1145: "Sky Color"

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rmsgrey
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Re: 1145: "Sky Color"

Postby rmsgrey » Wed Dec 12, 2012 5:41 pm UTC

orthogon wrote:
addams wrote:I blame Alice in Wonderland.
Would you step through the mirror?


Probably not. If I remember my A-level chemistry correctly, there would be serious risks: lots of food would be indigestible because it contained the wrong handedness of optical isomer, and I fear (though less sure about this one) that I would have no immunity to the mirror-image viruses and bacteria for similar reasons.

I never read Alice through the Looking Glass, and people don't talk about it as much as Alice in Wonderland. Presumably it was the usual substandard-sequel story?


I wouldn't worry about the mirror-image molecules - the mirror-image particles would finish you off before you could be poisoned...

One reason people talk more about Wonderland than Looking Glass is that the latter is later, and once you've talked someone into reading Wonderland, they'll know whether or not they're likely to enjoy Looking Glass. Aside from a slightly darker mood, and a theme of chess and mirrors rather than playing cards and animals, there's not a lot to choose between them.

Another reason people talk less about Looking Glass than Wonderland is that the movie versions, while some do draw upon Looking Glass, title themselves after Wonderland.

Similarly, people refer to elements from the Alice books as being from Wonderland without much concern for which book they originated in - Humpty-Dumpty, TweedleDum and TweedleDee, the Jabberwocky, and the Walrus and the Carpenter are all from Looking Glass.

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Re: 1145: "Sky Color"

Postby standingwave » Wed Dec 12, 2012 7:59 pm UTC

Professor of Physics Emeritus Walter Lewin (MIT) demonstrating Rayleigh scattering:

http://youtu.be/7vYOUPpUw50?t=27m50s

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Re: 1145: "Sky Color"

Postby Pfhorrest » Wed Dec 12, 2012 9:51 pm UTC

Jorpho wrote:You propose that because overstimulated photoreceptor cells interpret a lack of signal as a color, that under normal conditions the interpretation of a color inherently requires the lack of a signal?

All else aside, that is kind of true. The response of a photoreceptor to light is to decrease the strength of the signal it would otherwise be sending. Basically, the eyes are constantly sending a "there's no light [of this frequency] here" message, and when that message isn't received, the brain infers the reception of light. Quoth the wiki:

Wikipedia wrote:The rod and cone photoreceptors signal their absorption of photons via a decrease in the release of the neurotransmitter glutamate to bipolar cells at its axon terminal. Since the photoreceptor is depolarized in the dark, a high amount of glutamate is being released to bipolar cells in the dark. Absorption of a photon will hyperpolarize the photoreceptor and therefore result in the release of less glutamate at the presynaptic terminal to the bipolar cell.


The overstimulation happens because it takes a bit for the photoreceptors to return to their normal polarization and start sending normal amounts of neutrotransmitters again.
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Re: 1145: "Sky Color"

Postby stianhat » Wed Dec 12, 2012 11:08 pm UTC

Jorpho wrote:You propose that because overstimulated photoreceptor cells interpret a lack of signal as a color, that under normal conditions the interpretation of a color inherently requires the lack of a signal?


Well, not "inherently requires" but lack of a part of the sprectrum will be interpreted as the same as a monochromatic source of the reciprocal light. To a certain extent of course, but assuming normal light conditions, sure.The human brain is amazing at making this and other assumptions to intepret the visual stimuli it recieves.

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Re: 1145: "Sky Color"

Postby addams » Thu Dec 13, 2012 2:22 am UTC

Thank you. I am glad to know I am not the only person infected with the Alice meme.
I got mine from the movie. What a great up beat movie.
I am still thinking about the mirror.
I dimmly remember some of this from old photograhy books.
Yes. I used two mirrors to read a book.
Looked at my upside down self in a concave spoon.
And; I still don't know anything.

No longer avoiding mirrors.
Step through?
Yes. If you do I will.
You first.
-
rmsgrey wrote:
orthogon wrote:
addams wrote:I blame Alice in Wonderland.
Would you step through the mirror?


Probably not. If I remember my A-level chemistry correctly, there would be serious risks: lots of food would be indigestible because it contained the wrong handedness of optical isomer, and I fear (though less sure about this one) that I would have no immunity to the mirror-image viruses and bacteria for similar reasons.

I never read Alice through the Looking Glass, and people don't talk about it as much as Alice in Wonderland. Presumably it was the usual substandard-sequel story?


I wouldn't worry about the mirror-image molecules - the mirror-image particles would finish you off before you could be poisoned...

One reason people talk more about Wonderland than Looking Glass is that the latter is later, and once you've talked someone into reading Wonderland, they'll know whether or not they're likely to enjoy Looking Glass. Aside from a slightly darker mood, and a theme of chess and mirrors rather than playing cards and animals, there's not a lot to choose between them.

Another reason people talk less about Looking Glass than Wonderland is that the movie versions, while some do draw upon Looking Glass, title themselves after Wonderland.

Similarly, people refer to elements from the Alice books as being from Wonderland without much concern for which book they originated in - Humpty-Dumpty, TweedleDum and TweedleDee, the Jabberwocky, and the Walrus and the Carpenter are all from Looking Glass.
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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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Those that do not Know; Don't tell them.
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Re: 1145: "Sky Color"

Postby Jorpho » Thu Dec 13, 2012 2:38 am UTC

stianhat wrote:The sky is blue because of the colors it *lacks* not the colors it has - we percieve chlorophyll filled leaves as green because they lack the red light and correspondingly, as someone was close to earlier, we see the sky as blue because it lacks red light, but also has violet light ( violet = not yellow ) (not red = green). the resultant mix of (not yellow) + green = blue. The blue light that is already there just adds to this.
It occurred to me why this troubles me: given that the cones in the human eye are red/green/blue, shouldn't the opposite of violet and yellow be green and blue, respectively?

Image

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Re: 1145: "Sky Color"

Postby addams » Thu Dec 13, 2012 2:53 am UTC

Yes. Yes.
This stuff!
This is why humans lose color vision in the glooming. Dim light does not release enough transmiter. Right?
Cats see fine in the glooming.
Pfhorrest wrote:
Jorpho wrote:You propose that because overstimulated photoreceptor cells interpret a lack of signal as a color, that under normal conditions the interpretation of a color inherently requires the lack of a signal?

All else aside, that is kind of true. The response of a photoreceptor to light is to decrease the strength of the signal it would otherwise be sending. Basically, the eyes are constantly sending a "there's no light [of this frequency] here" message, and when that message isn't received, the brain infers the reception of light. Quoth the wiki:

Wikipedia wrote:The rod and cone photoreceptors signal their absorption of photons via a decrease in the release of the neurotransmitter glutamate to bipolar cells at its axon terminal. Since the photoreceptor is depolarized in the dark, a high amount of glutamate is being released to bipolar cells in the dark. Absorption of a photon will hyperpolarize the photoreceptor and therefore result in the release of less glutamate at the presynaptic terminal to the bipolar cell.


The overstimulation happens because it takes a bit for the photoreceptors to return to their normal polarization and start sending normal amounts of neutrotransmitters again.
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.

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Re: 1145: "Sky Color"

Postby Pfhorrest » Thu Dec 13, 2012 3:05 am UTC

Jorpho wrote:
stianhat wrote:The sky is blue because of the colors it *lacks* not the colors it has - we percieve chlorophyll filled leaves as green because they lack the red light and correspondingly, as someone was close to earlier, we see the sky as blue because it lacks red light, but also has violet light ( violet = not yellow ) (not red = green). the resultant mix of (not yellow) + green = blue. The blue light that is already there just adds to this.
It occurred to me why this troubles me: given that the cones in the human eye are red/green/blue, shouldn't the opposite of violet and yellow be green and blue, respectively?

Image

Unfortunately the physiology of color perception isn't consistent between the cones in the eyes and the nerves which carry those signals to the brain. The signals our brain receives aren't "how much red", "how much green", and "how much blue", but "how much more red than green, or vice-versa", and "how much more blue than red or green, or vice-versa". Quoth the wiki again:

Wikipedia wrote:Besides the cones, which detect light entering the eye, the biological basis of the opponent theory involves two other types of cells: bipolar cells, and ganglion cells. Information from the cones is passed to the bipolar cells in the retina, which may be the cells in the opponent process that transform the information from cones. The information is then passed to ganglion cells, of which there are two major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers. Parvocellular cells, or P cells, handle the majority of information about color, and fall into two groups: one that processes information about differences between firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones. The first subtype of cells are responsible for processing red–green differences, and the second process blue–yellow differences. P cells also transmit information about intensity of light (how much of it there is) due to their receptive fields.


(S cones are the ones that detect short-wavelength blue light, M cones detect medium-wavelength green light, and L cones detect long-wavelength red light)

So instead of a color wheel with red, green, and blue each 120 degrees apart from each other, our mental model of color is more of a color square with red at the top, green at the bottom, and blue on the right, with the yellows (between red and green) stretched out across the left side, and everything from cyan to magenta squished together on the "blue" side. (Flip and rotate this square as you like, directions arbitrarily chosen). In essence, our brains think in four primary colors: red, green, blue, and not-blue aka yellow.

All that aside, violet is not equivalent to magenta or any of the purples (colors on a color wheel between red and blue): violet is basically "ultrablue", a blue so blue that the M cones aren't getting any of the stimulation they get at the frequency where S cone response peaks (which is why violet appears dark, it's hardly stimulating any of the cones, but the S cones gets most of what little there is). We think of violet as like a purple because, sadly, the L cones pick up their sensitivity again down at those short wavelengths, so this "ultrablue" seems, perversely, a little bit reddish to us.



On a highly tangential note: it bothers me on an aesthetic level that our color vision is such a messy hodge-podge. In my fictional universe (see sig), I have an alien species which sees colors on a continuous spectrum of frequencies, and colors which go well together are ones with harmonic frequencies, much like our human hearing perception finds that sounds with harmonic frequencies sound good together. If only human vision were so elegant...
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Re: 1145: "Sky Color"

Postby Jorpho » Thu Dec 13, 2012 4:23 am UTC

Pfhorrest wrote:
It occurred to me why this troubles me: given that the cones in the human eye are red/green/blue, shouldn't the opposite of violet and yellow be green and blue, respectively?
Unfortunately the physiology of color perception isn't consistent between the cones in the eyes and the nerves which carry those signals to the brain. The signals our brain receives aren't "how much red", "how much green", and "how much blue", but "how much more red than green, or vice-versa", and "how much more blue than red or green, or vice-versa".
Of course, nothing is ever simple. But the general relation still holds, does it not?

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Re: 1145: "Sky Color"

Postby Pfhorrest » Thu Dec 13, 2012 4:47 am UTC

Jorpho wrote:
Pfhorrest wrote:
It occurred to me why this troubles me: given that the cones in the human eye are red/green/blue, shouldn't the opposite of violet and yellow be green and blue, respectively?
Unfortunately the physiology of color perception isn't consistent between the cones in the eyes and the nerves which carry those signals to the brain. The signals our brain receives aren't "how much red", "how much green", and "how much blue", but "how much more red than green, or vice-versa", and "how much more blue than red or green, or vice-versa".
Of course, nothing is ever simple. But the general relation still holds, does it not?

The opposite of yellow is blue, yes; and violet is essentially "ultrablue", so its opposite is still yellow. Violet is not magenta, so its opposite is not green, and due to the skewing of the color wheel caused by the opponent process we perceive colors by (blues squish together and yellows spread out), it's contestable to even say green is the opposite of magenta, since it can well be said that green is the opposite of red, as far as our final neurological response to light goes. (In mixing light or pigment, yes, green is the opposite of magenta and not red). There is, confusingly enough, an RGB color named "violet" as well (three parts blue to one part red, at hue angle 270, directly between true blue and magenta), and its opposite is the yellow-green called "chartreuse" (three parts green to one part red, at hue angle 90, directly between yellow and green).

For aesthetic purposes I like to rethink the opponent process in terms of the RGB color wheel with a "warm-cool" axis centered on orange (hue 30) and azure (hue 210), and a "green-purple" axis centered on green (hue 120) and magenta (hue 300). The top of the square thus has all the purples, everything from rose (hue 330) to "violet" (hue 270); the right side has all the blues from "true blue" (hue 240) to cyan (hue 180); the bottom has all the greens from spring green (hue 150) to chartreuse (hue 90); and the left side has all the oranges from yellow (hue 60) to red (hue 0). But I think that's just my own creation and doesn't have much scientific basis. You can see a chart I made a while back that's kind of similar to this (a diamond instead of a square, and it's upside-down compared to this description) here. (Incidentally, that little section of my site is a great way to see what your eyes think is the opposite of a color: click a color, stare at your screen for a minute, then click back to the white square in the middle. You will now see the white screen as whatever your eyes consider the opposite of the color you were just looking at).
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Re: 1145: "Sky Color"

Postby Dryhad » Thu Dec 13, 2012 10:01 am UTC

This question of why violet appears purple has long interested me and I have heard the "L cones have a second peak in the violets" explanation, but with the greatest of respect I would like to call [citation needed] because at least from what I've previously found there doesn't seem to be much of a consensus. It also raises a couple of questions for me: first, that graph shows a similar rise in M cone response in the same place so why doesn't violet appear cyan? Second, doesn't this explanation conflict with the characterization of violet as "not yellow" since the L cone's other peak is in the yellows?

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Re: 1145: "Sky Color"

Postby stianhat » Thu Dec 13, 2012 1:48 pm UTC

Pfhorrest wrote:On a highly tangential note: it bothers me on an aesthetic level that our color vision is such a messy hodge-podge. In my fictional universe (see sig), I have an alien species which sees colors on a continuous spectrum of frequencies, and colors which go well together are ones with harmonic frequencies, much like our human hearing perception finds that sounds with harmonic frequencies sound good together. If only human vision were so elegant...


And the "color spectrum" they see range over several, ehm, "octaves" then? human vision barely spans a single "octave" of light frequencies and thus the harmonic color thing really isnt that feasible for us measly homo sapiens since the deepest red is less than 2x the "brightest" violet we percieve. But assuming a several octave view of the spectrum and, assuming we do as with sound, several "blues" and several "reds" that would indeed be pretty interesting. Say, if the vision extended into the infra red range, then an object with a certain color could be harmonic with being at a certain temperature. A cup of coffee would go from golden to barf-colored depending on temperature? Would have saved me from many disgusting moments. I hate lukewarm coffee =/

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Re: 1145: "Sky Color"

Postby Klear » Thu Dec 13, 2012 1:56 pm UTC

In any case, the reason you cannot enter the mirror is because your mirror self is in the way. Thankfully, you are able to prevent him from entering our world as well, which is a good thing, since it's been proven that all mirror counterparts are evil, moustache or no moustache.

In any case, I propose that any successful attempt at entering mirrors has to begin with a ‮murder

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Re: 1145: "Sky Color"

Postby orthogon » Thu Dec 13, 2012 3:36 pm UTC

rmsgrey wrote:I wouldn't worry about the mirror-image molecules - the mirror-image particles would finish you off before you could be poisoned...

Why? Please tell me more!

Thanks for all the Alice background. I've downloaded Through the Looking Glass onto my Kindle, so I can see what it's like for myself. Interestingly, early on Alice says
Perhaps looking-glass milk isn't good to drink.
Apparently Carroll had opto-isometry in mind too.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1145: "Sky Color"

Postby rmsgrey » Thu Dec 13, 2012 4:10 pm UTC

orthogon wrote:
rmsgrey wrote:I wouldn't worry about the mirror-image molecules - the mirror-image particles would finish you off before you could be poisoned...

Why? Please tell me more!


CPT-reflected particles (mirror-image particles with reversed charge, moving the opposite way in time) are better known as antimatter.

According to a quick check on Wikipedia, simple mirror-reflection is no longer thought to produce antimatter, but rather matter that barely interacts with normal matter at all - in which case Alice would have found a mirror world less solid than smoke...

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Re: 1145: "Sky Color"

Postby Klear » Thu Dec 13, 2012 4:41 pm UTC

rmsgrey wrote:According to a quick check on Wikipedia, simple mirror-reflection is no longer thought to produce antimatter, but rather matter that barely interacts with normal matter at all - in which case Alice would have found a mirror world less solid than smoke...


This sounds interesting... please elaborate, if you can:
- Would this mirror world be visible? (that is, would photos interact with it?)
- Would gravity affect Alice there, thus making her disappear in the middle of the mirror-earth? Or would she levitate behind the mirror, possibly slowly drifting away from it?
- Would this phantom world basically be vacuum to Alice?
- Could she survive there in a spacesuit?

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Re: 1145: "Sky Color"

Postby Pfhorrest » Thu Dec 13, 2012 7:54 pm UTC

stianhat wrote:
Pfhorrest wrote:On a highly tangential note: it bothers me on an aesthetic level that our color vision is such a messy hodge-podge. In my fictional universe (see sig), I have an alien species which sees colors on a continuous spectrum of frequencies, and colors which go well together are ones with harmonic frequencies, much like our human hearing perception finds that sounds with harmonic frequencies sound good together. If only human vision were so elegant...


And the "color spectrum" they see range over several, ehm, "octaves" then? human vision barely spans a single "octave" of light frequencies and thus the harmonic color thing really isnt that feasible for us measly homo sapiens since the deepest red is less than 2x the "brightest" violet we percieve. But assuming a several octave view of the spectrum and, assuming we do as with sound, several "blues" and several "reds" that would indeed be pretty interesting. Say, if the vision extended into the infra red range, then an object with a certain color could be harmonic with being at a certain temperature. A cup of coffee would go from golden to barf-colored depending on temperature? Would have saved me from many disgusting moments. I hate lukewarm coffee =/

I do intend their color vision to be a bit broader than ours, including near-IR and near-UV, but yeah, I am aware of the problems of having such a limited range of frequencies and still having harmonics in them. My solution to this is to say that they don't directly perceive the frequency of light, but rather a difference in frequency from some central "carrier frequency". So if that carrier frequency was 500Thz, then they would see 510Thz, 520Thz, 540Thz, 580Thz, etc, as well as 490Thz, 480Thz, 460Thz, 420Thz, etc, as harmonics of each other.

You know, I hadn't thought about the possibility of combining color harmonics with sound harmonics with them; maybe they associate certain pitches with certain colors. Hmm...

rmsgrey wrote:According to a quick check on Wikipedia, simple mirror-reflection is no longer thought to produce antimatter, but rather matter that barely interacts with normal matter at all

Is this a plausible candidate for dark matter? Weakly interacting, yet still (presumably) massive?
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Re: 1145: "Sky Color"

Postby Zanthra » Thu Dec 13, 2012 9:23 pm UTC

Speaking of mirrors, does anyone else find it really annoying that Apple feels the iPhone (and all their other user facing cameras) should locally display their image with the horisontal axis flipped? When you use FaceTime, if you raise your left hand, the display that shows what is going to be sent to the other person shows what looks like your right hand being raised (as in a mirror) however the person on the other end sees it correctly. It's even more annoying as there is no option to turn it off!

Can someone with an Android and a user facing camera try this: Go to the camera app, and set the camera to the front facing camera, then raise your left hand. in the photo preview does it show what you would see in a mirror? Next actually take the photo, then go to the photos you took and see if it restored the horizontal axis to the CCDs axis? If they don't do this mirror image stuff, I might just get an android for my next phone.

Also speaking of fictional species, I one thought of one that saw color more or less like we do, but also saw polarization.

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Re: 1145: "Sky Color"

Postby makc » Thu Dec 13, 2012 10:14 pm UTC

SEE wrote:The trick on the mirror one is that there is no such reversal.
Ye, the mirror actually reverses front to back.

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Re: 1145: "Sky Color"

Postby zxcv » Thu Dec 13, 2012 10:58 pm UTC

makc wrote:
SEE wrote:The trick on the mirror one is that there is no such reversal.
Ye, the mirror actually reverses front to back.

well, you just have to put you mirror on the floor

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Re: 1145: "Sky Color"

Postby Jorpho » Fri Dec 14, 2012 3:39 am UTC

orthogon wrote:Thanks for all the Alice background. I've downloaded Through the Looking Glass onto my Kindle, so I can see what it's like for myself.
I hear Martin Gardner's "Annotated" editions are the way to go.

Zanthra wrote:Also speaking of fictional species, I one thought of one that saw color more or less like we do, but also saw polarization.
I think someone mentioned the mantis shrimp already?
http://en.wikipedia.org/wiki/Mantis_shrimp#Eyes

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Re: 1145: "Sky Color"

Postby sehkzychic » Fri Dec 14, 2012 5:02 am UTC

Spoiler:
cyberneticentomologist wrote:Many years ago, my mother called me up to say hello while I was in the shower, so my roommate answered the phone. In order to kill time, he asked my mom "Why is the sky blue", completely not expecting the half-hour science lecture that followed. So that's been kind of an inside joke for us ever since. Naturally, this comic was immediately forwarded to my mother, who sent the following reply, because my mom is awesome like that.

"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 – down to less than the scattering in the yellow and red wavelengths. Interestingly, the magnitude of scattering depends on the angle between the sun and the look direction, and there is much more violet at 90 degrees, which is why I suppose you get a more violet tinge to the sky at the zenith near sunrise and sunset, or near the horizon at noon. The human eye’s blue cones are maximally sensitive at 445 nm (corresponding to max blue scatter), still substantial at 400, but virtually 0 at 350. So, the sharp dropoff I will take as a function of atmospheric composition and the specific molecular sizes present, and that’s why the sky isn’t violet – not that we couldn’t see it if it were!"

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

Why does my mother know this off the top of her head? She's a professor of graduate-level remote sensing, and gets to play with lasers and satellites because she has an even nerdier job than my boring IT existence. Nerd runs deep in my genes.


Yay, expert answers! Now just turn your mom loose on the mirror problem and we can close the thread, lol. :D

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Re: 1145: "Sky Color"

Postby Edgar » Fri Dec 14, 2012 3:50 pm UTC

justusranvier wrote:Electron drift velocity is much slower than you think it is.

Um ... no. I am aware of how slow drift velocity really is.

The electrons in the wire are under constant acceleration but it's so small that you can't detect the radiation.

So your answer is, "It does radiate." That is correct.

That "you can't detect the radiation" is incorrect.

The whole answer is, to the best of my reckoning, "It radiates, at DC. Constant current in the loop induces a constant magnetic field. That constant magnetic field is the physical embodiment of the non-varying (0 Hz) electromagnetic radiation."

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Re: 1145: "Sky Color"

Postby rmsgrey » Fri Dec 14, 2012 6:50 pm UTC

Pfhorrest wrote:
rmsgrey wrote:According to a quick check on Wikipedia, simple mirror-reflection is no longer thought to produce antimatter, but rather matter that barely interacts with normal matter at all

Is this a plausible candidate for dark matter? Weakly interacting, yet still (presumably) massive?


It's a candidate. Plausibility is way beyond my paygrade... Apparently it explains certain observed phenomena, and makes testable predictions about other phenomena, so it's not unreasonable. Wikipedia page: http://en.wikipedia.org/wiki/Mirror_matter.

Shadow matter or ghost matter has turned up from time to time in science fiction for at least 30 years.

Klear wrote:
rmsgrey wrote:According to a quick check on Wikipedia, simple mirror-reflection is no longer thought to produce antimatter, but rather matter that barely interacts with normal matter at all - in which case Alice would have found a mirror world less solid than smoke...


This sounds interesting... please elaborate, if you can:
- Would this mirror world be visible? (that is, would photos interact with it?)
- Would gravity affect Alice there, thus making her disappear in the middle of the mirror-earth? Or would she levitate behind the mirror, possibly slowly drifting away from it?
- Would this phantom world basically be vacuum to Alice?
- Could she survive there in a spacesuit?


Like I said to Pfhorrest, this is beyond my paygrade. From the Wikipedia article, the mirror matter would have normal gravity. How strongly it interacted via other forces would depend on the mixing between normal and mirror bosons - my guess is it would be like the world wasn't there - just its gravity - but it could be more like a world made of... treacle - with light massively dimmed, and Alice sinking through the ground slowly. She definitely wouldn't be able to breathe the air, but, wearing a spacesuit, she could probably survive a fair while - the question would be whether the interaction is strong enough for her to burn up in the mirror-earth's core, or strong enough that she'd reach a depth where the ground is dense enough to support her, or weak enough that she'd just fall through...

Zanthra wrote:Also speaking of fictional species, I one thought of one that saw color more or less like we do, but also saw polarization.


I believe our eyes respond differently to light depending on its polarisation, so, while we may not be able to pick it out of the other effects that affect what we see, we do kinda see polarisation...

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Re: 1145: "Sky Color"

Postby addams » Wed Dec 19, 2012 5:50 pm UTC

http://www.mcescher.com/Gallery/gallery-back.htm

The mirror thing:
I don't have to understand it.
It is enough for me that he understood it.

And; He was able to communicate it in 2D.

Some of his work makes me dizzy. Some of it calms me and makes me sad.

Art books are all on the internet, now?
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.

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Re: 1145: "Sky Color"

Postby orthogon » Thu Dec 20, 2012 8:40 pm UTC

Edgar wrote:The whole answer is, to the best of my reckoning, "It radiates, at DC. Constant current in the loop induces a constant magnetic field. That constant magnetic field is the physical embodiment of the non-varying (0 Hz) electromagnetic radiation."

A 0Hz EM wave should have an electric field component at right angles of magnitude H x Z0. Is it the supposition of two 0 Hz EM waves travelling in opposite directions so the E fields cancel? That would fit with no energy being transferred. The sum of the Poynting vectors would be zero. Just a thought.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1145: "Sky Color"

Postby Edgar » Sun Dec 23, 2012 5:00 pm UTC

orthogon wrote:A 0Hz EM wave should have an electric field component at right angles of magnitude H x Z0. Is it the supposition of two 0 Hz EM waves travelling in opposite directions so the E fields cancel? That would fit with no energy being transferred. The sum of the Poynting vectors would be zero. Just a thought.

It's been way too long since I studied fields and waves for me to answer in any definitive fashion. But I don't think that there is no energy transfer -- there is energy in the magnetic field. Another thought that occurred to me was that the E field component might be whatever electric field produced the current in the loop in the first place.

As I said, it was such a novel question that even my professor was taken aback by it. So I, not being an expert in the field, can only offer conjecture.

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Re: 1145: "Sky Color"

Postby clambam » Sun Dec 30, 2012 2:58 am UTC

The sky actually is violet. The light waves are, if I recall correctly, about 60% violet and 40% blue. However, your eyes are much more sensitive to blue than they are to violet, so the violet is drowned out.

Next time you have a question about perception, consider asking an artist. Ask me about obscure artist's pigments some time.

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Re: 1145: "Sky Color"

Postby rmsgrey » Mon Dec 31, 2012 1:02 am UTC

clambam wrote:The sky actually is violet. The light waves are, if I recall correctly, about 60% violet and 40% blue. However, your eyes are much more sensitive to blue than they are to violet, so the violet is drowned out.

Next time you have a question about perception, consider asking an artist. Ask me about obscure artist's pigments some time.


What about the infra-red and near ultra-violet contributions? If you're going for the spectrum of electromagnetic radiation detectable at any point on Earth by pointing a detector at the sky, then you should include the entire spectrum, not just those wavelengths that the human eye can detect to some extent.

On the other hand, if you're limiting yourself to colours the human eye sees, then you should take account of the human eye's perception of those colours.

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Re: 1145: "Sky Color"

Postby pgimeno » Mon Jan 14, 2013 12:40 pm UTC

I'm surprised no one gave the most logical explanation to the mirror question.

Mirrors invert left and right because we're used to compare the images by turning our neck, which is a vertical axis.

If we put a figure under our chin (say, in our chest), and put a mirror in front of us, then when looking down at the figure and comparing it with the mirror image, left and right will be in their place, but up and down will be inverted as seen from our eyes.

That's because the comparison in this case is done by moving our head over an horizontal axis.

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Re: 1145: "Sky Color"

Postby J Thomas » Mon Jan 14, 2013 12:49 pm UTC

Pfhorrest wrote:All that aside, violet is not equivalent to magenta or any of the purples (colors on a color wheel between red and blue): violet is basically "ultrablue", a blue so blue that the M cones aren't getting any of the stimulation they get at the frequency where S cone response peaks (which is why violet appears dark, it's hardly stimulating any of the cones, but the S cones gets most of what little there is). We think of violet as like a purple because, sadly, the L cones pick up their sensitivity again down at those short wavelengths, so this "ultrablue" seems, perversely, a little bit reddish to us.


Does this mean that real life can give you color combinations that simply cannot ever be created on an RGB monitor?

Does that have any sort of important implication? I thought there should be one, but I'm drawing a blank when I think what it would be.
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Re: 1145: "Sky Color"

Postby oscar abraham » Mon Jan 14, 2013 1:13 pm UTC

J Thomas wrote:
Pfhorrest wrote:All that aside, violet is not equivalent to magenta or any of the purples (colors on a color wheel between red and blue): violet is basically "ultrablue", a blue so blue that the M cones aren't getting any of the stimulation they get at the frequency where S cone response peaks (which is why violet appears dark, it's hardly stimulating any of the cones, but the S cones gets most of what little there is). We think of violet as like a purple because, sadly, the L cones pick up their sensitivity again down at those short wavelengths, so this "ultrablue" seems, perversely, a little bit reddish to us.


Does this mean that real life can give you color combinations that simply cannot ever be created on an RGB monitor?

Does that have any sort of important implication? I thought there should be one, but I'm drawing a blank when I think what it would be.


I believe RGB monitos cannot display anything bluer than blue or anything redder than red (infrared). But having a lights that can stimulate all of the cones, RGB should be able to simulate any visible color for the human eye.

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Re: 1145: "Sky Color"

Postby jpk » Mon Jan 14, 2013 2:17 pm UTC

Jorpho wrote:
orthogon wrote:Thanks for all the Alice background. I've downloaded Through the Looking Glass onto my Kindle, so I can see what it's like for myself.
I hear Martin Gardner's "Annotated" editions are the way to go.


"The Annotated Alice" is a really glorious piece of work, yes.

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Re: 1145: "Sky Color"

Postby Jorpho » Mon Jan 14, 2013 2:28 pm UTC

J Thomas wrote:Does this mean that real life can give you color combinations that simply cannot ever be created on an RGB monitor?

Does that have any sort of important implication? I thought there should be one, but I'm drawing a blank when I think what it would be.
The term you are looking for is color gamut.

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Re: 1145: "Sky Color"

Postby rmsgrey » Mon Jan 14, 2013 3:20 pm UTC

Jorpho wrote:
J Thomas wrote:Does this mean that real life can give you color combinations that simply cannot ever be created on an RGB monitor?

Does that have any sort of important implication? I thought there should be one, but I'm drawing a blank when I think what it would be.
The term you are looking for is color gamut.


I find it interesting that an online article about colours that can't be displayed has illustrations purportedly showing colours that can't be displayed on a standard computer monitor...

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Re: 1145: "Sky Color"

Postby FishDawg » Tue Jan 15, 2013 2:55 am UTC

And a follow-up to the mirror question, why is your own image in the mirror reversed left-right but not top-bottom (the answer is different from why text is reversed)?

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Re: 1145: "Sky Color"

Postby rmsgrey » Tue Jan 15, 2013 1:14 pm UTC

FishDawg wrote:And a follow-up to the mirror question, why is your own image in the mirror reversed left-right but not top-bottom (the answer is different from why text is reversed)?


Because I'm standing sideways on? A mirror reverses front-back/near-far not left-right nor up-down nor north-south nor smile-frown (to borrow an example from Ian Stewart's book The Magical Maze) nor grue-bleen

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Re: 1145: "Sky Color"

Postby pgimeno » Thu Jan 17, 2013 2:09 pm UTC

FishDawg wrote:And a follow-up to the mirror question, why is your own image in the mirror reversed left-right but not top-bottom (the answer is different from why text is reversed)?

Same response:

pgimeno wrote:Mirrors invert left and right because we're used to compare the images by turning our neck, which is a vertical axis.

If we put a figure under our chin (say, in our chest), and put a mirror in front of us, then when looking down at the figure and comparing it with the mirror image, left and right will be in their place, but up and down will be inverted as seen from our eyes.

That's because the comparison in this case is done by moving our head over an horizontal axis.

If you have some words written in your shirt upside-down so that they look correct when you look down, and look at your reflection in a mirror, the mirror will preserve left and right and invert up and down.

Your image in a mirror is inverted left-to-right because it's psychologically less sound to compare it by turning your head over an horizontal axis. If I compare my legs with their reflection on a mirror, the up and down are inverted and the left and right are preserved.

Think of a camera turning over a vertical axis (as we do when turning our neck) and over an horizontal axis (as we do when we compare ourselves by looking down with our reflection on a mirror). In the vertical axis case, left and right will be inverted; in the horizontal axis case, up and down will.

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Re: 1145: "Sky Color"

Postby ThirdParty » Thu Jan 17, 2013 7:41 pm UTC

I've decided I want to take a stab at the mirror question:

As many people--phlip was first--have said already, the mirror flips the image along the front-to-back axis. (This is obvious when you think about it: what you see when you look in the mirror is your face, as opposed to seeing the room behind the mirror or even seeing the back of your own head.)

The front-to-back axis is special because it's the axis perpendicular to the plane of the mirror. Light traveling forward hits the mirror and bounces off, thereby getting reflected; light traveling upward or sideways doesn't hit the mirror. Light traveling diagonally will have its forward component reflected while the other components aren't reflected, like a billiard ball bouncing off the edge of a pool table: if the billiard ball was traveling NE and then hit an edge that was aligned N-S, the billiard ball will be traveling NW after the collision, because the E-W component of the ball's motion was the component perpendicular to the wall.

Reflection along any axis, including the front-to-back axis, reverses chirality. For example, if you hold a clock up to the mirror, the hands of the mirror-image will appear to be moving counter-clockwise. (Likewise, as Minibear Rex pointed out above, if you've got a watch on your lefthand wrist, your mirror image will appear to have his watch on his righthand wrist--regardless of whether you're looking in the mirror while standing up, lying down, standing on your head, or even standing on the mirror.) Depending on how you rotated the clock when you turned it to face the mirror, 12-o-clock may also no longer be at the top of the clock--but this will be true of the actual clock as well as of the clock's reflection, so isn't the mirror's fault.

There are only two chiralities--clockwise and counterclockwise--so there's no mystery about why those two chiralities are special and why the image doesn't have other chiralities flipped instead. We tend to describe a chirality reversal as a flip from left-to-right rather than a flip from up-to-down or a flip from front-to-back (probably because, as Minibear Rex mentioned above, many objects and people in our environment have approximate left-to-right symmetry), but that's just how we talk.

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Re: 1145: "Sky Color"

Postby pgimeno » Fri Jan 25, 2013 12:29 pm UTC

ThirdParty, while I agree with you in that mirrors reverse chirality (handedness, if you prefer), I don't agree on the reason you give for the left-to-right reversal. You just say:

ThirdParty wrote:We tend to describe a chirality reversal as a flip from left-to-right rather than a flip from up-to-down or a flip from front-to-back (probably because, as Minibear Rex mentioned above, many objects and people in our environment have approximate left-to-right symmetry), but that's just how we talk.

I am not convinced about that explanation. I was not convinced either when I read it in a Gardner book either.

First, in order to say that something is inverted, it needs a reference to compare with. The inversion is the result of a comparison. How do we tend to compare an image with its reflection in a mirror? Obviously we won't look at the source image upside down; we will be upright. And when we turn to the mirror to compare, we will still be upright. That turn is made around a vertical axis.

The "magic" of the left-right inversion vanishes when we compare the source image with the mirrored one by turning our eyes over an horizontal axis. Like watching the text in your T-shirt and comparing it with the mirror image: it will have up and down inverted, but not left and right. Or watching a mountain reflected in a lake: the mirrored mountain is upside down, because we compare the images by looking up and down (thus turning the eyes over an horizontal axis). If we tilt our head 90° left and then compare, then we have the situation of everyday mirrors: the peak will be to the right while the reflected image has it to the left, because we compare the images by turning our eyes over a vertical (relative to our head) axis.

Another example. Imagine that you stand about between the mirror and the person to compare (but not enough as to hide that person). When you look at the person, you are standing, but in order to look at the mirror, instead of turning towards it as you naturally would, crouch and look at it between your legs. What do you see? You see that in the mirror reflection left and right are preserved, but up and down are inverted. Why? Because the turn to compare was made about an horizontal axis.

And the effect holds no matter the symmetry of the figure being compared, which is why I don't buy the symmetry argument.

Of course, if you choose a non-orthogonal turn axis you will see e.g. bottom right inverted with top left etc.


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