Wikipedia:Reference desk/Archives/Science/2017 December 10

= December 10 =

Towed array
What is the minimum depth under the keel which is required to safely deploy a towed array without it ripping away from dragging on the sea bottom? Is this depth the same for submarines and surface ships? 2601:646:8E01:7E0B:F9B4:9A86:7938:FC5D (talk) 02:14, 10 December 2017 (UTC)


 * No they aren't the same. Civilian towed arrays are a bit more sophisticated than their military counterparts, and can be actively flown at different depths, handy if a ship wants to cross your stern during a survey. I believe military submarine streamers are slightly negatively buoyant, I don't know about military surface ships arrays. In the days before solid streamers a big part of deploying a surface streamer was ballasting it to neutral buoyancy, since if a section dived the sea pressure would force the paraffin up, emptying the tube, encouraging a deeper excursion. Very messy. This is the design I worked on, I think http://www.ldeo.columbia.edu/research/office-of-marine-operations/seismic-equipment-and-operations
 * The depth is typically 10 m below the sea surface for a surface array. Greglocock (talk) 02:42, 10 December 2017 (UTC)
 * There are definitely many different types and some likely technically independent from distance to the ground. However an array must be roughly in one straight line, because the real water column is divided into Thermocline or thermal layers which de- and reflect acoustic signals like a Mirage or Fata Morgana can de- and reflect light. A "hanging" towed array would make it a challenge to keep the array in one of these layers and therefor massively reduce the arrays capability to detect sound sources, their direction and distance reliable. --Kharon (talk) 06:32, 10 December 2017 (UTC)
 * OK, the array in question is a military underwater array for submarine detection and tracking (such as the TB-23). 2601:646:8E01:7E0B:F9B4:9A86:7938:FC5D (talk) 07:58, 10 December 2017 (UTC)
 * I think if the sub is stationary the streamer hangs vertically. But I don't know. As you know the density of water varies and the compression of the array varies with depth so it is not possible to have a neutrally buoyant streamer for all depths. It would be nice to put active depth keeping modules into the streamer but that is not compatible with a winched system (on a ship the deck crew can fix wings on as the streamer is deployed). Greglocock (talk) 19:41, 10 December 2017 (UTC)
 * Nope, modern civilian arrays can use various position sensing techniques so they know the shape of the streamer and can then adjust the signal processing to account for bends in the streamer.Greglocock (talk) 19:41, 10 December 2017 (UTC)
 * [un-indent] OK, does anyone happen to know the minimum safe depth for deployment? I'm researching for a manor-house detective novel which takes place aboard a nuclear submarine, and I need to know how far offshore they have to be in order to deploy the array!  (In Red Storm Rising, it says that 200 feet below the keel is too shallow for a towed array -- but that's a work of fiction, so I can't rely on that alone.) 2601:646:8E01:7E0B:F9B4:9A86:7938:FC5D (talk) 03:35, 11 December 2017 (UTC)

Submarine crew
How big is the engine watch on the Virginia-class submarines, and how many of them are officers and how many enlisted? (If this is classified, please say so and I'll cross this question out.) 2601:646:8E01:7E0B:F9B4:9A86:7938:FC5D (talk) 02:17, 10 December 2017 (UTC)
 * If it's classified, no one here will know the answer. ←Baseball Bugs What's up, Doc? carrots→ 03:19, 10 December 2017 (UTC)


 * People do things they regret, Bugs. https://www.theguardian.com/us-news/2016/aug/20/us-navy-sailor-jailed-for-taking-photos-of-classified-areas-of-nuclear-submarine http://www.foxnews.com/us/2017/01/25/pardon-me-navy-sailor-in-jail-for-submarine-photos-pleads-for-mercy-from-trump.html We can hope no one would share classified information, for their own sake. μηδείς (talk) 04:05, 10 December 2017 (UTC)


 * I googled "crew size of virginia class submarine" and found this top-secret US Navy website: ←Baseball Bugs What's up, Doc? carrots→ 04:50, 10 December 2017 (UTC)


 * Doesn't say how many of them work in the engine room, though (and I need that exact piece of info to determine the number of suspects). 2601:646:8E01:7E0B:F9B4:9A86:7938:FC5D (talk) 07:55, 10 December 2017 (UTC)
 * "Suspects"? ←Baseball Bugs What's up, Doc? carrots→ 06:46, 14 December 2017 (UTC)
 * Yes, it's for a detective novel and the first known clue about the saboteur is that he works in the engine room. 2601:646:8E01:7E0B:2508:122:A2DD:3288 (talk) 10:40, 15 December 2017 (UTC)

Nitrate testing chemical
Two questions:
 * 1) What chemical is used to conduct the nitrate test for freshwater from API?
 * 2) What chemicals (or class of chemicals) could create a false positive?

Background: I tested my hand-me-down aquarium's water and the test turned cherry red within 30 seconds (which is bad). The Miata should be like 20 ppm, not 200+, given the bio load and time since water change. My thought was that a terracotta pot leeched something into the water (silicate most likely). Fish show no signs of stress which makes me think it's not actually nitrate.  Eve rgr een Fir  (talk) 07:22, 10 December 2017 (UTC)
 * Link to help explain the API reference.--Phil Holmes (talk) 12:01, 10 December 2017 (UTC)

Glock pistol's walls design
Why are Glock pistols kind of square on the top? What advantage has such design? I assume a part won't be more resistant that at it's thinner spot, and it's easier to create uniform parts than to create parts with variable thickness. --Hofhof (talk) 13:37, 10 December 2017 (UTC)


 * If you do a Google search for Glock squared off slide or similar, you'll find lots of discussions on this point. Selecting one at random, there appear to be a number of plausible factors.  (You will also find plenty of flame wars.  Gun aficionados are as prone to bickering over minutiae as any other class of fanatic.)
 * Reduced manufacturing cost. Squared-off blocks are often easier and cheaper to make.
 * Deliberately increased weight. Since the Glock pistol has a fairly lightweight plastic frame, putting a bit more heft in the metal slide can reduce perceived recoil.  It also helps with durability of the slide&mdash;the increased mass means it doesn't move as fast when cycling.
 * Improved robustness of locking lug(s). The square slide permits use of a single large square lug, rather than multiple radial lugs.
 * Better grip. Some people find it's easier to grasp a square slide compared to a rounded one.
 * There's lots of inconclusive discussion about which factors guided the design and which ones were consequences. TenOfAllTrades(talk) 15:50, 10 December 2017 (UTC)
 * Its simply a design choice. No physics or magic behind it. Why are all US Pickups huge? Why does Toblerone sell chocolate in triangular bars? Customers get used to designs and features connected to brands and in general reject changes. So it is always a huge risk to change an established product.
 * Anyway, a SIG Sauer P226 does not look that much different. At least if you compare both for example with the iconic Mauser C96 which technically outmatches all of today's firearms. But with its outdated design no one would buy it anymore today. --Kharon (talk) 00:09, 11 December 2017 (UTC)

Because of the rear sights are removeable. It's a milled slot. Newer RMR sights require a flat surface as well. Search for "sight pusher" and you'll see the jigs used to remove the sight. Search for "RMR sight" and you will see the flat, large sight that attaches to the slide. Also, the barrel breach end is a square block of steel unturned when the barrel is made. The Glock design has the barrel tilt up with the square slide moving across the square breach. If the slide were round, the pistol would need to be bigger. Lastly, it's easier and cheaper to make a metal piece in a brake than a CNC mill. --DHeyward (talk) 23:15, 11 December 2017 (UTC)

mental activity during unresponsiveness
Last winter I underwent emergency abdominal surgery, followed by respiratory failure and severe sepsis. During the first 10 days in intensive care I was unresponsive, recovering gradually until after about a month I was completely aware of my surroundings. During this time I experienced an active mental life: feeling that I was on a difficult but purposeful journey filled with struggle and frustration, I passed through quite a few dreamlike scenes. At one point I had to choose between life and death; at another, I found myself completely alone in the universe. Gradually I began to incorporate incidents happening around me in the hospital, but these were incorporated into my private world. I'm now completely recovered.

I wonder if the medical literature describes any similar experiences? If it would be useful for me to share my experience with the medical world, how would I do that? --Halcatalyst (talk) 16:13, 10 December 2017 (UTC)


 * Glad to hear you have recovered. Think what you describe, is what goes on in the mind when placed in a induced coma, which would have been appropriate for your serious condition. Doctors are well aware of these dreamlike states where one is not sure which world or reality one is in. This is just one woman's experience: This Woman Explains What It’s Like to Be in a Medically Induced Coma. Think your experience is better telling to others that are just recovering. You can Google around for support groups where you can help give support to those that are wondering about this most bewildering experience. Aspro (talk) 20:27, 10 December 2017 (UTC)
 * In the non-medical literature, The Bridge (Iain Banks) and Pincher Martin (William Golding) use this as a plot device, so the phenomenon is not unknown. We _do_ have an article Life review, but it's written from a decidedly non-scientific perspective, so I can't really recommend it here. Tevildo (talk) 21:34, 10 December 2017 (UTC)


 * Delirium is a well-known phenomenon. The last time I was hospitalized the person in the bed next to me came in screaming of being attacked by unseen entities, was diagnosed with multiple organ failure, and died.  When they brought in the next patient for that bed at 3am in the morning without waking me or explaining what was going on I felt like I was in an alien-abduction episode of The X-Files with wires sticking out of me, bright lights, strange figures.  It took me quite some time (I was under sedation and on an IV) to realize my actual situation.  The same happened in the past when I was on a morphine drip and woke intubated and restrained from an abdominal surgery that was supposed to be laparoscopic but lasted 16 hours.  And when I had minor foot surgery (2 hrs.) and woke intubated I had to bang the bedrail to get the doctors to realize I was awake and aware, so they would remove the tube.


 * My point is, medical professionals will have seen this all, and although it might seem of great interest to the patient, unless you are part of a study on post-operative recovery, there's not much point in advising anyone of your delirious state after the fact. It would be unethical for them to induce delirium, and they won't learn anything from subjective reports after the fact that hasn't been known since the ancients. Our article: "Delirium is one of the oldest forms of mental disorder known in medical history." μηδείς (talk) 23:56, 10 December 2017 (UTC)


 * Of course, observations from random internet posters shouldn't be relied upon. Your doctor is your best bet for discussing the subject. 2606:A000:4C0C:E200:831:EE2:9FFB:76D0 (talk) 00:07, 11 December 2017 (UTC)


 * Mental activity of unconscious patients is well-documented. It appears that the question is asking if the content of the dream is important. Documentation of people divining life meaning from dream content goes back thousands of years. In the medical field, dream content is not important. 209.149.113.5 (talk) 18:01, 11 December 2017 (UTC)

Thank you all for your responses. I don't believe the "journey" story means anything, it was just the way I perceived/interpreted my experience.

The article you referenced was very interesting; the woman and I definitely had the same type of experience. Muscular atrophy, cognitive problems, etc. At one point with the speech pathologist I couldn't think of a single word that begins with S.

I did receive fentanyl, but I don't believe I had any of the symptoms of delirium. At one point the doctors told my family I was on the brink of death, but I don't identify with near death experiences as they are usually described. Nor with the Life Review article. --Halcatalyst (talk) 21:54, 11 December 2017 (UTC)


 * Delirium is a symptom and a broad spectrum; so certain aspects may apply, yet not others. It is certainly a response both to trauma and sedation, and confabulation (the making up of narratives) is typical.  I suggest you read the article, as much of it may apply, whether that word is one you would choose to use yourself.  Since you are not asking for a diagnosis, the choice of label is unimportant. μηδείς (talk) 03:23, 12 December 2017 (UTC)
 * Yes, I did read the article (thanks again for suggesting it), and looked up further information on delirium. The doctors didn't say they had induced a coma. As you say, the label isn't important. To me, looking back, the experience was interesting, maybe the more so because I didn't have any of the unpleasant aftereffects described by the woman in the article.

Secondary car mirror reflections
In my front view car mirror there are two faint reflections in addition to the main one - one above it and one below (the pic shows only the one above). What is the explanation for this? Gil_mo (talk) 16:40, 10 December 2017 (UTC)
 * If your car allows you to flip the mirror to a "night vision" setting which reduces the glare of car lights behind you, the mirror used for that setting is probably what you're seeing. ←Baseball Bugs What's up, Doc? carrots→ 16:52, 10 December 2017 (UTC)


 * (ec) A lot of those are rigged with a feature where you push them back somehow and get a weaker reflection in a different plane, to prevent the high beams of the guy behind you from being quite so annoying at night. This is the "prismatic" feature described at rear view mirror; apparently the angle is implemented as a glass prism. Wnt (talk) 16:57, 10 December 2017 (UTC)


 * This diagram may help. SemanticMantis (talk) 17:12, 10 December 2017 (UTC)
 * In short: the "daytime" reflection uses the mirrored surface from the rear of the glass; the "nighttime" reflection uses the plain front surface of the glass. The glass is wedge-shaped, and flipping the lever-thing changes the angle of the mirror accordingly. 2606:A000:4C0C:E200:CCFA:802A:8BDD:BA79 (talk) 19:09, 10 December 2017 (UTC) — Oops, I missed the "in addition to the main one" part of the query. [dynamic IP] :2606:A000:4C0C:E200:831:EE2:9FFB:76D0 (talk) 22:58, 10 December 2017 (UTC)
 * When I was a wee lad my parents' car had a "Danite double vision mirror," per the owner's manual. Having read Arthur Conan Doyal's "Study in scarlet," I wondered what  rear view mirrors had to do with Mormon terrorists.Edison (talk) 21:29, 10 December 2017 (UTC)


 * Our article: Rear-view mirror.
 * But this explains only one of the two faint reflections asked about. I've noticed it as well, and found that at night if I have the mirror properly adjusted while clicked in either position, then flicking it to the other position will give me an equivalent dim view.  Is this second dimmed reflection due to a partial internal reflection causing a portion of the light beam to make yet another pass through the mirror?  (Hmm, Internal reflection redirects to Total internal reflection.  There is such a think as a partial internal reflection, isn't there?) -- ToE 21:31, 10 December 2017 (UTC)
 * Yes, there is. 2606:A000:4C0C:E200:831:EE2:9FFB:76D0 (talk) 22:52, 10 December 2017 (UTC)
 * I think that's on the right track. Think of a mirror aligned with the headlights with a prism in front of it.  You turn it one way and now the surface of the prism is aligned perpendicular to the headlights, and you see the reflection off that.  You turn it the other way and now the reflection of the surface of the prism (apparently on the far side of the mirror!) is aligned with the headlights.  The only thing I need to make this a theory is for the partial internal reflection of light hitting glass straight-on to be the same as (or at least similar to) the partial external reflection of light hitting glass straight-on.  My gut feeling is that it has to be because a light path should be reversible -- if 96% of the light makes it through one way, I'd think that means 96% makes it through the other.  And if the glass doesn't absorb much light, it doesn't have any momentum to allow it to do anything but pass through or reflect, so... Wnt (talk) 03:38, 11 December 2017 (UTC)
 * Your feeling is kind-of wrong correct, but misleadingly formulated ; see Fresnel equations for the mathy explanation, and total internal reflection for an example where this goes horribly wrong. When the light strikes close to perpendicular to the air/glass surface, which is the case for the rear-view mirror, the coefficient of reflexion depends only on the refraction indices and not on the striking angle (Fresnel_equations), i.e. the transmission coefficient is the same for glass-air as for air-glass). But far from the normal, it is symmetrical with respect to light-return, but not space-symmetrical. Tigraan Click here to contact me 10:58, 13 December 2017 (UTC) (edited 11:02, 13 December 2017 (UTC))
 * Actually I don't think total internal reflection contradicts this idea, because the whole idea there is that there is no way for an external ray of light to get inside the refractive medium following the same path as the reflected ray. Wnt (talk) 11:35, 13 December 2017 (UTC)
 * Your idea that "a path of light A -> B (on surface) -> C has the same transmission coefficient as the path C -> B -> A" is correct. But the way I misread it at first (and I assume others could misread it as well) is "a path of light A -> B -> C has the same transmission coefficient as the path C' -> B -> (whatever), where C' is the symmetrical point to C with respect to the surface", which is wrong. I just wanted to clear that up (and provide links to the relevant article for transmission coeffs). Tigraan Click here to contact me 17:13, 13 December 2017 (UTC)



Article improvement opportunity: If anyone here is handy with Inkscape or the like, Rear-view mirror would be well served by an image similar to the one linked above by SemanticMantis. -- ToE 16:10, 11 December 2017 (UTC)
 * Tigraan Click here to contact me 17:32, 11 December 2017 (UTC)
 * The graphics files are File:Rear view mirror antiglare-day position.svg and File:Rear view mirror antiglare-night position.svg. Art is not my forte but hopefully the diagrams are clear enough they can be moved into the article as they are. I cannot do it right now, but I will get around it tomorrow if none has taken care of it. Tigraan Click here to contact me 18:10, 11 December 2017 (UTC)
 * Ha! I love the constricted / dilated pupil.  Great job!  I might be pushing my luck here since the result wouldn't really be applicable to the article, but I would also love to see a third image with the mirror tilted down from the glare position instead of up, showing the path via internal reflection, where the main light beam passes through the glass, reflects off the mirror, and then a small portion reflects off the interior of the glass, back to the mirror, and then out through the glass.  This is the second dim image asked about by the OP.  Angles as they are, I suspect that your first two images would be too crowded to include this third path, but with the mirror down so that this third path is reaching the eye, angles may be such ASkthat all three paths could be included.  -- ToE 20:13, 11 December 2017 (UTC)
 * ✅ Ask and you shall receive. I added the other two pictures to the article. Tigraan Click here to contact me 20:56, 12 December 2017 (UTC)
 * Tigraan! You're my hero!  And the illustrations in the article look great.  (I didn't even know about switcher!)  The sun / moon & stars are frosting on the cake.  -- ToE 23:28, 12 December 2017 (UTC)
 * OP here. Well done, Tigraan, well done everybody! Smarter every day... Gil_mo (talk) 07:44, 13 December 2017 (UTC)
 * OP again, here's a challenge - could this be simulated in Blender? Gil_mo (talk) 07:47, 13 December 2017 (UTC)

See the mirror here and how it reflects the exposed sunbeam. -- Hans Haase (有问题吗) 18:07, 13 December 2017 (UTC)