1145: "Sky Color"

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1145: "Sky Color"

Postby Quicksilver » Mon Dec 10, 2012 5:06 am UTC

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http://xkcd.com/1145
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?"
Another sky based comic? Could Randall be BHG?

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Re: 1145: "Sky Color"

Postby asdfzxc » Mon Dec 10, 2012 5:08 am UTC

...Good question.

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Re: 1145: "Sky Color"

Postby rhomboidal » Mon Dec 10, 2012 5:10 am UTC

If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.

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Re: 1145: "Sky Color"

Postby sardia » Mon Dec 10, 2012 5:11 am UTC

The first is probably due to the limits of human vision? maybe, my best guest. What's the answer to the 2nd one?
Last edited by sardia on Mon Dec 10, 2012 5:37 am UTC, edited 1 time in total.

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Re: 1145: "Sky Color"

Postby Jorpho » Mon Dec 10, 2012 5:14 am UTC

'Tis quite an infuriating answer indeed to stop with Rayleigh scattering. The more complete answer is, if I am not mistaken, that the human eye rather sucks at seeing purple. (Rayleigh scattering is also responsible for the bluish color occasionally observed in skim milk, and for the bluish color of veins in the skin; deoxygenated blood is not actually blue, as some may claim.)

See also this thread.

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Re: 1145: "Sky Color"

Postby Dopefish » Mon Dec 10, 2012 5:16 am UTC

I'm pretty sure the sky appears blue rather than violet because the sun emits much more intensely at the blue wavelength than violet.

I vaguely feel as if I heard some shinangans about water dipoles something something something and that's why the sky is blue during my electrodynamics class, but I prefer to stick to scattering based explanations.

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Re: 1145: "Sky Color"

Postby SEE » Mon Dec 10, 2012 5:16 am UTC

The trick on the mirror one is that there is no such reversal.

(Doubt me? Write something on clear plastic, then hold it up to a mirror.)

(Alternatively, hold a piece of paper horizontally, top of the text closest to a vertical mirror. Hey, now the letters are all upside-down!)

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Re: 1145: "Sky Color"

Postby phlip » Mon Dec 10, 2012 5:17 am UTC

My understanding is that the sky does scatter violet light... the colour we see is a culmination of various effects within the eye with how we process colours... a combination of the fact that the sky is also scattering green and even red light (though to a lesser extent) so we're getting the full spectrum, just weighted to one end (this is why it's a pale blue, not heavily saturated), and the fact that our vision response drops off for violet light (so the blue appears more prominently, even though there's less of it by photon count). Google finds this, of interest.

There's also the fact that oxygen is (faintly) blue, which has a nonzero effect on the whole thing. But IIRC this isn't the major component to what we see.

For the mirror question in the alt text, this is actually a trick question, as a mirror reverses neither of these axes.
Spoiler:
It actually reverses the third axis - in to and out of the mirror. Things on your left are also on your left in the reflection, things above you are also above you in the reflection, but things closer to the mirror than you will have that order reversed in the reflection.

The left/right effect is actually a result of what you're comparing. Say you hold up a book, vertically, so you can read it normally. Then you turn it around to face the mirror, and try to read it, finding it to be reversed left/right. However, that turning around is relevant - it is geometrically equivalent to reflecting it left-to-right, and reflecting it front-to-back (generally, two perpendicular reflections is equivalent to a 180 degree rotation). The mirror then undoes the front-to-back reflection and leaves the left-to-right reflection alone, so that is what you see in the mirror. So it all comes back to human habits, of living with gravity and treating the up/down axis as a privileged direction. If you rotated the book around a horizontal axis instead of a vertical one to face the mirror (ie ended up holding the book "upside-down" and facing the mirror) then the image in the mirror would be reversed vertically, not horizontally.
Last edited by phlip on Mon Dec 10, 2012 5:27 am UTC, edited 2 times in total.

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Re: 1145: "Sky Color"

Postby Otto » Mon Dec 10, 2012 5:19 am UTC

The cones in your eyes aren't adapted to seeing violet as much as they're adapted to seeing blue. Mainly you see red, green, and blue, and the brain just infers the rest. Also, the sun doesn't put out the same amount of light across the whole spectrum. There's a lot more green and blue coming out of the sun than there is violet.

Also, mirrors do reverse on the vertical axis. Turn your head sideways.

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Re: 1145: "Sky Color"

Postby Djehutynakht » Mon Dec 10, 2012 5:20 am UTC

rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.


If this was grounds for human extinction we'd all be long dead.


Anyways... this seems like something Randall would do with his potential children. Each day when they come home from school he immediately asks them what they are supposed to learn about in their science classes the next day, and then starts teaching them tricky stuff to ask.

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Re: 1145: "Sky Color"

Postby lemonickous » Mon Dec 10, 2012 5:30 am UTC

Obviously the answer to the Feynman conundrum is gravity. It works downwards, so up and down is special. Whereas horizontal is an equipotential plane so anything can happen.

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Re: 1145: "Sky Color"

Postby Flabbergasterisk » Mon Dec 10, 2012 5:35 am UTC

rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.

To call something blue when it's not, we defile it.
But aw what the heck. It's hard to rhyme violet.

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Re: 1145: "Sky Color"

Postby WIMP » Mon Dec 10, 2012 5:46 am UTC

I suspect the answer to part 1 is a combination of the sun's output and the human eye's receptors being strongly biased towards blue relative to violet, so that even a much stronger reflection of violet isn't sufficient to beat blue. Planck's Law falls off very rapidly past the wavelength peak, and human vision consists of three roughly Gaussian absorption peaks centered around red, green, and blue with moderate overlap.

It occurs to me that the purple of twilight is probably legitimately purple winning over blue as a result of more Rayleigh scattering, since light has to travel through more atmosphere at dusk (that's why the Sun's disk is red). In the extreme of maximal Rayleigh scattering, purple *does* overcome the Sun+eye bias.

I know the answer to the second is that mirrors invert along the axis connecting the onlooker and the mirror, not left-right or up-down. It's weird because we psychologically think of our reflection as us standing on the other side of the mirror, as though by turning around we ought to reproduce the mirror's transformation. It's not actually possible to reproduce what the mirror's doing, because it's pulling your front through your back, not doing any rotations that you can mimic in reality.
Last edited by WIMP on Mon Dec 10, 2012 5:51 am UTC, edited 1 time in total.

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Re: 1145: "Sky Color"

Postby Coyne » Mon Dec 10, 2012 5:51 am UTC

The answers I would give.

So why isn't the sky violet?


Like ultraviolet, most violet is absorbed in higher levels of the atmosphere. The violet portion of the solar spectrum is comparatively weak at ground level. This is compounded by the insensitivity of the eye to electromagnetic violet, which is at the upper limit of our visual sensitivity. Therefore, what violet is scattered is reduced in strength and poorly perceived, permitting the apparent blue color to dominate.

(The above would be reaching, since I'm really not sure. But it might be a decent ad-lib. It was written from my collected knowledge without reference, as I would do if I were giving a quick answer to a kid. I've since looked at Wikipedia and I think it's inadequate to explain the problem raised by the question. Here's hoping Randall didn't anticipate my answer and give the kid yet another question to ask...)

Feynman wrote: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?


"Reversed" and "horizontal axis" are misdirections in this case: The letters in the mirror are reflected to you in their exact spacial positions as you are holding the card. When you turn the card toward the mirror, you place the first letter on the card ("A" in "Ambulance") on your right-hand end of the card. Therefore, you see it in the mirror on your right-hand side; exactly the same way you would see it if you could look directly through the card. The letters are not reversed by the mirror and the horizontal axis is not special.

I suspect this was a newbie "orientation" question.
In all fairness...

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Re: 1145: "Sky Color"

Postby Patashu » Mon Dec 10, 2012 6:01 am UTC

I loved the question about the mirror. I had to think about it for a while before realizing what a mirror was doing was 'pulling' your back to your front and your front to your back, so there's no reversing at all - or rather, the reversing is perpendicular to the mirror's surface.

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Re: 1145: "Sky Color"

Postby searching » Mon Dec 10, 2012 6:12 am UTC


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Re: 1145: "Sky Color"

Postby jpk » Mon Dec 10, 2012 6:25 am UTC

Simplest answer to the Feynman: the mirror "reverses" front to back, not left to right.
Sorted.

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Re: 1145: "Sky Color"

Postby latrine » Mon Dec 10, 2012 7:08 am UTC

I think the word "reverse" is just confusing because it's hard to see what is reversed. I suppose if you order objects from a wall leading up to the mirror then the objects are in reverse order in the reflection from what appears to be the closest point in the mirror to the reflected wall. But this doesn't help you understand why text looks reversed. I think a better analogy is that the words in the mirror appear the same way they would if you held the page away from you up to a light and tried to read through the paper.

And yes, a reflection is not the same thing as a rotation. If a mirror gave you a rotated image then if you held a page up to the mirror it would appear the same as if you were looking at the front of the page directly.

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Re: 1145: "Sky Color"

Postby mikrit » Mon Dec 10, 2012 7:17 am UTC

rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.

Tell me why the stars do shine,
Tell me why the ivy twines,
Tell me why the skies are blue,
And I will tell you why I love you.

Nuclear fusion makes stars to shine,
Tropisms make the ivy twine,
Rayleigh scattering make skies so blue,
Testicular hormones is why I love you.

Second verse published but not written by Isaac Asimov.

(It's like this: Asimov cited it in his Treasury of Humor when he didn't know the author. In a later book, though, he had learned the author's name and gave him credit. But I cannot remember the name and I have returned the book to the library.)
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Re: 1145: "Sky Color"

Postby Archgeek » Mon Dec 10, 2012 7:22 am UTC

Now now, human cones can detect violet without too much trouble, and even ultraviolet to an extent, though the crystalin lens serves to shield the retina from that. There are documented cases of cataract surgery patients who've had their lenses replaced with a glass or plastic surrogate (without a certain coating found on modern variants) that reported seeing a layer of "whitish purple" light on things, notably including strange new patterns on flowers, like orchids, which have UV-reflective pigments to help guide insects. Though yeah, ozone absorbs the high-energy stuff, water vapour in the air absorbs a goodly amount of the rest, and what's left gets added to the blue and green and everything else that gets scattered to make the nice light sky blue instead of a pure blue.
As for the mirror, I'd say it's reversed because the observer has reversed it by turning it to face the mirror! Too bad one can't turn down the opacity of arbitrary objects.
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Re: 1145: "Sky Color"

Postby Dryhad » Mon Dec 10, 2012 7:33 am UTC

Clearly the sky is blue because it's moving towards us. Of course, it reverses direction every night, as can be briefly seen at sunset or dawn.

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Re: 1145: "Sky Color"

Postby FourTael » Mon Dec 10, 2012 7:39 am UTC

You know what's most interesting to me? Just how many different answers that question has gotten.

Now, we could use this to disprove science because clearly a random group of people (not entirely random, but enough for my point to stand) don't have 100% accuracy at answering questions, there is no answer and science must just be a bunch of people just coming up with random hypotheses that they make no effort to prove.

Spoiler:
You get all the points if you get what I'm referencing.


Or we can say that not everyone here is an expert or not all things are known, even some of the more basic things.

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Re: 1145: "Sky Color"

Postby sehkzychic » Mon Dec 10, 2012 7:54 am UTC

I'm just a biologist, but I think there's two reasons for the blue rather than violet. Violet light is scattered very efficiently by the atmosphere, but there is such a thing as too much scattering. If I'm not too mistaken, that's why the sky looks red at sunset: because of the shallower angle of the sun, the normal path length for light to reach us is much greater, and the blue light is scattered so much that it loses too much intensity by the time it reaches our eyes. So part of the explanation is that violet light is scattered too much.

The other reason has been mentioned already: sun emits more strongly towards the blue-and-green parts of the spectrum. More specifically that the sunlight that reaches the ground without being absorbed by the atmosphere is strongest in that range.

So, in short: over-scattering and the solar emission spectrum.

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Re: 1145: "Sky Color"

Postby Drowsy Turtle » Mon Dec 10, 2012 7:58 am UTC

We don't see much indigo in the sky because there isn't much indigo in sunlight (plus as others have described, indigo is readily absorbed in the atmosphere).

Image



The x axis isn't inverted in a mirror - things to the left of you appear to the left in the mirror, and things to the right appear to the right. What is inverted in the x axis (and also the y axis but not the z axis) is when we deal with another person, face-to-face. They are inverted in the x and y axes because they have have been rotated relative to us by 180 degrees, around the z axis. We are used to dealing with this inversion, so it confuses us when we have to deal with a mirror where the axes are unchanged in orientation (but vary in length).
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Re: 1145: "Sky Color"

Postby cantab314 » Mon Dec 10, 2012 8:06 am UTC

Yeah, my immediate answer to "Why isn't the sky violet?" was that the Sun doesn't emit as much violet. But the response of human vision I'm sure is also important.

A related one, it's clear the sun's output peaks in the green, so why isn't the Sun green? It turns out that a black body can never be green, whatever its temperature, so why not? My assumption is that there's enough red and blue as well so it looks white, but I have no fuller answer.

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Re: 1145: "Sky Color"

Postby sehkzychic » Mon Dec 10, 2012 8:12 am UTC

Regarding the mirror thing...there is a sort of reversal, I think. The illustration with a piece of clear plastic is a good one, but there is a sort of reversal: what you see in the mirror is not the same as what someone standing in front of you would see if they faced you. They would see the text on the paper you're holding written normally. (That's why the word "AMBULANCE" is written in reverse on the front of the eponymous vehicles. The text looks different in your rear-view mirror than it would if you turned around to look at it directly.) And yet both the mirrorworld and the person standing in front of you see your feet at the bottom and your head on top. Put it another way: if you hold up your hand while looking in the mirror, the mirrorworld you holds up what looks like a left hand (insert hand-chirality pun here). So yeah, I thought I understood this mirror madness and it seemed very intuitive. But the more I think about it, the muddier it gets. Help?

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Re: 1145: "Sky Color"

Postby da Doctah » Mon Dec 10, 2012 8:14 am UTC

Why isn't the sky violet? Because "violet" is just a pretentious made-up name for "really, really blue". We protanopes figured out that little dodge a long time ago.

Why does the mirror reverse left and right but not top and bottom? Because it's on the wall. Put it on the floor and step on it. Now your reflection has its head at the bottom and feet at the top, but its left and right hands are on the same side as yours. Voila, a mirror that reflects top and bottom but not left and right.

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Re: 1145: "Sky Color"

Postby Icalasari » Mon Dec 10, 2012 8:29 am UTC

Archgeek wrote:Now now, human cones can detect violet without too much trouble, and even ultraviolet to an extent, though the crystalin lens serves to shield the retina from that. There are documented cases of cataract surgery patients who've had their lenses replaced with a glass or plastic surrogate (without a certain coating found on modern variants) that reported seeing a layer of "whitish purple" light on things, notably including strange new patterns on flowers, like orchids, which have UV-reflective pigments to help guide insects.


What exactly happens to the eye if it doesn't filter ultra violet light?

Flabbergasterisk wrote:
rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.

To call something blue when it's not, we defile it.
But aw what the heck. It's hard to rhyme violet.


*Slow clap*

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Re: 1145: "Sky Color"

Postby gingermrkettle » Mon Dec 10, 2012 8:32 am UTC

Roses are red, violets are violet,
We have still not learned to describe the sky yet.

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Re: 1145: "Sky Color"

Postby Eutychus » Mon Dec 10, 2012 8:33 am UTC

sehkzychic wrote:That's why the word "AMBULANCE" is written in reverse on the front of the eponymous vehicles.

Surely I can't be the only one in this company who looks in my rear view mirror and instinctively thinks "what on earth is an ECNALUBMA"?
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Re: 1145: "Sky Color"

Postby StClair » Mon Dec 10, 2012 8:36 am UTC

For that matter, the reason why the "visible spectrum" is what it is is another of those chicken-and-egg things: our vision is, understandably enough, optimized to see by the wavelengths emitted by this star that penetrate this atmosphere. Alien life would similarly be optimized to see whatever portion of the light emitted by their star fits through the "window(s)" in their atmosphere.

(This may be "duh, obvious" to many, but I thought I'd post this just in case there's that one in whatever thousand reading this thread.)

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Re: 1145: "Sky Color"

Postby Jeff_UK » Mon Dec 10, 2012 9:09 am UTC

The horizontal axis is 'special' because our eyes are arranged along the horizontal axis.

Also, it's now eye awareness month. You're welcome. :shock:
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Re: 1145: "Sky Color"

Postby Alltat » Mon Dec 10, 2012 9:13 am UTC

StClair wrote:For that matter, the reason why the "visible spectrum" is what it is is another of those chicken-and-egg things: our vision is, understandably enough, optimized to see by the wavelengths emitted by this star that penetrate this atmosphere. Alien life would similarly be optimized to see whatever portion of the light emitted by their star fits through the "window(s)" in their atmosphere.

It's not really much of a chicken-or-egg situation: it's pretty clear that we see the way we do because that happens to work pretty well on this particular planet. It would be rather peculiar if the sun emitted light designed to fit our eyes, even if you're a creationist, as the sun was created before people in virtually every mythology.

I'd personally be tempted to answer the alt text question with "because brain". The confusion arises because your brain can't intuitively understand mirrors, leading to people having weird ideas about how they work.

Jeff_UK wrote:The horizontal axis is 'special' because our eyes are arranged along the horizontal axis.
Also, it's now eye awareness month. You're welcome. :shock:

Close one eye. Try again.

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Re: 1145: "Sky Color"

Postby mrb4 » Mon Dec 10, 2012 9:17 am UTC

Yet another unnecessarily complex and long answer to "why is the sky blue", *sigh*... The answer is simply:

Blue is the color of oxygen. (Yes, gaseous oxygen is very, very slightly blue, to a point it is sometimes incorrectly described as colorless.)

Similarly, when a kid asks "why are plants green", you reply "green is the color of chlorophyl, a pigment in plants", you don't talk about wavelengths, photons, absorption, scattering, etc.

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Re: 1145: "Sky Color"

Postby The Old Wolf » Mon Dec 10, 2012 9:17 am UTC

Thank Mogg for Google. As a lover of science, but one who is forever relegated to peering through the keyhole where the big boys and girls play because of an inability to learn higher math, the mirror problem vexed me. But YouTube has a whole series taken from a lecture with Feynman, explaining this and other problems. What a mind...
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Re: 1145: "Sky Color"

Postby AvatarIII » Mon Dec 10, 2012 9:28 am UTC

rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.


Roses are Red
Violets are Violet
I am your shoelace
and you are my eyelet.

yeah not hugely romantic

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Re: 1145: "Sky Color"

Postby Klear » Mon Dec 10, 2012 9:59 am UTC

Roses are Red
Violets are Violet
Knee deep in the dead
on ultra-violent

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Re: 1145: "Sky Color"

Postby Donk » Mon Dec 10, 2012 10:08 am UTC

AvatarIII wrote:
rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.


Roses are Red
Violets are Violet
I am your shoelace
and you are my eyelet.

yeah not hugely romantic


Well at least it's delightfully Freudian.

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Re: 1145: "Sky Color"

Postby Lawton » Mon Dec 10, 2012 10:22 am UTC

sehkzychic wrote:That's why the word "AMBULANCE" is written in reverse on the front of the eponymous vehicles.
Eutychus wrote:Surely I can't be the only one in this company who looks in my rear view mirror and instinctively thinks "what on earth is an ECNALUBMA"?
You aren't.

Also, the mirror question can be explained in simple terms. When you rotate a book to the mirror, you are the one who keeps up as up and down as down. If you flipped the book upside down then up and down would be flipped instead of left and right.

I almost forgot, left and right are special, as there is no possible way of defining them without left and right already being established. (just imagine trying to define "left" to someone over radio)

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Re: 1145: "Sky Color"

Postby honnza » Mon Dec 10, 2012 11:01 am UTC

Lawton wrote:I almost forgot, left and right are special, as there is no possible way of defining them without left and right already being established. (just imagine trying to define "left" to someone over radio)


There is. The weak interaction violates the parity symmetry. See http://en.wikipedia.org/wiki/Parity_(physics)#Parity_violation

So,
up = negative direction of the gravity force
front = an arbitrary direction defined by the center of the visual field.
left = the cross product of both, such that the coordinate system is chosen such that two specific interactions violate two of (CP symmetry, P symmetry, C symmetry) in predetermined ways.
Last edited by honnza on Mon Dec 10, 2012 11:03 am UTC, edited 1 time in total.


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