[Physics] When objects fall along geodesic paths of curved space-time, why is there no force acting on them

equivalence-principleforcesgeneral-relativitygravityreference frames

On cseligman.com, it is written that

So, we see things falling with an acceleration which we call the acceleration of gravity,and thinking that we live in a straight line , uniformly moving or stationary inertial frame, we attribute that acceleration to a force, the force of gravity. Whereas in reality,objects falling towards the Earth are moving along geodesic paths with no acceleration and according to modified version of law of inertia, have no force acting on them. They fall simply because the curved space-time near the Earth …

Now, why do the objects falling towards the Earth move along the geodesic paths with no acceleration? That means the objects don't have any force acting on them, but why? A body in a free-fall moves with acceleration $g$, so, why is it written like that? Why does the author use Law of inertia on freely falling body?? Law of Inertia can only be applied when no external force acts on the body. So, is the freely-falling body accelerates under force of gravity or moves uniformly while moving through geodesic paths as quoted by the author?

Best Answer

Suppose you and I start on the equator, a kilometre apart, and we both head exactly due North in a straight line, so we head off in exactly parallel directions:

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Now we know that in Euclidean geometry parallel lines remain the same distance apart. But if you and I measure the distance, $d$, between us we find that $d$ starts off at 1km but decreases as we head North and we eventually meet at the North Pole.

Se we have a paradox: we started out parallel but we moved together. The only explanation is that there is some force pulling us together. But we know there is no force really, it's just that we are moving on a curved surface.

This is what happens in general relativity, though as I'm sure you'd expect it's a lot more complicated (principally because time is curved as well). If you see a freely falling body accelerating towards the earth you'd say there must be a force acting between the body and the Earth, and you'd call that force gravity. But the general relativist would say the Earth and the object are both moving along geodesics, i.e. in a straight line, and it's just that because spacetime is curved the two straight lines converge just as we saw for motion on a sphere. There isn't really a force acting even though it looks like a force to us. That's why gravity is sometimes described as a fictitous force.

Actually there is an accelerating object involved in this, and it's you standing on the Earth's surface. How do you know you're accelerating? Well the Earth is pushing at the soles of your shoes and accelerating you upwards. Where there's a force there's an acceleration, so the conclusion must be that the surface of the Earth is accelerating you outwards while the freely falling object you're watching is not accelerating.

If you're interested, twistor59's answer to What is the weight equation through general relativity? explains how to calculate this acceleration, though you may find the maths involved a bit hard going.

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