SETI alien distance?

Question

How far can an alien be so we can "hear" it? Lets consider that there is a similar civilization as ours with the same amount of radiation

Before posting this question I found a similar one at
http://answers.yahoo.com/question/index;_ylt=AikXjDYdKQ89z3xiVjYifv4jzKIX?qid=20060619125432AADW8Ax

the calculations by Karman tell us that we wouldn't be able to validate any "good" signal to the nearest star.

If this is correct then what are we trying to listen?

I think I haven't made myself clear.
My problem is the strength of the signal, if like Gene states they are send high power broadcast , ok!

But probably isn´t what they are doing. At least for a Planets WWW to exist we would have caught it by now.

Answer 1

We can't assume the only thing they are transmitting might be hip hop FM leaking into space. They may be making high power broadcasts.

Answer 2

If the signal is strong enough we could detect it from pretty well anywhere in the universe. The catch is the farther away, the older the signal. We may receive a signal some day from a civilization that died out a billion years ago.
For an alien to hear us, we have sent radio broadcasts for about 150 yrs, so they would have to be 150 light years away or closer.
The stars Alpha Centauri A & B are roughly 4.35 light years away from us. Proxima Centauri is slightly closer at 4.22 light years. Our radio and television signals have reached them long ago.

Answer 3

Well, based upon the idea that the nearest star (not our Sun) is about 4 light years from us, aliens might be located on planets circling stars out around 10 light years.

Radio signals travel at the speed of light.

So a one way transmission would take 10 years to reach a possible alien civilization, and it would take another 10 years for a reply to come back to us.