Electromagnetic radiation (radio waves, light, etc.) consists of interacting, self-sustaining electric and magnetic fields that propagate through empty space at the speed of 299,792 km per second. Thermonuclear reactions in the cores of stars (including the sun) provide the energy that eventually leaves stars, primarily in the form of electromagnetic radiation. These waves cover a wide spectrum of frequencies. Sunshine is a familiar example of electromagnetic radiation that is naturally emitted by the sun. Starlight is the same thing from "suns" that are much farther away.
When a direct current (DC) is applied to a wire (conductor) the current flow builds an electromagnetic field around the wire, propagating a wave outward from the wire. When the current is removed the field collapses, again propagating a wave. If the current is applied and removed repeatedly over a period of time, or if the applied current is made to alternate its polarity with a uniform period of time, a series of waves is propagated at a discrete frequency. This phenomenon is the basis of electromagnetic radiation.
Electromagnetic radiation is propagated nominally in a straight line at the speed of light in a vacuum, and does not require a medium for transmission. It is slowed as it passes through a medium such as air, water, glass, etc. The amount of energy arriving at a detecting device of fixed area located at a given distance from an isotropic source is proportional to the amount of energy passing the surface of an imaginary sphere with a radius of the given distance.
Therefore, the amount of electromagnetic energy passing through a unit area decreases with the square of the distance from the source. 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 observable electromagnetic radiation decreases very rapidly as the distance from the emitter is increased. Whether the emitter is a 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.
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