In general relativity, it is assumed that spacetime is curved by the presence of matter (energy), this curvature being represented by the Riemann tensor.

In special relativity, the Riemann tensor is identically zero, and so this concept of “non-curvedness” is sometimes expressed by the statement Minkowski spacetime is flat.

According to Einstein,the fourth dimension was space-time, an elastic membraneous fabric,and mass grabbing the space-time would be telling it how to curve and space-time grabbing mass would be telling it how to move.This is the explanation of gravity by einstein.Now we know space-time is an elastic membrane and if einstein was right,his thoery of special relativity says that time dilation will occur if a body approaches relativistic speed(speed closer to the speed of light) and this means that as our velocity increases,time becomes relative. Similarly there’s contraction of space at relativistic speeds,so time dilation is due to the factor of length contraction,as lesser distance is travelled.So space and time have become relative phenomena for observers in inertial frame of reference.