Wikipedia:Reference desk/Archives/Science/2013 August 7

= August 7 =

Creating a Faraday cage without matter.
Greetings!

I've been studying the concept of Faraday cages and how they contain energies from certain frequencies of the electromagnetic spectrum. I have a question, though, as to how this relates to the properties of visible light apropos human sight. To wit, certain materials—such as the acrylic used to make a woman's swimsuit—remain impermeable to wavelengths longer than 400 nanometers, even when wet. (Otherwise, why would she even wear it?)

I clearly understand the concept as it relates to matter. Nevertheless, I cannot help but wonder: Could one achieve a similar effect with energy, or some other immaterial substance? Namely, is there any method known to science that can create an immaterial, two-dimensional barrier though which Gamma Rays, X-Rays, and Ultra Violet can pass, but not Visible Light (or, for that matter, Infrared, Microwaves, or Radio)?

If so, how would somebody go about doing so, and what would the risks and implications (if any) would such a device entail? Thank you.Pine (talk) 03:47, 7 August 2013 (UTC)


 * Gravity perhaps? I'm visualizing an array of microsized blackholes in a vacuum. If the curvature of space-time can that be calibrated, it should allow only certain frequencies to pass. As for how to create such a device, I have no answer. Plasmic Physics (talk) 03:58, 7 August 2013 (UTC)


 * Pine, you seem confused about a couple of things. A faraday cage works by surrounding the protected area with electrical conduction paths (typically either wire mesh or conductive sheets).  It works by forcing the electric field within to be such that no voltage can be measured or defined between any point on a object within, and any point on the inside of the shield.  For any frequency not zero, subject to some practical limitations, faraday shields cancel out any external magnetic field because such fields will cause a circulating current in the faraday cage, which will set up another magnetic field equal and opposite.
 * Things that are not transparent, such as swimsuit cloth, are NOT faraday cages. Such things work by attentuation - as the electromagnetic waves (light) penetrate into the material, the interact with the material molecules and are converted to heat.  Another example of attenuation is the absorption of light in pure water.  The greater the distance into the water, the dimmer the light gets.  This has nothing to do with faraday cage operation - pure water does not conduct electricity.
 * All manner of substances are available that attenuate visible, light, xrays etc. For xrays metals are commonly used.  Lead is very effective.  In medical xray rooms, good thick concrete walls are used to protect the operators.
 * What do you mean by "immaterial"? If you mean blocking radiation by use of more radiation, it cannot work, as energy can neither be created nor destroyed - only converted from one form to another - as in the converstion of light or radio waves into heat.  Such conversion requires the use of physical matter.  Faraday cages are by definition made of matter, which supplies the electrically conductive paths.  If you mean "force fields" a la Star Trek, that's just science fiction nonsense.
 * 1.122.160.132 (talk) 04:18, 7 August 2013 (UTC)


 * I noticed those confusions as well, however, I ignored the bulk of the query and only focused on the question actual at the very end. With regards to 'immaterial', I think that the OP is referring to 'not of matter'. Plasmic Physics (talk) 04:23, 7 August 2013 (UTC)


 * You're right, I am confused.
 * Somebody told me that the protective mesh on the door of a microwave oven acts as a Faraday cage allowing visible light (400 nm to 700 nm) to pass through, but not the microwaves themselves (12 cm, or so). i.e. One can see his food being cooked, without himself being cooked in the process.


 * Yes, by 'immaterial,' I mean something not of matter that would produce a similar effect, but would also block out the visible spectrum.


 * Does this violate the laws of thermodynamics? Or can it theoretically happen? Pine (talk) 04:35, 7 August 2013 (UTC)


 * No, nor really. If you take my above gravitational solution into account, you can deflect or absorb the unwanted frequencies. When microsized blackholes evaporate, they release only x-rays and γ-rays. Plasmic Physics (talk) 05:22, 7 August 2013 (UTC)


 * While one could argue about semantics, it is correct to say that the mesh in the door of a microwave oven acts as a sort of faraday shield in reverse, working with the magnetically doped door seal (positioned within just the right dimensioned metal parts) and the inner metal surface of the oven to keep the microwaves safely inside. (A true faraday shield is an enclosing electrically conductive surface that keeps outside energy getting inside.) The mesh is a good electrical conductor, and the microwaves set up circulating currents in it, causing the microwave enegery impinging on the mesh to be reflected back into the oven.  The key to understanding this is that the microwave energy is reflected back in, not converted into heat as it passes through.
 * To eliminate radiated energy from a given region without the use of matter to do the conversion or reflection is not possible. It would indeed violate an important thermodynamic principle:  As I said above, energy can neither be created nor destroyed, but only converted from one form to another - the so called First Law of Thermodynamics.
 * 1.122.207.51 (talk) 06:05, 7 August 2013 (UTC)


 * Blackholes do not destroy energy, so no it does not. Plasmic Physics (talk) 07:21, 7 August 2013 (UTC)

KDKA's weakened radio signal
When my father was a child, he says KDKA (AM) was audible throughout the Eastern United States at night. The article Greg Brown (broadcaster) agrees it was audible ~175 miles away. Finally, I can confirm it too: as recently as 2004, I picked it up clearly in South Carolina.

But it is no more! Even in different parts of Central Pennsylvania, I can no longer hear it. Not even a small flicker even fades in. And it can't hold a candle up to the easily audible WBZ (AM) and KYW (AM).

What happened? I can't find anything written about a signal reduction. Magog the Ogre (t • c) 04:53, 7 August 2013 (UTC)


 * All three staions have similar frequency and power output, so in general, coverage should be about the same. What is your location now?  Perhaps you are closer to WBZ than you are to KDKA.
 * In general though, AM distance reception is not what it was in earlier decades. The technology of modern solid state receivers with synthesised or digital tuning results in poor intermodulation performance - this results is weak signals being overiden by what appears to be white noise.  Transistor radios made up to the 1980's and tuning with variable capacitors can be a lot better in this regard.  Well designed tube radios better again.  As well as that, with the increased use of electrical appliances of all kinds, there is more noise to drown weak signals out nowadays.
 * Local geology can affect reception. AM travels well over water and can be affected by monazite etc.  HV powerlines also affect it.
 * Lastly, an anecdote: I once worked in a similar 50 kW AM "clear channel" station.  Over a few years, more and more complaints arrived from distant (300 to 700 km) listeners that our signal was dropping.  Eventually we realised that some villain was sneaking in to the unattended transmitter site and bit by bit digging up the copper earth mat and stealing it (a buried mesh of thick copper wire extending over several acres.  As well as causing power loss in the ground, it altered the intended direction of radiation.
 * 1.122.207.51 (talk) 06:23, 7 August 2013 (UTC)
 * I am in central Pennsylvania. I travel within parts of central Pennsylvania. Magog the Ogre (t • c) 05:05, 8 August 2013 (UTC)
 * Interesting. KDKA transmits from Alison Park, at the extreme western end of Pennsylvania.  KYW transmitts from Lafayette Hill, at the extreme eastern end.  WBZ transmitts from Hull in Mssachusets, much futher from you than either KDKA or KYW.  However KYW has a directional antenna.  I was unable to find out on the web the preferred direction, but westerly would seem likely.  So all three should be equally good or equally bad for you.  Perhaps you could email the station engineers for their view. If you use the form provided under "Contact" in KDKA's website, be sure to put in the first line "For Station Engineer" - you'll get a good answer then, and not one from some PR or sales person.1.122.190.3 (talk) 09:14, 8 August 2013 (UTC)
 * In the United States, we can get such information from the FCC, including their extensive collection of online databases. For example, you can use the online AM Broadcast Station database to find data and diagrams, including antenna radiation pattern for the KYW licensee (and even a graphical plot plus detailed information about both antenna masts).  Nimur (talk) 17:15, 8 August 2013 (UTC)

Melting and boiling points of the heavy alkali metals
Caesium has melting point 28.44 °C; francium has melting point ~27 °C; ununennium is predicted (see the article's infobox) to have a melting point of 22–24 °C. Why are they so close to each other? (The lighter alkali metals have a more normal trend.) Can anyone offer a (relativistic) explanation?

The boiling points also behave similarly: Cs 671 °C, Fr predicted to be 677 °C(!), Uue predicted to be 655–669 °C.

(A lessened effect seems to occur in group 2; Ubn may at the high extreme have a melting point higher than Ba and Ra (why?), but everything else conforms to normal periodic trends.)

Standard Double sharp disclaimer: Please give sources if possible, because I want to include this into an article (in this case, alkali metal). Double sharp (talk) 07:43, 7 August 2013 (UTC)
 * The 1978 Bonchev full-text (cited as the source for the data in the ununennium infobox) is available here (note Table 1 and figures 6 and 7). The properties of Cs and Fr are quite similar to each other, and the predicted properties of Uue are extrapolated based on a variety of parameters derived from empirical data, which are periodic. They are not based on first principles/relativistic considerations. -- Scray (talk) 22:29, 7 August 2013 (UTC)
 * Interesting. And yet Fricke (1971), which is relativistic (I think), also gives a melting point of 0–30 °C and a boiling point of 630 °C for Uue. The lower value of 0 °C is all right, but is there any mechanism that would give the higher value of 30 °C? Double sharp (talk) 11:38, 8 August 2013 (UTC)


 * Note that the francium value is certainly theoretical rather than empirical, because a macroscopic amount of francium has never been collected in one place, and if you did, it would give off so much heat from radioactive decay that it would essentially instantly explode, with the radiation killing anyone nearby. So even the meaning of a melting point for francium is a bit obscure.  I suppose it means something like "if by pure chance, one chance in $$10^{10^\textrm{large value}}$$, it so happened that not too many francium atoms in your sample decayed in the interval of observation, but the laws of probability worked otherwise as expected except for that, this is the melting point you'd see". --Trovatore (talk) 21:38, 8 August 2013 (UTC)


 * Why has no one ever tried to deposit a francium vapour onto a cooled surface to form a nanolayer, which is then slowly heated to test francium's melting point. Plasmic Physics (talk) 23:54, 8 August 2013 (UTC)
 * Well, I can't tell you for sure why (or even that) no one has ever tried that. But I can tell you that the largest amount of francium that our article mentions being collected in one place is 300,000 atoms, which works out to about 10^&minus;16 grams.  If you think you can measure the melting point of that, even in a monolayer, go ahead and write a grant proposal.  How much can you learn about the melting point of a 3D material from a monolayer, which is effectively 2D?  Doesn't strike me as the same thing. --Trovatore (talk) 00:16, 9 August 2013 (UTC)


 * OK. FYI: nano- and mono- are not the same thing, a nanolayer can consist of any number of layers with a cumulative thickness of less than a micron. Plasmic Physics (talk) 00:20, 9 August 2013 (UTC)


 * I think you're vastly underestimating the difficulty of assembling and manipulating a macroscopic quantity of francium&mdash;even a microscopic macroscopic quantity. The largest quantity ever collected has been about 300,000 atoms, and that only in the vapor phase.  If you made a solid cube out of it, it would be less than 70 atoms on a side–and it would be vigorously heated by more than 2000 radioactive decay events every second.  And even if you were able to measure a melting point using such a small quantity of material, it wouldn't be 'correct' anyway.  Melting points are depressed sometimes by very significant amounts when you deal with very small particles; see melting-point depression.  Essentially, the unusual or quirky behavior of the atoms at the surface or edges dominate the 'bulk' material behavior that you're trying to measure, resulting in a very sensitively size-dependent melting temperature.  TenOfAllTrades(talk) 00:21, 9 August 2013 (UTC)


 * That is why I suggested a nanolayer and not a nanodot. Is it not possible to use the depressed melting point data to improve current estimates of the bulk melting point? Plasmic Physics (talk) 00:44, 9 August 2013 (UTC)
 * You'd have to correct for self-heating from the radiation. (In fact this is probably part of the reason why the Fr melting point is so close to the Cs melting point). I've seen a value of 23 °C given for Fr before; presumably some corrections were applied to the value after the experiment there. (Is the 27 °C value even experimental? The very fact that it is so close to the Cs value suggests to me that it is.) Double sharp (talk) 06:55, 9 August 2013 (UTC)

Relative motion..
A driver was sent to traffic court for speeding. The evidence against the driver was that a policeman observed the driver's car alongside a second car at a certain moment, and the policeman had already clocked the second car as going faster than the speed limit. The driver argued, "The second car was passing me. I was not speeding." The judge ruled against driver because, in the judge's words, "If two car were side by side, you were both speeding." In this case, how to argue the case in term of physics in the favour of accused driver?

AmRit GhiMire &#39;Ranjit&#39; (talk) 14:15, 7 August 2013 (UTC)
 * This sounds like a homework question aimed at testing your understanding of concepts and the articulation thereof, and unwarranted deductions based on flawed interpretation of wording. On the other hand, my experience of the legal system makes this kind of abuse of logic (often deliberate on the part of the practitioners manipulating the outcome, being protected by the shear weightiness of the process of fighting it) pretty standard in the application of "justice". — Quondum 14:24, 7 August 2013 (UTC)


 * As stated, the judges' decision is clearly wrong. You could be parked, stationary with another car passing you at the instant the policeman observed you.  Your statement of the story is that the policeman observed the action "at a certain moment"...ie, over a period of zero elapsed time.  Since the measurement of speed requires knowing the distance travelled over time, (even if it's distance travelled relative to another car) - having a zero-time snapshot tells you nothing whatever.  If that were literally true then he has no clue how fast you were moving.


 * But if this is what the claim hinges on then it's a ridiculously fabricated situation - nobody could see an event like this in a literal instant.


 * In reality - a policeman doesn't get an instant, zero-millisecond snapshot picture of the event. He'd easily be able to tell whether the two cars were or were not going at the same speed from a one or two second glance...and if we trust his testimony then both drivers were indeed speeding and the judge made the right call.  If we don't trust the police officer's testimony then all bets are off and we have no way to know whether the accused was speeding or not.


 * The only other possible inference (and one that this contrived story may be trying to elicit from you) is that the two cars might have been driving around a tight bend in the road - with the accused driver on the inside of the curve and the speeding car on the outside. Both cars could then remain side-by-side with the car on the outside of the curve traversing a longer distance than the one on the inside.  Thus the outside car could be exceeding the speed limit while the inside car was not.


 * But that's one hell of a stretch. In any practical situation, the difference in speed would have to be tiny - probably less than the threshold of error that the police would allow for in using a speed gun.  I don't buy it - if that's it, then in any practical situation, the driver is guilty as hell!


 * SteveBaker (talk) 16:58, 7 August 2013 (UTC)
 * If I were the driver's lawyer, I would argue that the policeman's account of the relative speed of the two cars is not reliable, because his attention was focused on the first car. I would also look for anything in the law that required a radar speed record, and if there were such, I would argue that, even if the policeman's account is reliable, it is not codified in the law that the policeman is allowed to make inferences (even valid ones) from the speed of one car and its relative speed to another car.  That's not really an answer to a science question, but it might be an answer to the original question in some other context. --Trovatore (talk) 21:50, 8 August 2013 (UTC)

Why no industrial scale production of human breast milk using transgenetic cows?
More than 25 years ago I thought that by now we would be living in a World where genetic modification of animals would have led to many applications. While you could expect that some predictions would turn out too difficult to realize, (e.g. genetically engineered pigs to grow human organs has faced problems due to retrovirusses), it's rather strange that almost nothing has changed in the food industry. So, what is preventing us to make progress with genetic modification of animals? Count Iblis (talk) 19:42, 7 August 2013 (UTC)
 * How do you propose getting human milk from a cow? ←Baseball Bugs What's up, Doc? carrots→ 21:04, 7 August 2013 (UTC)
 * It's difficult enough convincing the population that tiny modifications in the genes of cereals are safe to eat, so can you imagine the reaction of mothers asked to feed their babies milk from cows with human udders? Research is continuing, but slowly.   D b f i r s   21:18, 7 August 2013 (UTC)
 * It's not so much the udder, it's human nutrients in human milk which are alleged to be better for baby than cow's milk is, provided the mother is producing adequate and good milk, which not every woman can. So it would require engineering cows to alter the chemistry of their milk to simulate human milk. Given that some of the benefit of human milk is alleged to be the ingestion of antibodies which the mother has in her system, it would be a very tall order to make a cow produce that. They would be better off focusing on what the negatives are with cows' milk, and trying to fix that. ←Baseball Bugs What's up, Doc? carrots→ 01:58, 8 August 2013 (UTC)
 * I believe (sorry, can't track down references right now) that I have also read about research showing that the mother's milk changes composition over time as the baby's needs change, and can react to things such as an infant's illness or stress as well. Obviously those are benefits an artificial milk will never produce, but I suppose it would still be a big step up from formula - many mothers pump and freeze milk because their schedules (or their baby) don't allow for a regular nursing schedule. But like Dbfirs points out, it will likely be a very long time till people accept cows with human genes. Human insulin is produced on a large scale by genetically modified bacteria, but our article on Humulin doesn't really make it clear if there are definitely human genes added to the bacteria. 209.131.76.183 (talk) 11:36, 8 August 2013 (UTC)
 * Researchers have worked on exactly this project, as of 2011. www.cbsnews.com/8301-504763_162-20071923-10391704.html I have limited internet access at work so I cannot provide more links, but googling 'cow human gene' and similar terms will be productive. 198.190.231.15 (talk) 13:42, 8 August 2013 (UTC)


 * Our article on genetically modified organisms contains the following paragraph:


 * "In 2011, Chinese scientists generated dairy cows genetically engineered with genes for human beings to produce milk that would be the same as human breast milk.[56] This could potentially benefit mothers who cannot produce breast milk but want their children to have breast milk rather than formula. Aside from milk production, the researchers claim these transgenic cows to be identical to regular cows.[57] Two months later scientists from Argentina presented Rosita, a transgenic cow incorporating two human genes, to produce milk with similar properties as human breast milk.[58] In 2012, researchers from New Zealand also developed a genetically engineered cow that produced allergy-free milk."


 * So clearly people are working on that sort of thing, but all the progress has been very recent. Looie496 (talk) 14:08, 8 August 2013 (UTC)


 * Note that these advances involve changing to human style lysozyme and lactoferrin and removing beta-lactoglobulin. They make the milk "more human-like" but by no means human.  Basic parameters like the overall fat content are different.  It is no easy task to change everything, hard even to tell if you've missed something.  Also, I worry about putting human-style immune functions (the lysozyme and lactoferrin) into an otherwise bovine context - you might get infections that don't bother the cow, but adapt to defeat the human antibacterial measures, and do bother the humans.  But that is IMHO. Wnt (talk) 15:02, 8 August 2013 (UTC)

Most abundant organic polymers
According to the respective articles, the #1 abundant organic polymer is cellulose and #2 is lignin (which apparently employs 30% of non-fossil organic carbon). Is there a longer list available (say, a top 10), particularly one with estimates for the global mass and the % of non-fossil organic carbon employed? List of most abundant organic polymers would be an interesting article... ManyQuestionsFewAnswers (talk) 22:27, 7 August 2013 (UTC)


 * There are chitin (fungi, protostomes) and keratin (vertebrates). You seem to be aiming for long-chain structural polymers. μηδείς (talk) 23:43, 7 August 2013 (UTC)


 * I suspect keratin is nowhere near as abundant given that vertebrates form a relatively small % of biomass, but I could be wrong. My not particularly well-informed guess would be that several hemicelluloses would be higher on the list. There are other polysaccharides (e.g. pectin), as well as polypeptides (presumably the most abundant being RuBisCO) and nucleic acids, which I suspect would rank lower. But I'd love to see some hard data. ManyQuestionsFewAnswers (talk) 01:27, 8 August 2013 (UTC)