Today, we see why time goes only from then
to now. The University of Houston's College
of Engineering presents this series about the
machines that make our civilization run, and the
people whose ingenuity created them.
Students have little trouble
with the First Law of Thermodynamics -- the
idea that energy is conserved. But the
Second Law is a huge obstacle. And I think
that might be the result of discontent more than
any complexity. For what the Second Law says is
that order becomes disorder. It says that no
spontaneous process can ever be completely undone.
It tells us the sun will eventually burn out and
all living things will one day die. Mother Goose
said it wonderfully well when she sings about a
broken egg:
Humpty Dumpty sat on a wall.
Humpty Dumpty had a great fall.
All the Kings Horses and all the Kings men,
Couldn't put Humpty together again.
The odd thing about all this is that it's rooted in
probability. The Second Law says nothing about
individual molecules. Molecules aren't bound by it.
The law comes about because vast numbers of
molecules have to go from less probable
arrangements to more probable ones. Our awareness
doesn't reach down to the scale of molecules
either. So the Second Law mirrors human
consciousness.
As the Second Law follows probability to greater
disorder it becomes a companion to our subjective
perceptions of order and disorder. It's been called
time's arrow because we only experience
events in one direction. In one of his hymn texts,
the seventeenth century mystic Isaac Watts caught
this directionality of time in the haunting words,
Time, what an empty vapor 'tis;
and days how swift they
are.
Swift as an Indian arrow flies;
or like a shooting
star.
Yet a closer look tells us that the true state of
affairs is not so dreary. The less well-known
principle of Le Chatelier and Braun limits the
Second Law. It says that when natural processes go
to greater disorder, they at least summon up
resistance to their own completion. The best-known
example is chemical reaction:
The hotter a flame becomes, the less complete
combustion will be. Instead of racing to
completion, burning opposes its own action. Rust
likewise tends to cover metals with protective
coatings that slow the process of rusting.
Spontaneous processes degrade things, but nature
always invokes processes that retard degradation.
Nature protects us. It slows and inhibits the
inevitable. It grants us time. You've probably
heard Shakespeare's invocation of the Second Law in
his play Cymbeline,
Golden lads and girls all must,
As chimney sweepers come to dust.
But remember that our coming to dust is muted by
the Le Chatelier-Braun principle. We can grow old
gracefully. The Second Law aims time toward
disorder and decay, but Le Chatelier and Braun tell
us that time's arrow is slowed -- that it's
possible to sustain and enjoy a measure of beauty
and order along the way.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
The Second Law of Thermodynamics is detailed in any
undergraduate thermodynamics text. I especially
recommend Reynolds, W.C., and Perkins, H.C.,
Engineering Thermodynamics. Garden City, NJ:
Doubleday and Co., Inc., 1966, Chapter 11.
My favorite sources for the principle of Le
Chatelier and Braun are Epstein, P.S., Textbook
of Thermodynamics. New York: John Wiley &
Sons, Inc., 1937, Chapter 21. Or Callen, H.G.,
Thermodynamics and an Introduction to
Thermostatistics. New York: John Wiley &
Sons, 1985, Sections 8.4 and 8.5.
The original formulation of the Second Law of
Thermodynamics was made by Sadi Carnot. See:
Carnot, S., Reflections on the Motive Power of
Heat. (tr. R. Thurston) New York: ASME, 1943.
This is a greatly modified version of old Episode 121.
Puisque tout passe, faisons la melodie
passegere,
celle qui nous desaltere aura de nous raison.
Chantons ce qui nous quitte avec amour et art;
soyons plus vite qui le repide depart.
Since all is passing,
Retain the melodies that wander by us.
That which assuages when nigh us,
Shall alone remain.
Let us sing what will leave us
With our love and art.
Ere it can grieve us,
Let us the sooner depart.
Ranier Maria Rilke;
from Poemes
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