Word
Gems
What is a
man but the sum of his thoughts?
Time:
Freeman
J. Dyson:
Gravity
Is Cool
IN THE 19TH CENTURY natural historians of the earth were divided into catastrophists
and uniformitarians. Catastrophists believed time molds the earth by means of sudden
catastrophes, most famously Noah's flood. Uniformitarians believed time works slowly,
molding the earth by the same processes we observe at work today.
We now know that both sides in the debate were right.
- Time loves to sit quietly for millions of years and then to
pounce suddenly in a single hour of fury. Nothing is permanent, but the illusion of
permanence can last for a long time.
We see in the Channeled Scabland of Washington State the traces of a flood more violent
than Noah's, when a huge lake formed by the melting North American ice cap broke through
an ice dam and destroyed everything in its path as it roared down to the Pacific. And we
see, a few hundred miles to the north, the Burgess Shale, where delicate fossils of our
ancient ancestors have been marvelously preserved in rocks that have been largely
unchanged for 500 million years.
When we move from the earth to the heavens, the contrast between the quiet and
the stormy faces of time becomes even starker. In our quiet little corner of the
universe, the sun and moon and planets serenely ride in their orbits, the stars shine
steadily for billions of years. Elsewhere, in places remote from us in space and time,
sudden cataclysms fill the sky with violence as heavenly bodies are born or die. The birth
of our moon was such a cataclysm, more than 4 billion years ago, when the earth was young.
Another planet, smaller than the earth but larger than the moon, collided obliquely with
the earth. In a few minutes the incoming planet was disrupted, half of it plunged into the
earth, the other half formed a ring of orbiting debris that condensed to make our moon.
Whatever structures existed on the earth before the collision were totally obliterated.
The heat of impact made the earth shine white-hot with an atmosphere of vaporized rock.
Afterward the earth cooled down and became the planet we now know.
No object of comparable size has collided with the earth in more recent times. Later
impacts were smaller and did only superficial damage. But we see, across the universe,
cataclysms vastly more violent than the collisions of planets - supernova explosions in
which the core of a massive star collapses in a fraction of a second and the envelope of
the star is ejected trillions of miles into space. We know that such an explosion must
have occurred in our neighborhood shortly before the earth and the sun were born to
produce the mixture of heavy atoms that we see on earth today. And we see other cataclysms
even more abrupt and energetic than supernova explosions. We call these events gamma-ray
bursts, but nobody knows how or why they happen.
The universe is full of cataclysms. Luckily for us, they are separated by immense
distances and by aeons of time. During the intervals between cataclysms, time shows her
quiet face and life can survive and prosper.
The existence of life depends crucially on the fact that time has these two faces, the
quiet and the violent, cleanly separated front each other. The violent face created us.
The quiet face sustains us and allows us to evolve. It is important to understand the two
reasons - one superficial and one fundamental - why these faces exist. The superficial
reason is that the universe is extravagantly large. Distances are so great that collisions
hardly ever happen. Planets can run like clocks, with orbits undisturbed by close
encounters with alien stars, for billions of years.
The fundamental reason for the two faces of time is that the universe is dominated by
the force of gravitation. Gravitational energy is quantitatively the largest reserve of
energy, and qualitatively the least disordered. Because of its superior quality,
gravitational energy can change easily and irreversibly into other forms of energy. Other
energy sources are associated with disorder and heat, but gravitational energy is cool.
This is why gravitational energy can drive turbines in a hydroelectric power station
with almost 100% efficiency while other kinds of power stations, run by coal or oil or
natural gas or uranium, struggle to mach 50%.
The gravitational energy of the North American ice cap waited quietly for centuries,
until the thaw and the break in the ice dam released it. It changed into the turbulent
energy of the flood that excavated a trillion tons of rock and formed the Scabland.
Similarly, the gravitational energy of the planet that collided with the earth stayed cool
for millions of years and then suddenly changed into heat at the moment of impact when the
moon was born. All over the universe, when conditions are right for gravitational energy
to be released, it can change instantly into heat and radiation, and a cataclysm results.
- The two faces of time are a consequence of the two faces of
gravitation. Gravitation is the ordering principle that holds our earth together as a
stage for us to walk on, and gravitation is the ultimate reservoir of energy that can
smash our world to pieces.
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