Wikipedia:Reference desk/Archives/Science/2015 September 13

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September 13

Difference between a large multiple star system and a small globular cluster?

Is there some magical number of gravitationally bound stars that marks the boundary line here? Anybody got a reference? Hcobb (talk) 03:31, 13 September 2015 (UTC)

This has a table, globular cluster minimum 10000 , open cluster between 10 and 10000, solar masses that is... The text mentions that the smallest open clusters contain fewer than a dozen stars. The known multiple star systems seem to go up to seven only. Ssscienccce (talk) 06:19, 14 September 2015 (UTC)

Mostly cloudy/partly sunny

The weather prediction for here on Sunday is "mostly cloudy". For Monday it is "partly sunny". What is the difference between these? Bubba73 ^{You talkin' to me?} 05:31, 13 September 2015 (UTC)

"Mostly cloudy" is slightly cloudier than "partly sunny". These are subjective terms by weather forecasters, not rigorously objective terms. Cullen³²⁸ Let's discuss it 05:43, 13 September 2015 (UTC)

Resolved

Thanks. Bubba73 ^{You talkin' to me?} 05:45, 13 September 2015 (UTC)

I beg to differ: Mostly Cloudy and Partly Sunny are both standardized technical vocabulary used by civilian weather forecasters in the United States. In civil weather services, these terms ("partly sunny," "mostly cloudy," ... and so on) are used, and they are based on octants of cloud coverage. In aviation weather services, the corresponding terms "scattered" and "broken" are used. Consider reading Sky Conditions; FAA Handbook 7900 Surface Weather Observations; and of course, AC 00-45G Aviation Weather Services. The terminology is not only carefully defined; sky condition is (nowadays) typically detemined by machine, using ASOS sky condition equipment, and reported in standard format.

"Mostly cloudy" means 5 to 7 oktas will be covered by opaque cloud; "partly sunny" means 3 to 5 oktas. The first of these forecasts implies the existence of a ceiling; the second forecast indicates that a ceiling is less likely.

Nimur (talk) 07:43, 13 September 2015 (UTC)

That helps clear it up. In the table, partly cloudy and partly sunny are in the same category. I had assumed that partly sunny was more like mostly cloudy. Bubba73 ^{You talkin' to me?} 15:39, 13 September 2015 (UTC)

Note: The following answer was accidentally deleted in the edit where I made an answer. I apologize for my error, which was not deliberate. Cullen³²⁸ Let's discuss it 05:17, 14 September 2015 (UTC)

• read this First link in a google search for "difference between mostly cloudy and partly sunny" --Jayron32 05:38, 13 September 2015 (UTC)

...which illustrates the important distinction between a "weatherman" and a meteorologist! Not everybody who reads out the weather report on television news is a scientifically trained forecaster. People who care about the accuracy of their weather information rely on products of the National Weather Service (at least, in the United States). Although there is a plethora of copycat websites and TV reporters, with fancy graphs and data visualization eye-candy, there is no substitute for hearing the report from an actual meteorologist or briefer!

I would go so far as to say that television news - let alone a web search engine result - does not constitute a reliable encyclopedic source for weather information. We have a National Weather Service, and it publishes all its reports, as well as several great books on weather theory and practice, at no cost. Nimur (talk) 07:35, 14 September 2015 (UTC)

You may find this perspective interesting. Short Brigade Harvester Boris (talk) 13:15, 14 September 2015 (UTC)

Learn how to read METARs and you'll never have to worry. They even have decoders. Here's current METAR locations and conditions in U.S. [1] --DHeyward (talk) 05:57, 16 September 2015 (UTC)

Source gated transistor

The BBC reported today about a school student who had done research on source gated transistors. I don't know what these are (and am not asking for a direct answer to address that). Despite Google returning lots of links to information about them, it seems that Wikipedia has never heard of them either. Is there an article in which the term "source gated transistor" can be placed so that this apparent ignorance goes away? Bazza (talk) 09:29, 13 September 2015 (UTC)

Here's the article that is referenced by BBC News: Self-Heating Effects In Polysilicon Source Gated Transistors (2015). It cites five other publications when it defines "source-gated transistor," and it looks like the term has been used in publications since at least 2003. Nimur (talk) 15:04, 13 September 2015 (UTC)

Thanks. I don't doubt the validity of the term; but where in Wikipedia should it be referenced? This is not a subject area I am anything like familiar with: I came to Wikipedia hoping to read a sentence or two on why this type of transistor differs from others. Is there someone who knows better who can either start a new article, or include mention of this in an existing one? Bazza (talk) 16:36, 13 September 2015 (UTC)

Could be added to thin-film transistor. This google book has some info: Thin Film Transistor Technologies, other info here, and here.

Like a FET, the current between source and drain is controlled by the gate voltage, but in a FET this happens across the length of the channel, in an SGT the current is controlled at the metal-semiconductor contact at the source. As I understand it, the SGT seems to be a slower but more efficient, more robust, alternative for the present TFT (used in for example LCD displays). The source-controlled nature reduces the impact of process variability, making them cheaper to manufacture. Possible applications include large-area electronics made with inexpensive but imprecise patterning techniques. Ssscienccce (talk) 20:29, 13 September 2015 (UTC)

Added to FET article for now. Hcobb (talk) 21:53, 13 September 2015 (UTC)

Resolved

Excellent, thank you. Wikipedia Search now looks not quite as ignorant ;-). Hopefully someone will feel the need to expand that section sometime in the future. Bazza (talk) 10:32, 14 September 2015 (UTC)

Looking forwards to listing practical applications. Hcobb (talk) 13:39, 14 September 2015 (UTC)

Over-sampling in archaeology?

in List of bog bodies, "Esterweger Dose Child", "Girl of the Bareler Moor" and "Ballygroll Child" each mention "oversampling", "over-sampling" or "over sampling" (respectively), as a potential cause of loss of remains post-excavation. I'm kind of at a loss as to what, exactly, this would entail. Surely even a few hundred years ago there wasn't such a great need for destructive testing of bog bodies that more than half of the bones of a body would be destroyed? Or is oversampling something else?

Has anyone even seen this phrase used in the context shown here? Is this a common term in the field? 97.90.151.30 (talk) 09:34, 13 September 2015 (UTC)

I have heard this term ("oversampling," "destructive sampling," and so on) extensively in the context of archaeology.

For lack of a better resource, here is a pamphlet, Archaeology: Science and the Dead. It's published by the Anglican Church, but it's a very neutral publication that outlines the ethical, scientific, and legal issues related to scientific testing on human remains. In fact, the specific topic of disrupting ancient "bog-bodies" in England is a significantly-debated topic: the Advisory Panel on the Archaeology of Burials in England (APABE) was specifically formed by the Church of England in response to a government request to more appropriately deal with ancient and prehistoric human remains archaeology in the British islands.

Destructive sampling affects non-human remains, too. I recall that one of the great debates over the Shroud of Turin was whether it should be cut up for submission to, say, radiocarbon dating: some people considered it too valuable to destructively sample. Even ordinary artifacts must balance the utility of destructive sampling against its effects on irreplaceable artifacts. As an example, here are published guidelines for one museum: Destructive Analysis Policy and Procedures for the archaeology museum at the University of Michigan.

Nimur (talk) 15:17, 13 September 2015 (UTC)

Single-letter amino acid representation

I am familiar with the single-letter representation of amino acids. I've come accross a description of an SmD peptide which is described as:

AARG-sDMA-GRGMGRGNIF

(1) I understand what the capital letters represent, but not what the lowercase "s" represents.
(2) What do the hyphens in "-sDMA-" mean?

Reference: Mahler et al, 2005 - PMID 15642993.

Thanks, --NorwegianBlue ^talk 10:16, 13 September 2015 (UTC)

According to that article, "a symmetrical dimethylarginine (sDMA) residue", and cites PMID 15642139 regarding this idea. We have an asymmetric dimethylarginine article but no symmetric dimethylarginine article. The symmetric variant has one methyl group on each of the two pendant N of the arginine sidechain rather than both methyl groups on one. The hyphens set off "sDMA" as a single residue rather than suggesting that "sD", "M", and "A" are regular one-letter codes in the chain.DMacks (talk) 10:26, 13 September 2015 (UTC)

Thanks! --NorwegianBlue ^talk 10:33, 13 September 2015 (UTC)

Resolved

Time traveller's age

Could someome remind how the problem of time traveller's reversed age while travelling to the past is solved in sci-fi? For instance, unless the time travel occurs in a localized bubble of present, a 29-year old time traveller from 2090 travels backwards to 2062 which would render him 1-year old and making the return impossible (not to mention older times before his birth). From what I see, Novikov self-consistency principle assumes that you would meet yourself, and according to grandfather paradox, the only other possible solution, assuming the possibility of time travel, is a parallel universe. Maybe Asimov or Gamov tackled this? Brandmeister^talk 17:32, 13 September 2015 (UTC)

There are literally dozens of ways that this is handled in different science fiction tropes - no one of them is 'definitive' or 'correct', they are just fiction. Since mainstream science doesn't admit the possibility of time travel, there is no meaningful answer to your question. (This may not prevent various 'fringe' types from coming here and babbling on about wormholes and tachyons and other things that are about as real as unicorns and fairies.) SteveBaker (talk) 18:04, 13 September 2015 (UTC)

Yes. Backward time-travel is impossible, as far as we know. Assuming it is possible makes for entertaining plot lines in fictional stories. As one of my old math teachers said, "If you start with incorrect assumptions, you're liable to get interesting results." ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:49, 13 September 2015 (UTC)

As far as science goes, the closest thing to reverse time travel might be under the many worlds hypothesis, if you could find a parallel universe just like our own, but with the desired time offset, and just jump right into it. That wouldn't create any time paradoxes, as no actual time travel occurs, it only appears to. StuRat (talk) 22:17, 13 September 2015 (UTC)

The notion of parallel universes seems only slightly less far-fetched than unicorns and spaghetti monsters. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:24, 13 September 2015 (UTC)

Actually, no - the notion of the Many-worlds interpretation of quantum mechanics is actually one of the leading theories right now. A 1995 survey of 70 leading experts came out with 58% of them saying that they expected it to be true. What is as far-fetched as unicorns and the FSM is the idea that you, as a person, can jump between parallel universes (although you do continually get replicated into near-identical universes). So you're right that StuRat's approach to getting some kind of time travel is unreasonable - but you're very, VERY wrong about parallel universes being on the level of unicorns, etc. SteveBaker (talk) 16:05, 14 September 2015 (UTC)

(Caveat: If parallel universes exist - then unicorns almost certainly exist in an infinite number of them...but not in our universe.) SteveBaker (talk) 16:13, 14 September 2015 (UTC)

Why would you assume a time traveler becomes younger when traveling to the past? According to general relativity, if traversable wormholes exist, time travel would be possible. Ssscienccce (talk) 04:11, 14 September 2015 (UTC)

Yeah - and if unicorns existed, you could rub their horns and make arbitrary wishes. Which is precisely as likely as worm-holes existing that let you teleport/time-travel/unlock-the-secret-to-eternal-youth/whatever. There are a large and very annoying number of so-called documentaries on the Science and Discovery channels that insist on talking about wormholes as if they were some kind of established science - and they just aren't. To the contrary, in fact. SteveBaker (talk) 16:05, 14 September 2015 (UTC)

I don't understand the question. The "localized bubble of present" is part of the definition of time travel. What travels through time is your 29-year-old body including your 29-year-old brain with its encoded memories of the year 2090. You can come up with not-quite-100%-absurd ways that that might happen (such as wormholes or rapidly rotating cylinders). Regressing in age is something else entirely. It's magic or super-advanced rejuvenation technology, not physics. -- BenRG (talk) 03:54, 14 September 2015 (UTC)

And if you jumped back 10 years, with you somehow magically became 10 years younger as a result - then your body (and therefore your memories) would be exactly what they were 10 years ago...then you'd have no way to tell that you'd ever been in our present. That wouldn't be time travel...it would be completely indistinguishable from normal reality. SteveBaker (talk) 16:09, 14 September 2015 (UTC)

It would still be different if either 1) You original position after travelling is where you time travelled from; or 2) There are now two of you. Well 2 would normally imply 1, otherwise you end up with a rather nasty death. If you have 1 without 2, then this also means the original you disappears.

I'm not sure if you can come up with a reason why you will age backwards etc, but will either not affect the other "you" or at least would end up in the same position as what you started from, particularly since "position" is actually a fairly tenous concept when you're talking about time travel. But then again, a lot of the rules appearing in time travel fiction don't make much sense, and I say this as someone who enjoys soft science fiction including the occasional time travel story.

I do agree reverse aging etc isn't what's normally meant by backwards time travel in fiction etc anyway. Probably at least partially because it leads to far less interesting scenarios. (Time travel does normally include 1 and 2, if the time travelled is short enough for them to come up.)

Nil Einne (talk) 13:07, 15 September 2015 (UTC)

(2) without (1) would probably result in a rather messy explosion as the density of normally incompressible fluids instantly doubles. This is also a problem when teleporting a'la StarTrek because at the instant you arrive, you have to somehow displace all of the air that was previously at that location. Many details of these processes are left to guesswork...the soles of your shoes would have to be pre-compressed to accomodate small rocks under your feet - or else you'd have to teleport in an inch or two above the ground and have a weird shock as you drop down onto it.

The other issue with time travel is that people generally seem to materialize in the same place that they were in when they activated the machine (think "Back to the Future", for example)...but the Earth will have revolved and moved in it's orbit and the Sun will have moved around the galaxy...so where exactly would you be if you *only* travelled into the past? In the vacuum of space, I think...although with considerations of relativity and the expansion of the universe...I'm not so sure what would happen. So you need a machine that can not only move you in time - but also in space. Also, is momentum conserved? Because you're on the surface of a rapidly spinning planet - if you aren't very careful, you'll materialize with a velocity sufficiently high to vaporize you and everything nearby! And now you have all the problems of the StarTrek transporter all over again.

Then there is the issue of what happens if you change something in the past? Science fiction never seems to agree on this one (often, as with StarTrek, they aren't even consistent within one fictional universe). So if you go back to the time of the dinosaurs and stomp on a butterfly...

1. You can't return to the present, so what happens there is moot.
2. When you return to the present, it's as though you never stomped the butterfly.
3. When you return to the present, everyone evolved from lizards, everything is different - but you haven't changed.
4. When you return to the present, everyone evolved from lizards - but everything else in the world is more or less the same, we still have cars made by Ford and the (lizard) president is still called "Obama".
5. When you return to the present, everyone evolved from lizards - and so have you.
6. When you return to the present, everyone evolved from lizards - and now there are two of you (one is a bit "lizardy", the other not).
7. When you return to the present, everyone evolved from lizards - but everyone remembers how it used to be before you changed it, so you get the blame for the "lizard-thing".
8. When you return to the present, it's as though you never stomped the butterfly, but you remember that you did.
9. When you return to the present, it's as though you never stomped the butterfly, you remember that you did - but your on-board computer mysteriously didn't capture any information about that.
10. When you return to the present, you don't remember a thing about the trip - but your on-board computer contains information about you stomping the butterfly.
11. At the instant you stomp the butterfly, the future you changes into a lizard-man, so the 'you' in the dinosaur era also changes - so everything stays perfectly consistent. But maybe you create oscillations in the timeline by changing the future in such a way that lizard-you doesn't stomp the butterfly - so things return to how it was if you hadn't stomped the butterfly - so now human-you DOES stomp the butterfly - so now you're lizard-you and...Aaarrgggghhhh!
12. You can't stomp on the butterfly, no matter how hard you try - something always happens to stop you.
13. You stomp the butterfly, cause a temporal anomaly and you instantly cease to exist.
14. You stomp on the butterfly, cause a temporal anomaly and the entire universe ceases to exist.
15. You stomp the butterfly, causing the time machine's anomaly-detector circuit to blow up, trapping you in the past.
16. You change the future, and S-L-O-W-L-Y the effect of that catches up with you (pictures of loved ones fade over hours in Back to the Future).
17. You end up creating a fully consistent alternate timeline/universe.
18. The consequences of stomping the butterfly gradually blend back into normality - so the future is never changed measurably (the opposite of "The butterfly effect"!)
19. The time cops come with a replacement butterfly and sort everything out.
20. The time cops have the forethought to arrest you just BEFORE you stomp the butterfly.
21. Events unfold in such a way that you cancel out the effect of stomping the butterfly - so the present is unchanged.

I'm sure there are many more. SteveBaker (talk) 15:01, 15 September 2015 (UTC)

Rainbow width

I saw a rainbow today and it looked wider than other rainbows that I recall seeing in the past. Does the larger width necessarily mean that the conditions that were causing it were closer to me than other rainbows? Is there any correlation between rainbow width and other factors? Dismas|^(talk) 22:09, 13 September 2015 (UTC)

Some other editors are bound to correct me if I am wrong but the width of rainbows are always the same due to the physics involved. The reason this rainbow may have appeared wider is due to the juxtaposition of other things in the landscape which gives the mind a sense of scale. So on a prairie, it looks normal but if the Rocky Mountains are in view it looks wider. Much like the moon appears larger when very close to the horizon.--Aspro (talk) 23:25, 13 September 2015 (UTC)

There are, of course secondary and tertiary rainbows, pastel-coloured supernumerary rainbows and twinned rainbows (I've seen several of these at the same time), but you are unlikely to have mistaken these for a single wider rainbow. As Aspro said, the colour width is a fixed angle (about two degrees for the primary and three degrees for the secondary), but the brain interprets this as different widths depending on the surroundings. Hold your thumb at arm's length to make a comparison. Dbfirs 07:12, 14 September 2015 (UTC)

See moon illusion. --65.95.178.150 (talk) 14:23, 14 September 2015 (UTC)

Thank you. Dismas|^(talk) 23:25, 14 September 2015 (UTC)