Today, we do experiments in our heads. 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.
The late 19th-century German
philosopher Ernst Mach added a phrase to our
language. It was "thought experiment." It's a
wonderfully contradictory idea.
One scientist observes and measures nature. Another
writes mathematical theories. Experiment and
thought seem to be separate roads to learning. Mach
walked the experimenter's road. But he said we can
also experiment in our heads, not just in nature.
He said, look what Galileo did. Aristotle thought
heavy things should fall faster than light ones. He
thought a stone should fall faster than a feather.
So Galileo said, "Let's glue a feather to a stone
and drop it. Together, they weigh more than either
the feather or the stone alone. If Aristotle's
right, they'll fall faster together than either
would fall alone. But who could believe the feather
wouldn't slow the stone? Aristotle has to be
That's what we call a thought experiment. Galileo
did the experiment in his head. Mach made a funny
argument to justify doing that. He said our
intuition is accurate because evolution has shaped
it. We shrink from the absurd. That's what Galileo
was doing when he said a feather can't speed the
fall of a stone.
Enter now the grand master of thought experiments,
Albert Einstein. Einstein was Mach's disciple. He
created a great theater of thought experiments to
explain relativity. But relativity flies in the
teeth of our intuition. He talked about throwing
balls and reading clocks on railroad trains and
elevators. His conclusions had nothing to do with
Mach finally cracked under the weight of Einstein's
admiration. Relativity theory was, he wrote,
"growing more and more dogmatic." For Mach, a
theory was only a framework for our measurements.
Einstein figured that you first wrote correct
theories. Then the data would simply fall into
In 1919 Einstein shrugged off an important
telegram. Someone wired that they'd seen rays of
light bend under the influence of gravity. That's
what relativity predicts. Einstein was unimpressed.
If light hadn't bent, he said, then "I'd have been
sorry for the dear Lord. The theory is correct!"
So Einstein took Mach's thought experiments and
used them in ways Mach never intended. He used them
to show us a world that's far more subtle and
surprising than anything our intuition ever
prepared us for. Mach's ideas about thought
experiments finally faded because the world is much
stranger than our capacity to imagine it.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds