If an astronomical source of electromagnetic radiation is observed to flicker, with intensity changes in as short a time as 1/30 s, how large is the emitting region?

Question

Quizplus · Accepted Answer

The size of the emitting region can be estimated using the light travel time argument. If the intensity changes in as short a time as 1/30 s, then the emitting region cannot be larger than the distance light travels in 1/30 s, which is approximately 10,000 km. This is because any changes across the region must be communicated at or below the speed of light, limiting the size of the region that can change in unison to the distance light can travel in the observed flicker time.

If an Astronomical Source of Electromagnetic Radiation Is Observed to Flicker