Wikipedia:Reference desk/Archives/Science/2013 March 6

= March 6 =

Silent Supernovae
Is it right to say that when a star goes supernova, it is an acoustically silent event, even if one could (hypothetically) witness it from a close distance? And because sound cannot travel through space, this would also apply to events such as Gamma Ray bursts, and even the Big Bang itself, correct?Honeyman2010 (talk) 04:50, 6 March 2013 (UTC)


 * This seems to be a common question. Sound can and does travel through space, as it is not a complete vacuum.  However, due to it being a pretty good vacuum, the sound propagated is not what human ears could detect, even if the human could function in that vaccuum.  You would need absolutely immense ears.   Go to the top of this page, and try "sound in space" and similar strings in the Search Reference Desk archives field, and you'll be entertained for quite a while.  Wickwack 120.145.35.223 (talk) 05:17, 6 March 2013 (UTC)


 * And, when the particles blasted off during the supernova hit you, they would make quite a racket, indeed. StuRat (talk) 05:29, 6 March 2013 (UTC)


 * Not to mention, vapourise you in the process. I mean planets in orbit have a difficult time surviving such an event, what chance have a mere human? If our sun were to go supernova, Jupiter would be stripped entirely of its atmosphere and be left a metallic white-hot cinder only just larger than earth. The outer gas giants beyond should fare better. Plasmic Physics (talk) 08:06, 6 March 2013 (UTC)


 * If you were in that star system, sure. But if you were far enough away, you're spaceship would just get noisy when the particles hit. StuRat (talk) 16:30, 6 March 2013 (UTC)
 * Or do it the Native American way: touch your ear to the dark side of a planet. The chances of survival are still somewhat low... ;) - ¡Ouch! (hurt me / more pain) 08:04, 8 March 2013 (UTC)
 * As for the Big Bang, yes, you would hear it, but it would not sound like a bang. It would sound more like a very, very ,very loud, high pitched hum, which would decay both in volume and pitch in an exponential way. You'd hear it because space was not as vacuous as it is today. Plasmic Physics (talk) 07:24, 6 March 2013 (UTC)


 * After this much time, it has decayed to such an extent, that it is not discernable from background noise. The sound would initially have been audible for millions of years. Plasmic Physics (talk) 07:28, 6 March 2013 (UTC)


 * Considering a gamma ray burst, would it not create secondary sound as it irradiates the listening device, creating a large amount of static. Plasmic Physics (talk) 08:09, 6 March 2013 (UTC)

Big Bang and light
Is it true that all matter can be converted into light? If so, is the coverse true? If that is also true, could the primeval atom have been pure light in a metastable state, which spontaneously condensed into matter, such as particle-antiparticle production from gamma radiation. How many electron volts would a single photon have to represent to contain the total universal energy and all its mass. Plasmic Physics (talk) 07:51, 6 March 2013 (UTC) "Let there be light." Plasmic Physics (talk) 07:54, 6 March 2013 (UTC)
 * If matter fell into a black hole, or quantum tunneled into a black hole, the hole may then decay into Hawking Radiation, which may be mostly photons. So the future of matter may the formation of cool photons.  You will have to say how big is the universe to get an answer to the total energy in the universe.  As the universe ages the visible universe expands, so we do not actually know the size or mass of the universe. Graeme Bartlett (talk) 09:33, 6 March 2013 (UTC)


 * Well, the most obvious problem is that there's no such thing as light before electroweak symmetry breaking. If you believe inflation, the energy of all the particles in the universe came from the energy of the field that caused inflation, which is similar to the present-day Higgs field or dark energy. So they did come from pure energy, in a way.


 * According to Google the density of the universe is around 5·10−30 g/cm³, and the radius of the visible universe is about 46 billion light years, and it's spatially flat, so the total mass in the visible universe is around (5·10−30 g/cm³) (4/3) π (46 Gly)³ ≈ 1090 eV/c². But that's just the part we can see. -- BenRG (talk) 22:13, 6 March 2013 (UTC)


 * What would happen to such an energetic photon if released into our universe today? Plasmic Physics (talk) 08:14, 7 March 2013 (UTC)

Some questions about the Universe
I have some questions about the Universe.

1. Is the Universe finite or infinite? Universe is the sum total of all matter and energy. The unknown part of the Universe, i.e. the part that extends beyond the observable universe, may be finite or may be infinite. What are the current opinion regarding this?

2. If the Universe is finite, then it must have edges or boundaries. I want to know what is beyond that boundary? Is that pure vacuum?

3. If the Universe is infinite, then why is it said that the Universe is flat. An infinite object is not supposed to have any shape.

4. According to the Big Bang theory, the Universe, at its beginning (the very early Universe), was rapidly expanding and cooling, and then expanded gradually. I want to know is there any explanation how did this very early Universe originate? --PlanetEditor (talk) 13:31, 6 March 2013 (UTC)
 * (Q4) If I'm not wrong, Stephen Hawking said something like at the very very early stages of the Universe, it (the Universe) was just like a very small balck hole. As time does not exist in a black hole, there was no time before the Big Bang. There is no cause or origination for the Universe because there was not time for a cause to exist in. It just "popped up" like a proton, which IS possible according to some elaborate Science laws. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 13:44, 6 March 2013 (UTC)
 * If you're talking about the stuff in A Brief History of Time, that was never mainstream; it was just a description of his own (highly speculative) research. -- BenRG (talk) 21:53, 6 March 2013 (UTC)
 * re Q2: finite does not imply edges or boundaries. The surface area of the earth is finite, for instance, but has no edges, (according to modern theories). Imagine a 4th dimensional sphere analog that we're on the 3d surface of. Now imagine that sphere expanding, and now you have the expanding universe we experience, where there is no particular center to the expansion which is uniform all over. Gzuckier (talk) 18:15, 6 March 2013 (UTC)
 * (Q1) According to the Shape of the universe article, the infinite flat model is currently the most popular of the models of the universe that fit the currently available data. Red Act (talk) 21:54, 6 March 2013 (UTC)


 * 1. Nobody knows. If you asked cosmologists to guess, I suspect a majority of them would say infinite, but that doesn't mean much. 2. As Gzuckier said, it doesn't imply there's a boundary, but there could be a boundary. There would be nothing outside of it, which is not the same as vacuum. Light can't propagate through nothing. 3. An infinitely large object can have a shape. For example an infinite plane and an infinite paraboloid have different shapes. 4. Nobody knows, but if you asked cosmologists to guess, most of them would guess some kind of inflation. -- BenRG (talk) 21:53, 6 March 2013 (UTC)

Thank you everyone. --PlanetEditor (talk) 10:12, 7 March 2013 (UTC)


 * However I would have liked a bit more explanation of Q4. The religionists use this issue as an excuse to prove the existence of God. --PlanetEditor (talk) 10:20, 7 March 2013 (UTC)


 * If you haven't seen it already, Big_Bang. You really are stuck in a bind, there. Either you have something exist before the Big Bang, and then have to explain where that came from; or you have a theory such as Hawking's where a beginning is (allegedly) not required. Neither class of theories is likely to satisfy. Someguy1221 (talk) 10:45, 7 March 2013 (UTC)


 * Hawkins' explanation that time is compressed inside a black hole - and that's (effectively) what the early universe was like - may or may not be correct, but at least it offers one possible explanation for the "Origin of Everything" question that doesn't result in an infinite regress. The idea that this is an argument for the existence of God is a silly one because one only has to ask the question "So...what created God then?" to demonstrate that adding a supernatural being as the creator of the universe doesn't help to answer the ultimate question If the early universe was "caused" by something, then you're in the infinite regress of asking what that cause was and what was the cause of the cause and so forth.  Hawkins' approach is elegant in that by pointing out that time inside a black hole is crunched into nothingness and therefore there is no "Before" in the case of the big bang - it was there for a literally infinity amount of time.  He might not be right - but at least there are possible "ultimate origin" explanations that do not require gods and other mystical junk to make them work. SteveBaker (talk) 22:04, 7 March 2013 (UTC)


 * Thank you Someguy and Steve. --PlanetEditor (talk) 09:07, 8 March 2013 (UTC)

Statistics of hysterectomies
Why are hysterectomies so common in women above tha age of 50 if its such a major operation? Clover345 (talk) 14:48, 6 March 2013 (UTC)
 * I think the article on Hysterectomy actually does a good job of going into some of the issues. The major downside of the surgery (aside from risk) is loss of the ability to bear children, which is usually a moot point by the age of 50.  The upshot to the surgery, such as benefiting cancer risk or severe pain, will often outweigh that downside, although for sure there are a lot of these being performed. ~  Amory ( u  •  t  •  c ) 15:24, 6 March 2013 (UTC)
 * From personal experience in the UK, if there is no good reason to remove a uterus then they won't. Fibroids or adenomyosis stop responding to oestrogen as less and less is being produced, and shrink of their own accord. I'd like to see up to date statistics on this. --TammyMoet (talk) 16:23, 6 March 2013 (UTC)

Prove relation
Given $$P(x,t) = |\Psi(x,t)|^2$$,

and $$J(x,t) = -\frac{ih}{2m}\left(\Psi^*(x,t)\frac{\partial}{\partial x}\Psi(x,t)-\Psi(x,t)\frac{\partial}{\partial x}\Psi^*(x,t)\right)$$

I want to prove the following relation

$$\frac{\partial}{\partial t}P(x,t)+\frac{\partial}{\partial x}J(x,t) = 0$$

I denote complex conjugate with asterisk. I believe I can think of examples of functions Psi for which it's not true. Here is my working so far anyway.

$$=\frac{\partial}{\partial t}|\Psi(x,t)|^2-\frac{ih}{2m}\frac{\partial}{\partial x}\left(\Psi^*(x,t)\frac{\partial}{\partial x}\Psi(x,t)-\Psi(x,t)\frac{\partial}{\partial x}\Psi^*(x,t)\right)$$

$$=\frac{\partial}{\partial t}\left(\Psi(x,t)\Psi^*(x,t)\right)-\frac{ih}{2m}\frac{\partial}{\partial x}\left(\Psi^*(x,t)\frac{\partial}{\partial x}\Psi(x,t)-\Psi(x,t)\frac{\partial}{\partial x}\Psi^*(x,t)\right)$$

$$=\Psi^*(x,t)\frac{\partial}{\partial t}\Psi(x,t)+\Psi(x,t)\frac{\partial}{\partial t}\Psi^*(x,t)-\frac{ih}{2m}\frac{\partial}{\partial x}\left(\Psi^*(x,t)\frac{\partial}{\partial x}\Psi(x,t)-\Psi(x,t)\frac{\partial}{\partial x}\Psi^*(x,t)\right)$$

It's not clear at all that this is zero to me.

150.203.115.98 (talk) 15:19, 6 March 2013 (UTC)


 * maybe this is a homework question, so we should be wary. I don't know much about this area, but I'd try integrating J first. Let me know if I'm out on a limb. IBE (talk) 17:19, 6 March 2013 (UTC)
 * You should differentiate the expression in parentheses (by x) then you should substitute derivatives by time from the 1D Schrodinger equation in free space. After that everything will be obvious. Ruslik_ Zero 18:15, 6 March 2013 (UTC)

Reverse osmosis semipermeable membranes
Semi-permeable_membrane says that membranes used in reverse osmosis are made of polyimide while Thin-film composite membrane says that they are made of polyamide.

What's correct?--Gauravjuvekar (talk) 16:42, 6 March 2013 (UTC)
 * Looking through a number of suppliers of such membranes, most just describe them as "Thin Film", but Dow's datasheet states that their membranes are polyamide. On the other hand, this patent is for a polyimide membrane.  I think it's safe to say that both _can_ be used. Tevildo (talk) 22:36, 6 March 2013 (UTC)
 * This site lists lots of polyamide membranes, a few "Thin Film" membranes, and a few cellulose acetate membranes. Polyamide seems to be the most common material. Tevildo (talk) 22:45, 6 March 2013 (UTC)

Meteor fragments and aircraft
There is LOTS of discussion about meteors/comets/etc impacting the Earth, the History of the big ones, etc. HOWEVER, I heard somewhere that 40 TONS of material impact on the Earth's atmosphere daily. ALL discussions I've read say something like "most" of this material burns up in the Earth's atmosphere, leaving particles "smaller than grains of sand" etc. When most folks read this phrase, they say "no problem". HOWEVER, I would imagine that a particle not much bigger than a grain of sand could have a disastrous effect should it strike the wing of a flying aircraft. How come there aren't more recorded (or hypothesized) cases of aircraft being affected by meteor fragments? Thanks, Jack Murray p.s.- This is a WORLDWIDE question p.p.s. - How do I know what to search on in Wikipedia to find out if anyone addressed this? — Preceding unsigned comment added by Jaqmur (talk • contribs) 17:14, 6 March 2013 (UTC)


 * Regarding your last question, "How do I know what to search on in Wikipedia to find out if anyone addressed this?", knowing what search terms to use to finds the answer to a question is known as Google-fu, and it's a skill you learn through practice. If you meant where do you search, there's a search bar at the top of every reference desk page where you can search the desk archives. 90.193.232.228 (talk) 17:43, 6 March 2013 (UTC)


 * Particles the size of a grain of sand will rapidly decelerate to free-fall speed, due to air resistance. A grain of sand at that speed is not a problem for an aircraft.  However, many such grains of sand are, and we get those by flying through a volcanic plume. StuRat (talk) 18:00, 6 March 2013 (UTC)


 * Just in case you're still struggling with Google, I had a go myself. I typed aircraft "meteor strike" into the search bar (the words within the "quote marks" means that Google will only show an exact match for that phrase). I found a data analysis blog called Revolutions - How much of a threat are meteors to aviation? Using some complicated calculations, the author states that the chance of a meteor strike on a single aircraft are "about 1 in a billion, give or take". However, given the number of aircraft using the sky, "there's about a 4.3% chance of a meteor strike on at least one airliner in the next 20 years". Alansplodge (talk) 18:16, 6 March 2013 (UTC)


 * It's called terminal velocity, not "free fall speed". μηδείς (talk) 23:28, 6 March 2013 (UTC)


 * "Free fall speed" is a synonym for "terminal velocity". StuRat (talk) 05:08, 7 March 2013 (UTC)


 * Aircraft regularly encounter hail stones. Hail is capable of stripping paint and abrading windshields so pilots use their weather radars to avoid heavy rain and hail. Despite that, damage more serious than stripped paint and abraded windshields occurs occasionally. A meteor fragment the size of a grain of sand, falling at its terminal velocity, is nowhere near as hazardous to an aircraft as a hail stone falling at its terminal velocity. Dolphin  ( t ) 00:14, 7 March 2013 (UTC)


 * How do we know the fragments would be at terminal velocity? Objects only reach terminal velocity in free fall; a meteor may strike the earth at many orders of magnitude faster than its terminal free-fall velocity, and it may take some distance to slow down to terminal velocity.  A meteor could be moving quite fast when it strikes the plane; there's no requirement that it be moving as slow as terminal velocity.  -- Jayron  32  06:05, 7 March 2013 (UTC)
 * The OP has prefaced the question with most of this material burns up in the Earth's atmosphere, leaving particles smaller than grains of sand. The question excludes particles much larger than grains of sand. Meteors disintegrate in the Earth's upper atmosphere. By the time a fragment the size of a grain of sand falls to the altitude at which aircraft operate, a long time will have elapsed and it will be at its terminal velocity. True, the geological record shows evidence of very large meteors having reached the Earth's surface, leaving large craters, but the question doesn't extend to consideration of such large meteors. Dolphin  ( t ) 06:17, 7 March 2013 (UTC)
 * Gotcha. Yeah, in that case a grain of sand at its terminal velocity is still a grain of sand at terminal velocity, and carries very little energy, certainly not enough to damage anything on an aircraft.  The origin if the sand itself is irrelevant.  If the sand were really hauling ass, then maybe, but sand in free fall is still sand in free fall, even if it came from space.  -- Jayron  32  06:23, 7 March 2013 (UTC)
 * I interpreted the OP to be asking about a high-speed aircraft striking a particle that has no horizontal velocity and almost zero vertical velocity. Dolphin  ( t ) 06:37, 7 March 2013 (UTC)
 * Re your last question. I read that as "How do I find out if anyone has answered this very question I'm asking here?".  You just search for this page by its name, Wikipedia:Reference desk/Science, using the search function.  Then scroll down to find the right thread.  --   Jack of Oz   [Talk]  00:41, 7 March 2013 (UTC)


 * From playing around with numbers at this impact simulator, it looks like any meteor smaller than about 50 cm in diameter is going to be moving at its terminal velocity by the time it reaches the typical airplane. For a piece of sand, that terminal velocity is a few miles per hour, so most of the impact speed will come from the airplane's movement, not the rock's movement. --Carnildo (talk) 01:56, 7 March 2013 (UTC)

Thanks for all the info. Jaqmur (talk) 14:35, 7 March 2013 (UTC)


 * The thing to bear in mind when thinking about meteors and such is that if you halve the diameter of a roughly symmetrical object, you reduce it's cross-sectional area by a factor of four and it's mass by a factor of eight. So the aerodynamic drag forces on small particles (which depends on their cross-sectional area and not on their mass) are much larger compared to their initial kinetic energy (which depends on their mass) than for big rocks.  So these sand-grain sized meteors slow down to their terminal velocity very quickly - where a mountain-sized meteor hardly slows down at all.


 * Do a mental experiment - take a rock the size of your fist and imagine how far you can throw it - 20 feet maybe? How far do you think you could throw a grain of sand?  A couple of feet maybe?  Both objects leave your hand at about the same speed just as large and small meteors are entering the atmosphere at about the same speed.


 * So for meteor dust, it's going to be moving at terminal velocity - which is when the gravitational force (which is proportional to its mass) balances the drag force (which is proportional to it's cross-sectional area) - with very tiny mass and less tiny cross-section, it's moving very slowly. But the hailstone is obviously moving a lot faster because the ratio of mass to cross-sectional area is much higher than for the dust.  More importantly, since kinetic energy (which is what goes into damaging the plane) is proportional to the SQUARE of the speed multiplied by the mass - these low-mass, low-speed sand-grains don't have enough energy to do much damage...and as Carnildo pointed out - the speed of the airplane is by far the biggest contributor in terms of relative velocity.


 * Hitting meteor dust is much less damaging than hitting even a raindrop or a snowflake...it's about as bad as hitting the smoke particles from the airplane a mile or two ahead of you.


 * SteveBaker (talk) 13:21, 8 March 2013 (UTC)

Film problems
Not sure if this should go here or on the entertainment desk, but...

While watching The Aztecs (Doctor Who), I noticed two problems:

1. In episode 1, the image jerks upwards for a fraction of a second and then returns to normal.

2. There is considerable background noise, sounding a lot like rain.

Why do these effects occur? Whoop whoop pull up Bitching Betty 20:19, 6 March 2013 (UTC)


 * 1) This sounds like a problem I've sometimes seen when projecting a film. Presumably it was copied into a digital format, but, if that process involves using a film projector, and the operator is rather careless, this could be the result.  Or, perhaps that happened when filming.  Were they doing any hand cranking:  ?  (This techniques was sometimes used to change the recording speed of the film, manually.)


 * 2) This might be an original issue with the sound recording. Wind noise can sound like that, if they don't cram the microphone inside a dead cat to prevent it.  British TV of that era had notoriously low production values, so it wouldn't surprise me if this happened.  StuRat (talk) 20:34, 6 March 2013 (UTC)


 * 1) Then why is the image stable for the entire episode with the exception of that momentary jerk? Whoop whoop pull up Bitching Betty 20:39, 6 March 2013 (UTC)


 * 1) Probably a torn sprocket hole or two messed up the timing, then it was able to recover. This type of thing happens more often than you might think.  It's just that in a Hollywood production, as bad as the plots are, they do an extraordinary job of fixing any production quality problems like this which crop up.  Watching foreign films and TV, I've been surprised to see microphones repeatedly in the frame, characters calling each other by the wrong names (possibly by the actor's name), and worse mistakes, all making it onto the final film.  In a Hollywood production, this type of thing ends up on the blooper reel. StuRat (talk) 20:43, 6 March 2013 (UTC)


 * A 1964 Dr. Who? I wouldn't be surprised to see one of the crew wandering around the back of the frame eating a sandwich. Gzuckier (talk) 21:03, 6 March 2013 (UTC)


 * In Terrence Malick's The Thin Red Line, if you watch the DVD copy on a computer, you can see a crew member standing about staring at the camera holding a clipboard on the far edge of the frame in one of the love scenes. Can't be seen on a TV. – Kerαu noςco pia ◁ gala xies 03:15, 7 March 2013 (UTC)


 * No period after "Dr". See Dr. Who (the Peter Cushing Dalek movies). Tevildo (talk) 22:53, 6 March 2013 (UTC)


 * Our article on negative cutting doesn't say anything, but sometimes a negative cutter will foobar a splice. Projectionists have been known to cut out single frames of their favorite shots, using cheap splicing tape or cement to piece the film back together. Also damage happens to film, sometimes a single frame or sprocket can become damaged and either cut or, if unnoticed, left in the film. If the Dr Who episode was made from a print, rather than a negative, this could be what happened, though a solid jump up and down probably means there was a splice. (The thickness of the cement/tape jerks the film about in the telecine or projector.) By the way, regarding projectionists cutting out frames, there shouldn't be a problem with sound sync anymore (it's been a while) since I think sound CDs are now synchronized to code printed on the actual film edge. In the old days, though, if someone cut out a splice, you would hear a pop/jump in the sound about a second later. – Kerαu noςco pia ◁ gala xies 03:15, 7 March 2013 (UTC)


 * Did the BBC actually spend the money in 1964 to shoot Dr who on film? It would have been cheaper to shoot on video tape. The first programs were lost, and The Aztecs is from several months later. I just watched it on Netflix, and it had the look of videotape. Perhaps it got converted to film at some point for distribution and rebroadcast. Doctor Who (series 1) says that the series only transitioned from videotape to film in the 2005 relaunch, which they perversely call "Series 1." Edison (talk) 04:23, 7 March 2013 (UTC)


 * Our article on that episode makes it clear that it was first recorded using videotape, then converted to film so that the tape could be reused (because video tape was extremely expensive at that time), then later converted back to a video format using the VidFIRE process. Looie496 (talk) 06:22, 7 March 2013 (UTC)

Genetic disorders that come out due to inbreeding.
The article on the Amish states "Since almost all Amish descend from about 200 18th-century founders, genetic disorders that come out due to inbreeding exist in more isolated districts (an example of the founder effect)." I have always thought that inbreeding, while it does increase the incidence of certain disorders, does not create new ones by itself. Am I correct? If not, can anyone of you provide examples of genetic disorders that come out due to inbreeding? --190.19.83.213 (talk) 02:19, 7 March 2013 (UTC)


 * You are correct, but I think you are misreading that quotation. It is basically saying the same thing you did. StuRat (talk) 02:29, 7 March 2013 (UTC)

"Come out" is a misleading term. By far the most common type of genetic disease associated with inbreeding is autosomal recessive enzyme or other protein deficiencies. If the carrier (heterozygote) rate in the US population for an autosomal recessive mutation that causes blue hair disease in homozygotes is 1%, but the carrier rate in an intermarrying subpopulation, like Amish, or Ashkenazi Jews, or Yupik Eskimos, is 6%, then the likelihood of a carrier marrying another carrier is 6x higher, and the rate of homozygous blue hair births in that population will be 36x higher than in the general US population. alteripse (talk) 02:41, 7 March 2013 (UTC)

Jumping from a airplane with wingtip sails


This picture is taken from the humanities.

My question is:

Is it safer to jump out from an airplane with wingtip sails because of reduced wingtip vortices? -- Toytoy (talk) 03:18, 7 March 2013 (UTC)


 * Maybe if the chutes are going to open immediately, as they seem to in the pic. On the other hand, if the chutes wait a bit to open, then they will be clear of the vortices before opening. StuRat (talk) 03:23, 7 March 2013 (UTC)


 * Our article, Wake turbulence, doesn't mention hazards to parachutes, but does say that vortices are generated when the wings are creating lift. I suspect that the pilot would take this into account when people are jumping out of his or her aircraft, but perhaps another editor has some experience in this field. Note that the RAF has been dropping paratroopers since 1941, but has never possessed one with wingtip devices until it acquired some C-17s in 2002. Alansplodge (talk) 18:01, 7 March 2013 (UTC)


 * On large aircraft such as the one pictured here, the wingtip vortices are too far outboard in any case to affect parachutists jumping out of that aircraft. FWIW 24.23.196.85 (talk) 04:57, 9 March 2013 (UTC)