How can radio waves continue to travel for ever in outer space?

How can radio waves continue to travel for ever in outer space?


On earth the radio waves from your favorite radio or TV station will only go 100 miles or so at the very most, but NASA and Astronomers say that they will travel almost for ever, or at least Billions and Trillions of miles with out stopping.


How is that possible?|||Voyager 1 is so far from Earth in 1998 that it takes 9 hours 36 minutes for a radio signal traveling at the speed of light to reach Earth. Voyager's signal, produced by a 20 watt radio transmitter, is so faint that the amount of power reaching NASA's antennas is 20 billion times smaller than the power of a digital watch battery. Think that they will eventual fade into the noise of background radiation. We don't know if our signals can leave the Sun's magnetic heiloshpere|||On earth, propagation is limited by the earth's curvature. A television transmitter throws hundreds of kilowatts to go thirty miles or so, but an aircraft can talk to an ATC radio a hundred miles away with only eight watts of power because there is nothing in the way. At large distances, 1/r^2 becomes important, but with suitable antennas we can talk to spacecraft a billion miles away: there is nothing but empty space in the way.||| Radio waves on earth are either line of sight (FM or TV) or broadcast. The former can only be picked up when the earth has not curved out of their path. The latter in spreading soon become weak enough that the other signals being generated overwhelm them.


All of these signals, in theory, can go on forever in space, but in practice become so weak that they can not be detected. An indication of how far they can go is the very low wattage of the transmitters on the Mars orbiters and landers, which are sent and received with dish antennas with digital correction for lost bits. Harder to find is the wattage of the distant transmissions from the satellites that went beyond Neptune, which had to be received with very large dish antennas.|||Remember that radio waves and light waves are the same phenomena: electromagnetic radiation. The light you see from stars traveled thousands and probably millions of light years. You can see them, because:





1. stars give off intense light


2. there is nothing blocking your view of light





A similar story holds for radio waves.