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### WHAT IS AN EVENT? THE RELATIVITY OF SIMULTANEITY

An “event” is anything that happens at some definite locations at some definite time. Typical examples are your birth, assassination of Abraham Lincoln etc. In contrast, a forest fire that sweeps across 10000 acres in 5 days does not constitute an “event” because the fire is spread out in space and time.
The adjective “definite” means   “distinct” or  “limited” for any one observing the happening.

THE RELATIVITY OF SIMULTANEITY
• Spatially separated events that are simultaneous in one frame are, in general, not simultaneous when viewed from other reference frame.
• Simultaneity is a relative concept, but not an absolute one.
• The concept of simultaneity between two events in different space points has an exact meaning only in relation to a given inertial system i.e. “Each frame of reference has its own particular time”.
• To measure the length of an object means to locate its end points simultaneously. As simultaneity depends on frame of reference, the length measurements will also depend on frame of reference.
• Thus, “The length i.e. Space is a relative concept, not an absolute one”.
• Thus there is no such thing as an absolute, global “now”.