No. 1765:
The Speed of Gravity
Audio

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 created them.

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 speed?"

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 Principia.)

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 work.

(Theme music)

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 321.

For more on Newton and gravity, see Episode 606.

I wrote this episode in 2003. Soon after, new measurements of the speed of gravity bracketed the speed of light closely as to remove any doubt that gravity and light move at the same speed. See details in the Wikipedia page on The Speed of Gravity.

From the 1925 Boy Scientist, an illustration of how gravity bends light, in accordance with Einstein's theory

Also from The Boy Scientist, an illustration of the Solar System showing how the distance from Earth to Jupiter's moons can vary as the planets orbit.