These pages are an experiment in making an exposition more
readable by taking advantage of hypertext.
The first version of my
explanation of why the sky is blue has been up for years. I've
gotten a variety of different comments on it.
One reader threatened to do me bodily harm unless I compensated
him for his wasted time. Some other people have liked it, but I can
never tell whether they really understood it or not. A couple of
years ago I saw another Web page that gave mine a glowing
recommendation, but summarized it completely incorrectly.
These people weren't dumb, they were just mistargeted. I
originally wrote the article to fill a perceived gap between
elementary and advanced, highly mathematical treatments of the
subject. I was imagining my audience as consisting of undergraduate
physics majors or engineers, who had learned some electromagnetism
and maybe even read a textbook treatment of this already, but still
didn't really know why the sky was blue.
The fact is, though, that everyone wants to know why
the sky is blue. I didn't want to remove the physics detail that I
had already put in, though, since that was the original motivation
for the page. I needed an explanation that would scale well to many
different kinds of readers, without talking down to them.
As I pondered how to redesign the page, I read some articles by
Jakob Nielsen about writing for
the Web. These stressed the value of short pages, inverted-pyramid
style (like a newspaper column), and heavy use of hyperlinks. It
occurred to me that if I chopped up the explanation in this way, it
would scale better; people could read as much or as little detail
as they wanted. If you don't care about the electromagnetic scalar and vector potentials,
you can take those details on faith and just get the big picture.
These ideas of Nielsen's are probably his most controversial;
they've put him at odds with many Web creators who argue that there is
a place on the Web for longer, more linear writing styles.
Obviously I've got a lot of material on my pages that is written
more traditionally (such as the page you are now reading), and I do
revert to a more linear style in the goriest depths of the blue-sky
exposition, but I think that the short, inverted-pyramid,
link-heavy style is particularly suited to the topmost part of the
document.
A reader recently pointed out that my pages don't explain
why the sky is blue, just how it is blue, or,
more precisely, how known scientific theories explain its
blueness.
In these pages I talk a lot about electromagnetism. You could
well ask "Why do electromagnetic fields work that way?" and I
wouldn't be able to give a very satisfying answer. I could go a
couple of steps further back and talk about quantum electrodynamics
or the electroweak force, but nobody has the slightest idea why
that behaves the way it does. This is how scientific
theories work; they always have some unexplained statements in them
that are justified only by the theory's power to predict results.
If you look for a completely self-contained explanation of
anything, you will be disappointed.
Of course, it's not just science that has this property. Most
kids discover that they can annoy their parents by asking "why?"
over and over at an early age! Four or five whys in a row, and
nobody can give a straight answer.
However, I think that my title is still justified-- if only
because it has a question mark! In these pages I explain why the
sky is blue, given the structure of air molecules, the known behavior of electromagnetic fields and potentials, and
the known behavior of the human visual
system. Not everything is known about these subjects (the human
visual system is by far the most mysterious of the three!) But I
can confidently say that these are the subjects that people need to
learn more about, in order to understand more thoroughly why the
sky is blue.
