## 1145: "Sky Color"

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DougDean
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### Re: 1145: "Sky Color"

How do you get a mirror to reverse top and bottom? Just stand on one lying on the floor (or under one mounted on the ceiling).

Klear
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### Re: 1145: "Sky Color"

Lawton wrote:
honnza wrote: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.
I don't quite buy into this, but I'll admit I don't have a solid argument against it.

I find it odd that in a thread about a comic about explaining science to children, everyone is objecting to my simplistic explanation because it doesn't involve the Z-axis.

Okay, here's an even shorter explanation: Left and right are relative to the observer, but up and down aren't.

The asymmetry of parity is one of the more unsettling parts of physics, because it's extremely unintuitive. It's not as well known, but as strange as relativity, but it's even harder to prove to a layman, I believe.

BTW, I am such a layman. I have absolutely no idea how it is possible, I just know that it is.

score_under
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### Re: 1145: "Sky Color"

Icalasari wrote:
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*

Spoiler:
Read it again, as a couplet

DougDean
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### Re: 1145: "Sky Color"

"Why is blue light scattered but not violet?" For the same reason that things get white-hot rather than green-hot. While hot objects are radiating green they are also still radiating some red/orange and other colors, which combine to make white light. Only when things get really hot does the red end fall off, but by then you've passed right by green and get blue. Similarly, the sky scatters both blue and violet light, but the net result looks blue because it dominates, violet being near the edge of the visible spectrum and accordingly harder to see.

Archgeek
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### Re: 1145: "Sky Color"

Icalasari wrote:
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?

If I recall correctly, given enough exposure, retinal sunburn...where no lotion can reach.
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keithl
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### Re: 1145: "Sky Color"

finity wrote:... It's like blaming things on the Coriolis effect...

Wow! Mirrors reverse left to right the other way in the southern hemisphere???

DougDean
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### Re: 1145: "Sky Color"

Dryhad wrote: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.
Maybe we should nickname you "Lucy"

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### Re: 1145: "Sky Color"

For more on the color of the sky, there's an episode of the public radio show "Radiolab" which covered color perception. The "Colors" episode from May of 2012 had a relevant segment titled, "Why the Sky Isn't Blue?" I have not listened to it for some time, but it brings up the question of how people have/have not perceived the color blue over time.

Fire Brns
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### Re: 1145: "Sky Color"

Mirrors don't reverse images, they reflect. Cameras hooked up to flatscreen displays at Best Buy reverse images.
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aledonne
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### Re: 1145: "Sky Color"

When talking to kids, it's best to go with the simplest true answer.

Why is the sky blue? Because air, like water, is blue.

Then you show the kid a diving pool that has its walls painted white. The water is then quite obviously blue - it just takes a bunch of water to see that fact. Same with air. You don't see its blueness until you're seeing light through miles of it.

Rayleigh scattering. Pff.

obfpen
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### Re: 1145: "Sky Color"

Aelfyre wrote:I always thought Oxygen was faintly green? From all those deep space nebula photos where they enhance it so it is visible to the naked eye? And also the Aurora Borealis.. I have been told the green light comes from excited oxygen.

A different process is responsible for the colours of excited atoms and molecules. For most objects (in the non-excited, ground state), the colour seen is the colour of the light that that object doesn't absorb (reflect or transmit). But excited oxygen is different, because it's not reflecting/transmitting light, but actively luminescing, emitting a new photon as it returns to the ground state. And emitted colours are more like the absorbed ones (state changes are similar, but in opposite directions), so there's no reason to expect the same, reflected/transmitted colour. Actually, if the emission and absorption spectra were identical we could expect the emitted colour to be the opposite of the reflected colour.

cantab314 wrote: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.

JoeZ gives a nice answer to the black body question, but it should be further noted that we don't see by examining the whole spectrum in one sweep, making a 1:1 link between wavelengths and colours. We sample three overlapping regions with varying sensitivity, and then infer colour from those data.
Consequently, as far as perceived colour goes, the spectral peak for the incoming light is less important than the spectral peaks for the absorption spectra of the opsins in our eyes. So you could change the profile of the light and move the peak, yet retain the same perceived colour (this helps TVs work) or keep the peak in the same place but change the relative intensities at other wavelengths and produce a differently perceived colour.
Also, we're adapted to an environment bathed in light with essentially the same spectral profile as the sun, and have come to regard that as white light.
Also also, our perceptions are altered by context. Surrounded by the blue sky, we'd likely perceive the sun as at least a little orange-yellow even if it were actually white.

Ubertakter
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### Re: 1145: "Sky Color"

Here's the best explanation I've found.
http://www.patarnott.com/atms749/pdf/blueSkyHumanResponse.pdf

A model of human eye color detection was built utilizing humans' response to various wavelengths of radiation (in the visible spectrum). Various experiments, as well as a "Rayleigh sky" and actual sunlight intensity measurements were used to show why the average person would classify the sky color as blue.

In short, our eyes aren't very sensitive to very short wavelengths (id est, violet colors) and a combination of inputs of various wavelengths and intensities work together so the average person will see the sky as blue.

Edit: I also meant to mention that sky color also depends on where the light source (the sun) is and how close to it you are looking.
http://en.wikipedia.org/wiki/Rayleigh_scattering
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cooperph
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### Re: 1145: "Sky Color"

Mirrors appear to reverse left and right because your reflection is facing the opposite way to you, and while left and right are designations that depend on which way you are facing, up and down do not.

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### Re: 1145: "Sky Color"

Might as well ask them why, if they put an imprint of their hand in the mud, the vertical axis isn't backwards.
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Flumble
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### Re: 1145: "Sky Color"

Dryhad wrote: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.

Good ol' Doppler - explains it so easily. No need for complex 'scattering' theories.

score_under wrote:
Spoiler:
Read it again, as a couplet

Really, it took me 2 replies to get the joke?

Minibear Rex
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### Re: 1145: "Sky Color"

Why do you think the mirror is inverting the y-axis? When you look at yourself in a mirror, if you have a watch on your left hand, the mirror will show you what appears to be a person wearing a watch on their right hand. But what happens if you lie down on your side facing a mirror? It still looks like the person in the mirror has a watch on the right hand. The mirror doesn't treat the two axes any differently; you just interpret the image you see in a peculiar way because the human body has (approximately) symmetry across the y-axis.

DougDean
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### Re: 1145: "Sky Color"

WIMP wrote: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.
The T-1000 in Terminator 2 actually does this, late in the movie. If you blink you'll miss it. Of course it helps if you're composed of a metal alloy which is at the triple-point at Earth temperature and pressure, enabling you to switch from solid to liquid and back at will.

ike
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### Re: 1145: "Sky Color"

cyberneticentomologist wrote:And there you have it. That's why the sky isn't purple.

did you know that there is no single frequency/wavelength of light coorisponding to the color purple?

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### Re: 1145: "Sky Color"

DougDean wrote:...Of course it helps if you're composed of a metal alloy which is at the triple-point at Earth temperature and pressure, enabling you to switch from solid to liquid and back at will.

triple-point would be pretty problematic and movie-breaking, it's not a vampire after all. You meant the solid-liquid phase boundary.

bzakharin
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### Re: 1145: "Sky Color"

UnstableMongoose
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### Re: 1145: "Sky Color"

Question, extremely hypothetical:

If our eyes were on a vertical axis rather than a horizontal one, how does everyone think it would change our perception of the mirror problem?

Edgar
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### Re: 1145: "Sky Color"

During one of my undergrad Fields and Waves classes, I asked the professor the following question: "If an electron radiates any time it is accelerated, and anything that travels in a circle is under constant acceleration, then why doesn't a DC current loop radiate?"

The professor handed me the chalk (still used chalk back then) and said, "There is a tradition that any time a student asks a question that the professor cannot answer, the professor must allow the student to take over the lecture."

xtifr
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### Re: 1145: "Sky Color"

"Raleigh scattering" is still a horsecrap answer. "Why is my shirt blue?" "Well, you see, certain chemicals interact with photons of different frequencies, because of the sizes of the gaps between the atoms. Longer freqencies don't resonate with the atoms as well, and are more likely to be reflected instead of absorbed." "Then why is your shirt red?"

Technically, all that stuff about scattering is true, but it completely misses the point, and ignores the fact that a nitrogen atmosphere would not be blue.

Cyberneticentymologist came close, saying:
"the answer is that there is a sudden drop in actual scattering (reading from long to short wavelengths) at about 450 nm, a slight rise at about 410 nm, then a very steep dropoff to almost nothing at 400 nm

But why is there a sudden drop at those frequencies? Answer: because oxygen is blue! Other substances have different drop-offs, or none at all. Just as the dyes in my shirt have drop-offs in reflectiveness at certain frequencies. So you might as well short-circuit the whole thing (remember, we're talking to kids here) and simply say that oxygen is blue! Hold off on the explanations of scattering and absorbtion until the kid asks "why do things have colors?"

Reference: http://amasci.com/miscon/miscon4.html#blue
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wolfgang.rupprecht
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### Re: 1145: "Sky Color"

Mirrors don't reverse things. You reverse things to make them face the mirror. You can either flip the page horizontally to face the mirror, in which case it will appear flipped left-right, or you can flip the page vertically and the page appears flipped vertically in the mirror. No mystery.

For completeness, if you write something on a clear sheet of plastic so that the writing is visible in the mirror though the back and you don't need to flip it then the reflection isn't flipped vertically or horizontally.

Viltris
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### Re: 1145: "Sky Color"

When I was in Vegas, the sky was gray. And not because it was cloudy in Vegas. It was a bright summer August afternoon, and the sky was gray. So I'm convinced that Rayleigh scattering is either wrong or is an incomplete answer. (Of course, in the outdoor photos I took, the sky came out blue, so it might just be a human perception thing.)

Regarding the mirrors thing, I got asked that by my high school physics teacher while we were studying optics, and I drew a diagram and realized that mirrors don't flip left-to-right, they flip front-to-back. Our brain is looking at the image from "behind" and simply misinterpreting it as being flipped left-to-right. (If you don't believe me, draw a diagram.) A better question is why our brains perceive the image as being flipped left to right rather than top-to-bottom. That's more a neuro-science question than a physics/optics question, and prolly has to do with how our brain perceives symmetry or something.

paulrowe
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### Re: 1145: "Sky Color"

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.

I was surprised not to see this quote (or something similar) already posted:
HHGTTG wrote:Vogon poetry is of course the third worst in the Universe. The second worst is that of the Azgoths of Kria. During a recitation by their Poet Master Grunthos the Flatulent of his poem "Ode to a Small Lump of Green Putty I Found in My Armpit One Midsummer Morning" four of his audience members died of internal hemorrhaging, and the President of the Mid-Galactic Arts Nobbling Council, survived by gnawing one of his own legs off. Grunthos is reported to have been "disappointed" by the poem's reception, and was about to embark on a reading of his twelve-book epic entitled My Favorite Bathtime Gurgles when his own major intestine, in a desperate attempt to save life and civilization, leaped straight up through his neck and throttled his brain. The very worst poetry of all perished along with its creator, Paula Nancy Millstone Jennings of Greenbridge, Essex, England, in the destruction of the planet Earth.

Klear
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### Re: 1145: "Sky Color"

UnstableMongoose wrote:Question, extremely hypothetical:

If our eyes were on a vertical axis rather than a horizontal one, how does everyone think it would change our perception of the mirror problem?

It wouldn't change. The axis on which we have eyes have nothing to do with it, as has been explained several times before in this thread. To be more precise, if we had eyes one above the other, we would still consider up and down special, because of gravity.

Drowsy Turtle
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### Re: 1145: "Sky Color"

An example of why the mirror problem isn't a problem:

Try writing on tracing paper (or write on normal paper so heavily that you can see it from the back) and holding it up to a mirror.

Notice anything?

The view through the paper is the same as the view in the mirror - so how exactly is the x axis inverted?
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baf
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### Re: 1145: "Sky Color"

This is the first time I've seen the mirror writing thing connected to Feynman, but I've seen it described as inspiration for Lewis Carroll.

The answer is that the mirror of course doesn't treat left-right any different from up-down, but we do, in ways that we're not aware of when we do them.
Stand in front of a mirror, holding a book in front of you, with the text facing you so that you can read it.
Now turn the book so that it's facing the mirror.
In order to do this, you had to rotate the book 180 degrees. The important question, then, is: what axis did you rotate it around?
If you chose a vertical axis, the text in the mirror will be reversed left-right. If you chose a horizontal axis, it will be reversed top-bottom.
But you almost certainly chose a vertical axis, because that's what's natural for us as creatures living in a gravity field that keeps us oriented vertically.

Now that I see this has already been argued a fair bit, and what the replies have been like, I think it's worth making an additional point:
It isn't really about the act of rotating an object. It's about what orientation we'll accept.
Suppose you don't start with the book facing you. Suppose you just pick up the book and hold it facing the mirror, and observe the results.
If the text is reversed top-bottom, you're likely to just think "Oh, that's just because the book is upside down". You might even flip it in order to "correct" your "mistake", because that's what you do when you look at a book and see upside-down text.
But if the text is reversed left-right, it affords no such easy explanation and remedy. We look at it and conclude that the mirror has done something weird.
But it has done nothing weirder than it did when you held the book upside-down. The difference is only in your reaction. Holding the book upside-down is wrong? Sure. But consider this: If you look (not using a mirror) at a book that's been turned upside-down, that book is also reversed left-right. It's just rotated 180 degrees from what you expect, and that rotation affects both top-bottom and left-right orientation. But looking at it in the mirror lets you see the side facing away from you, and thus lets you replace one of these reversals with a front-back reversal. If you see the book as still wrongly-oriented if it's reversed front-back and top-bottom, but see the mirror as doing something strange when it's reversed front-back and left-right, that distinction is in you, not in the mirror.
Last edited by baf on Mon Dec 10, 2012 11:58 pm UTC, edited 1 time in total.

rmsgrey
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### Re: 1145: "Sky Color"

I like to describe what a mirror does as reversing "near" and "far"

JetstreamGW
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### Re: 1145: "Sky Color"

Harry Voyager wrote:
mrb4 wrote: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.

A fun demonstration:

I really, really wish they'd have done one guy and then the other. It was SUPER annoying that they keep bouncing between them. I want to watch one guy talk, do his experiment, then switch to the other. Why the cuts?

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### Re: 1145: "Sky Color"

Code: Select all

`Technically, all that stuff about scattering is true, but it completely misses the point, and ignores the fact that a nitrogen atmosphere would not be blue...  But why is there a sudden drop at those frequencies? Answer: because oxygen is blue!`

You are aware, of course, that the atmosphere has almost four times as much nitrogen as oxygen. Also, quite obviously, when the sunlight passes through the atmosphere at a lower angle, such as at sunrise or sunset, it passes through even more oxygen, so the resulting color should then be even more blue, not orange, if the color of the oxygen were the primary factor responsible for the sky's blue appearance.

Moonfish
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### Re: 1145: "Sky Color"

The sky isn't always blue, sometimes it's black with white speckles.
Last edited by Moonfish on Tue Dec 18, 2012 7:28 am UTC, edited 1 time in total.

justusranvier
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### Re: 1145: "Sky Color"

Edgar wrote:During one of my undergrad Fields and Waves classes, I asked the professor the following question: "If an electron radiates any time it is accelerated, and anything that travels in a circle is under constant acceleration, then why doesn't a DC current loop radiate?"
Electron drift velocity is much slower than you think it is. The electrons in the wire are under constant acceleration but it's so small that you can't detect the radiation.

Jorpho
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### Re: 1145: "Sky Color"

mrb4 wrote: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.)
The next obvious question then is, how can the sky be blue if the atmosphere is only 21% oxygen?

xtifr wrote:Technically, all that stuff about scattering is true, but it completely misses the point, and ignores the fact that a nitrogen atmosphere would not be blue.
How so? Rayleigh scattering just depends on the presence of particulate matter, does it not? Even http://amasci.com/miscon/miscon4.html#blue says as much and places no particular emphasis on the presence of oxygen. As I said earlier, skim milk will appear blue under the right conditions, and veins also appear blue due to scattering through the skin.

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### Re: 1145: "Sky Color"

Donk wrote:
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
and you are my eyelet.

yeah not hugely romantic

Well at least it's delightfully Freudian.

That poem is delightful. Yes. It is Freudian. That is more in the head of the guy that noticed it than in the poets head.
Yet; Now that we know; I can't unsee it. Funny stuff.

I'll make a try at The Sky is Blue.

I had to do it, once.
THINKING!

The light comes in full flavored. White light. As it interacts with the chemicals in the atmosphere the light is broken and some is absorbed.

There is more; My one liner was,
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We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
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WIMP
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### Re: 1145: "Sky Color"

Klear wrote:The asymmetry of parity is one of the more unsettling parts of physics, because it's extremely unintuitive. It's not as well known, but as strange as relativity, but it's even harder to prove to a layman, I believe.

BTW, I am such a layman. I have absolutely no idea how it is possible, I just know that it is.

Consider a massless particle. It is impossible to get into that particle's rest frame because it travels at the speed of light. Now imagine that the particle is spin-1/2, so that it has exactly 2 spin states. Either it's spinning so that the spin vector points more along its direction of travel than back, or more in the reverse than forward. The first is called right handed, the second is called left handed.

Now remember that the particle is massless. All observers agree on the relative orientation of its spin and velocity, because no one can go so fast that the particle seems to be going in the opposite direction--you'd have to be going faster than c. So a left handed massless particle is left handed in every reference frame, and a right handed massless particle is right handed in every reference frame. At this point, we have something like electric charge. A positive particle is positive in every frame, and a negative particle is negative in every frame. So, left and right handedness are like charges (for massless particles): a particle with left hand "charge" is just a *completely different particle* from one with right hand "charge".

In the Standard Model, for technical reasons, all particles start out massless. There is a left handed massless electron, and a right handed massless electron, and they are as different from each other as the regular electron is from the positron. There's nothing more special about electric charge than "handedness charge" for massless particles. It turns out that the left handed electron feels the weak force and the right handed electron does not. Why? Because the left handed electron has a counterpart under the weak force called the neutrino, but the right handed electron does not. Why is there no right handed neutrino (or at least, why is the right handed neutrino so different from the left that we haven't detected it)? Good question. No one knows the answer for sure, although there are many possibilities.

It turns out that the Higgs ties the left handed and right handed electrons (and other fermions) together, so that the resulting state we call "the electron" has mass and can be either left handed or right handed. However, this doesn't remove the fact that the weak force only interacts with the underlying left handed piece and ignores the rest. Thus, parity violation.

CP violation is way more complicated and much more subtle.

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### Re: 1145: "Sky Color"

I can't compete with knowing what spin electrons have. I am pleased to know they are.
I ran out of room.
My one liner was, The sky is not blue. It is vibrating red. The blue is what gets through.

The red skies at night has to do with more atmosphere between the observer and the light source.

The way water absorbs red is different from the way smoke and other airborne crap absorbs blue.
ok. That was my guess. I got it from some books. Oh, and from watching sunsets with Science guys.

There is a city hundreds of miles away. I was told the air from the city makes pretty sunsets.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

rockachu2
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### Re: 1145: "Sky Color"

the answer to the mirror question is that we mentally measure horizontal direction in polar, so by looking in the mirror we alter our viewpoint so our right eye sees what a persons left eye woukd see if they faced the mirors direction. so depending on what direction into the mirror we face, our eyes are plac differently and we percieve being in different orientations, some quite awesome.

Zanthra
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### Re: 1145: "Sky Color"

Speaking of the second question, I like to imagine that if someone grew up their entire life in a zero gravity environment where you have a left hand wall and a right hand wall, and your left hand always stays on the left wall, and right hand always stays on the right wall, looking in a mirror would look flipped on the vertical axis.