I’ll preface this by saying that I have a very good understanding of Newtonian mechanics, but only an amateur’s understanding of Einstein’s relativity.
I understand that Newton’s law of gravitation is insufficient to accurately describe and predict certain physical phenomena, and that Einstein’s relativity “fixes” this. I don’t understand, however, why we must do away with the model of gravity as a force to build a better model. Couldn’t Newton’s law of gravitation be amended to account for the discrepancies? It looks to me as if it’s a question of which mental model we prefer. Saying that gravity isn’t a force, it is a curvature, or vice-versa, sounds to me like saying that positive charges are actually negative and negative charges are actually positive, i.e., a matter of convention. Whether gravity >is< one or the other seems to me much more a matter of philosophy than physics properly.
So why is this such a central point?
In: Physics
Gravity is a fictitious force like centrifugal force. If you are spinning around on a merry go round and then the merry go round stops you feel a “force” pushing outward from the center of rotation. This is a centrifugal, an away from the center, force. Except you aren’t actually experiencing a force, you’re experiencing Newton’s first law. Going in a circle like on a merry go round requires a constant centripetal, going toward the center, force. If you swing a ball around your head you have to keep putting in work to keep it spinning. But once you stop putting in that work that force goes away and it will have some velocity tangent (next to) the circle in whatever direction it happens to be moving at that exact instant. And an object in motion stays in motion, so the ball will fly out of your hand in that direction with whatever speed it was spinning around with. That is the centrifugal force, it’s just inertia. Gravity is the exact same way. It seems like a force, but it’s actually just inertia in curved spacetime. It just seems like a force from certain points of view. When that ball flys out of your hand it looks like some new force was acting on it to make it do that, but it wasn’t. When you are pulled to the ground by gravity it seems like there is some force causing that motion, but there isn’t.
We can’t modify Newton to be more in line with our observations because it makes a fundamentally incorrect assumption, that gravity is a force. If you try you will inevitably lead to contradicting yourself.
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