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No, the source of gravity is energy-momentum, which includes many more things other than just mass. For example, and electromagnetic field (in otherwise empty space) would also be a source of gravity, as would be stresses and strains in the interior of a planet (e.g.). It needs to be spacetime, not just space. It is not meaningful, in the context of gravity, to separate space from time, and vice versa. As to what spacetime is - it is quite simply the set of all events, meaning the set of all spatial locations at all instances in time. It is thus a mathematical model. No. When any test particle - irrespective of whether it has mass or not - is affected by gravity (and only gravity, for simplicity), then that means that its world line in spacetime is a geodesic of that spacetime. It is a purely a geometric phenomenon. When space is expanding, that means that the separation between any two points within that space increases over time. However, locally those points remain at rest - you can attribute a relative velocity to these specific points, but not to space (that would be meaningless). Relative motion is not a source of gravity, so it does not 'warp' spacetime. What GR does is model the motion of test particles in the presence of sources of energy-momentum; as such, its predictions are quite physical indeed. This is just what Newtonian gravity does, and such a model works quite well in the low-velocity, weak field domain. However, once you venture further into the strong field regime, the predictions of Newtonian gravity are no longer accurate. And even in the everyday low energy domain - consider putting an accelerometer into free fall (drop it off a tower etc). It will read exactly zero at all times while it is falling - and zero acceleration means no force is present. And yet, the falling accelerometer is very clearly still affected by gravity. So gravity cannot be a force in the Newtonian sense. There are also deeper, more technical reasons why gravity cannot accurately be modelled by a vector field. You cannot accelerate a massless test particle. You have every right to be, because the way GR is generally presented does indeed make it confusing, once you give it more than just a passing glance. Spacetime is not a mechanical medium, so curvature is not any kind of mechanical 'bending'. As mentioned above, spacetime is simply the set of all events, and the geometry of spacetime can be thought of as how these events are related to one another. If the geometry is flat, then that means the relationship between any pair of neighbouring events will be the same, regardless of where/when in spacetime you are (like on a flat sheet of paper). If spacetime is curved, then this is no longer true - the relationship between a given pair of neighbouring events depends on where that pair of events is located in space and time. That's the meaning of curvature - a change in the relationship between events. It's a geometric property, not a mechanical action. This is analogous to the longitudinal lines on a globe - at the equator, they are spaced apart by a specific distance, but as you go north (or south), that distance will change, even though these lines are perfectly straight within the surface. That's because the relationship between points on those lines changes depending on where you are, since the surface has intrinsic curvature. Spacetime is the same, just in two more dimensions.

Why do you not get rid of the fraction first ?

...make a computer game and put VR helmets on players' heads... Disable players' memory so they don't know they're in a simulation and simulate their entire lives in Career Mode. Load them onto a spaceship flying to other planets in space. The creator, administrators and moderators will properly punish misbehaving players, spoiling the careers of other players. ps. Think big Asimov, think big!