I understand that a clock on the surface of the earth is observed to run more slowly than a clock in orbit or a clock "at infinity". What about a clock situated at the center of the earth. Would it be slowed because of the concentration of mass? Or would it run normal speed because there is no net effect of gravity? Is there a slow down simply because it is in a gravity well? How does a clock situated at the cener of the earth run compared to a clock on the surface and compared to a clock at "infinity"? And (briefly) why? Thanks.