Observed Time vs Observed Duration

1. Jan 27, 2008

gonegahgah

Obviously when we observe an event it has not occured at the time that we observed it occuring as light travel time must be taken into account.

However within our frame is the observed duration of an event (ie a star exploding) considered to be the actual amount of time the event took to occur; even when it event is moving away or towards us?

Question in short: observed time <> actual time; observed duration = actual duration?

2. Jan 27, 2008

Staff: Mentor

Right. Unless the observer was co-located with the event.

In order to interpret any observations one must take into account light travel time.

3. Jan 27, 2008

Staff: Mentor

And the equations of relativity apply to those "corrected" times and intervals, except of course for the equations that are used to calculate those corrections in the first place (e.g. the relativistic Doppler effect equation).

4. Jan 27, 2008

gonegahgah

Thanks. So observed duration <> actual duration in our frame?

5. Jan 27, 2008

Staff: Mentor

Again, it depends on how you are doing your observations. In relativity thought experiments, frames are imagined to have observers everywhere (each with his own synchronized clock)--so there's always "someone" present at the location of any event. If you do your observations "remotely", then you need to account for light travel time.

6. Jan 28, 2008

gonegahgah

So if you are remote than observed duration <> actual duration in our frame?

7. Jan 28, 2008

JesseM

It depends if the two events marking the beginning and end of the process happened at equal distances from you (in which case observed duration=duration in your frame), or if the second event happened farther from you than the first (in which case observed duration > duration in your frame), or the second event happened closer to you than the first (in which case observed duration < duration in your frame).

8. Jan 28, 2008

yogi

The spacetime interval is the same in all frames - in your frame the spacetime interval can be considered as composed only of a temporal component - in the other frame in uniform relative motion wrt you, the same interval will have both a time component and a space component. The time component of the interval in the other frame is the duration of the event in the other frame, and accordingly the duration in your frame will in general not be equal to the duration in the other frame