Gravity is not a Force?

Question

I don't know much about this topic, but I read something saying that gravity is not a force using an example of inertial observation. I started thinking about the topic again when I was researching photon spheres and thought why does gravity (if it is a force) curve light when photons have no mass. The only possible answer would be that gravity is not a force but rather the warp of space around mass. Would this be right?

Answer 1

You are correct that gravity is not a force in the normal sense of the word. As others have said, gravity is caused by the warping of spacetime. But more specifically by the warping (or dilation) of time. Nobel Lauriat Kip Thorne referred to what he calls “Einstein’s Law of Time Warps”. He said, “Things like to live where they age the most slowly. Gravity pulls them there. And so as an application, the Earth's mass warps time according to Einstein. It slows time near the surface of the Earth. And this time warp is what produces gravity.”

So now that you understand that mass causes a slowing (or dilation) of time, you can start to look at the bending of light differently:

You know that when you shine a light into a prism, it bends the light. We are often told that it is the denser medium of the glass prism that causes the bending. But in reality it is the slowing of the speed of light in that denser medium that causes the bending. So what happens when light from a distant star passes by a star or galaxy? Well that light is slowed by the time dilation near that star. It is this slowing of the speed of light that causes the bending or refraction of the light. No mass of the photon is required as you said.

Answer 2

The best theory on gravity that we know is General Relativity, where those concepts of curved space-time are part of it. But even Newtonian gravity can be understood as involving a curved space-time. When the mass of one of the bodies is much greater than the other (like earth orbiting the sun), and the only 'force' acting on the smaller mass is gravity: $$F = ma = m\frac{d^2\mathbf r}{dt^2} = \frac{-GMm\mathbf{\hat r}}{r^2}\implies \frac{d^2\mathbf r}{dt^2} + \frac{GM\mathbf{\hat r}}{r^2} = 0$$

The last equation can be easily modified to have the same form of a differential equation known as geodesic equation. Geodesics are a kind of extension of the notion of straight line in a more generic case. So, the movement of planets can be understood as a geodesic in a 4-D space-time, even in Newtonian theory.