It is sometimes argued that time travel violates conservation laws. For example, sending mass back in timeincreases the amount of energy that exists at that time. Doesn't this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of theequations of physics imply local conservation. A local conservation law tells us that the amount of stuffinside a small volume changes only when stuff flows in or out through the surface. A global conservation lawdrops the assumption that the stuff must flow through the surface: it integrates over all space and assumes thatthere is no flow in or out at infinity. If this integral cannot be performed, then global conservation doesnot follow. So, sending mass back in time might be all right, but it implies that something strange ishappening. (Why shouldn't we be able to do the integral?)


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