 To make the notion of curvature pathology more precise, we will use the manifestly physical idea of tidal force. Tidal force is generated by the difference in intensity of the gravitational field at neighboring points of spacetime. For example, when you stand, your head is farther from the center of the Earth than your feet, so it feels a (practically negligible) smaller pull downward than your feet. Inertial Frames.) -->  Tidal forces are a physical manifestation of spacetime curvature, and one gets direct observational access to curvature by measuring the resultant relative difference in accelerations of neighboring test bodies. For our purposes, it is important that in regions of extreme curvature tidal forces can grow without bound. 