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> The author hasn't neglected to consider light cones She describes them, but does not consider their implications for the structure of spacetime. > you could
by pdonis 4y ago
> The author hasn't neglected to consider light cones
She describes them, but does not consider their implications for the structure of spacetime.
> you could always (hypothetically) boost to another frame in which the disconnected past becomes the disconnected future (and vice versa).
No, you can't. That's the whole point of looking at light cones instead of spacelike "now" slice. Light cones are invariant; they're the same in all frames. Spacelike "now" slices are not. Also, you can't do a Lorentz boost that turns "future" into "past" or vice versa.
- jamesmaniscalco 4y agoI think OP means “disconnected past” as the spacelike-separated region south of the “now” slice. Boosting could transform points in this region into the spacelike-like separated region in the “future” (i.e. the “disconnected future), no? The points are not transforming across the light cone boundary, just across the arbitrary “now” plane.
- panda-giddiness 4y agoYes, this is what I meant. By "disconnected" I meant causally disconnected, i.e. spacelike separated. Sorry for not making that clearer! And yes, you can always boost to a frame such that the "order" of spacelike separated events will change (i.e., reverse which event is above and which is below a line of simultaneity). Since you can't objectively say which event happened first, they must be unable to influence each other, hence "causally disconnected".