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Q & A: why is Newtonian gravity inconsistent with fixed light speed

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Most recent answer: 01/30/2017
Q:
why was the Newtonian understanding of gravity inconsistent with Einstein's concept of the speed of light being the "cosmic speed limit"?
- Aisha (age 18)
LA,CA,USA
A:

That's a very deep question.  Here's a first try at an answer. We'd be happy to follow up if needed.

The fixed speed of light is just part of the whole program of special relativity, that the laws of physics are exactly the same for observers moving at fixed velocity relative to each other, and that the equations (Maxwell's) that give the speed of light are among those laws of physics. Special relativity runs into very bad science-fictiony problems if any signals can travel faster than that speed, "c". 

Newton's gravity acts instantly at a distance. So if some mass moved around, it would instantly change the gravitational field far away. That's a signal traveling faster than c. Whoops. So some other theory is needed in which changes in the field themselves move around no faster than c.

Mike W.

 


(published on 01/30/2017)

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