The Inverse Square Law
Electromagnetic energy decreases as if it were dispersed over the area on an expanding sphere, expressed as 4πR2 where radius R is the distance the energy has travelled. The amount of energy received at a point on that sphere diminishes as 1/R2. This relationship is known as the inverse-square law of (electromagnetic) propagation. It accounts for loss of signal strength over space, called space loss.
The inverse-square law is significant to the exploration of the universe, because it means that the concentration of electromagnetic radiation decreases very rapidly with increasing distance from the emitter. Whether the emitter is a distant
spacecraft with a low-power transmitter or an extremely powerful star, it will deliver only a small amount of electromagnetic energy to a detector on Earth because of the very great distances and the small area that Earth subtends on the huge imaginary sphere.
