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Re: World's noise



At 10:52 PM 9/6/01 +0300, Muhsin Ogretme wrote:
One of my students in the sixth grade science class asked the following
question;
"Is it true that the earth creates a big noise while it is rotating? Why
cant we hear it? Why is there such a sound?"

Well, there are several ideas in there; let's take them separately.

A1) If we take the word "while" literally, then the answer is that the
world does make a big noise !while! rotating, and we most certainly can
hear it. I can hear waves crashing on the seashore. I can hear wind
whistling through the trees. I can hear animal activity and human
activity. Et cetera. All this goes on !while! the world is rotating.

A2) Perhaps the student meant to ask if the world makes noise !because! it
is rotating. To approach this question, consider each of the
aforementioned terrestrial noises. Does such noise arise !because! the
earth is rotating?

This is a much trickier question. Answering it depends on having a
reasonable notion of what "causation" is. There are some very widespread
misconceptions about this. For help sorting this out, see:
(1) General discussion:
http://mailgate.nau.edu/cgi-bin/wa?A2=ind0010&L=phys-l&P=R32554
(2) Application to F=ma
http://mailgate.nau.edu/cgi-bin/wa?A2=ind0010&L=phys-l&P=R23906

As suggested in reference (1), to really discuss causation, we need to
imagine the counterfactual: What would happen if the earth were rotating
differently? Answer: Most of the known noise sources would continue,
although the details would change. The winds would be different, but there
would still be winds. Animal activity would be different, but there would
still be animal activity. From this we conclude the following:
-- Rotation is not the direct or primary !cause! of the world's noise in
any scientific sense. (Unscientific figurative senses will not be
discussed here.)
-- Rotation could be considered the indirect !cause! of some secondary
details of the worlds's noise.

A3) Perhaps the student meant to ask not about the real world but an
imaginary solid ball lacking the "obvious" noise-makers such as uneven
solar heating. In that case I would ask
-- Is there any theoretical reason to believe that rotation _per se_
causes noise?
-- Is there any experimental evidence that rotation _per se_ causes noise?

Answers should be obvious.

============================

The question about "a big noise that we can't hear" again depends on how
literally we want to take things:

B1) If we take a reasonably general physical notion of the term "noise",
then the world does make huge noises that we can't hear. For instance,
waves crashing on the shore make treeeemennnnndous amounts of infrasonic
noise. We can easily detect this with instruments, but we can't hear it
because of well-known limitations of human ears.

B2) If we take a narrow notion of "noise" namely "audible noise", or if we
take a broad notion of "hear it" namely "detect it", then the question is
grossly self-inconsistent and not worth delving into. We can detect very
tiny noises, so any noise that is hard to detect can't be a very big noise.

B3) There are some interesting questions and surprising answers about why
the earth's !surface! is not as noisy as it could be. For instance, if you
are only a few miles from the seashore you (normally) can't hear the surf,
even though a simple energy calculation suggests that you ought to be able to.

The trick is that the air is (normally) warmer near the
ground. Temperature affects the speed of sound, and therefore the
refractive index of the air. Noises produced near the ground and initially
moving horizontally are refracted upwards, so that people on the ground
even modest distances away can't hear them.

Humans in a hot air balloon can hear the surf from a hundred miles
away. It is believed that migratory birds use the world's noise as a
navigational reference.