The Universe of Disco

Facts you should know
The Minneapolis Star Tribune offers an article on Why is the sky blue? Facts you should know, subtitled "Scientists offer 10 basic questions to test your knowledge". [ The original article has been removed; here is another copy. ] I had been planning to write for a while on why the sky is blue, and how the conventional answers are pretty crappy. (The short answer is "Rayleigh scattering", but that's another article for another day. Even crappier are the common explanations of why the sea is blue. You often hear the explanation that the sea is blue because it reflects the sky. This is obviously nonsense. The surface of the sea does reflect the sky, perhaps, but when the sea is blue, it is a deep, beautiful blue all the way down. The right answer is, again, Rayleigh scattering.)

The author, Andrea L. Gawrylewski, surveyed a number of scientists and educators and asked them "What is one science question every high school graduate should be able to answer?" The questions follow.

1. What percentage of the earth is covered by water?

   This is the best question that the guy from Woods Hole Oceanographic Institute can come up with?

   It's a plain factual question, something you could learn in two seconds. You can know the answer to this question and still have no understanding whatever of biology, meteorology, geology, oceanography, or any other scientific matter of any importance. If I were going to make a list of the ten things that are most broken about science education, it would be that science education emphasizes stupid trivia like this at the expense of substantive matters.

   For a replacement question, how about "Why is it important that three-fourths of the Earth's surface is covered with water?" It's easy to recognize a good question. A good question is one that is quick to ask and long to answer. My question requires a long answer. This one does not.

2. What sorts of signals does the brain use to communicate sensations, thoughts and actions?

   This one is a little better. But the answer given, "The single cells in the brain communicate through electrical and chemical signals" is still disappointing. It is an answer at the physical level. A more interesting answer would discuss the protocol layers. How does the brain perform error correction? How is the information actually encoded? I may be mistaken, but I think this stuff is all still a Big Mystery.

   The question given asks about how the brain communicates thoughts. The answer given completely fails to answer this question. OK, the brain uses electrical and chemical signals. So how does the brain use electricity and chemicals to communicate thoughts, then?

3. Did dinosaurs and humans ever exist at the same time?

   Here's another factual question, one with even less information content than the one about the water. This one at least has some profound philosophical implications: since the answer is "no", it implies that people haven't always been on the earth. Is this really the one question every high school graduate should be able to answer? Why dinosaurs? Why not, say, trilobites?

   I think the author (Andrew C. Revkin of the New York Times) is probably trying to strike a blow against creationism here. But I think a better question would be something like "what is the origin of humanity?"

4. What is Darwin's theory of the origin of species?

   At last we have a really substantive question. I think it's fair to say that high school graduates should be able to give an account of Darwinian thinking. I would not have picked the theory of the origin of species, specifically, particularly because the origin of species is not yet fully understood. Instead, I would have wanted to ask "What is Darwin's theory of evolution by natural selection?" And in fact the answer given strongly suggests that this is the question that the author thought he was asking.

   But I can't complain about the subject matter. The theory of evolution is certainly one of the most important ideas in all of science.

5. Why does a year consist of 365 days, and a day of 24 hours?

   I got to this question and sighed in relief. "Ah," I said, "at last, something subtle." It is subtle because the two parts of the question appear to be similar, but in fact are quite different. A year is 365 days long because the earth spins on its axis in about 1/365th the time it takes to revolve around the sun. This matter has important implications. For example, why do we need to have leap years and what would happen if we didn't?

   The second part of the question, however, is entirely different. It is not astronomical but historical. Days have 24 hours because some Babylonian thought it would be convenient to divide the day and the night into 12 hours each. It could just as easily have been 1000 hours. We are stuck with 365.2422 whether we like it or not.

   The answer given appears to be completely oblivious that there is anything interesting going on here. As far as it is concerned, the two things are exactly the same. "A year, 365 days" it says, "is the time it takes for the earth to travel around the sun. A day, 24 hours, is the time it takes for the earth to spin around once on its axis."

6. Why is the sky blue?

   I have no complaint here with the question, and the answer is all right, I suppose. (Although I still have a fondness for "because it reflects the sea.") But really the issue is rather tricky. It is not enough to just invoke Rayleigh scattering and point out that the high-frequency photons are scattered a lot more than the low-frequency ones. You need to think about the paths taken by the photons: The ones coming from the sky have, of course, come originally from the sun in a totally different direction, hit the atmosphere obliquely, and been scattered downward into your eyes. The sun itself looks slightly redder because the blue photons that were heading directly toward your eyes are scattered away; this effect is quite pronounced when there is more scattering than usual, as when there are particles of soot in the air, or at sunset.

   The explanation doesn't end there. Since the violet photons are scattered even more than the blue ones, why isn't the sky violet? I asked several professors of physics this question and never got a good answer. I eventually decided that it was because there aren't very many of them; because of the blackbody radiation law, the intensity of the sun's light falls off quite rapidly as the frequency increases, past a certain point. And I was delighted to see that the Wikipedia article on Rayleigh scattering addresses this exact point and brings up another matter I hadn't considered: your eyes are much more sensitive to blue light than to violet.

   The full explanation goes on even further: to explain the blackbody radiation and the Rayleigh scattering itself, you need to use quantum physical theories. In fact, the failure of classical physics to explain blackbody radiation was the impetus that led Max Planck to invent the quantum theory in the first place.

   So this question gets an A+ from me: It's a short question with a really long answer.

7. What causes a rainbow?

   I have no issue with this question. I don't know if I'd want to select it as the "one science question every high school graduate should be able to answer", but it certainly isn't a terrible choice like some of the others.

8. What is it that makes diseases caused by viruses and bacteria hard to treat?

   The phrasing of this one puzzled me. Did the author mean to suggest that genetic disorders, geriatric disorders, and prion diseases are not hard to treat? No, I suppose not. But still, the question seems philosophically strange. Why says that diseases are hard to treat? A lot of formerly fatal bacterial diseases are easy to treat: treating cholera is just a matter of giving the patient IV fluids until it goes away by itself; a course of antibiotics and your case of the bubonic plague will clear right up. And even supposing that we agree that these diseases are hard to treat, how can you rule out "answers" like "because we aren't very clever"? I just don't understand what's being asked here.

   The answer gives a bit of a hint about what the question means. It begins "influenza viruses and others continually change over time, usually by mutation." If that's what you're looking for, why not just ask why there's no cure for the common cold?

9. How old are the oldest fossils on earth?

   Oh boy, another stupid question about how much water there is on the surface of the earth. I guessed a billion years; the answer turns out to be about 3.8 billion years. I think this, like the one about the dinosaurs, is a question motivated by a desire to rule out creationism. But I think it's an inept way of doing so, and the question itself is a loser.

10. Why do we put salt on sidewalks when it snows?

    Gee, why do we do that? Well, the salt depresses the freezing point of the water, so that it melts at a lower temperature, one, we hope, that is lower than the temperature outside, so that the snow melts. And if it doesn't melt, the salt is gritty and provides some traction when we walk on it.

    But why does the salt depress the freezing point? I don't know; I've never understood this. The answer given in the article is no damn good:

    Adding salt to snow or ice increases the number of molecules on the ground surface and makes it harder for the water to freeze. Salt can lower freezing temperatures on sidewalks to 15 degrees from 32 degrees.

    The second sentence really doesn't add anything at all, and the first one is so plainly nonsense I'm not even sure where to start ridiculing it. (If all that is required is an increase in the number of molecules, why won't it work to add more snow?)

    So let me think. The water molecules are joggling around, bumping into each other, and the snow is a low-energy crystalline state that they would like to fall into. At low temperatures, even when a molecule manages to joggle its way out of the crystal, it's likely to fall back in pretty quickly, and if not there's probably another molecule around that can fall in instead. At lower temperatures, the molecules joggle less, and there's an equilibrium in this in-and-out exchange that results in more ice and less water than at higher temperatures.

    When the salt is around, the salt molecules might fall into the holes in the ice crystal instead, get in the way of the water molecules, and prevent the crystal from re-forming, so that's going to shift the equilibrium in favor of water and against ice. So if you want to reach the same equilibrium that's normally reached at zero degrees, you need to subtract some of the joggling energy, to compensate for the interference of the salt, and that's why the freezing temperature goes down.

    I think that's right, or close to it,and it certainly sounds pretty good, but my usual physics disclaimer applies: While I know next to nothing about physics, I can spin a line of bullshit that sounds plausible enough to fool people, including myself, into believing it.

    (Is there a such a thing as a salt molecule? Or does it really take the form of isolated sodium and chlorine ions? I guess it doesn't matter much in this instance.)

    I think this question is a winner.

    [ Addendum 20060416: Allan Farrell's blog Bento Box has another explanation of this. It seems to me that M. Farrell knows a lot more about it than I do, but also that my own explanation was essentially correct. But there may be subtle errors in my explanantion that I didn't notice, so you may want to read the other one and compare.]

    [ Addendum 20070204: A correspondent at MIT provided an alternative explanantion. ]

The questions overall were a lot better than the answers, which made me wonder if perhaps M. Gawrylewski had written the answers herself.

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