Why is it that, when we observe an extragalactic source whose diameter is about 1 light-day, we are unlikely to see fluctuations in light output in times shorter than about 1 day?
A)because the light from different parts of the source will be Doppler shifted by different amounts, allowing us to see only an average shift
B)because light from the back of an object, 1 light-day further from us than the front, cannot be received by us earlier than 1 day after the front
C)because absorption of light by intergalactic matter will smooth out rapid fluctuations within the beam
D)because it is inconceivable that a source of this size could vary on such a short time scale

Question

Quizplus · Accepted Answer

This is due to the finite speed of light. Light from the back of the object will take 1 day longer to reach us than light from the front, so any fluctuations in the light output from the back cannot be observed until at least 1 day after the fluctuations occur.

Why Is It That, When We Observe an Extragalactic Source