Re: How far can radio waves travel?

Christopher Hodges asks, "How far can radio waves travel?  As I understand
it, radio and TV waves travel through space and are  detectable if you
have the proper equipment. However, after a certain point, those waves
will "break up" and fall apart. How far can they  travel before this 
occurs? Supposing you planted a receiver/transmitter  out in space, could 
you then theoretically pick up these radio waves and them beam them out 
again to extend their "life", sort of like a  relay station? Could you 
even have a series or network of relay stations so that a radio wave 
originating on earth could be picked up and read hundreds of light years 
away?" 

Dear Mr. Hodges,
	Radio waves travel forever and do so at the speed of light
(c in Einstein's famous equation, e = mc^2), 300,000 kilometers per second.
Radio and TV waves, light, x-rays, gamma rays, ultraviolet and infrared
are all electromagnetic waves which move through space at the speed of
light and obey the same laws.  Electromagnetic waves don't "break up",
they appear to weaken as they are received further and further away from
their source.  

Here's why: At the broadcast antenna, the wavefront is very small.  
But as it travels, it spreads out to cover a greater and greater area.  
Think of a light bulb in a small room- it shines brightly on every wall 
and the ceiling.  But the same light bulb in an Olympic stadium shines 
only very, very dimly on the distant seats and roof. 

Now area is basically calculated by multiplying length times width
which is why we measure it in square units- square centimeters, square 
meters, etc. So the further away from the source we measure the radiation,
the more square units the area it is illuminating.  Hence we say the 
illumination is inversely proportional to the square of the distance.
 
If your space ship travels twice as far away, the strength of the radio
waves will be only 1/4 as great.  At ten times the distance, the strength
of the signal would only be one hundredth as great.

How far can we broadcast?  The Pioneer 10 space probe is now some 
66 A.U.(an A.U. is the average distance from Earth to sun,
149,000,000 kilometers) from Earth, outside the solar system. 
Our deep space network can pick up its weak signal, although it is
heavily distorted with static.  Eventually Pioneer's signal will be
weaker than the static which fills the universe and we will no longer be
able to detect it. If we could send a relay station out that far, we
certainly could boost the signal.  But we'd better send it soon-
Pioneer was launched 35 years ago and we have not developed any 
technology to get a vehicle that far away any more quickly. On the other
hand, we can detect signals from quasars 10 billion light years away. 
A light year is the distance an electromagnetic wave travels in one year,
6 trillion miles.  So with a black hole with a mass of tens of thousands
of suns, we can broadcast from one end of the universe to the other.  
The catch is that it takes the lifetime of the universe for the signal to
travel the whole distance.