Observed Time vs Observed Duration

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Discussion Overview

The discussion revolves around the concepts of observed time and observed duration in the context of relativity, particularly how these relate to actual time and duration of events such as a star exploding. Participants explore the implications of light travel time and the effects of relative motion on observations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that observed time differs from actual time due to light travel time, but question whether observed duration equates to actual duration.
  • Others emphasize that the interpretation of observed duration depends on the observer's frame of reference and whether they are co-located with the event.
  • It is noted that corrections for light travel time must be applied to observations, and the equations of relativity are relevant for these corrections.
  • Some participants propose that observed duration may not equal actual duration if observations are made remotely, depending on the distances of the events being observed.
  • One participant introduces the concept of spacetime intervals, suggesting that these intervals differ across frames, affecting the perceived duration of events.

Areas of Agreement / Disagreement

Participants express differing views on whether observed duration equals actual duration, with some suggesting it does under certain conditions while others argue it does not, indicating an unresolved debate on the topic.

Contextual Notes

Limitations include the dependence on the observer's frame of reference and the specific conditions under which observations are made, which remain unresolved in the discussion.

gonegahgah
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Can somebody please help me with this question?

Obviously when we observe an event it has not occurred at the time that we observed it occurring 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?
 
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gonegahgah said:
Obviously when we observe an event it has not occurred at the time that we observed it occurring as light travel time must be taken into account.
Right. Unless the observer was co-located with the event.

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?
In order to interpret any observations one must take into account light travel time.
 
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).
 
Thanks. So observed duration <> actual duration in our frame?
 
gonegahgah said:
So observed duration <> actual duration in our frame?
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.
 
So if you are remote than observed duration <> actual duration in our frame?
 
gonegahgah said:
So if you are remote than observed duration <> actual duration in our frame?
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).
 
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
 

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