Re: The Value of Science: Explanation vs Prediction

J. J. Lodder wrote:
David Ewan Kahana <dek@xxxxxxx> wrote:

Robert Carnegie wrote:
David Ewan Kahana wrote:
Thanks for the correction, and apologies
to Robert Carnegie, who had the right idea.

But I have no knowledge of these other people whom you mention,
Rayleigh and so forth ;-)


It should be Lord John Rayleigh to the likes of you and me ;->

(born John Willliam Strutt.)

Just Lord Rayleigh, or Sir John, had he happened to be a Sir.

...something about why the sky is blue? - which Genesis gets confused
over... daylight is created by God separately from the sun several days
later ;-)

Yup, you're remembering correctly, it's the same man.

Lord R. was a man of many talents ... he also worked out the
first steps toward the explanation of the blue color of the
sky.

He put the colour down to the (eponymous) phenomenon of
`Rayleigh scattering' of light by small particles, dust,
water droplets and so on, that were supposedly in the
atmosphere. He showed that small particles would
preferentially scatter light of short wavelength (blues and
violets), and worked out that the dependence on wavelength
was very strong (inverse fourth power of wavelength).

In clean air, the scattering is from statistical density fluctuations.
From no more than a look at Mt Everest from the distance Rayleigh
calculated a fairly good estimate of Avogadro's number.


Do you have a cite for that? I had thought that this suggestion
was due to Smoluchowski, who had looked at the phenomenon
of critical opalescence, and thought it might be related to the
atmospheric colour, and that it was Einstein who first
did the quantitative calculation of the scattering off of the density
fluctuations, and got the Avogadro's number in a different way.

The Einstein paper I can get easily enough. Rayleigh's
work would take me a bit longer ...

Rayleigh also proved that evolution is impossible,
by estimating the age of the earth
to be at most 100.000.000 years,
from cooling by conduction.
This seems to have bothered Darwin a lot.


No.

That particular `blunder' was due to William
Thomson, Lord Kelvin. His estimate started
at closer to 10,000,000 years, but geologists
and Darwin both correctly cried: `No way is
that old enough!'

Kelvin apparently listened, and he gradually
found ways to push the cooling time up, ending
somewhere near 100,000,000 years.

Of course, he didn't know about radioactivity, and his
calculations were actually pretty reasonable, given
that no heat source was inside the earth.

R. did live to see the discovery of radioactivity,
but lacked the greatness of mind
to retract his by then untenable statement,


I don't know about whether he retracted publicly
or not. But it was obvious that one of his basic
assumptions had been wrong. There was no
real way for him to have known that. So in one
sense, at least, there was nothing really there for
him to retract.

In any case, Kelvin was without doubt a great
thinker and he did critical work in the foundations
of thermodynamics.

There is an apocryphal story that I got from my
father: Rutherford supposedly gave a
seminar on the subject of radioactivity, which
Kelvin attended; he was very nervous about
bringing up the subject of Kelvin's calculation
of the cooling time of the earth, which was obviously
impacted by the new information about radiation.
Rutherford reported that the old boy appeared to
sleep until he got to the point at which he discussed
the issue of the estimate of the earth's age,
and then he woke up and started to listen.
Rutherford was very careful to point out that Kelvin's
calculation had been correct, but that clearly the new
heat input changed everything. Kelvin harrumphed,
and then went straight back to sleep ;->

I don't know if it's true, but it makes a nice
story in any case.

Best,

David


Jan

.

Relevant Pages

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