Special Relativity in 14 Easy (Hyper)steps
7. Definition of an event: measurements
specify intervals between events
We use the term "event" to mean something quite specific
in the context of Special Relativity: an event is something which happens
at a single point in space at a single instant in time.
You need to specify both
the position and the time of an event
to define completely its coordinates.
Ignoring the finite sizes of the Nostromo and Sulaco
clocks, the following are events:
-
Nostromo clock passes Sulaco's forward clock
(length measurement which only used one clock)
-
Nostromo clock passes Sulaco's aft clock (length
measurement which only used one clock)
-
Nostromo clock 11 is passed by Sulaco's nose
(length measurement which required at least two clocks)
-
Nostromo clock 3 is passed by Sulaco's tail
(length measurement which required at least two clocks)
The following are not events:
-
Nostromo crew eats breakfast (doesn't happen in an
instant, or at a single point in space)
-
Nostromo crew measures the length of Sulaco
(either takes place over a non-zero time interval [one-clock length measurement]
or a non-zero space interval [two-clock length measurement])
Usually, measurements specify the intervals
(differences in spacetime coordinates) between pairs of events.
The determination of the length of Sulaco is a
measurement which requires knowledge of the space and time coordinates
of
two distinct events:
-
For the length measurement which used one clock:
-
position of (and time displayed by) Nostromo's clock
as it passed Sulaco's nose
-
position of (and time displayed by) Nostromo's clock
as it passed Sulaco's tail
-
For the length measurement which used clocks 3 and 11:
-
position of Nostromo's clock 11 as it came into alignment
with Sulaco's nose (just as it read time
0)
-
position of Nostromo's clock 3 as it came into alignment
with Sulaco's tail (just as it read exactly
the same time as clock 11)
(If the positions were determined
at different times, the length measurement would be inaccurate.)
