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No. 121:
Time's Arrow

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.

My wife tells me she wants no part of the Second Law of Thermodynamics. The Second Law tells us that order naturally moves to disorder -- that no spontaneous process can ever be completely undone -- that the sun will eventually burn out and that every living thing must eventually die. Mother Goose says it best when she tells us 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 funny thing about all this is that it's rooted in probability. The Second Law says nothing about individual molecules. It comes about because great numbers of molecules have to go from less probable arrangements to more probable ones. Our awareness of things is on a scale much larger than molecules. So our consciousness follows probability to greater disorder. The Second Law has been called "time's arrow" because we experience events only in that one direction. Isaac Watts caught this directionality of time in one of his hymn texts:

Time, what an empty vapor 'tis;
and days how swift they are.
Swift as an Indian arrow flies;
or like a shooting star.

Yet I come back to my wife's discontent with this dreary state of affairs. She has cause to question it. The less known principle of LaChatelier 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.

I'll give you an example. You know the old saying, "Three on a match is unlucky." The idea is that if soldiers light three cigarettes on one match, they give an enemy sniper enough time to draw a bead on the light. It takes time for a match to burn. It doesn't go all at once. That's because, as a flame heats the air and wood, the rate of burning slows down. The match is degraded into smoke and ash, but nature slows that degradation. All natural processes do that in one way or another -- to some degree or another. Nature protects us. It slows and inhibits the inevitable. It grants us time.

Shakespeare had his own version of the Second Law. He said:

Golden boys and girls all must,
As chimney sweepers, come to dust.

But our coming to dust is muted by the principle of LaChatelier and Braun. We can grow old gracefully. The Second Law aims time toward disorder and decay, but LaChatelier and Braun tell us that time's arrow is slowed -- that it's possible to sustain, and enjoy, some 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)

This episode has been greatly revised as Episode 1564.


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 Français