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Comment by CollinEMac | Hacker News Reader

> The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.

Maybe I'm misunderstanding as it's been a while since physics class but I'm pretty sure gravity does cause acceleration. One of the few numbers I still remember memorizing in college 9.8m/s2 -- the acceleration by gravity on Earth.

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If you let go of something, it will stop accelerating because there is no longer a force on it, and start following its geodesic. You have to apply a force to make it not do that (like, by standing on the Earth so the ground can push you upward). The natural path (geodesic) of an object near the Earth plotted from a purely space-like (Newtonian) perspective accelerates toward the Earth. But the point of GR is to not have a purely space-like perspective.

(I made up the waffly term “purely space-like” to not have to explain what an inertial frame is, not sure if that’s going to help.)

That's the acceleration you feel on the surface of the earth. But that's not an inertial reference frame. In an inertial reference frame gravity doesn't cause acceleration (kind of by definition).

The argument is that gravity doesn't cause acceleration, resisting gravity does. Kind of how spinning an object doesn't cause a centrifugal force, the real force is whatever forces it to stay on a circular path instead of continuing straight

> Kind of how spinning an object doesn't cause a centrifugal force, the real force is whatever forces it to stay on a circular path instead of continuing straight

Let's say the Moon is in a circular orbit around the Earth (close enough), what's the real force that's forcing it to stay on that path? If it's not gravity, what is it?

The Moon moves not in circles but rather along a straight line in curved spacetime. It doesn't require any force to stay on this path — it is in free fall.

Great question. The answer is nothing. There is no force making the moon follow a circular path. The moon "thinks" it's moving in a Newtonian, inertial "straight line." Because the spacetime around the earth and moon is curved, the moon moves in a straight line through that curved space.

Caution: The circular path we see the moon follow is not the curvature of spacetime itself. Rather it's a zero-force iso-line along that 4D spacetime. This is also called a geodesic.

That’s relative to the surface of the earth

That's also true for, say, electromagnetism. The strength of the force follows the inverse square law, so the acceleration applied on an charged object depends not only on the intensity of the charges but also the distance between the two charges. The only difference between classical gravity and electromagnetism (in a mathematics sense) is that gravitational "charge" is mass, which means the mass term of the accelerated object can be canceled out since it is on both sides of the equation. For an object with a fixed mass (like the earth), that means you can say the acceleration is purely a function of distance rather than distance and mass of the other object. But I don't see how that makes the acceleration term invalid or implies that gravity doesn't actually cause acceleration.

This is because classical gravity is wrong. The easiest way to notice this is to build an accelerometer, and show that while it is in freefall towards the Earth, it measures the same reading as it does while it is in space far away from any massive body. However, when it is resting on a table on the earth, it will measure an acceleration away from the table, upwards. The same direction of acceleration it will measure if put on a rocket in the direction of movement of the rocket.

You yourself are such an accelerometer. The feeling you get while falling is the same feeling you'd get if you lived on the International Space Station. If you wanted to add a module on the ISS to make you feel just like on earth, you'd a module that generates a force on your feet up towards you head, not a force that pushes on your head up towards your feet.

So, Newton's universal force of attraction is quite clearly wrong, and there is no equivalent force in reality.

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