Wikipedia:Reference desk/Archives/Science/2015 May 24

= May 24 =

A rocky start
First post of the day. Well... I have a small collection of stones and minerals, see, that I've picked up over the years. I took some pictures of two earlier. I was wondering if anyone would be able to identify them, or at least give an educated guess. — Chris Woodrich (talk) 10:53, 24 May 2015 (UTC)

The second could be andesite or formed from fused pyroclastics likely of Holocene age. It could have come from the local volcano, or from Krakatoa. Graeme Bartlett (talk) 11:37, 24 May 2015 (UTC)


 * They're lovely clear photos Chris - just a tip, but including a scale is always helpful. I'd also recommend you check out Mindat.org - Mindat, some very helpful and knowledgeable people there. DuncanHill (talk) 12:30, 24 May 2015 (UTC)


 * The first one could be calcite, or fluorapatite. DuncanHill (talk) 12:40, 24 May 2015 (UTC)


 * Identifying minerals from photos can be difficult - would you be able to test the streak and hardness - this page has a scale of everyday objects which you could use for comparison. An estimate of density is also helpful - even as rough and ready as "it's heavy for its size" can tell us something. DuncanHill (talk) 12:51, 24 May 2015 (UTC)
 * I'd have to reshoot for a scale, but since I've got ready access that would be easy (I was somewhat disappointed with the sharpness of the first one anyways). I'll see what I can do about the streak and hardness, though as a lit major I wouldn't be able to do much. — Chris Woodrich (talk) 14:00, 24 May 2015 (UTC)
 * Graeme, thanks for the suggestion of andesite. I'd considered that, but I wanted a second opinion. — Chris Woodrich (talk) 14:06, 24 May 2015 (UTC)
 * I can't see the crystal shape of the mineral (and it's been so long since I did geology that it probably wouldn't help if I did). But apart from that, the two mot basic tests we would do with minerals when I was a geology student were "does it react with acid?" and "how hard is it?  (Specifically "can you scratch it with your fingernails?" and if not "can you scratch it with a pen knife?").  If it is harder than a stainless steel penknife and unreactive, it might be some sort of quartz or silica (there are plenty of other minerals it could be, but quartz/silica are more common).  If its softer than steel but harder than a fingernail and reactive, it could be calcite or a related carbonate.  If its softer than your fingernail, it might be something like gypsum (but I don't think it looks much like that). 109.155.253.73 (talk) 22:25, 24 May 2015 (UTC)
 * Harder than my fingernails. — Chris Woodrich (talk) 23:34, 24 May 2015 (UTC)


 * If I'm not to late to answer, I direct you to this helpful chart it might be a good start.(see what I did there start? Get it?) Mr.Magik-Pants (talk) 19:13, 28 May 2015 (UTC)

Wow! Signal
Wow! signal article says the signal would have required a 2.2-gigawatt (2,200,000 kW) transmitter, vastly more powerful than any on Earth in the speculation on the signal's origin section, referring to a signal which might be sent to the origin of the signal. It was labeled with a why template which I replaced with a cite. A small discussion was undergoing the way in the talk page. What exactly is the truth and if some beam is sent, why would it require such a high energy (energy, I presumed from the unit Watt)? Can I demand a suitable explanation that can be added to that article? -The Herald (Benison) • the joy of the LORD my strength 16:40, 24 May 2015 (UTC)
 * I could be wrong but I think the commentators on the talk page are right. The power of the hypothetical transmitter will depends significantly on the distance from earth. That being the case, our article is problematic since it seems to simply it depends only on the size of the transmitter. I'm not even sure whether size (at a resonable distance) will make a significant difference on the power required as opposed to the directionality of the transmitter. Does the source you add provide no clarification as to these points? (This isn't really the place to discuss this but I do differ with the commentators in that I don't think giving a properly explained example is necessarily wrong if it's well sourced.) Nil Einne (talk) 18:04, 24 May 2015 (UTC)


 * Knowing the received power, we'd need two or perhaps three other pieces of information in order to determine the source power:
 * Distance. The received power is proportional to one over the distance to the transmitter squared.  Since we really don't know where the signal came from - we can't know that.
 * Beam width. If you broadcast out a signal in all directions at once (like a typical TV or radio station) - then you need a LOT more energy than if you are shooting a narrow beam (like a laser or a very narrow-beam flashlight).  We don't know whether the beam was constrained to (say) 1/1000th of a steradian - or whether it was omnidirectional - that's a difference of 12,000:1 in source power.
 * Intervening dust & debris. This could attenuate the signal - implying a much stronger transmitter than you'd otherwise guess.
 * There are far too many unknowns - and for a number that quoted to two significant digits, I very much doubt that calculation is valid. Sadly, the source for that statement is behind a paywall - so it's hard to know what assumptions were behind the 2.2GW number.  I'm very dubious about using that number in our article without listing the assumptions made in the original article. SteveBaker (talk) 00:35, 25 May 2015 (UTC)
 * Thanks. So shall I assume it as superfluous? -The Herald (Benison) • the joy of the LORD my strength 04:27, 25 May 2015 (UTC)
 * Not necessarily. As I said the full source for that statement is a pay-to-read scientific paper.  Since I'm not about to pay to read it, it's hard to judge what's going on.  There are several possibilities:
 * The paper doesn't say how it arrives at that number - in which case it's not a valid reference for the fact and we should delete it.
 * The paper makes some assumptions about the distance, beam size and intervening attenuation - in which case our article should explain those assumptions because a different set of assumptions would produce a wildly different answer.
 * The paper demonstrates proof of the source and beam size of the signal (I don't see how!) in which case we should be quoting a heck of a lot more of the paper than just a line from the abstract.
 * The paper references some other source for this information and we should go and find out what THAT source says.
 * We're taking the number out of context...which seems unlikely because it's mentioned in the abstract of the document.
 * Without knowing any of those things, I'd personally be deeply suspicious about a number with two digits of precision. If the article had said "Between 1 and 10GW" - then maybe I'd give it some credence.  However, if the body of the referenced article says something like "Assuming that the source is in the north-west quadrant of the Messier 55 galaxy and assuming that it was an omnidirectional signal then..." - we'd find that much more credible.  Acta astronautica published that paper - they say that they do a "fast peer-review" - which isn't too hopeful. What Wikipedia should do is to remove the 'fact' and have someone go and read the full article and report back.  What someone reading the article should do is to ignore that information until such time as someone goes in there to fix it. SteveBaker (talk) 14:08, 25 May 2015 (UTC)
 * , I got a copy of the paper, expecting it to outline some of the nature of the assumptions that Steve lists above. However, even the title alone should be an indicator something went wrong - the paper itself is fine - "DETECTION OF THE EARTH WITH THE SETI MICROWAVE OBSERVING SYSTEM ASSUMED TO BE OPERATING OUT IN THE GALAXY" - but it is about
 * "Now that the SETI MOP is well defined, it becomes possible to extend these calculations to consider specifically how well a system similar to the MOP could detect the Earth."
 * It basically considers a sort of reverse question e.g. something like: "If equipment similar to ours was out in the galaxy, could it detect Earth?" I'd be happy to email anyone who's interested a copy of the article to read for themselves, it's fairly short. As far as I can tell, the strings '22' and '2.2' do not appear in the article, nor does the string 'wow'. I can only assume the intent was to say something like this in our article: "If something like the SETI MOP, were outside of the solar system, we would have to be emitting a signal with a power of at least 2.2 gigawatts to expect it to be detected". Or maybe not. Anyway, I can't see any easy way to fix/explain this.
 * In fact, the most relevant bit seems to be a quote from a much earlier paper in Science

A now classic paper addressing "The Radio Signa- ture of the Earth" was published in Science in 1978 [3] by Sullivan, Brown and Wetherill. They performed a very detailed analysis of all radio transmissions from the Earth, and concluded that the most detectable signals would be those emanating from UHF television stations with effective isotropic radiated power (EIRP) levels of 5x10^6, and from the ballistic missile early warning system (BMEWS) radar systems with power levels of ca 2x10^11 W.
 * Though neither of those numbers are in the giga range. Anyway, I can see how some of this info conceivably add some good context to the article, but this 1992 article does not claim anything like the sentence it is attached to in our article. I have blanked that sentence, and put a note to this thread in the edit summary and on the talk page. SemanticMantis (talk) 15:54, 25 May 2015 (UTC)
 * Wow! Thanks...I kinda thought there was something suspicious going on here. Oh well, just another typical day on Wikipedia!  :-) SteveBaker (talk) 16:03, 25 May 2015 (UTC)

Reynolds number
I know for turbulent flows, Reynolds number is independent of wavelength but is there a relationship between them for laminar flows? 90.198.210.104 (talk) 19:49, 24 May 2015 (UTC)


 * Relationship between what? Re can be calculated for laminar flows. Short Brigade Harvester Boris (talk) 20:22, 24 May 2015 (UTC)
 * Between Re an wavelength. — Preceding unsigned comment added by 90.198.210.104 (talk) 22:33, 24 May 2015 (UTC)


 * The wavelength of what? Vortex train? Greglocock (talk) 01:35, 25 May 2015 (UTC)