Why is it that we will not see fluctuations in light output in times shorter than about 1 day when we observe an extragalactic source whose diameter is about 1 light-day?

Question

Quizplus · Accepted Answer

Arrival times of different parts of the source will vary, causing a smoothing effect on short-term fluctuations. This is known as the light travel time effect, where light from different parts of the source travels different distances to reach the observer, resulting in a time delay. As a result, we will not see rapid fluctuations in light output in times shorter than about 1 day for an extragalactic source whose diameter is about 1 light-day.

Why Is It That We Will Not See Fluctuations in Light