ellis@spar.UUCP (Michael Ellis) (11/20/85)
[Note- this is a partial response to a flame from Mr. Gwyn concerning
a previous article of mine..]
>Consider simultaneity: before Einstein, it was assumed that simultaneity was
>an inherent property of the relationship between two events; either they
>were simultaneous or they were not. But Einstein pointed out that
>whether two events are perceived as simultaneous depends upon the location of
>the observer. *Any* two different events will be appear to be simultaneous
>to an observer at the right location. - Jim Balter
This is an overstatement -- only two events separated by a SPACELIKE
4-distance can be seen as `simultaneous' by an appropriately chosen
observer.
BTW, two events which can be causally connected must be separated
by a TIMELIKE (or NULL, in the limiting case) 4-distance.
4-distance is invariant to all observers, whereas the spatial or
temporal separations vary according to observers. The formula
for 4-distance between two events is:
------------
\/ x*x - t*t
where x is the spatial distance (in terms of the speed of light)..
t is the amount of time..
..separating the two events. 4-distances are categorized:
>0 Spacelike
=0 Null
imaginary Timelike
Timelike and null intervals are crucial to modern notions of
causality. Since 4-distances are invariant, if I throw a
rock thereby breaking a window, all observers will agree
on the order of events (I threw the rock, then the window broke).
-michaelgwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (11/29/85)
> [Note- this is a partial response to a flame from Mr. Gwyn concerning > a previous article of mine..] > ... > >to an observer at the right location. - Jim Balter Uh, "Jim Balter" is not a pseudonym of mine, thanks just the same. I tried to figure out what this message might nonetheless have to do with anything I ever "flamed" about, but since it didn't say anything I didn't already know I think you have simply misattributed the posting to which you were responding. The only thing I recall in this regard was a statement that Hume is not the final word on causality. Unless you were perhaps talking about frictionless monkeys?