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Q & A: Is gravity a force?

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Most recent answer: 01/01/2018
Einstein said there is no such thing as a gravitational force. Mass does not attract mass over a distance. Instead, it curves spacetime and then follows the curved spacetime geodesic as if there is no force on it. If there is no gravitational force, then how does a mass on a spring pull and extend the spring in a gravitational field? On the other hand, if there IS a gravitational force, then why not just say that the acceleration is proportional to the gravitational force?
- Khushro (age 55)

Let's say Einstein is right. Then that mass on the spring, following the no-force "geodesic" spacetime path, would bump into the Earth, also following its own geodesic path, if the spring were not attached to some mount. A chain of genuine forces (e.g. in steel bars) keep the spring from following its geodesic, and the spring forces keep the mass from following its.

What's wrong with saying that gravity is a force? It works well for familiar objects in what we imagine to be a flat spacetime. If you try to calculate the amount light bends as it goes past a star, however, that picture only gives half the bend. It also doesn't give any gravitational waves (ripples in spacetime) and thus cannot explain the recent waves from colliding black holes and neutron stars. Which reminds me, it can't explain black holes, which involve such extreme distortions of spacetime that they can't be described by a flat spacetime with an extra force to account for the curved behavior. Many other such behaviors, including major timing effects on GPS systems, also follow from Einstein's picture but not from a gravity-force picture.

Mike W.

(published on 01/01/2018)

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