Science Puzzle
If the Sun Vanished, When Would We Know?
Imagine that in a single instant the Sun simply vanished. No explosion, no warning: one moment it's there, the next it's gone. This can't really happen, which is why it makes such a useful thought experiment.
Earth is about 150 million kilometres from the Sun, and light travels at about 300,000 kilometres per second. How long would it be before our sky went dark, and when would Earth stop following its curved orbit?
The Answer
Both at the same moment: about 8 minutes and 20 seconds later.
The darkness is the easier half. Divide 150,000,000 km by 300,000 km per second and you get 500 seconds, which is 8 minutes 20 seconds. Sunlight already on its way would keep arriving for all that time, so the sky would stay bright and nobody on Earth would notice a thing.
Gravity is the surprising half. In Newton's picture gravity acts instantly, so Earth would fly off the moment the Sun vanished. Einstein's general relativity says otherwise: a change in gravity spreads out through space as a ripple travelling at the speed of light. So for 8 minutes 20 seconds Earth would go on curving round an empty patch of space, still steered by the Sun's gravity from 8 minutes earlier.
This has been tested. In 2017 detectors picked up gravitational waves from two neutron stars colliding 130 million light years away, and telescopes saw a burst of gamma rays from the same crash just 1.7 seconds later. After a journey of 130 million years, the two had kept pace to better than one part in a hundred trillion.
When the last light and the last pull of the Sun's gravity reached us, the sky would go dark and Earth would carry on in a straight line in whatever direction it happened to be heading, at about 30 kilometres per second.
The principle: Nothing travels faster than light, not even a change in gravity. News of any event, whether it arrives as light or as a change in gravitational pull, takes time to cross space: from the Sun to Earth, about 8 minutes 20 seconds.