Wikipedia:Reference desk/Archives/Science/2012 December 16

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December 16

Restoring a faded receipt

I occasionally spend my own money on items for a voluntary organisation I'm part of, then bundle a heap of receipts together and seek reimbursement from the organisation's treasurer. My problem is that one receipt I currently have was one printed on that glossy paper with less than permanent "ink", and it has faded so much that it's almost unreadable. (Is that Thermal paper?) One bit that's definitely missing is the amount.

Is there a simple way of restoring what used to be there? HiLo48 (talk) 03:02, 16 December 2012 (UTC)

Yep, that's thermal paper all right. Holding it up to a light might help you read it, but actually getting a copy you could use for proof of purchase will be more of a challenge. I've also found that highlighters wipe the ink right off and darken with time, so avoid those. StuRat (talk) 03:14, 16 December 2012 (UTC)

You should find various suggestions online (I know because I've seen them when I looked). One risk is most of them could potentially make things worse. One option if everything is still visible but some are so faint to be bearly readable is to try scanning the receipt (scanning is likely better then photographing) which generally should not significantly damage/ruin any chances of recovery. From experience it's always best to make a copy, photograph or scan any thermal receipt ASAP to avoid such problems if you want to need them for a while. Particularly if you're not storing them out of sunlight, heat and away from moisture or particularly oils (including skin oils) such as in a wallet. Although even if stored well they could easily fade within a year. Nil Einne (talk) 06:58, 16 December 2012 (UTC)

Barely rather than bearly. Kittybrewster ☎ 11:29, 17 December 2012 (UTC)

Unless Nil is a cousin to Smokey the Bear. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:18, 17 December 2012 (UTC)

"Bearly" is a perfectly good word: "I bearly ate dinner" (meaning I broke into my neighbor's home, ripped the door off the refrigerator, stuffed my head inside, and ate everything). StuRat (talk) 16:54, 17 December 2012 (UTC)

"Barely" is a perfectly good word, too: "I barely went to work last week, but they complained that I'm expected to wear clothes, even on casual Fridays". StuRat (talk) 16:54, 17 December 2012 (UTC)

You, sir, win this discussion. :D -Chris (talk) 10:34, 20 December 2012 (UTC)

I'd scan the image at the highest res I could then manipulate it in an image processing program to increase the contrast. If it's legible, you might also take it to the vendor and ask him to give you a replacement for it. μηδείς (talk) 21:57, 16 December 2012 (UTC)

That's a good plan. I've done that from time to time. Keep the original, so the vendor can see what the problem is. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:16, 17 December 2012 (UTC)

I think the best advice is to go to the customer service desk of the vendor, and ask for a handwritten receipt to replace the original. Very few businesses are actually hoping to dick you out of a few bob because your receipt is fading. μηδείς (talk) 03:20, 18 December 2012 (UTC)

Microfibre 8 848 metres long weights 0.4 grams?

Hi all,
The swing tag on a waterproof bicycle jacket I bought a little while ago claims

"(name) is a sportswear fabric woven from a fine, light microfiber - so light that the amount needed to reach from sea level to the top of Mt Everest would weigh only 0.4 grams."

I assume the the claim was about the height of Everest, not its distance from the nearest ocean. So I sat down do the math:

• Height of Everest above sea level is 8 848 metres
• The fabric is polyester, so the monomer would be an ester of some kind
• Guess this has probably got something to do with Avogadro's number somewhere in the calculations
• Err... OK, I'm completely lost now

Claim plausible?
--Shirt58 (talk) 09:32, 16 December 2012 (UTC)

Let's assume the jacket just barely floats on water, i.e. Density = 1 g/cm^3. For a filament 884,800 cm long, a total volume of 0.4 grams is accomplished by 0.4 cm^3, which is a cube of 884800 cm * sqrt (0.4/884800 cm^2) = .000672 cm = 6.72 nmum (square) [or 5.96 nmum (circle)] I suspect this is worked backward from a "6 nm micron filament" figure. Wnt (talk) 09:52, 16 December 2012 (UTC)

I don't think you mean a "cube", but rather a (highly) rectangular solid with a square cross section, or a cylinder. StuRat (talk) 18:47, 16 December 2012 (UTC)

Erm, yes, rectangular prism and cylinder. The correction I applied to circle the square was the square root of pi*1^2/4. Haste, waste, etcetera. Wnt (talk) 03:37, 18 December 2012 (UTC)

You made an error. 0.000672 cm = 6.72 μm, not nm! Ruslik_Zero 19:25, 16 December 2012 (UTC)

Thanks! No excuse for that keyboard fart. Wnt (talk) 03:34, 18 December 2012 (UTC)

Thanks for the answers so far. Lets make this more specific. Consider not a very thin fibre of those that are woven together to make the fabric of the jacket, but at the atomic level. Would a single strand of polymerised (aliphatic, to be most generous to the claim) ester monomers 8 848 metres long weigh 0.4 grams? I lack the vocabulary to describe this in other than laypersons terms: would a chain of ...=<ester polymerisation monomer>=<ester polymerisation monomer>=<ester polymerisation monomer>=... 8 848 metres long weigh 0.4 grams? --Shirt58 (talk) 09:22, 17 December 2012 (UTC)

Let's take poly-(R)-3-hydroxybutyrate (P3HB) (an aliphatic ester). This article, on page 26 gives the length of two repeat units (in stretched configuration) as 0.92 nm, so 0.46 nm per repeat unit. If we had a completely stretched single P3HB molecule with a length of 8 848 m, it would contain (8848 m)/(0.46 * 10^-9 m) = 1.92 * 10¹³ units. Each unit contains 4 C's, 2 O's and 6 H'2, thus 4*12+2*16+6 = 86 atomic mass units (u). So the total mass of the chain is 1.92 * 10¹³ * 86 u = 1.65 * 10¹⁵ u, which is (1.65 * 10¹⁵ u)/(6.02 * 10²³ u/g) = 2.7 * 10^-9 g = 2.7 ng. In practice, of course such a long molecule would never be in a completely stretched configuration, but even if one would need 1000s of times the number of repeat units to reach the required length, its mass would still be far below 0.4 g. - Lindert (talk) 13:22, 17 December 2012 (UTC)

To cross-check these two methods, 0.4 g/ 2.7 ng = 1.48 x 10^8 parallel filaments in a bundle. So if the bundle has a 6.72 um square cross-section, and the filaments are packed in a grid fashion, then there are 12172 filaments along a side, each 0.55 nm (really) = 5.5 Angstroms apart. Which seems a little far (especially since the packing could be better) but we seem in the right ballpark, anyway. Wnt (talk) 03:45, 18 December 2012 (UTC)

(xkcd) why is text in the mirror backward, but not upside-down? what's so special about the horizontal axis?

(xkcd) why is text in the mirror backward, but not upside-down? what's so special about the horizontal axis?

This is the question. I wonder if the TRUE answer isn't any of the baloney that you Google but instead (struck out to avoid spoiler, if you're just now thinking about this or googling it, read following when finished) The mirror doesn't reverse text in any direction. However, if you see a page you can't see it in the mirror in front of you, since it's facing you, not the mirror. - so, to see it, you would have to turn the page toward the mirror. If you turn it around horizontally the text becomes flipped horizontally, and if you turn it around vertically the text becomes flipped vertically. You're the one who flipped the image, so you could view it in the mirror. This seems to me to be a far better answer than the baloney I read while googling it. What do you guys think? Have I unlocked a deeper answer? 178.48.114.143 (talk) 18:27, 16 December 2012 (UTC)

Seems like a good answers, but why did you strike it out ? StuRat (talk) 18:42, 16 December 2012 (UTC)

So you can think about it and/or google it before reading "my" answer. I've updated "(spoiler)" to "(struck out to avoid spoiler, if you're just now thinking about this or googling it, read following when finished)". As you can see from Googling it, most answers are far more complicated and, in my opinion, wrong. 178.48.114.143 (talk) 18:46, 16 December 2012 (UTC)

Something else that might support this way of thinking about it is to remove the mirror entirely. That is, just hold up a transparency so it's facing away from you. Now, depending on how you rotated it to get it there, the text should either appear backwards or upside-down (but not both, as rotating it both ways brings it back facing you). StuRat (talk) 18:54, 16 December 2012 (UTC)

The mirror doesn't reverse right-to-left. It reverses your view of things front-to-back. Just write a word on a transparency and look it from behind. You'll see the same as the image in the mirror. OsmanRF34 (talk) 18:56, 16 December 2012 (UTC)

The question is, specifically, "Why does text appear flipped on the horizontal but not vertical axis in a mirror?" Are you saying that this experience is not descriptive of most people's experience of viewing text in a mirror? (That it so appears?) I offer the answer to that "question" but you seem to be questioning its premise. In fact, in common experience I would say most people know what I'm talking about, as they do have that visual recollection of their interaction with reading things in mirror :) 178.48.114.143 (talk) 19:00, 16 December 2012 (UTC)

Yes, I question the premise. There is no flipping, just a projection. OsmanRF34 (talk) 19:18, 16 December 2012 (UTC)

As mentioned elsewhere in this thread: a transparency held up in front of you (normally) and seen through a mirror looks fine. This is an experience of looking identical. You've never had the "experience" of "mirror" writing looking, instead of this, like a picture that in Microsoft Paint you choose "flip horizontally" on? (Note: we are only describing the experience, not its explanation.) 178.48.114.143 (talk) 19:51, 16 December 2012 (UTC)

Osmond! I just realized that where you write "The mirror doesn't reverse right-to-left. It reverses your view of things front-to-back. Just write a word on a transparency and look it from behind. You'll see the same as the image in the mirror" your suggestion is NOT well-defined! After "writing a word on a transparency" to follow the step of "looking at it from behind" you must now get around it. There are an infinite number of simple ways to get around it - i.e. one for each arc that your face can follow from the point in front to the point behind. Each one shifts the image in a DIFFERENT way!!! It is not at all true that "you'll see the same as the image in the mirror" as that image might have been created following a different flipping-path. The REASON, then, that we think of mirrors as flipping things horizontally is because THIS IS THE AXIS OF GRAVITY: out of the infinite number of arcs that your face can follow to get around to the other side, the only two that make sense are the arc heading 'due' left and the arc heading 'due' right: any other arc (such as the arc in between, heading due 'up') is not a very practical way to get around to the other side. As for why we flip OBJECTS such as a book, along the plane perpendicular to gravity, hence causing an apparent flip over pure horizontal (as oppoesd to the infinite otherp ossibilities) it's because, where this another person in the mirror, they could then see it along the same straight orientation that everyone is accustomed to reading. So, it would seem, the FULL answer is this: mirrors flip on the horizontal because we turn objects toward them on the horizontal. We do this because, where the person in the mirror a separate person instead of ourselves, this is the orientation in which they would be accustomed to reading. What do you think 178.48.114.143 (talk) 21:31, 16 December 2012 (UTC)

Another factor is how we rotate our heads. We don't stand upside-down to see what's behind us. This might come into effect when they write the word AMBULANCE "backwards" on the front of ambulances, since they assume we will have rotated our heads in the usual way to see it in the rear-view mirror of our cars. If we all rode in our cars upside down, facing forward, then they would need to change how they write AMBULANCE. StuRat (talk) 19:01, 16 December 2012 (UTC)

• It simply depends on the location of the text in relation to your orientation to the mirror, which can be demonstrated quite easily. Place an object like a license plate or bumper sticker oriented so that you can read it normally on the counter in front of you in your bathroom, and then look in the mirror. Your line of sight is going through the letters from top to bottom to the mirror, so the text itself will be flipped top-o-bottom: i.e., upside down. Now place the plate on the wall in the normal reading position to your right between yourself and the mirror. Now the line of sight is going along the plate from side to side in the direction of the mirror, so now the text will be reversed side to side: i.e., backwards. Now, rotate the plate on the wall 90% counterclockwise so that the text is facing you running up the wall instead of parallel to the floor. Now the line of site will be going along the letters from top-to-bottom again, so the letters will once again appear upside down in the mirror. μηδείς (talk) 19:37, 16 December 2012 (UTC)

• Osman has it. Richard Feynman explains the question and answer in his characteristic style here: [1]. He also gives a psychological interpretation for why he thinks people come to the wrong answer. (His answer starts at ~0:45, but it's only a 3 minute clip). SemanticMantis (talk) 19:39, 16 December 2012 (UTC)

• I don't think Feynman addressed why having your face inside out appears to cause a left-right reversal, but not an up-down reversal. StuRat (talk) 20:01, 16 December 2012 (UTC)

• He does, although he might be more explicit. The core cause is that left and right are usually understood as relative directions, while up and down are absolute directions. If you wave your right hand, the hand on the right hand side of the mirror waves back. But from the perceived position of the person in the mirror, that is his (or her) left hand. This becomes clear if you, as Feynman does, go to completely absolute coordinate system. You wave the east hand - so does your mirror image. No flipping. --Stephan Schulz (talk) 20:15, 16 December 2012 (UTC)

This reminds me that there is (I've read) a language in northern Australia where absolute directions are strongly preferred: "John passed me the gourd with his west hand." —Tamfang (talk) 19:45, 26 June 2013 (UTC)

This is precisely my contribution: a transparency viewed from in front appears neither flipped vertically nor flipped horizantally. Viewed from behind, however, it depends on how you got around it: did you follow a 180 degree arc around horizantally or fly up over vertically? So it is not at ALL true that the "core cause is that left and right are usually understood as relative directions while up and down are absolute positions". That's not it AT ALL, whatsoever, even a little bit. Whether you flip horizontally, vertically, or a different amount if you end up taking a different path depends on just how you get around to the other side. 178.48.114.143 (talk) 00:04, 17 December 2012 (UTC)

Your head doesn't appear upside down because, like in my experiment, your head would have to be above or below you between your eyes and the mirror, like the license plate on the counter in front of you. It's impossible. (You could place a dummy head on the counter facing upwards and the face in the mirror (which would have to be titled) would appear to look down out of it.) My suggested experiment explains this in full. I am not sure what Osman means by no flipping, just projection, unless he's making the same sort of point one might make who said there are no colors, just wavelengths of light. His claim a mirror only reverses things front to back is also wrong, as my experiment shows, unless I misunderstand what he means to say. The bottom line is that whether an image appears upside down or backwards to an observer depends on the placement and orientation of all three objects with one another. μηδείς (talk) 20:34, 16 December 2012 (UTC)

I would agree with Osman that it's a matter of front-back reversal (i.e. mirroring). Imagine that Bob's mirror image were a physical (3-dimensional) object. Now if I as an outside observer would see both Bob and his mirror image, the difference between the two would be a front-back reversal, i.e. to make Bob and his image identical, I would need to bring both of Bob's eyes to the back of his head, moving them exclusively on the front-back axis, and likewise for all body parts. Now I could get the same effect by first rotating Bob 180 degrees around a vertical axis, and then mirroring him in a left-right direction, but the same is achieved by first rotating him 180 degrees around a horizontal (left-right) axis (so that his head pointed down), and subsequently mirroring him in an up-down direction. In fact, I could conduct this procedure with any two axes that are perpendicular to one another and to the front-back axis, so there is nothing special about the left-right direction. I sincerely hope this makes sense to anyone but myself. - Lindert (talk) 00:34, 17 December 2012 (UTC)

Calling what a mirror does "mirroring" is a neat trick. Pointing out that whatever is closest to the mirror will appear closest in the reflection to the observer is also true. But the fact remains that some text appears upside down but not backwards, and some text backwards but not upside down, and this is explained by the relation of the observed object and the mirror to the line of sight. Osman hasn't addressed the position relative to the observer so far as I understand, and instead of explaining why certain text appears upside down, and other backwards, seems to have denied the difference exists. μηδείς (talk) 04:51, 17 December 2012 (UTC)

That's because the relative position of the observer and the mirrored object is irrelevant. No matter if you see text in a mirror that is above you, below you, beside you or even between your eyes, it always seems to be backwards but not upside down, but this is not actually the case. Scratch that, I think 91.120.48.242 explained it best (see below). - Lindert (talk) 10:15, 17 December 2012 (UTC)

Lindert, how about this. Normally it is NOT possible to see an object in a mirror: the text you're reading, or Bob, is facing you and so is the mirror. If you want to see it in the mirror you have to (relatively) turn it toward the mirror. The direction you relatively turn it toward the mirror in determines the axis of reflection. 91.120.48.242 (talk) 08:34, 17 December 2012 (UTC)

Yes, that's exactly right. - Lindert (talk) 10:15, 17 December 2012 (UTC)

So I think Feynman isn't exactly right in his explanation. If we happen to notice text in a mirror, it can be flipped along one of many directions. It's only when we start to fumble with turning something toward the mirror that we select a "preferred wrong axis" (the left-right flip that keeps up the same direction), perhaps because people rarely flip head-over-heels (so if we show something to someone else we do so by keeping it right-side-up) and also our horizontal symmetry means that if we catch a glimpse of ourselves it's easy to assume there's a mirror person who got there by turning. If we were as asymmetric left-right as we are up-down we might not prefer to flip text horizontally by that trick of the eye. But make no mistake (as Feynman does) that this perception causes the horizontal flip: it's just our reaction to it, turning objects preferably horizontally only. 91.120.48.242 (talk) 12:12, 17 December 2012 (UTC)

For those wondering about xkcd in the heading, the question is based on http://xkcd.com/1145/ which was published December 10. Hover your mouse over the drawing to see the alt text "Feynman recounted another good one upperclassmen would use on freshmen physics students: When you look at words in a mirror, how come they're reversed left to right but not top to bottom? What's special about the horizontal axis?" xkcd forum thread about the comic: [2]. PrimeHunter (talk) 00:36, 17 December 2012 (UTC)

Conventional mirrors don't flip anything, they merely reflect it straight back. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:14, 17 December 2012 (UTC)

I am astounded at the length of this thread.165.212.189.187 (talk) 13:55, 17 December 2012 (UTC)

Well, keep in mind that the obvious can prove elusive to some folks; like the ones who couldn't pour water out of a boot with the instructions written on the heel. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:17, 17 December 2012 (UTC)

So there is no left-right preference in the mirror or your eyes or your brain. Arguments that start with: "You have two eyes that are horizontally placed" fail because you still see the reversal with one eye closed.

For proof of that, do an experiment. Write the word "MIRROR" onto a piece of paper and rotate it 90 degrees so that the first letter is at the top and the last is at the bottom. Now look at that in the mirror - what you get is shown in the image at right. You can see that it's messed up - but now you can tell that you can fix it so that you can read the word EITHER by a left-right flip OR a top-bottom flip. There is no "preferred direction" here.

The thing is that in order to see text in the mirror, the real-world object already has to be being viewed from behind. So the mirror doesn't flip it either left-right or top-bottom - it flips it near-far...in depth. Holding the paper backwards so you can see it in the mirror is what flips it. The mirror flips near-to-far, so the thickness of the paper is what is reversed, effectively moving the text from the side of the paper that's furthest from you to the side that's nearest. What makes text flip horizontally but not vertically is simply our choice as humans as to how to rotate the paper so we can see it in the mirror...the fact is that we seem to always choose a left-right rotation and not a top-bottom one when we do that!

SteveBaker (talk) 14:23, 17 December 2012 (UTC)

My God, SteveBaker!!! You have PRECISELY described how DEEPLY rooted your IMPLICIT step is, which is the ONLY one that causes the flip!!!! The IMPLICIT step happens RIGHT HERE: "For proof of that, do an experiment. Write the word "MIRROR" onto a piece of paper and rotate it 90 degrees so that the first letter is at the top and the last is at the bottom. Now look at that in the mirror - what you get is shown in the image at right" -- WRONG!!!! There are TWO ways (that I list) to "now look at that in the mirror". You can flip the paper around horizontally (top of page remains top of page, bottom of page remains bottom of page) or you can flip it around head-over-heels (left side of page remains left side of page right side of page remains right side of page). Your idea of "LOOKING AT IT IN THE MIRROR" so deeply includes the IMPLICIT step of flipping it horizontally that you think it's part of the definition of "looking at it in the mirror." NOPE! I have bested you and Feynman!  :)  :) :)  :)  :) 91.120.48.242 (talk) 15:12, 17 December 2012 (UTC)

OOps. Tricky guy, you do mention that in the final sentence. But, trickily, you pretend that "now look at it in the mirror" is as well-defined as "rotate it 90 degrees so that the first letter is at the top and the last is at the bottom". That's a clear direction. "Now look at it in the mirror" is NOT.  :) The ONE AND TRUE answer to this riddle or conundrum is:

“ The reason mirrors prefer to flip text and images horizontally but not vertically - an empirically true characteristic everyday mirrors "actually" possess - is that to look at items in the mirror, we flip them around horizontally, instead of vertically or around some skewed axis. Mirrors don't flip the items - we do. ”

91.120.48.242 (talk) 15:16, 17 December 2012 (UTC)

Just in case someone thinks this discussion is not long enough, you can look at [3], which I found referenced on Talk: Mirror image, which also discusses this in the untitled opening section. Duoduoduo (talk) 15:29, 17 December 2012 (UTC)

Interesting article. For this picture - http://upload.wikimedia.org/wikipedia/commons/5/52/Mirror.jpg - how would you check how the front of that jug actually looks, so you can compare it with what we see in the mirror... would you walk around it? Not me, I'd fly up over it, ending up with my head on the ground, since that way I can see what it really looks like. So you see, that mirror flips everything vertically!!!! 91.120.48.242 (talk) 15:50, 17 December 2012 (UTC)

I still don't get what the continuing problem is here. The text does appear upside-down when the reflected object is below and between you and the mirror, not backwards. A book held oriented so that you can read it in the normal reading orientation as you look at a mirror that reflects both you and the book will show the text upside down, not backwards. Is this unclear, or does anyone dispute it? The same holds with a mannikin head. Place it before you on a bathroom counter on its back peering up into a mirror; its reflection will seem to gaze downward out of the mirror. Does anyone dispute this or find it confusing? Objects above or below the line of sight of an observer will appear reversed upside-down, not backwards in a mirror. Is this confusing or disputed? μηδείς (talk) 03:12, 18 December 2012 (UTC)

Take http://upload.wikimedia.org/wikipedia/commons/5/52/Mirror.jpg - we see the back of the jug "in" the mirror. How does what we see "in" the mirror defer from what the actual back of the jug looks like? 91.120.48.242 (talk) 08:15, 18 December 2012 (UTC)

I am amazed by the length of this discussion. Have none of you actually tried going to your bathroom mirror, and holding a page of wrinting so that you first see the writing reversed, and then manipulating the page so the that the mirror image is upside down (it will then not be transposed left to right). Trust me, if you did, you would instantly understand what is going on, and realise that Poster 91.120.48.242 had it right, as did Steve Baker, and Medeis/μηδείς in his last post is off in fantasy land. Ratbone 60.230.205.13 (talk) 01:31, 19 December 2012 (UTC)

I have suggested the experiment that you just suggested twice already and given an explanation. (Search the word bathroom and see if ayone besides myself in this thread has used the word.) I have done the experiment. Hold a book in front of you, open on the counter and facing up the way you would normally read it, and the text in the mirror in front of you will appear upside down. If you hold the text beside you with the words running parallel to the floor, or tilt up the book so that the cover text faces the mirror, the image of the text will be backwards, not upside down. That amounting to "fantasy land" sounds like crazy talk if you ask me. Perhaps I confused you? μηδείς (talk) 03:40, 19 December 2012 (UTC)

I was well aware of the previous suggstions to go to the bathroom and try it. I posted because it strongly seems that none of the other posters actually went and tried it. Its one of a multitude of things that are a bit difficult to reason out in one's mind, but a few seconds manipulation in front of an actually mirror makes it immediately clear. In your explanantion in your post (that I referred to) you have it all confused - did you actually try it with a mirror? I think not. Your second and penultimate sentences are wrong - it depends on orienation of the object, not on whether it is positioned to the left (or right), or below (or above) your line of sight. Enough of this. If anyone still hasn't figured it out, go to a mirror now and try it. Like another poster said, hold teh text in front of you so you can read it directly. Now, rotate it about a vertical axis so you can read it in the mirror. Viola! It looks transposed left to right. Then, rotate it back again, and now rotate it about a horizontal axis so yoy can again see it in the mirror. Viola! Its now not transposed left to right, its upside down. Move it about left to right, up and down, round about, without changing the orientation. Stays upside down, doesn't it? Nothing to do with lines of sight. Ratbone 124.178.37.15 (talk) 05:45, 19 December 2012 (UTC)

Yuck. You can illustrate these things without images.

Right to Left:ЯOЯЯIM
Upside Down : ЬMИED.

p.s. Are there any reserved bits for non-standard orientation in any of the xx-bit charsets? - ¡Ouch! (^{hurt me} / _{more pain}) 06:24, 21 December 2012 (UTC)

Has anyone considered that the answer is because of how our brains process visual information? What we see is divided into a right and left visual field, but does not have an "up" and "down" visual field. Mirrors reflect the light the way mirrors do, but once the light hits our retinas the brain takes over. There's nothing special about mirrors, or any axis in particular, it's all a quirk of our nervous system.Quietmarc (talk) 14:59, 23 December 2012 (UTC)

Just having scanned through quickly, I don't see anyone mentioning one arbitrary choice of symmetry breaking, namely that the mirror is implicitly assumed to face horizontally rather than vertically? Place a mirror flat, face-up on your desk (or face-down on the ceiling). Hold a book oriented upright the opposite side of it, and inspect the writing in the reflection. It appears to be flipped vertically, not horizontally. This doesn't answer anything, just though I'd mention the oversight. — Quondum 14:36, 24 December 2012 (UTC)

Evolutionary fate of siblicide

Is it reasonable to expect that throughout evolution siblicide and similar behavior in animals will disappear as disadvantageous traits by means of producing a single offspring (so that Nazca Booby for example would lay only one egg instead of two)? I also wonder whether such behaviour (including eating of males after copulation in spiders) creates a gender disbalance in population, ultimately leading to difficulties in finding a mating partner. Brandmeister^talk 20:12, 16 December 2012 (UTC)

Very interesting questions, but they touch on many complex topics and phenomena. For the first, see selective pressure and r-k selection. Basically, what is and isn't beneficial for a species is highly context-dependent, and many factors are integrated by the evolutionary process. In some instances, e.g. infanticide_(zoology) could be beneficial for an organism. For the second question, see sexual conflict, and think of how that interacts with allee effects. SemanticMantis (talk) 20:49, 16 December 2012 (UTC)

Yes, as said it will depend on the selection pressure. As it is, birds with this egg laying pattern lay two because it is a small investment (mass of the egg and difference in labor warming two eggs--which is negligible) compared to the risk of laying one egg that is sterile or cracked or lost before hatching and losing a whole breeding season. One egg inevitably hatches first, and since the math shows that one fat healthy chick is more than twice as likely to flourish than two underfed chicks, the inevitable happens.

Were there, however, to be a long string of really good years where those rare nests with two surviving chicks did well, then the trait that leads one sibling to oust the other will slowly be ousted by the friendly-chick trait. Just a few chicks with the friendly trait would do so much better over time that in a colony of a million birds it could become the dominant pattern over a geological instant. There seems to be some constraint with ground laying colony birds, however, that favors the one chick. It's probably lack of real estate.

In any case, whichever trait leads to better survival in the long run will win out, since it does a friendly chick no good if he and his sister both die before reproducing. You should read Richard Dawkins's The Selfish Gene. which makes this all quite clear. Just don't fall for his notion that genes, and not organisms are the objects of selection. Ernst Mayr's What Evolution Is also covers the topic more briefly but with a much more accepted viewpoint on selfish genes. μηδείς (talk) 21:47, 16 December 2012 (UTC)

I think one of the best examples where siblicide has been apparently selected for is the case of oophagy, or intrauterine cannibalism. Basically, the first embryo to develop to the point it can eat will consume the remaining embryos, potentially including other embryos that have also developed that far. From an evolutionary perspective, it seems like a very uncomplicated way to provide one's offspring with extra nutrition so it can develop further before leaving the womb. Someguy1221 (talk) 02:17, 17 December 2012 (UTC)

It should be noted in oophagy that most are simply unfertilized eggs intentionally produced for the first fertilized egg to consume. But yes, this is pretty radical siblingslaughter when it happens. μηδείς (talk) 03:09, 18 December 2012 (UTC)