Friday, October 06, 2006

Cirque du Lune

October 6th was the Harvest Moon. The Harvest Moon is the full moon that falls nearest the autumnal equinox. It happens right around harvest time, and since the Harvest Moon-rise is in near synchronicity with the sunset, it has given ancient cultures an extra few hours of light to work with. But there's more to the Harvest Moon than just antiquated superstition and primeval pragmatism. The HM steals colors. A recent NASA article explains it best.

Moonlight steals color from whatever it touches. Regard a rose. In full moonlight, the flower is brightly lit and even casts a shadow, but the red is gone, replaced by shades of gray. In fact, the whole landscape is that way. It's a bit like seeing the world through an old black and white TV set.

The reason for this lies in the human retina. We have two basic kinds of light receptors: rods and cones (we have three kinds of cones--but more on that later). The rods are sensitive to faint light, and the cones can differentiate colors. During the day, our cones supply enough information to the brain for it to build a color model of the world we see. But in the moonlight, rods are king. Our cones just cannot generate enough data for us to see a technicolor world.

The above photoshopped image is my attempt to simulate the effect. It is a blend of 15% moonlight color (see below) and 85% grayscale. But this is not the only way the Moon steals color from us. Besides being fainter, moonlight has a different spectrum than sunlight. It doesn't reflect 100% percent of the Sun's light, it absorbs some--and not all wavelengths are absorbed equally. Furthermore, the Harvest Moon is renowned as being an Orange Moon. There are two reasons for this: at this time of year the Moon stays lower on the horizon, and there is more dust in the atmosphere during the HM. Put together, it means that moonlight must pass through a much thicker and denser atmospheric dust filter during HM than at other times of the year. As a result, blue wavelengths get filterted out preferentially. It turns out that the sky is blue for the same reason that the Sun and Moon appear yellow/orange when near the horizon.

This got me thinking that if I were to do an extended exposure where the "cones" got to see the HM moonlight, I should see different colors (less blue). The picture below is the result of that experiment. Besides the apparent yellowing of the painting, the long exposure reveals a moon-glare that was invisible to my diurnal human eyes.

But obviously all this talk of the Moon "stealing colors" is metaphoric. Of course the colors are still there; we just can't see them because of the light. Well, actually no. If you can't see the colors, then they don't exist. Let me say that again: colors you don't see, don't exist.

"What kind of solipsistic nonsense is that?" you ask. Or perhaps you're asking "What is 'solipsistic'?"

Solipsism is a group of varied philosophic tendencies that state that "the self" or "the mind" is the truest (or only true) reality: Cogito Ergo Sum. The most extreme case--metaphysical solipsism--claims that nothing outside of the mind is real. My introduction to solipsistic thinking was the paradox of Schrödinger's cat. This thought experiment is an extension of the observed and reproducible phenomenon from Quantum Mechanics that a quantum entity which can exist in one of two quantum states, exists in neither and exists in both (quantum superposition), until the moment it is observed. In the experiment, a cat in an opaque box is set to be killed if an atomic nucleus (with a 50% chance of decaying) decays. After the prescribed period, until someone looks into the box and observes, the cat is neither dead nor alive. It is in quantum superposition of being both dead and alive. In other words, the definite fate of the cat (which happened in the past) does not exist until it is realized by the mind of an observer. We can call this Quantum Solipsism.

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Naturally, I rejected this and decided that solipsists were either clowns or lunatics. It would be years later (I was still in high school) before I learned about epistemological solipsism, so I shall come back to it later.

What started me on my road to giving solipsism a second look was my study of Neoplatonism, Middle Platonism and Plato. (That's right. I studied them in reverse order)

Neoplatonism is epitomized in the figure of Augustine of Hippo. Augustine believed that the sensible realm (the world we know) is transitory in nature, and that abiding realities could only be found in the intelligible realm, with God as its source. I really had trouble swallowing that circus act.

Middle Platonism is exemplified by Philo of Alexandria. Philo's Platonic reading of the Jewish scriptures, along with the transcendence of God and the abasing of the physical body, layed the groundwork for the future theological fondations of Christianity. Who are the crazies? I decided to check out the source.

Plato deserves much more attention than I can give him here, so forgive me if my summary is a bit curt. Plato seems to have also believed in the dichotomy of the sensible and intelligible. The intelligible is where the ideal exists and the sensible is but a poor imitation. This can be seen in his example of the chair.
He thought that everything had a sort of ideal form, like the idea of a chair, and then an actual chair was a sort of poor imitation of the ideal chair that exists only in your mind.

The trouble with this is that the mind (ie. - the brain) itself is fluid and not invariable. Today's ideal chair need not be tomorrow's, and mine (an EZ recliner) is surely not yours. Plato had it backwards: the "form" that exists in the mind is a contrived model based on incomplete sensory input. And this applies to all the "reality" that we know. As an example, let's re-examine the issue of color.

While it's true that some materials preferentially absorb or reflect certain wavelengths of electromagnetic energy, the "colors" that you and I are so familiar with are constructed in the brain. They do not exist outside of our awareness! Take a look at the retina (since rods and cones are types of neurons, I feel safe calling the retina part of the brain). Below is a graph showing wavelength sensitivity of the three types of cones in the human retina.

Each type of cone detects a different range of wavelengths. The peak of each curve represents the wavelength that gives the strongest signal for that cone type. Each type of cone reports only the intensity of the signal it receives. It cannot differentiate between a weak signal close to its "peak" wavelength and a strong signal further off on a tail of the curve. By having different cone types with overlapping sensitivities, the brain is able to put the information together and construct a color.

You may notice in the above graph two of the cone types (in the "red" region of the spectrum) are very close together. This means that our brains can more precisely discriminate between wavelengths in this neighborhood of the spectrum. To explain this, we need a quick review of evolutionary history. Most non-mammalian vertebrates--including birds--have four types of cones.

Some time ago, the ancestor of all mammals lost two of those cones. This is most likely because this animal was nocturnal and so wasn't well served by such fantastic color vision. Today, non-primate mammals only have two cone types. Which brings us to primates and their three. There are two ways this could have happened. If the "lost cones" were in fact just dormant, they could've been reactivated. This was not the case. Instead, a new cone type was created by a process called duplication. First, a copying error caused there to be an extra copy of the gene for "red" cone. Later, mutations caused the duplicate cone to become sensitive to somewhat different wavelengths. This is why their sensitivities are so close. Although there is a theory that this evolved as a way to better distinguish pinks--an indicator of fertility and emotion in primates. I'm all flush just thinking about it.

The thing to remember here is that the brain is not seeing wavelengths. It is getting sensory signals and constructing colors from them. A combination of different wavelengths of light that activated the cones in the same manner as a single pure wavelength would be seen as the exact same color. In the Scientific American article What Birds See, Tim Goldsmith tells of an experiment he did with birds . He trained birds to react to a certain wavelength of violet light and not others. He then demonstrated the the right blend of (92%) blue and (8%) UV light was indistinguishable from the training light to the birds. We would naturally be able to tell the difference because we don't have UV cones; we would see blue or violet. But for most of the spectrum, birds have a broader range of colors than we do.

It's not that the brain is being fooled into seeing the wrong color. Color is a creation of the brain. It's not out there in nature. It's only in the mind. When you take in all the leaves on the autumn trees, remember that those colors aren't actally there: they only exist in your head! When you're brought to joyful tears by the blushing pinks on your baby's cheeks, remember that those colors aren't actually there: they only exist in your head! And while you're marvelling at the magnificent works of art at the local museum, remember that those colors aren't actually there: they only exist in your head!

Everything that you know as reality is a construct of your brain. You cannot directly know the reality outside of your own awareness. That's not to say that reality is just an elaborate simulation, Occam's razor makes that quite remote. We just don't "know" any more about the outside reality other than it correlates well enough with our own realities that we can satisfactorily interact with it. This philosophy is otherwise known as epistemological solipsism.

Perhaps it's time for me to join the circus.


Jennifer Ouellette said...

A truly excellent and informative post. not an easy feat to combine a real-world observable event like the Harvest Moon with how we see, and segue into a discussion of the philosophy of "reality" -- well, the result is a truly worthy endeavor. :)

The Science Pundit said...

Wow, thanks! That's quite a complement coming from someone whom I consider one of the most entertaining and informative science writers on the web (and beyond).

The relationship between how we see and the philosophy of "reality" is a topic I've been kicking around for a while now. If I were to write down all my thoughts on it, I'd be well on my way to finishing a book. (I can see it now. The preamble would be The Parable of the Blind Men and the Elephant.) When I read about the upcomming Harvest Moon, it all just seemed to "fall" together.