Today, a practical use for an abstract theory. 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.
First, a simple fact of
physics: If you average the velocity of one
molecule of the air in your room, as it collides
from one molecule to the next, that average comes
out the same as for all molecules in the room at
Now stretch that idea a little: Suppose we want to
know how the instant average of a set of molecules
fluctuates from moment to moment. This time, we can
imagine many similar sets of molecules and then
calculate how their averages differ at any instant.
In broader terms, physicists find they can simplify
their calculations if they replace history with
That's what we call a thought
experiment -- one we do only in our minds as an
aid to calculation. Now let's pretend that our
universe really is replicated over and over -- that
our world is just one realization along with all
Let's think about how our lives might fluctuate in
those other worlds. Remember that time you almost
had an accident -- how you just happened to turn
your head at the right moment? Remember that one
time in a hundred when you zigged instead of
zagging? Our lives are an accumulation of such
If such alternate universes really existed they'd
spin themselves out alongside ours with differences
that don't come from molecular fluctuations alone,
but from free will choices as well. Remember when
you teetered over a decision -- buying a house,
accepting a job, or just deciding which way to come
home from the store.
This thought model isn't a matter of
science-fiction speculation. The physicist uses it
to predict the most probable state of the universe,
along with its fluctuations. But the point is that
all those worlds fluctuate. In one, the Germans
don't bring Hitler to power. In another, no one
invents penicillin. In a third, I zag instead of
zigging after all.
We gain perspective by imagining parallel universes
out there, with our other selves making other
choices, or having different luck. If we hadn't
dropped the Hiroshima bomb, full scale nuclear war
with Russia might've looked viable in 1962. Someone
would've used the accursed thing sooner or later.
If you hadn't messed up in 1970, you might be a
less worthwhile person today.
The laws of probability say those parallel
universes only differ in the small stuff. If your
other self out there made one crucial choice you
wish you'd made, how different from you would he
be? We give too much weight to a few decisions that
loom large. Actually, we're formed in a thousand
undramatic day-by-day choices.
Physicists call these parallel universes
ensembles. I call that a fine reason
for rejoicing in the hand life deals us. Ensemble
theory reminds us that those other selves would be
doing not much better or worse than you and I -- in
the only universe we know.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds