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Response to Gravitational Forces When the Speed of Gravity is Equal to the Speed of Light

-- Adam | December 3, 2019

EHT image of event horizon in the central supermassive black hole of M87

Question:

If the speed of gravity is roughly the speed of light, why does the earth respond to where the sun is now rather than where it was 8 minutes ago? Or more significantly, why does one side of our galaxy respond to where the other side is now, rather than where it was 100 million years ago since it is over 100 million light years across?.

-- Adam

Answer:

Note that the theory of special relativity predicts that the constant c is not just a description of the speed of light. It is in fact the highest possible speed for any interaction in nature. Therefore, the information that an object receives through gravitational interaction is limited by this speed. I am not aware of any measurements of the Earth-Sun or Milky Way galaxy gravitational interactions which disagree with our understanding of the speed of gravity and its equivalence to the speed of light.

-- Jeff Mangum

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