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If the Sun Suddenly Disappeared Would We Notice It Immediately?

-- | February 24, 2016

Question: Hi National Radio Astronomy Observatory people! My name is Israel Romero and I am a Software Engineer. So, I have this hypothetical experiment. Let’s say that suddenly, the Sun disappears in space… it just vanishes. My assumption is that the Earth and all objects bound to its gravity would instantaneously be lost into space at a huge tangential speed. However, the Earth is about 8 light-minutes away from the Sun, therefore we would still see the Sun for 8 more minutes… then all darkness after that. So, the question: is gravity faster than light? Thanks! — Israel

Answer: The General Theory of Relativity, which is the one that Einstein formulated to explain a lot of things, including gravity waves, predicts that the speed of gravity, or the speed of a gravitational wave, is equal to the speed of light. I believe that the recent LIGO detection of gravity waves from a merging pair of black holes is consistent with this prediction. Applying this to your thought experiment, if the Sun disappeared instantaneously, we poor folks on the Earth would not notice it until about 8 minutes and 20 seconds after it had happened due to the light (and gravity wave) travel time from the Sun to the Earth.

Jeff Mangum

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