Wikipedia:Reference desk/Archives/Science/2013 May 10

= May 10 =

collars and tie outs
Why are dog collars not supposed to be used with tie-outs?Curb Chain (talk) 00:43, 10 May 2013 (UTC)
 * What's a tie-out? ←Baseball Bugs What's up, Doc? carrots→ 03:41, 10 May 2013 (UTC)
 * I think it's British English for "a chain you nail to the ground via a stake to keep your dog from running out of the yard and eating small children." In general, I believe that it is often recommended that you don't attach a leash or a chain to the dog's collar, for the potential to choke or injure the dog that way.  It is usually recommended that a Dog harness is used instead if you're going to be tying him up in the yard.  -- Jayron  32  04:55, 10 May 2013 (UTC)
 * Not a common British usage - I've never heard of it. Alansplodge (talk) 07:15, 10 May 2013 (UTC)


 * Googling "tie-out cable" returns heaps of sites that show that it is a cable or chain used to tie up a dog - the name being particularly used in the USA and Australia. It appears to be recently introduced marketing terminology.  This Australian, who is a dog owner who would never tie a dog up, as it is not nice for the dog (dog owners should have an appropriately sized and fenced yard) had bever heard of the term before seeing it in this question.  Wickwack 121.215.25.35 (talk) 07:56, 10 May 2013 (UTC)

Why have there not been visitors from the future?
If time travel is possible, and we can visit the future, why hasn't anyone from the future ever visited us yet? — Preceding unsigned comment added by 72.70.202.28 (talk) 03:35, 10 May 2013 (UTC)
 * How do you know they haven't? Whoop whoop pull up Bitching Betty 03:37, 10 May 2013 (UTC)


 * Ergo, time travel is not possible. QED.  That was easy.  -- Jayron  32  03:37, 10 May 2013 (UTC)
 * Specifically, backward time travel is not possible. Time only goes one direction. ←Baseball Bugs What's up, Doc? carrots→ 03:40, 10 May 2013 (UTC)
 * Those both depend on the assumptions that A: if time travel was possible, it would not be considered a military secret (unlikely); B: time travellers would be easily recognisable as such (also unlikely); and there are no obvious time travellers (how do we know what time travellers would look like)? Your argument is as holey as a colander.
 * Also, tell me what if anything precludes the possibility of time travel, Jayron Pond and Baseball Williams. Whoop whoop pull up Bitching Betty 03:50, 10 May 2013 (UTC)
 * 1)If time travel were possible, the Bible would mention that the Crucifixion was observed by millions of strangely clad people speaking strange languages, for just one example.
 * 2)We tend to think of a time machine like a car; you get in and drive to whatever time you want. But it may be more like an elevator, in that it covers a defined span; from the time it was built to the time it is destroyed, and you can no more go to a time before it was built than you can ride an elevator to a floor that is below the bottom floor the elevator goes to. This would be true for the Tipler cylinder, for instance.
 * 3) If time travel does indeed allow the past to be altered which then alters the future, which then causes a difference in the time travel to the past which alters it yet again, which then alters the future yet again, etc.; then the only stable reality where this will finally stop is one where time travel never gets invented, and everything then continues unchanged. Gzuckier (talk) 03:59, 10 May 2013 (UTC)
 * The problem with using Tipler cylinders (or any other kind of closed timelike curve) for backward time travel is that they're hard to stop. That is, they are essentially self-contained, and there isn't even a hypothetical way of "jumping off" and actually being able to do anything of consequence in the past. I believe there are also some serious ontological issues with creating them (I remember reading something years ago about Kerr black holes as a possible solution to that, but I don't recall the specifics). Evanh2008 (talk&#124;contribs) 04:12, 10 May 2013 (UTC)
 * Tipler culinders might as well be "wave your magic wand and say presto!" as far as practical solutions to time travel go. After all, a cylinder of infinite length would take an infinite amount of time to construct, and would reach the entire breadth of the universe. -- Jayron  32  04:51, 10 May 2013 (UTC)


 * No, it wouldn't reach the breadth of the universe - it would only tend to the breadth of the universe. (Infinity is not a number.) Although, the number 'orez', which I invented, is. (Not that anyone cares.) Plasmic Physics (talk) 05:00, 10 May 2013 (UTC)
 * If the cylinder is infinite, then it's infinite &mdash; that's just a tautology. As for "infinity is not a number", that's meaningless.  Infinity is not, say, a real number or a complex number, but there is no well-defined notion of "number, full stop" from which infinity can be excluded.  Complex infinity is, for example, an extended complex number.
 * As for the "tending to" thing, see potential infinity versus actual infinity. Pre-Georg Cantor, actual infinity had a poor reputation, but actually infinite structures are now well accepted. --Trovatore (talk) 05:05, 10 May 2013 (UTC)


 * Then what was all that about on the Maths Reference Desk, when I raised the topic of infinity? Plasmic Physics (talk) 05:20, 10 May 2013 (UTC)
 * The problem is not the existence of infinite structures, the problem is the construction of infinite structures in a finite amount of time. That's why Tipler cylinders are magic: if you don't have one already, you need to build one, and you'd need magic to do so.  And if you have magic, why not use your magic to time travel and save yourself the trouble of building an infinite length cylinder.  Tipler cylinders may be an interesting thought experiment, and may be mathematically consistent with current physics in allowing time travel of a sort, but that doesn't mean that we can actually build one and use it for time travel.  People seem to always forget the step where they have to actually make the object and then use it to send someone back to, I don't know, kill Hitler or try to convince your father to not be so much of a dweeb or something.  In the end, there's still no practical way yet proposed to do this, so it's a moot point.  Fun to sit around and bullshit about, but it's really not anything more than magic wearing a physics disguise.  -- Jayron  32  05:22, 10 May 2013 (UTC)
 * Well, maybe there already is one somewhere. Or maybe Hawking is wrong about the "weak energy criterion" or whatever he claims keeps very long finite cylinders from working. --Trovatore (talk) 06:05, 10 May 2013 (UTC)
 * Undoubtedly. If the universe is infinite in scope and infinite in matter, then every possible object that could exist does already exist somewhere right now.  However, the issue is still  a practical impossibility.  In an infinite universe, the Tipler cylinder does already exist, but it would take an infinite amount of time to search the infinite universe to find it, and then an additional sufficiently long period of time past infinity to travel to it to actually use the thing to travel backwards in time.  Still pointless to consider it a practical time machine.  -- Jayron  32  13:48, 10 May 2013 (UTC)
 * If the Tipler Cylinder does exist in an infinite universe, it doesn't follow that finding it is infinitely hard. It could be right over there... 202.155.85.18 (talk) 09:05, 13 May 2013 (UTC)
 * As usual, Mr Munroe has an answer: http://xkcd.com/1203/ 196.214.78.114 (talk) 08:03, 10 May 2013 (UTC)
 * Time travel FORWARDS is possible. People do that everyday when they sit on a bullet train. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 12:24, 10 May 2013 (UTC)
 * People do that every day sitting still. -- Jayron  32  13:48, 10 May 2013 (UTC)
 * Yes, and in that sense. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 09:08, 11 May 2013 (UTC)
 * I'm afraid I don't get what the cartoon is trying to say, but I should take a moment to mention negative mass as a crucial concept.
 * Another notion: any use of time machines to rewrite history is going to keep going on and changing things until... what? Until nobody ever invents a time machine.  In this model we'd occasionally get hit by one-off shrapnel of future time travellers, but only if their efforts have the effect of preventing time travel from being invented, which would tend to skew the odds of their actions to be heavily in the favor of not telling us the technical details. Wnt (talk) 12:28, 10 May 2013 (UTC)
 * Like your statement, the cartoon is a bit of a paradox. Self-contradictory, and therefore the world implodes. The end. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 12:32, 10 May 2013 (UTC)


 * "I'm afraid I don't get what the cartoon is trying to say": There's a wiki for that. AndrewWTaylor (talk) 12:44, 10 May 2013 (UTC)


 * Explanation here: http://www.explainxkcd.com/wiki/index.php?title=1203 196.214.78.114 (talk) 12:46, 10 May 2013 (UTC)


 * Would have been clearer without the third panel. Clarityfiend (talk) 20:05, 10 May 2013 (UTC)
 * The third panel is showing the backwards time travel that he's experiencing. ←Baseball Bugs What's up, Doc? carrots→ 22:18, 10 May 2013 (UTC)
 * Sometimes less is more. This is/was/will be one of those times. Clarityfiend (talk) 05:13, 11 May 2013 (UTC)
 * Well, the thing is, either he is in exactly the same state as when he first the lever the first time, in which case he experiences no confusion and is ready to flip the lever again... or he is a different entity from the one that threw the switch the first time, in which case he and the previous he are standing in the same spot (insert unpleasant teleporter accident anecdote here) or maybe the previous he vanishes the moment the future he materializes, in which case he never threw the switch, or ... Wnt (talk) 07:21, 11 May 2013 (UTC)

Time doesn't exist in the sense we intuitively think about it. The past and the future simply exist, they are the present moment for the people who live there. The question should really be formulated as why we can explain the present state of the universe in terms of only a past initial condition instead of a mixed one (the other possibility of explaining it in terms of only the future would de-facto redefine the future as the past and vice versa). My view on this is that what we're actually doing here is to take our present state and then find a set of rules that allows the information in our present state to be compressed. This then explains our present states in terms of hypothetical "past states" that should be considered to be alternate universes. This gives rise to a one parameter family of parallel universes, the parameter can then be identified with what we conventionally call "time". Entropy then decreases in the negative time direction by definition. We then perceive time in a way that makes it look like we're traveling in time in only one direction, in this intuitive picture, time travel to the past is impossible. Count Iblis (talk) 13:09, 10 May 2013 (UTC)
 * Time flies like an arrow. Fruit flies like a banana.  -- Jayron  32  13:43, 10 May 2013 (UTC)
 * and Green flies like a lettuce. SteveBaker (talk) 16:35, 10 May 2013 (UTC)

Some time i visit the past and have my de ja vu thanks water nosfim  — Preceding unsigned comment added by 81.218.91.170 (talk) 18:28, 10 May 2013 (UTC)

I wonder why people rarely seem to wonder about time travellers from the past visiting us. The past is a rich and fascinating place; everything that has ever happened in the universe happened in the past. Maybe that's what's behind the malaise affecting the world: many of us are actually from the past and feel we just don't belong in the modern world, but we can't quite put our finger on why, because the obvious answer is just too impossible, according to those who obviously know better. -- Jack of Oz   [Talk]  20:26, 10 May 2013 (UTC)


 * They do visit us, they are believed to come from 13 million years ago, the fastest ones arriving here in only 21 minutes of time travel. Count Iblis (talk) 22:03, 10 May 2013 (UTC)


 * Ah ha! That would explain all those people who go about their daily lives uttering "Oh my God" at as many random moments as they can fit in.  Nothing else possibly could.  --   Jack of Oz   [Talk]  22:48, 10 May 2013 (UTC)
 * Time travelers from the past would probably experience a culture shock. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 09:08, 11 May 2013 (UTC)
 * OMG! Double sharp (talk) 08:13, 14 May 2013 (UTC)

Jayron32's hypothesis (that there is no time travel), Baseball Bugs's addendum (not backwards), and Whoop whoop pull up's observation (you might not notice time travellers) don't between them cover all the scenarios. You also have to consider that a) there are no people in the future (nuclear war, global warming, rise of the Kirbys); b) the future is too stupid to discover time travel (Idiocracy); c) we're boring - no-one in the future cares about us ; and d) there is no future (universe implodes, particle accelerator mishap). All we know is that the future is an impregnable veil of uncertainly; history shows only that those who pretend to imagine what lies beyond it are deluded. -- Finlay McWalterჷTalk 22:35, 10 May 2013 (UTC)
 * You should really cite Arthur C. Clarke here, as you obviously ripped off the idea from All the Time in the World. --Trovatore (talk) 22:42, 10 May 2013 (UTC)
 * insert smiley --Trovatore (talk) 22:43, 10 May 2013 (UTC)
 * Yes. He might came back from the past in a time machine, and file a suit. ←Baseball Bugs What's up, Doc? carrots→ 22:44, 10 May 2013 (UTC)


 * It's pretty simple, from the scientific viewpoint: There is no evidence that demonstrates backwards time travel has ever occurred or ever could occur. That squares with the notion that backward time travel doesn't make logical sense. ←Baseball Bugs What's up, Doc? carrots→ 22:44, 10 May 2013 (UTC)
 * I think that's a bit too simple. The math of Tipler cylinders seems to work.  There may not be any such cylinders, but that's a contingent fact rather than an a priori one, so from the point of view of conceptual analysis it doesn't really matter.  Hawking seems to think finite cylinders can't work and that this squares with his "chronology protection conjecture", but to my naive eye these arguments seem awfully roundabout to be the basis of something so fundamental, and again from a conceptual-analysis POV don't address the problem that, contingently, there could be an infinite cylinder anyway, and then what? --Trovatore (talk) 22:54, 10 May 2013 (UTC)
 * Well, that's obvious. When it gets to the end of the infinite universe, its front end connects to its back end.  They made a movie about it, Torus! Torus! Torus! .  --   Jack of Oz   [Talk]  23:07, 10 May 2013 (UTC)


 * Perhaps backwards time travel is impossible. This seems like the most obvious and likely answer.
 * Perhaps the time machine has to be invented, built and even switched on before a visitor from the future can step out of it. This is the premise of the excellent (if mind-bending!) time-travel movie Primer.  In the world of Primer, since we have not yet invented a time machine, we can't get visitors from the future.  This would be the case for most of the (highly dubious) quasi-physics-based suggestions involving wormholes and other such non-existent things.
 * Perhaps the dangers of paradox-creation become well understood before time machines are invented and it's realized that it's simply too dangerous to use some future machine.
 * Perhaps a time machine is only a time machine - meaning that you arrive at the same point in space that you left...which would be unfortunate because the earth has rotated, moved through it's orbit around the sun - which in turn has moved through it's orbit around the galactic core - and the galaxy itself has proper motion. So perhaps you arrive at a point so far from the earth that it requires an entire spacecraft to be brought back with it.
 * Perhaps the universe avoids paradoxes by dumping the time traveller back into a parallel universe, identical-looking to ours. This could mean that only one time traveller can exist in each of a possibly infinite number of parallel universes.  One time traveller might easily be unnoticed...or dismissed as a crank.  We can even tell you the name of that guy...John Titor!
 * Perhaps the energy requirements of time travel make it such a difficult thing to do often, that only a handful of people can ever do it, and they choose not to be noticed.
 * I'm sure there are plenty of Sci Fi stories about situations where people come back in time and create havok by buying up lottery tickets and the shares of soon-to-be-successful companies. This causes so much chaos that in future, "Time Cops" will come back and patch up whatever changes they made to history...so we'd never notice anything untoward happening.
 * I bet we could come up with another dozen of these. SteveBaker (talk) 05:56, 11 May 2013 (UTC)
 * also, conservation of mass. at some point in time, mass X vanishes; at a different point in time, mass X suddenly is created. ?? Gzuckier (talk) 08:43, 11 May 2013 (UTC)
 * At least one SciFi time machine fixed that problem by taking X amount of mass from the past and bringing it to the present to precisely balance the mass from the present that would appear in the past. SteveBaker (talk) 14:22, 11 May 2013 (UTC)
 * Backwards time travel is very hard to achieve because of entropy increasing. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 11:01, 11 May 2013 (UTC)
 * But because entropy is only increasing within closed systems, you can create small pockets of increased or decreased entropy so long as you balance it out someplace else...so just like the conservation of mass argument, so long as your time machine exchanges entropy from the past into the present at the same time that it exchanges it from the present into the past - no conservation laws are broken. I agree that not violating conservation laws at both target and destination time-frames would be difficult, it doesn't seem impossible. SteveBaker (talk) 14:22, 11 May 2013 (UTC)
 * One in a billionth's chance – Boltzmann's theory. ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 14:28, 11 May 2013 (UTC)


 * Actually, we are all of us, except IP 72, from the future, and he just doesn't realize he's lagging behind us. μηδείς (talk) 17:41, 11 May 2013 (UTC)

Mountain prominences and parentage
Yesterday's main page linked to lists of mountains which made me think about peaks and cols and heights and prominences (yet) again. I find it confusing. So I have drawn a diagram:

In particular each col (lowercase letter) joins two peaks (uppercase letter) together and "ownership" below the col is assigned to the higher peaked mountain, which is deemed the parent of the other mountain. Prominence is the vertical distance from the peak to the col which joins that peak to its parent peak. The mountain height is the vertical distance above sea level (I suppose above "a" and "n" here). The colours in the diagram show "ownership" and how the tallest mountain gradually gets to include everything.

Is this diagram "correct"? Have I misunderstood anything? -- SGBailey (talk) 07:39, 10 May 2013 (UTC)
 * I think you have this right. However, prominence is measured above the key col (as you say) but the key col is with respect to any terrain higher than the peak in question. "Parent peak" is not essential to the concept. Indeed there are several definitions of "parent peak" with various advantages and disadvantages. I find the easiest way to think about prominence is as it says in Topographic prominence, "Suppose that the sea level rises to the lowest level at which the peak becomes the highest point on an island. The prominence of that peak is the height of that island. The key col represents the last isthmus connecting the island to a higher island, just before they become disconnected." What the mountains are like on this other island does not matter provided the land is higher somewhere. Thincat (talk) 14:49, 10 May 2013 (UTC)


 * OK That seems to match. Thanks. -- SGBailey (talk) 16:38, 10 May 2013 (UTC)
 * Neat diagram. From "The colours in the diagram show "ownership"" I would have expected, for example, A, B, C, and D to all be some shade of pink.  Instead it looks like peaks whose color is other than blue have blue peaks as daughter peaks (and blue peaks have no daughter peaks), but which peak is the daughter peak (ownership) is not indicated by color.--Wikimedes (talk) 17:31, 10 May 2013 (UTC)


 * All the blue peaks (ACDEGJKM) are never parent peaks. I was just economising on colours. I could have made each blue peak a different colour. Indeed, I will and I'll repost it. -- SGBailey (talk) 20:34, 10 May 2013 (UTC)
 * Because your diagram shows the (encirclement or island) parentage situation rather well, it doesn't show the difficulty with the concept. The diagrams in the article do not illustate the problem either. If col "k" were slightly higher (higher than "m"), the island parent of M would be H whereas many people might intuitively think it still ought to be L. Of course, your yellow would fill across to M and make the situation clear. So, what I am thinking is that, if by changing your diagram you could illustate the obvious and the non-obvious island parentage, it would be worth adding to the article. Thincat (talk) 22:06, 10 May 2013 (UTC)


 * m (and M) lowered and text adjusted to suit. M now has H as a parent. ((Is this worth adding to the prominence page?)) -- SGBailey (talk) 10:44, 12 May 2013 (UTC)


 * I was only looking at the peaks to code for parentage and I should have been looking at the bases as well. I see now that everything above the yellow base is a daughter (descendent?) of peak H, everything above the pink base is a daughter peak B, etc.--Wikimedes (talk) 08:23, 12 May 2013 (UTC)
 * I was going to say "yes" but I see you have gone ahead. I have added a bit of commentary. Thincat (talk) 23:23, 12 May 2013 (UTC)

List of Products that contain triclosan
Why hasn't anybody responded to my talk page section re: this topic?165.212.189.187 (talk) 19:13, 10 May 2013 (UTC)


 * I'll respond. AndyTheGrump (talk) 19:16, 10 May 2013 (UTC)


 * All brands of anti-bacterial soap, for one thing; toothpaste, for another; some kinds of antibiotic ointments, and I'm sure lots of other stuff too. 24.23.196.85 (talk) 02:19, 11 May 2013 (UTC)

Double minor
What do you think would be the best minor for an environmental geologist? — Preceding unsigned comment added by 149.152.23.34 (talk) 19:29, 10 May 2013 (UTC)

Law or political science.165.212.189.187 (talk) 20:04, 10 May 2013 (UTC)

why? — Preceding unsigned comment added by 149.152.23.34 (talk) 20:34, 10 May 2013 (UTC)
 * There isn't really a best. Minors allow you to show a more diverse education experience to graduate programs and future employers. You could choose something you like that has no relation to your major (like law or political science) or something close to your field that you want to explore (like physics or chemistry). Here is a NY Times article about choosing your minor. uhhlive (talk) 21:09, 10 May 2013 (UTC)


 * Or business management if available. For the same reasons - a more broader view and more flexibility in employment.  However, particularly for undergrads with no professional work experience, there's a heck of a lot of benefit in choosing something that for you will be the most fun - something that you'll like.  That will give you some easy high marks without wasting study time best spent on harder stuff, which will give you confidence and get you higher marks overall.  And, all other things being equal, employers of graduates take note of high marks.  Just what you got high marks in is often less important.  Wickwack 121.215.63.160 (talk) 04:54, 11 May 2013 (UTC)
 * Why do we entertain these stupid questions? Shadowjams (talk) 06:26, 11 May 2013 (UTC)
 * We? Richard Avery (talk) 06:56, 11 May 2013 (UTC)
 * Why do we entertain people complaining about questions they don't wish to answer. You know, it takes no energy or time at all to not type anything.  -- Jayron  32  17:58, 11 May 2013 (UTC)
 * Not quite true, Jay. Biting one's tongue does require considerable courage and a certain amount of mental and physical energy, depending on how strong the knee-jerk response desire is in any particular case.  Fortunately, service as a volunteer around here comes with a free and ongoing course in Tongue-Biting 101, as anyone who's been here more than a few micro-seconds will confirm.  In fact, I've entirely stopped speaking in RL, as my tongue has become so badly damaged over the years from hanging around here, it ain't worth a damn anymore.  Btw, anyone who thinks I can be a little talkative here sometimes, should know just how much I choose NOT to say.  Lots. --   Jack of Oz   [Talk]  23:27, 11 May 2013 (UTC)
 * For many high school students who are thinking about what to do next, this question does not seem as silly as it does to an older person. When I was in high school (1950's) we all gave a lot of thought to to further education and future careers, visited places, talked to older family members, obtained handbooks, lots of things.  But today's generation of teenagers doesn't do that. They tend more to react rather than plan.  While the OP should go to better sources than an unknown and uncalibrated bunch of volunteers who might or might not have good advice (ie us Wikipedians), at least he's giving it some thought, and he has asked.  Don't piss him off.  Wickwack 120.145.194.10 (talk) 02:54, 12 May 2013 (UTC)
 * Well, why don't we ask, why it is a stupid question? Perhaps it is a stupid question. Have we ruled out the possibility that it is a stupid question? Bus stop (talk) 03:07, 12 May 2013 (UTC)