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Q & A: speed and gravitational deflection

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Most recent answer: 05/02/2014
Hello. A friend of mine claims that the amount of gravitational deflection experienced by a small projectile moving past a large massive object depends on the speed of the projectile. His argument is that gravity is a progressive force, acting over a period of time. If the projectile moves towards the object at a low speed, it takes a long time for it to move past the massive object, resulting in a specific angle of deflection. If the projectile moves faster, it spends less time near the massive object, giving gravity less time to operate, and therefore the resulting angle of deflection is smaller. I seem to recall that the effects of gravity do not depend on the speed of the objects interacting, so I am not sure my friend is correct, but I am not sure how he is incorrect either. Can you please shed some light on this? Thank you very much.
- Diego Sebastián (age 39)
Madrid, Spain

Your friend is right. Let's look at an extreme case. Throw a ball up slowly. It will "deflect" so much that it falls right back to Earth. Throw it up very fast. Ignoring air friction, it will keep going up forever, never falling back to Earth. Those are really different paths!

I think perhaps what you remember is that the path doesn't depend on the mass of the ball, assuming it's small compared to that of the planets etc. around. In general relativity one says that the "geodesic" path doesn't depend on the mass of the object. Those geodesic paths, however, are in four dimensions, including time. So if two objects share a piece of geodesic path, that means that they're moving with the same velocity.

Mike W.

(published on 05/02/2014)

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