Einstein predicted the speed of gravity equals the speed of light, but it's never been proven.
Albert Einstein
Sunlight takes 8 1/3 minutes to travel to Earth. Scientists believe gravity has the same property.
"If you had some monster or deity able to pick up the sun and completely move it away from our solar system instantaneously, what would happen to the Earth?" asked Ed Fomalont, an astronomer with the Radio Astronomy Observatory, on CBC Radio's As It Happens.
Fomalont said the Earth would continue in its orbit around the sun for another 8 1/3 minutes until the sun and its gravity disappeared – the gravitational time lag would equal the time lag of light. Then the Earth would go in a straight line.
Since no one can move the sun, astronomers had to find another way to measure the speed of gravity.
Jupiter meet quasar
Fortunately, on Sept. 8, Jupiter passed close to the light coming from a quasar, a star-like object billions of light-years away. The alignment offered astronomers their chance.
Astrophysicists used the alignment and their modern telescopes to test Einstein's assumption that light and gravity travel at the same speed.
'You can't bet against Einstein. I'd bet 100 to 1 that he's right'
-Ed Fomalont
Radio and optical astronomers are able to measure the direction to celestial objects in the sky very well. They knew the quasar was going by as Jupiter happened to be almost in front.
Jupiter's gravity should have nudged the light coming from the quasar, causing it to appear to shift its position slightly in the sky.