Wikipedia:Reference desk/Archives/Science/2014 May 16

= May 16 =

Patterns That Change When Tilted
Hello. I do not know how to best word my question but here is a start. I would like to know the field that is concerned with designing patterns that change drastically in appearance when the paper on which it is printed is tilted so slightly. I am not looking for a specific change in appearance; this is be open-ended. Thanks in advance. --Mayfare (talk) 00:37, 16 May 2014 (UTC)
 * It sounds like you're asking about lenticular printing. Red Act (talk) 01:13, 16 May 2014 (UTC)


 * Iridescence also is affected by viewing angle, and diffraction gratings can be used to produce angle-dependent effects. —Quondum 04:45, 16 May 2014 (UTC)


 * There's also holograms of course. It might help if you could link to an example.--Shantavira|feed me 08:53, 16 May 2014 (UTC)

Is there a method that does not change the properties of the paper? --Mayfare (talk) 11:04, 16 May 2014 (UTC)


 * Anamorphosis is a sort of optical illusion involving an image that appears distorted from one angle, but is intelligible from another angle. The Ambassadors (Holbein) famously uses this technique. ZMBrak (talk) 13:49, 16 May 2014 (UTC)


 * Doing this on normal paper sounds difficult - paper is a very bumpy material at microscopic scales - so the orientation of the surface to the eye changes across every fraction of a millimeter across the surface. Hence any straightforward "ink" approach isn't going to work because the orientation of your eye to the surface is uncontrollable.  If you're prepared to go with very smooth plastic films and such - then it gets easier.  There are printable holographic approaches - and of course metal films like the ones on many credit cards.  Simplest of all is to use a lenticular film over a printed image - which is a technology that's been around for 80 years or more.  I think we need to understand more about the application for this.  SteveBaker (talk) 18:21, 16 May 2014 (UTC)


 * I'll suggest moire pattern since that ended up being the answer to a similar question I asked. μηδείς (talk) 18:24, 16 May 2014 (UTC)


 * In theory paper should be compatible with a diffraction grating - in a really quick search I get which points me at Optically variable inks, Optically variable pigment (whatever the difference is), multi-diffraction grating, pixelgram as (perhaps, I'm not sure) methods of printing diffraction gratings on currency to make copying harder.  I don't know how much modification they involve from regular paper though... would be interesting to see more. Wnt (talk) 20:33, 16 May 2014 (UTC)


 * It looks like they just added this feature to the US $100 bill. It now has a strip on it with images that shift between 100's and Liberty Bells, when tilted. StuRat (talk) 03:15, 18 May 2014 (UTC)

radical surgery on antarctic ice using lasers
Would a high powered laser be able to amputate the part of the sliding ice, if any, that overhangs into the ocean? I was thinking that would slow the slide of the remaining ice into the ocean, giving us more time to deal with it. I know the ice is very thick and hard to cut with a laser, but the overhanging ice would tend to open up any laser cut further, like when a leaning tree is cut on the upper side with an axe. The power level of such a laser would be immense, im sure, but what would an estimate for that power level be? (So that if 50 or so years from now, such a very powerful laser became feasible.)--(If not much of the ice is currently already over the ocean, the report is that it will be in a few years.) Thanks.Rich (talk) 04:31, 16 May 2014 (UTC)


 * Methinks high explosives (as used, e.g., for avalanche control) would be much more cost-effective. 24.5.122.13 (talk) 04:52, 16 May 2014 (UTC)
 * It could depend on location. An orbital laser would have an enormous initial cost but could use natural nuclear power, which is close to free, and once in orbit, there wouldn't be any travel overhead, no matter if it's an antarctic glacier, or closer to populated areas.
 * A pulsed laser to which ice is opaque would probably be the optimal tool for the job. At high power, it would crack, rather than melt, the ice without much heat transfer.
 * OTOH, wouldn't it be more useful to cut through cultures of this or that, with a laser that big? Preferably while they're being harvested, for added evulz? - ¡Ouch! (hurt me / more pain) 09:22, 16 May 2014 (UTC)


 * Hmmm...what kind of orbit is this laser in?  If it's a polar orbit, then you won't have much time to do your work in each orbit...if it's in a low equatorial orbit, then it wouldn't even  see the poles - and if you put it all the way out in a geosynchronous orbit, the accuracy and focussing issues would be amazingly difficult.  Plus, I'm not sure how other countries would feel about a laser out there in space with the capacity to slice through half a kilometer of ice at a positional accuracy down to a fraction of an inch...that would make a really good death-ray! SteveBaker (talk) 18:05, 16 May 2014 (UTC)


 * Why should we even bother to ask them, when we have the means and the ability to do it unilaterally? But I agree that it won't work for cutting ice (although it CAN work against Iranian nuclear missiles). 24.5.122.13 (talk) 22:42, 16 May 2014 (UTC)


 * Are you sure of your assumption that overhanding glaciers pull the rest down with them ? Aren't they supported by seawater ?  I believe the floating ice shelves act more as a cork, stopping the rest from sliding off the land.   StuRat (talk) 06:13, 16 May 2014 (UTC)


 * I'm puzzled as to why you would want to "deal with it", but anyway a laser would be completely ineffective as the hole it creates would continually fill with water which would block the beam.--Shantavira|feed me 08:45, 16 May 2014 (UTC)


 * That's why Rich made it "high powered". The water would be blocked out by a bubble (or possibly a jet) of water vapor. Not sure how much power you'd need. Also, this. - ¡Ouch! (hurt me / more pain) 09:22, 16 May 2014 (UTC)


 * Yea, getting the water vapor out of the hole, before it froze back to the sides, would be also be problematic. You'd really need some frequency that would pass through air, liquid water, and water vapor, and only heat up ice. StuRat (talk) 15:27, 20 May 2014 (UTC)


 * The notion of a destructive laser orbiting the globe would likely be met with very vigorous opposition, and in fact might already be banned by existing treaties. ←Baseball Bugs What's up, Doc? carrots→ 15:28, 16 May 2014 (UTC)


 * Weapons in orbit are already banned, but "It's not a weapon, it's weed control!"
 * ...or maybe not; this used to be the article that banned permanent blinding via laser, but right now, "Quoth the browser, '404'." - ¡Ouch! (hurt me / more pain) 10:41, 19 May 2014 (UTC)


 * To Nimur below, click the xkcd link to have a look at the power I had in mind: more than the YAL, and definitely pulsed, to avoid wasting too much energy into melting ice.
 * Even more clearly, it'd take a JamesBondVillainesque, KillSatirical, WaveMotionGunnical amount of power. I'd call it an EleventhHourSuperPower but that would be an Incredibly Lame Wave Motion Pun. - ¡Ouch! (hurt me / more pain) 10:41, 19 May 2014 (UTC)


 * You'd need a powerful laser. If you can't grok the orders of magnitude involved in this problem, then perhaps a more visual demonstration will help build intuition: go grab the most powerful laser you can find, and try to melt a block of ice with it.  (Unless you're SteveBaker, who owns a personal CO2 laser cutter - but he knows enough to be careful).  Everyone else: good luck.  Your ice will melt from ambient room temperature long before you etch away anything.  Between the immense heat-capacity of water ice, and the high reflectivity of ice to wavelengths of visible light, and the other inefficiencies related to making laser light, this whole exercise becomes very impractical when you scale it up to iceberg-sized objects.  Even if you put the laser and ice-cube in the freezer - so the ice doesn't heat up from the room - and let the laser shine on for days - every time it melts a little bore-hole, the water will re-freeze and seal it up pretty quickly.
 * For anyone interested in real-world ice boring, you might want to read about the IceCube Neutrino Observatory. Conventional technology that we use for drilling into materials like solid rock does not work on Antarctic ice.  (The ice flows into the bore-hole, moving as a fluid; and on contact, the pressure re-liquifies and re-freezes!)  So the ice  just seals up the holes you've cut.  The AMANDA and IceCube projects avoided "drilling" and "cutting" and instead relied on "melting" using a steady stream of hot water - and a very wasteful fossil-fuel plant to generate that water!  And here's an awesome review from Schlumberger - the oilfield service company - on "Drilling Through Ice".  Nimur (talk) 15:33, 16 May 2014 (UTC)


 * There are two or three significant problems with doing this. So let's talk a bit about my laser cutter...and then we'll see how this idea extends to cutting a glacier.
 * My laser puts out about 100 watts of energy - no more than an incandescent light bulb. That surprises most people when they see it slicing through a half inch of plywood like it wasn't there!  But take all of the heat from a 1000 watt bulb and put it into a beam the diameter of a pencil - and the result will set fire to wood and paper...but it won't cut it.  To get it to put enough energy into the material to cut it, you have to focus that pencil-thick beam into something about three hundredths of a millimeter across.  When you do that, it'll make most organic materials and many plastics simply "go away"!   A light bulb has about 100 square centimeters of area to radiate those 100 watts.  But that same amount of energy emitted as a one-square-centimeter beam is 100 times more energy per square centimeter than a lightbulb - but focus it down to 0.03mm and you get about 10 million times as many watts per square centimeter as that light bulb...and that's enough to zap just about anything that absorbs it.
 * However, my laser can't cut metal...even a thin sheet of kitchen foil is utterly impenetrable. That's because the metal reflects almost all of the energy away.   Because it's an infra-red laser, it also can't cut anything that's transparent to IR light because the beam goes right through it without the energy being absorbed.  There are lasers that can cut metal - but they have to be around 3000 watts and they aren't usually
 * The laser cutter has big fans to extract the smoke and hot gasses produced by the cutting process - and a high pressure air jet that squirts into the slot that the laser is cutting. These are there to remove smoke and debris from the path of the laser beam.  Anything like that that get in the way attenuates the beam so badly that it won't cut.
 * The trouble with slicing through a glacier is that you can't focus the beam thin enough once you're more than a few centimeters down into the ice. So you need VASTLY higher wattages.  If I wanted to cut wood with an unfocussed beam, I'd need a 100,000 watt laser rather than a 100 watt device!  Those kinds of laser exist...but they are massive, difficult, dangerous, fragile and horrendously expensive!   I suppose you could consider dynamically focussing the beam, increasing the depth of the focus into the ice as the beam cut deeper - but focussing is problematic.  My lasers focussing lens is made of Zinc selenide with a gold coating that's just 2 atoms thick!  That 1" lens costs $300 and is about as hard as candle-wax, so it scratches if you so much as look at it.  You can't use just ordinary lenses because if they aren't super-transparent, they get hot enough to melt in very short order!   Worse still, if you cut with a focussed beam, the "kerf" (the width of the slot) is about the same width as the diameter of the focal point of the beam...but that's not wide enough to fit the unfocussed beam as it enters at the top of the material.   For that reason, I can't cut wood that's more than a half inch thick without getting a much longer focal length lens.
 * Then the light frequency is a problem. Ice is both transparent and incredibly shiny at optical wavelengths...so you'd need to choose a frequency that the ice would strongly absorb and not be either transparent or reflective.
 * Finally, as you hit the ice, it's probably going to flash into steam...and you get a LOT of steam from a very small chunk of ice. Where that steam goes is a problem...if it simply drifts up the slot that you're cutting, then it'll absorb laser light and attenuate the beam...or it may condense back onto the sides of the slot...causing liquid water to fill the slot and have to be boiled away again.   You'd need to extract the steam as you cut if you wanted to make any progress.  Worse still, the steam will drift sideways back along the slot you've been cutting...it'll first condense and then re-freeze...so in all probability, your nicely lasered slot would just fill up with new ice again.
 * So I don't think a laser is the best way to do this.  We know that ice is drilled using hot water jets...and that's probably what you want here. SteveBaker (talk) 17:59, 16 May 2014 (UTC)


 * I have a hard time believing this could have any chance. I don't think of glacial ice having much tensile strength - after all, deep crevasses are all over the place. Wnt (talk) 15:40, 16 May 2014 (UTC)


 * And if all the reasons already given don't convince you, also consider that glaciers are constantly moving, and different layers move at different rates, so the hole would move and skew as you are trying to cut it. Therefore, you'd need to be able to cut the whole thing off quickly, so the movement wouldn't be significant. StuRat (talk) 18:06, 16 May 2014 (UTC)


 * I am reminded of Noah of Pontium who, reasoning that the Mediterranean was about to catastrophically overtop the Isthmus of Bosphorus, decided to dig just a little trench a few feet deep to let Mediterranean water slowly leak into the Black Sea, rather than in a flash flood that would kill or drive off millions of inhabitants of the area to their deaths. Of course, if we only had sharks with lasers on their noses, they could get under the glaciers and attack them from just the right point, where nothing could possiblye go wrong. μηδείς (talk) 16:59, 17 May 2014 (UTC)


 * You're talking about a massive quantity of water flowing through a narrow channel. That would rapidly erode it.  It would last longer, at least, if lined with thick stone. StuRat (talk) 23:30, 17 May 2014 (UTC)

Staged fire alarms
Why don't many public buildings use staged fire alarms? Some shopping malls and airports do but many places still seem to use the "if alarm goes off, evacuate building" method. In the staged alarm, there is a first stage where the occupants of the building are made aware that an emergency has been reported in the building and that people should prepare to evacuate, if necessary. During this first stage, building staff normally investigate the situation and make a decision as to whether evacuation is necessary. Normally this is a very quick process so doesn't delay evacuation significantly, in case it is necessary whilst also avoiding mass evacuatiation and system shut downs, if it is false alarms. The staged alarm can also be used for partial evacuation. So why don't many buildings opt for this staged alarm to minimise disruption in the case of false alarms? 82.40.46.182 (talk) 19:36, 16 May 2014 (UTC)


 * Because people don't get sued for over-reacting to a potential emergency. As building manager, if you purposefully cause a delay in evacuation and it then turns out that someone was killed, the notion that more people might have died in the stampede to the exits will not matter much to the jury. Nor will it matter if the delay wasn't the cause of the person's death. The risk of being sued to smithereens is way too high. Matt Deres (talk) 13:46, 17 May 2014 (UTC)


 * In which case what's the reason some buildings do use the staged alarms? Mostly large public buildings such as hospitals, airports, large stations, shopping malls, and large schools or colleges seem to use them. 82.40.46.182 (talk) 20:38, 17 May 2014 (UTC)


 * It depends on how quickly people can exit the building. With tall buildings like the World Trade Center, it took hours to evacuate everyone, so you had to just evacuate a few at a time, starting with those on the fire floor and directly above, to avoid people being trampled to death.  In a place with many exits and not many floors, like a mall, you should be able to evacuate everyone at once. StuRat (talk) 00:23, 18 May 2014 (UTC)
 * The risk analysis differs between different types of building as well. In a hospital, there is a risk of people being injured or killed by an unnecessary evacuation, which balances the risks of delaying evacuation in the event of a true emergency.--Srleffler (talk) 17:18, 19 May 2014 (UTC)

Why aren't magnets used to make pushing colenoids?
To make a pulling colenoid, you just need an iron core that is pulled by a coil. To make a pushing colenoid, you need plastic attached to the core, more length, and a spring to go back to the original situation. It seems much simpler to make the coil push the core by using a permanent magnet as core, where the coil makes a magnetic field in the opposite direction. Nobody seems to use that option, so my question is: why? I rewrote this a few times but I really can't explain it better than this. Sorry for that. Joepnl (talk) 22:11, 16 May 2014 (UTC)


 * I've never heard of "colenoid" - do you mean solenoid? I suspect that the article relay might answer, but I'm not sure. --ColinFine (talk) 23:05, 16 May 2014 (UTC)


 * The reason is strength. If you apply a weak magnetic field (as from a coil) to a soft ferromagnetic material (eg iron), yoy get a weak pull.  If you increase the strength of the field, the pull gets roughly proportionally stronger until the iron saturates (it is fully magnetised).  Increasing the field even more does not increase the strength of the pull, but it does not decrease it either.  Solenoids are generally designed to just saturate the iron - this reliably furnishes maximum strength, even if the current is not a precise amount, without wasting too much electrical energy.
 * To repell a magnet, the solenoid field has to be less than the permanent magnet field. If the current is a bit stronger than needed for balance, the "push" is cancelled out and the coil will pull the magnet in anyway, as the magnet field is simply overwhelmed.  Engineers always strive to make things non critical.
 * Further, there is a choice of materials to make magnets out of. If a metal alloy, such as alnico (a common permanent magnet material), its going to be a lot more costly than a soft iron and a spring, and the coil field will reverse its magnetisation so that it pulls anyway.  Metal alloy magnets are made by subjecting the alloy to a very strong coil, and the magnetisation is always in the "pull" direction.  The other choice is a ferrite.  The strength of magnetisation of ferrites is way below that of metal alloys.
 * 121.221.156.103 (talk) 02:25, 17 May 2014 (UTC)
 * (ec) A solenoid exerts a strong pulling force when there is a low Reluctance path for its magnetic field, with a gap across which the force is exerted. Typical magnetic materials are iron or ferrite; they form a small gap and the force direction is the same for either direction of current. Pulling solenoids need a spring or gravity to return them to the no-current position. It is possible to make a solenoid both pull and push a permanent magnet core but this has disadvantages:
 * Attainable force is limited by the strength of the permanent magnet, i.e. the Coercivity of its material which costs more than iron. High temperature, mechanical shock, excessive repelling current or a strong external magnetic field can all degrade its magnetism.
 * It is less easy to construct the curved low reluctance path from these materials.
 * Any loose ferromagnetic particles will attach themselves to the magnet.
 * Two examples of solenoids in permanent magnet fields are moving coil ammeters and the Voice coil in a Loudspeaker. The direction of force on these solenoids is reversible by reversing the current flow. 84.209.89.214 (talk) 02:51, 17 May 2014 (UTC)
 * I note that the two examples involve very weak forces. The pointer deflection force in a moving coil meter is obviously minute.  In order to assure long term accuracy and a deflection linear wrt electric current, the coil field strength is much less than the magnet field.  Loudspeakers are notoriously inefficient, typically converting only about 5% of the electrical input energy into sound energy. In order to have acceptable harmonic distortion, the coil field strength is quite small compared to the magnet field strength.121.221.156.103 (talk) 05:00, 17 May 2014 (UTC)
 * Yes. Linear deflection wrt electric current is achieved by the combination of 1) a return spring whose force obeys Hooke's law with 2) a permanent magnet constructed to give a uniform magnetic field strength throughout the range of motion of the coil. 84.209.89.214 (talk) 13:34, 17 May 2014 (UTC)

wage slavery around the world
hello, are there stats as to what percentage of (fit) people in a given country derive their livelihood from hired labor (as opposed to self-employment (such as keeping a shop), civil service, subsistence farming etc)? I presume it's roughly two thirds pretty much everywhere, but still? also, what makes the difference between the average worker and the "big" CEO's etc, who are but employed workers like everyone else (a fact of which they as a class always seem to make a big point), yet are hardly wage slaves. I only want to know just how prevalent this model, which afaik is pretty recent (300 years?), has become. thanks Asmrulz (talk) 23:58, 16 May 2014 (UTC)


 * I think the whole idea that "employee" = "slave" is just completely wrong. There are many highly compensated employees, such as the CEOs you mentioned, sports and movie stars, etc.  There are also many dirt poor self-employed people, like subsistence farmers, especially in poor nations.  The protest that began the Arab Spring was even started by a self-employed man (Mohamed Bouazizi) who was systematically abused by the government.  StuRat (talk) 00:37, 17 May 2014 (UTC)


 * Can arbitrary and capricious harassment be properly described as "systematic"? I'll accept "systemic". —Tamfang (talk) 06:19, 17 May 2014 (UTC)


 * I don't believe it was arbitrary, there was a concerted effort to harass anyone who didn't pay the bribes, so as to increase their take. StuRat (talk) 15:23, 20 May 2014 (UTC)


 * And from what I have read, it wasn't even harassment in the legal sense of the word -- just nitpicky law enforcement, and it wasn't so much the nitpicking per se that enraged him, but the fact that it was a woman who was laying down the law to him! In fact, one news report (which may or may not qualify as a reliable source) even claims that he was the one who physically assaulted the woman cop, and that's what started the whole thing. 24.5.122.13 (talk) 06:31, 17 May 2014 (UTC)


 * I don't think that started it, that was a result of the harassment. And the corruption was such there that nobody could get the proper licenses to operate legally without knowing somebody in the government and/or giving them a hefty bribe. StuRat (talk) 13:02, 17 May 2014 (UTC)
 * Before we go further... is this actually a science question? Wage statistics are really a matter of economics, or political theory, or history.  This question, and its responses, might fit in better at the humanities desk.  But we've apparently digressed into a discussion of various people's opinions about the "Arab Spring" - and thus far, nobody has actually connected their diverse opinions back to the topic of wage politics.  The topic is fascinating, and I'm sure many of us have strong and well-informed opinions on those topics - but this is not the correct place to have the discussion.
 * The OP might start informing his question with conceptual backgrounds and factual data by reading about industrial and labor relations; employer-worker relationships, minimum wage in the United States, ... Our encyclopedia contains much information on this topic, as pertaining to the U.S. and other parts of the world.
 * If the OP actually wants statistics - as they asked for - they might find that the Bureau of Labor Statistics (a branch of the United States Department of Labor - makes the most thorough and complete records available at no cost. Other countries may have similar government agencies.  Private-sector and academic research may also exist to provide such data, but is usually available at cost.
 * If the OP is interested in the perspective that employment and oppression are intermingled, he might find Howard Zinn's influential book, A People's History of the United States, a worthwhile read. It is well-informed and factual; it has been called exemplary and inflammatory.  Anybody who is interested in wage politics would do well to read it, even if you do not agree with Zinn's views.  Nimur (talk) 18:17, 17 May 2014 (UTC)