag at the altitude of the ISS. The ISS is boosted every few months, if the garbage isn't boosted it will eventually burn up. You don't actually want to push it towards Earth, you want to push it backwards along the line of the orbit. This will slow it down so it goes into a lower orbit where the drag will be more significant. I'm not quite sure what would happen if you threw it straight down - it certainly wouldn't go straight down since it would still have all its sideways momentum. I think you would actually increase the height of the orbit (well, it would become elliptical with the height at the point you threw it the same as it was before and higher on the other side of the planet). --Tango (talk) 17:30, 8 August 2009 (UTC)

Come on Tango, the question was clearly wheather a person would have enough arm power to do it, not wheather it is theoretically possible and as Finlay pointed out the delta-v is pohibitivily high. BTW the orbit you described at the end of your post would happen if the person through the object forward, not earthward as you described.Dauto (talk) 18:23, 8 August 2009 (UTC)

If you're already in a low enough orbit that you experience some atmospheric drag, then the garbage is already on a trajectory that will send it burning up eventually. Any way you throw it with any amount of force will work. It would be impossible for a person to throw it hard enough that it fell straight to Earth (completely stopping the object) but I don't think that's what the OP was asking.

As for the orbit resulting from throwing it in the direction of Earth, if the final velocity toward Earth is small compared to the angular velocity, the new orbit would be elliptical but with roughly the same total energy. Since the push is perpendicular to the direction of motion it doesn't significantly affect the kinetic energy. You would see it falling away from you, but as it did it would accelerate in the direction of the orbit, and as it accelerated, the speed of it's fall would slow, until eventually it would start rising again. Rckrone (talk) 19:14, 8 August 2009 (UTC)

The retro-rocket fire slows the speed by about 1% to cause reentry. Atmospheric drag does the rest of the delta v. Any delta-v in the retro direction would hasten reentry a bit, but shoving something, throwing it or pushing it would be far less than 1% or orbital velocity. For the ISS. 1% of velocity would be 77 m/sec. A baseball player might achieve a 100 mph or 45 m/s fastball. A cricket ball might reach the same speed [2]. It would clearly deorbit sooner than if not thrown. A tossed sack of garbage might be 1/10 of that speed or less. A kicked U.S. football might hit 27 m/sec [3]. A golf ball might hit 78 m/sec [4] and a jai alai ball could hit 84 m/s [5], and either should de-orbit nicely. Edison (talk) 19:19, 8 August 2009 (UTC)

Something on the order of 1% is needed if you want to land in the next few minutes or hours, but for disposing of garbage it doesn't matter if it takes days or weeks to burn up (much longer than that would be annoying because you would need to keep tracking it to avoid collisions). --Tango (talk) 19:34, 8 August 2009 (UTC)

It's not just a matter of it being possible - it has actually been done. I can't remember which EVA it was, but I do recall reading about a broken part that was being replaced (or something like that) being disposed of by the astronaut throwing it away and letting it burn up over the next few weeks). --Tango (talk) 19:34, 8 August 2009 (UTC)

The astronaut "throwing" has nothing to do with it; indeed, the astronaut didn't "throw", they just let go. Things in the ISS orbit lose speed, and thus altitude, all by themselves, due solely to atmospheric friction. Left alone the ISS itself would burn up for the same reason - it loses 100m of altitude per day, and requires an occasional kick of +ve delta-v from Progress or ATV - this burn gave it an additional 7km. -- Finlay McWalter • Talk 23:24, 8 August 2009 (UTC)

They do throw things - otherwise there is a risk of them hitting the ISS. --Tango (talk) 23:37, 8 August 2009 (UTC)

If an astronaut pushed something towards earth, then wouldnt she/he see it continually coasting towards earth at a constant speed? As it got closer to earth, then it would be slowed down by atmospheric friction, and accelerate downwards due to gravity? That is how the astronaut would see it - from a static viewpoint it would seem to spiral down towards earth. 89.240.34.84 (talk) 22:50, 9 August 2009 (UTC)

It wouldn't go straight down. The orbital period would not remain the same as the astronaut's. It would only spiral down because of drag, if it was a high enough orbit that drag was negligible it would go into a periodic orbit, just a more eccentric (elliptical) than before. --Tango (talk) 23:22, 9 August 2009 (UTC)

Tyres and Dry-Rot

Hello! I have a "project car" in my garage, and I've got a lead on a good deal for new tyres. But - The car won't be road ready for maybe a year or 2 at the rate I'm working on it! So if I buy these new tyres, and they go on the car now, will they dry-rot if they just sit there not rolling for extended periods? I usually roll the car forward and back 10-20 feet or so for various access while working, but nothing more than that. They are new-new as in 2008-2009 manufacture ,and not new-old, if that makes any difference. Cheers! —Preceding unsigned comment added by 71.62.88.123 (talk) 16:50, 8 August 2009 (UTC)

Tires bought 2 years ahead of when you will start driving the car will obviously not give as many miles of use as if you bought them when you needed them. But the "good deal" might make up for that, I suppose. Edison (talk) 19:05, 8 August 2009 (UTC)

Store your new tyres in a cool, dry, shaded place until you need them. Stack the tyres flat on their sides. Remember to check the tyre pressures when it's time to use them. Cuddlyable3 (talk) 22:50, 8 August 2009 (UTC)

It is completely normal to keep using the same tires for a decade or more, e.g. if you have snow tires that you put on the car for just a few months a year and don't drive much during those months. Snow tires are usually left in the garage during the summer too. 67.117.147.249 (talk) 01:58, 9 August 2009 (UTC)

I'd keep them in a cool, dry, dark place - resting on their sides, not inflated and not mounted onto wheels. I think they'll last you a good few years that way. (So what are you restoring? My current 'baby' is a 1963 Mk I Mini...tyres are no problem...but a replacement engine block is!) If the tyres you need are still being made - and if you don't actually need them yet - then I'd hold off buying them. There's bound to be another good deal coming along in a couple of years time - and since about 80% of restoration projects are never completed, you might be wasting your money. I rarely pre-buy anything - I get just the parts I'm going to use in the next month or two. SteveBaker (talk) 13:55, 10 August 2009 (UTC)

Detecting nuclear weapons

Lets say a terrorist has a suitcase nuclear bomb. Does that bomb emit any particles that can be detected at all? Perhaps neutrinos? This is all before the bomb detonates, not after. ScienceApe (talk) 19:51, 8 August 2009 (UTC)

I'm almost certain the short answer is "yes", but I don't believe the radioactivity of bomb materials (plutonium and/or enriched uranium) is really that great (I don't believe proneness to an actual chain reaction is too closely correlated with radioactivity levels, though if someone wants to correct ME I'd be interested). Also, the casing will presumably be lead, which will block most of that radiation. As for the neutrinos, well, they're pretty hard to detect as they only interact weakly (the weak interaction).--Leon (talk) 19:57, 8 August 2009 (UTC)

Mostly it would be gamma rays that could be detected externally. In principle, the gamma rays could be blocked or reduced below background radiation levels but it would probably take several centimeters of lead shielding on all sides - at the very least, and the "briefcase" would easily end up weighing as much as a small car. Yes, lead is really that dense! 69.140.12.180 (talk) 20:25, 8 August 2009 (UTC)Nightvid

Here is an excellent article on this topic. --Sean 20:43, 8 August 2009 (UTC)

Neither neutrinos nor gamma rays are produced by U235. Neutrinos are produced during beta-decay while gamma rays are produced during a gamma-decay (hence the name). U235 decays through a alpha-decay chain producing alpha-particles which can be easily stopped by a sheet of paper for instance. Dauto (talk) 20:47, 8 August 2009 (UTC)

Isn't there some beta decay eventually down the chain? Would that produce a detectable amount of beta radiation? I guess it's kind of moot though since beta particles are easy to block too and neutrinos are too hard to detect. Rckrone (talk) 21:14, 8 August 2009 (UTC)

The uranium and plutonium used in nuclear weapons are very pure and therefore practically free of radioactive decay products. Also, they have a very long half-life, so an appreciable amount of decay products simply cannot build up. So the answer to your question is a definite "no". 98.234.126.251 (talk) 21:26, 8 August 2009 (UTC)

In the case of ²³⁵U, even if it's totally pure, the daughter nuclid ²³¹Th decays emitting beta radiation and it has a half life of only 25.5 hours. The short half life means that you approach the equilibrium at which there is one ²³¹Th decay for each ²³⁵U decay pretty quickly (after 25.5 hours there will be 1/2 ²³¹Th decay for each ²³⁵U decay, after 51 hours 3/4 etc.). But beta radiation isn't difficult to shield, only the neutrinos are, but they are also too difficult to measure. By the way, the uranium and plutonium in nuclear weapons are maybe very pure chemically, but certainly not very pure with respect to unwanted isotopes. Icek (talk) 17:45, 9 August 2009 (UTC)

That is an excellent, not to mention worrying article. I am surprised that the detectors they have in place seem to be so ineffective (although it seems that it is not really possible to construct effective ones), but I am amazed by the idea that a gunlike nuclear weapon could be so easy to build (even to the point that just dropping one piece of enriched uranium onto another could potentially create a 1 kiloton explosion). I had always assumed that you would need the technological infrastructure of a state to build such a weapon. I can see now why people can be so very concerned about smuggling of enriched uranium. Bury me in a Y shaped coffin (talk) 23:01, 8 August 2009 (UTC)

The single hardest step of building an atomic bomb is the enrichment. Dauto (talk) 01:40, 9 August 2009 (UTC)

Yes, it would seem so. This article from a few years back seems to suggest that designing a crude weapon would certainly not be beyond the ability of a group that could recruit a couple of PhD students. Of course, the last pieces of the puzzles are the fabrication and assembly of the bomb components. I wonder what level of expertise and equipment is required for that. Bury me in a Y shaped coffin (talk) 10:43, 9 August 2009 (UTC)

I guess that one bit of good news is that the quality of enriched uranium being produced by states that are more likely to lose it (or deliberately provide it to groups) is likely much lower than that discussed in the Scientific American article (which they state could cause a 1 kt explosion just by dropping one piece onto another), meaning that a bomb that used it would need to be more sophisticated (in terms of the accuracy of the components) and therefore less easy to construct. Bury me in a Y shaped coffin (talk) 10:48, 9 August 2009 (UTC)

The hard parts of making a "suitcase" nuclear bomb are not the conceptual parts (which any Ph.D. student would have no trouble grasping). Probably, most college sophomore engineering students can handle all the conceptual details of the designs. But the execution of the extreme precision engineering required for all parts of the device are very hard to do. Refining the radioactive elements is the most commonly cited "difficult engineering task" - conceptually simple, but actually manufacturing and refining pure U235 or plutonium is so nontrivial that it really does take a sizeable operation. But this is only one single part of the process! While reading today about the Nagasaki bombing, I read about the fuzing device - an Exploding-bridgewire detonator (designed by a Ph.D. student at Berkeley, no less). But the precision engineering required on the Nagasaki bomb required a timing control on the chemical reaction of the detonators on the order of microseconds - else the shockwave does not properly yield the explosive lens effect necessary for achieving the the critical density for fission. Surely there are thousands of other extremely precise mechanical, chemical, nuclear, and electronics details. Even with a small team of highly trained experts, there's not a very good way to get these engineering details worked out - a large infrastructure base is required for the kinds of details necessary. In brief, there is a huge difference between understanding the process to make pure U235, and being able to make it. The same applies for every other engineering detail of these weapons - they're really fragile, balanced on a knife-edge betwen functional- and dud- so any little slip-up would result in a heavy lead-lined suitcase with some sort of hazardous stuff in it (but no nuclear weapon). Nimur (talk) 19:28, 9 August 2009 (UTC)

See Radioactive Boy Scout. Apparently it isn't too hard for a 17-year-old. -RunningOnBrains^(talk) 06:31, 10 August 2009 (UTC)

You're confusing a tiny "reactor" of sorts with an actual bomb. Not the same, at all. If he had run his reactor for the rest of his life, he wouldn't have had an atomic bomb. More relevant article: Nth Country Experiment. --98.217.14.211 (talk) 00:30, 11 August 2009 (UTC)

Make sense, although the bomb dropped on Nagasaki was of an implosion design rather than a gun-type weapon. I understand that implosion weapon are many times more difficult to create because of the timing issues that you mention. Not sure if you have had a chance to read the article linked by Sean above, but it suggests that gun-type designs are much easier to fabricate and, if the uranium is sufficiently enriched, you could potentially even get a detonation by just dropping one piece on the other. In a gun type weapon you are basically just slamming two pieces of uranium together, but of course the engineering required is still going to be slightly complex, I just wonder how complex. Bury me in a Y shaped coffin (talk) 20:00, 9 August 2009 (UTC)

You'd need a gun that can shoot uranium at high speeds; that's easy enough to make. You'd also need neutron reflectors to cause as many fission events as possible before the bomb disintegrates, but I don't see anything challenging about surrounding the gun with reflector material. Whether all of this can fit in a suitcase, I have no idea. --Bowlhover (talk) 22:52, 9 August 2009 (UTC)

Probably, if you have the right materials and know what you are doing. They used gun-type weapons in developing nuclear artillery shells which are about the size of a large suitcase. --98.217.14.211 (talk) 00:30, 11 August 2009 (UTC)

The theory is only that you have to take two or more sub-critical-mass chunks and push them together to make a critical mass chunk and kaboom! The practical problem (which is why the guys at the Manhattan project had such a hard time making something that actually worked) is that when the two sub-critical mass pieces get close - the amount of heat and radiation goes up very rapidly, and this can be enough to rip the bomb to shreds without it actually going critical and producing the expected yield. The call this a "fizzle". So simply dropping one chunk of material onto another doesn't produce the large yield you need to take out a city...although it might be enough to spread radioactive material over a few city blocks...a "dirty bomb". You could do as well by just wrapping some radioactive material around a few sticks of dynamite. The whole business of experimenting with "gun" and "implosion" designs was to solve the question of how to get the fissile material close enough together and to hold it there for long enough for the chain reaction to really get going. Slamming the parts together RAPIDLY seems to have been the key - and for that you need either a "gun" or an "implosion" device...but small, subtle details of the design - and high engineering tolerances - seem to be a vital part of making something that actually works without fizzling. Given the ruinously expensive cost of the fissile material, this kind of thing can't really be done experimentally by anyone short of a major governement - so you are left with needing to steal someone else's design and doing some really first class precision machining - or employing a LOT of very smart people to figure out a new design with a real chance of working the very first time you try it. SteveBaker (talk) 13:48, 10 August 2009 (UTC)

I just want to note that it's easy to deride a fizzle as not being very powerful, but if you set off a .5kt explosion in a major city, it would without a doubt be the most deadly terrorist attack ever perpetrated. When you start talking about nuclear yields, it's easy to fall into the "if it's smaller than Hiroshima, it's a dud" mindset, but if you take the time to calculate how bad that would actually be, it's pretty bad if you get an appreciable yield at all (which for U-235, you probably would, if you dropped 1.5 crits together in a crude way). The Oklahoma City bomb was only .002 kt. If you can get anything appreciable nuclear yield, then you're still talking about a relatively large explosion.

I also want to object that only governments could come up with a gun-type design that would work. I suspect a small team of people with the right backgrounds (some physics, some explosives, some engineering) could easily come up with a Hiroshima-style weapon IF they had the U-235. It's a shop-class sort of project, and you don't need to waste U-235 to experiment about it (they didn't during the Manhattan Project, and things are a lot better known today than they were then... you can get a million times more information about fission processes through a nuclear engineering course than they ever had during the war). Implosion designs are much, much harder, and require the whole team of very smart people to work on (even if it is not as revolutionary as it once was). For U-235, the only significant hurtle is obtaining the material—which is why it is a much, much more dangerous element from the point of view of terrorism than plutonium. --98.217.14.211 (talk) 00:30, 11 August 2009 (UTC)

On detecting nuclear weapons: Robert Oppenheimer was asked by a Senator once, in Congress, what instrument he would use to detect an atomic bomb. "A screwdriver," he replied—to open every suitcase coming in. --98.217.14.211 (talk) 00:30, 11 August 2009 (UTC)

. Nimur (talk) 21:05, 11 August 2009 (UTC)

Bird and Sherwin, American Prometheus, p. 349. A great book, incidentally. --98.217.14.211 (talk) 18:55, 12 August 2009 (UTC)

Nobody mentioned muon radiography. While this LANL press release says that "efforts to shield nuclear materials with lead or similar heavy metals make a smuggled object easier to detect with muons" you would of course only see that there are these high-Z materials like lead. Icek (talk) 08:43, 12 August 2009 (UTC)

Antenna length

(apologies if I'm not on the right desk) I have one of those portable TVs with a whip antenna attached. (You can extend the antenna to a length of about 30 inches, and retract it to about 5 inches.) What is the best length of this antenna - half or quarter wavelength? Xenon54 (talk) 21:41, 8 August 2009 (UTC)

What frequency or station are you trying to tune to?83.100.250.79 (talk) 22:21, 8 August 2009 (UTC)

Specifically, it's WJZ-TV on channel 13 (211 MHz). Quarter-wavelength is 13.3 inches (I think), but something's telling me "No! It's half!" and I can't remember which it is. Xenon54 (talk) 22:54, 8 August 2009 (UTC)

Dipole antenna is 1/2 , Whip antenna is 1/4 which must 'be grounded'. (I get about 13.9" for 1/4) I assume the article is right? 83.100.250.79 (talk) 23:47, 8 August 2009 (UTC)

At 211MHz, the wavelength = 1.42 meters = 56 inches. 1/4 wavelength is 14 inches.
Generally speaking, a longer antenna always 'intercepts' more energy. I'll go with the half-wavelength (28") being 'better' for reception ... 'in free space' at least.
Twang (talk) 08:44, 9 August 2009 (UTC)

Responders have assumed that "best" means maximum sensitivity. That could be true when seeking a distant station. It's more usual to have adequate signal strength but receive a picture that is degraded by "ghosting" due to delayed signal(s) from reflection(s). A whip aerial has little or no directivity but you can experiment with its length and position to minimise ghosts. Cuddlyable3 (talk) 12:42, 9 August 2009 (UTC)

A bit of OR here, I've had good results with ghost removal in a weakish reception area, by resting the tip of the antenna against an aluminium venetian blind.-KoolerStill (talk) 18:25, 9 August 2009 (UTC)

Dipole antennas for televisions are very weakly tuned anyway, (and they must pick up a reasonably wide-band channel, with various different wavelengths). The "quarter-wavelength" or "half-wavelength" concept is usually less relevant in this situation than other factors - most importantly, the dipole is directional, so changing its orientation will affect your signal reception capability. Also, the total received power will vary (roughly linearly) with the length of the antenna - so a longer antenna, which de-tunes from the correct frequency, may still yield better total signal reception due to better received power. Your specific dipole antenna circumstances may vary... Nimur (talk) 19:17, 9 August 2009 (UTC)