[emptybits]

Wild. My layperson mind goes to a simple example, which may or may not be possible, but please tell me if this is the gist:

Alice and Bob, in different reference frames, both witness events C and D occurring. Alice says C happened before D. Bob says D happened before C. They're both correct. (And good luck synchronizing your watches, Alice and Bob!)

[hinkley]

Yes that definitely happens. People orbiting Polaris would be seeing two supernovas explode at different times than us due to the speed of light. Polaris is 400 light years away so the gap could be large.

But when you are moving you may see very closely spaced events in different order, because you’re moving toward Carol but at an angle to Doug. Versus someone else moving toward Doug at an angle to Carol.

[mrkeen]

That will be the case when Alice stands close to where C happens, and Bob stands close to where D happens.

It's a little trickier to imagine introducing cause-and-effect though. (Alice sees that C caused D to happen, Bob sees that D caused C to happen).

I think a "light cone" is the thought-experiment to look up here.

[ianburrell]

There is distinction between seeing when events happened, and when they really happened. The latter can be reconstructed by an observer.

In special relativity, time is relative and when things actually happened can be different in different frames. Casually linked events are always really in the same order. But disconnected events can be seen in different orders depending on speed of observer.

[emptybits]

> 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?

[hinkley]

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.

[ianburrell]

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.

[hinkley]

We still haven’t proven whether some quantum effects do or don’t follow this. So there may be a loophole where information can move faster than light but the carrier for that information can not. Which might make ansibles possible some day, with the caveat that you can only have so many conversations per ansible before you need a refill with new entangled matter. In which case you have to divide the information by the travel time to determine your aggregate data rate. And the travel time will be at a fraction of the speed of light.

So you might be able to consult on which planet to terraform but you’re not going to video call the wife and kids unless you’re the richest person in the galaxy.

[thaumasiotes]

> 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.

[hinkley]

If Bob and Alice are moving at half the speed of light in opposite directions.