One of the things that I love about science is that it frees us from our parochial, human-centered world view. It allows us to extend our range of experiences far beyond our limited innate faculties. After all, there is no good reason why the sensory apparatus on an east-african-plains ape should come close to sampling what’s really out there.
In Kurt Vonnegut’s Slaughterhouse 5, the Tralfamadorians try to describe what human experience must be like to each other. They imagine it must be something like spending your entire life with a steel ball on your head, with only one eyehole that was welded to six feet of pipe.
The analogy is fairly accurate.
For example, the colors that we can see are only a tiny slice of the rich electromagnetic spectrum. If we could see the sky with eyes sensitive to microwaves, we could look up at the sky and see the cosmic background radiation — light emitted by the universe when it was only 379,000 years old. (Neat fact : if you switch on an older TV that’s not receiving any signal, what you see is grey static noise. The noise comes from the microwave spectrum, and about 10% of what you see is caused due to the CMB.) Fortunately, we can see the sky with different eyes.
But why can we only see the colors we do? Well, it turns out that if you shine light through water, how transparent it appears depends on the color of the light. In particular if you measure the opacity of water across all colors (by plotting absorption coefficient vs. frequency) this is what you get:
A huge peak, meaning that water is generally pretty opaque. Then there’s that sharp tiny valley, those are the range of frequencies for which water is transparent. And the vertical dotted lines represent the visible spectrum of light — that band of frequencies that we can see with our eyes. See how nicely they overlap? As Jackson himself remarks, “Mother Nature has certainly exploited her window!”
Our eyes are not just limited when it comes to color. When physicists study a light wave they talk about its frequency (color), its amplitude (brightness), and its phase and polarization. While our eyes give us information about the first two attributes, they tell us nothing about the others.
Well, here’s news that came as a surprise to me. Studies on Mantis shrimp have shown that they can not only detect the polarization of light, they can even tell in whether it is linearly or circularly polarized! PZ Myers has an excellent writeup on these findings. To quote:
This is powerful stuff; mantis shrimps are moving through a visual world rich with novel details beyond our imagination, able to detect qualities of light outside our experience.
This is especially useful if your prey in transparent. As light passes through a transparent object, it acquires a polarization in some direction. If the predators’ eyes can detect polarization, so much for invisibility!
Cephalopods are really fascinating creatures. While all vertebrate eyes are afflicted with a blind spot, the cephalopod eye has no such problem since their optic nerve approaches the photodetectors from behind. To learn more about them, head over to Pharyngula. Here are some good articles by PZ to get you started:
Cephalopod camouflage, or turning invisible is easier than it looks. (check out the amazing video)