The tidal force stretches an object in one direction while compressing it in a perpendicular direction. An example of a longitudinal tidal force is the moon's effect on the earth's oceans to give the tides. It occurs along the line of sight. Another example of a longitudinal tidal force is if an individual fell feet first into a black hole. A gravitational wave, on the other hand, exerts a transverse tidal effect.
General relativity explains gravity as the curvature of space- time by mass.
Any particle, such as a ball, within the gravitational field of another mass, such as the earth, follows what is called a geodesic. In flat space-time, this path is a straight line. On a curved two-dimensional surface such as the earth, it is a great circle. From Einstein's point of view, there is no tidal force. Instead, the earth curved the space-time so that the two geodesics came together.