3 ms·

There is a guarantee of the order of causally related events, just not an absolute timing. All observers will agree that Alice throws a ball which Bill later ca

There is a guarantee of the order of causally related events, just not an absolute timing. All observers will agree that Alice throws a ball which Bill later catches, but different observers will disagree on when these events occurred exactly. They will however all agree that the order is Alice throws, Bill catches, and none will see Bill catch a ball that Alice later throws. To use the lightning bolt example from Einstein, different observers will disagree on the timing of the strikes, and whether or not they’re simultaneous. If one of the bolts strikes Alice and she goes flying though, everyone will agree that the order of events insofar as Alice is struck before she’s launched. This all assumes no FTL or CTCs of course, with either you would lose causality and events separated by spacelike intervals could influence each other.

pentops 8y ago
> All observers will agree that Alice throws a ball which Bill later catches I think this needs qualification. If I am stood directly behind Alice or Bill then it is possible I never see the interaction at all even though I am an observer, just as if I hold a two identical pencils together and look at them from the correct angle I observe only a single pencil. The important thing is in all cases, observers "see" space-time interactions as the geodesic of the event projected on to their own observational frame. Through that projection it is possible to lose degrees of freedom which can be space-time etc. It's through this projection that HUP emerges since if in the observational frame the time dimension is limited, the projected time axis of the event tends toward zero.