alt Hacker News> But disconnected events can be seen in different orders depending on speed of observer.
What are "disconnected events"? In a subtle but still real sense, are not all events causally linked? e.g. gravitationally, magnetically, subatomically or quantumly?
I can understand that our simple minds and computational abilities lead us to consider events "far away" from each other as "disconnected" for practical reasons. But are they really not causally connected in a subtle way?
There are pieces of space time that are clearly, obviously causally connected to each other. And there are far away regions of the universe that are, practically speaking, causally disconnected from things "around here". But wouldn't these causally disjoint regions overlap with each other, stringing together a chain of causality from anywhere to anywhere?
Or is there a complete vacuum of insulation between some truly disconnected events that don't overlap with any other observational light cone or frame of reference at all?
Replies
Causality moves at the speed of light. Events that are farther apart are called spacelike and aren't causally connected.
I think you might be confusing events that have some history between them, and those are influence each other. Like say right now, Martian rover sends message to Earth and Earth sends message to them, those aren't causally connected cause don't know about the other message until light speed delay has passed.
> But wouldn't these causally disjoint regions overlap with each other
Yes.
> stringing together a chain of causality from anywhere to anywhere?
No? Causality reaching one edge of a sphere doesn't mean it instantaneously teleports to every point in that same sphere. This isn't a transitive relationship.
> What are "disconnected events"?
The sentence you're responding to seems like a decent definition. Disconnected events are events which might be observed in either order depending on the position of an observer.
We now know that gravity moves at the speed of light. Imagie that you aretwo supernovas that for some unknown reason, explode at essentially the same time. Just before you die from radiation exposure, you will see the light pulse from each supernova before each supernova can 'see' the gravitational disruption caused by the other. Maybe a gravity wave can push a chain reaction on the verge of happening into either a) happening or b) being delayed for a brief time, but the second explosion happened before the pulse from the first could have arrived. So you're pretty sure they aren't causally linked.
However if they were both triggered by a binary black hole merger, then they're dependent events but not on each other.
But I think the general discussion is more of a 'Han shot first' sort. One intelligent system reacting to an action of another intelligent system, and not being able to discern as a person from a different reference frame as to who started it and who reacted. So I suppose when we have relativistic duels we will have to preserve the role of the 'second' as a witness to the events. Or we will have to just shrug and find something else to worry about.