we all have heard about general relativity. Something related to space-time curvature...
At the time of Einstein (around 1905), only two fundamental forces were known (Newtonan law of gravitation and Maxwell laws of Electromagnetism). Today, we know two others (weak and strong interactions). The special relativity had focused on electromagnetism and solved the problem of constant speed of light in every inertial referentials.
After his theory of special relativity, Einstein still had a problem. The Newtonian law of gravitation was not relativistic. He first emitted his principle of equivalence which will eventually lead to the theory of general relativity, the only theory at that times able to predict the existence of blach holes !
But let's just make a pause on the principle of equivalence. Basically, this principle states that it we are in a rocket without window to see the outside, it is impossible to guess with physical experiment (mecanics, electromagnetism) if the rocket is accelerated in outer space or if it is landed at the surface of a planet creating the equivalent gravitational acceleration. For exemple, at the surface of the earth this acceleration is about 9.8 meter per second square, ie a speed variation of 9.8 m/s per second.
Let's now go a little bit further, In outer space, we feel weightlessness if the rocket is inertial (engines are off). The principle of equivalence states that if we are in an imaginary lift dropping in the gravitational field (without air in the elevator shaft), we will feel the same a few seconds before the crash. So if we had enough time to make physical experiments in this dropping elevator, we would obtain the same results than people in the ISS. For exemple, if we fire a horizontal light in the elevator cage, we will see the endpoint exactly at the horizontal of the source "in the elevator". But during the time necessary for light to travel the distance from source to endpoint, the elevator has accelerated. That means that from the point of view of the earth surface, the light ray has been curved...
All this to say that the inertial referential is not the earth surface (it is accelerated upwards), but the dropping lift which goes at a constant speed in a curved space-time due to the presence of mass.
Amazing, isn't it ???
Hope you enjoyed this little trip in the world of theoretical physics