Today, the speed of gravity. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
In 1609, Galileo began using
his new telescopes to radically change our view of
the heavens. Astronomy moved very quickly after
that, and seventeenth-century astronomers didn't
realize right away that they were about to resolve
another ancient question: "Is light transmitted
instantaneously, or does it move at a finite
Galileo believed that light took time to travel. So
did Moslem scientists before him, as well as Roger
Bacon, who worked on basic optics in the thirteenth
century. But Kepler, Descartes, and others believed
that light was transmitted instantaneously.
You might think we had to wait for modern physics
to give us an answer, but Danish astronomer Ole
Roemer measured the speed of light as early as
1676. He was timing the orbits of
Jupiter's moon Io. Starting in the fall of
the year before, he found each rotation a little
slower than the one before it. By spring, Io had
lost fifteen minutes in its orbit. Was the
solar system slowing down?
Then Roemer realized: Earth was hundreds of
millions of miles farther away from Jupiter in the
spring. Light had to travel that extra
distance, and it took extra time to make the trip.
Roemer put pencil to paper and found that light
moved at 192,500 miles per second. He had the speed
of light within three percent -- only sixty-six
years after Galileo first turned his telescopes to
the heavens. And Isaac Newton was still young.
I mention Newton because, when Roemer made his
measurement, Newton was on the trail of another
elusive quantity: gravity. He'd already
formulated most of his Principia, but he
was sitting on it.
Then Halley, of Halley's Comet fame, asked Newton
about gravity. Newton answered by showing how his
law of universal gravitation could be used to
predict elliptical planetary orbits. (After that,
he got to work and finished the
Now gravity was out on the table, wanting to be
explained. Newton ducked when he was asked
why his law was true. "I don't make
hypotheses," he said. But he did have an
opinion about the speed of gravity. Newton thought
about gravity the way Pascal had thought about
light -- that it was transmitted instantaneously.
Our first instinct is to agree. It seems surprising
that, if the sun were somehow to magically
disappear, we'd have to wait around for many
minutes before the lingering gravitational pull of
the sun released us to spin off into the void.
Einstein, on the other hand, believed that gravity
traveled at the same speed as light. Only now
have astronomers finally announced that
they've measured how light from a distant quasar
bent as it passed around Jupiter. From that,
they've deduced an actual speed of gravity. They've
concluded, within only twenty percent error, that
light and gravity really do go at the same speed.
I find that exhilarating. For, if the measurement
proves to be valid, it puts such a fine finishing
flourish on a four-hundred-year-old quest.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
For more on Ole Roemer's calculation of the speed of
light, see Episode 682.
For more on Galileo's telescope observations, see
Episodes and 266 and
For more on Newton and gravity, see Episode 606.
This episode was written in 2003. Since then the experimental verification
has been subject to some questions, although few people doubt that gravity
does indeed travel at the speed of light. See, e.g., the Wikipedia page
on The Speed of Gravity.
From the 1925 Boy Scientist
illustration of how gravity bends light, in
accordance with Einstein's theory
Also from The Boy Scientist
illustration of the Solar System showing how the
distance from Earth to Jupiter's moons can vary as
the planets orbit.
The Engines of Our Ingenuity is
Copyright © 1988-2003 by John H.