As I watched the storm approach, I thought about one of the most fascinating bits of weather trivia I ever heard: the weight of rain clouds in terms of elephants [ABC has unfortunately removed the 2003 news article - Ed.].

Every cloud is made up of moisture, of course. On a day like today, a single fluffy cloud contains about 550 tons of water. Your average elephant weighs about six tons, so that means that those happy spring clouds are equivalent to around 100 elephants.

A bigger storm cloud, more like the one I saw barreling into the area recently, is more like 200,000 elephants in terms of weight. That’s quite a jump, and it’s kind of humbling, funny, and awesome to think of 200,000 elephants stampeding across the sky.

No such emotions attach to the numbers associated with a hurricane, however. Instead of awe or humor, now we’re talking absolute terror. Instead of 100 elephants, or even 200,000 elephants, the water in a hurricane is equivalent to 40,000,000 – yes, forty million – elephants. Forty million elephants in the sky, bringing destruction and fear. Forty million elephants.

Next time you look at a cloud, think about how many elephants are in it. You had no idea that a cloud was so massive, and no idea that a creature so huge and seemingly earth-bound can be used to understand something so apparently light and ephemeral, did you?

Suppose this is the case: we’re inside a simulation designed by a freckle-faced superkid for extra credit in her fifth grade science class. Is this something we could discover, or must it, like so many aspects of Theory 2, be forever hidden from our scientific investigation? Surprisingly, this variety of Theory 1 is quite amenable to experiment: neither revelation nor faith is required. What would we expect to see if we inhabited a simulation? Well, there would probably be a discrete time step and granularity in position fixed by the time and position resolution of the simulation—check, and check: the Planck time and distance appear to behave this way in our universe. There would probably be an absolute speed limit to constrain the extent we could directly explore and impose a locality constraint on propagating updates throughout the simulation—check: speed of light. There would be a limit on the extent of the universe we could observe—check: the Hubble radius is an absolute horizon we cannot penetrate, and the last scattering surface of the cosmic background radiation limits electromagnetic observation to a still smaller radius. There would be a limit on the accuracy of physical measurements due to the finite precision of the computation in the simulation—check: Heisenberg uncertainty principle—and, as in games, randomness would be used as a fudge when precision limits were hit—check: quantum mechanics.

In that case, the nearly ‘impossible’ exact necessary value for the cosmological constant to an accuracy of 120 digits could simply be an effect of a simulation with limited precision …

The author of this paper goes one step further and adds the human color vision system with the three different color receptors (cones) to the picture. Due to this restriction to three types, there is the possibility for metamers, i.e. different mixes of light frequencies leading to the same color as perceived by the human eye. An example is the mixing of red and green light resulting in a perceived yellow exactly like the single frequency yellow light.
Armed with that background, he tries to identify a simple frequency spectrum which will be perceived the same as the frequency spectrum resulting from the sun and Rayleigh scattering. As model he adds a very narrow frequency range to a background of white light of all frequencies. Fitting the frequency of the narrow band to result in the same perceived color, one gets a background of white light with a narrow blue frequency range – corresponding to unsaturated blue. Ergo, we see the sky as blue not violet!
Nice paper and very readable!

For me, I recognized another important thing: one usually says that all colors are in the rainbow – but this is not true! Due to the fact that there are three different types of color receptors, a small frequency range will always produce some special combination of responses from the three receptor types – but not all possible combinations. For example: with a small frequency range, one cannot excite the low and the high frequency receptors without the medium frequency receptor.
But with a suitable mixture of various light frequencies I can get any combination of responses from the three types. I.e. there are many more perceived colors than the rainbow contains!

Corollary: for color blind people with only two types of color receptors the rainbow indeed contains all possible perceived colors.

Reference:

Smith, G.S. 2005. Human color vision and the unsaturated blue color of the daytime sky. American Journal of Physics 73(July):590-597. Abstract available at http://dx.doi.org/10.1119/1.1858479.