How far would alien world be for SETI to detect radio signal and recognize as intelligent?

Question

If an alein world broadcast a TV signal. How far would they be for SETI to detect them at minimum strenght and still recognize it as intelligent.

So Karman, if an earth standard TV signal strength broadcast from Alpha centari, we would not be able to recognize it (as an intelligent signal) with the most powerfull antena on earth?

Answer 1

I think your question is a bit vague. If they had a TV transmitter that was very powerful, we could receive the signal no matter what the distance. Of course the technology is not conceivable for us. But what if they modulated a star? We could receive it from millions of light years away. But, since our galaxy is some 60k lyr across, it would be more likely to receive something from less than that.

As to the power required to transmit a TV signal to say Alpha Centauri (4.35 lyr) given our current technology, let see. Here is a quick free space loss equation we will calculate for a microwave link to Alpha Centauri:

Free space loss = 36.56 + 20Log10(Frequency) + 20Log10(Distance in miles)

X dB = 36 + 20*L10(2.4Ghz) + 20*L10(4.3*5.8x10^12)

and shazam! ------> 491 dB loss

Throw in a few high gain antenna and we get at some 350 dB loss. The equivalent power required is in the terawatt range. We currently use about 2 terawatts electricity worldwide. In other words, a high bandwidth connection to the nearest solar system is not within our reach with wireless radio.

Answer 2

SETI works by listening, not by sending. What we hope to find is a signal that is sent out by some other civilization, either as a general broadcasts -- to be detected -- or as a message to another civilization -- and we would happen to intercept it. It is true that if we receive such a signal, it would have been sent a long time ago. However we could still learn quite a few things. Some basic things to learn are: -we are not alone -the nature of the carrier or of the message might give us a clue about the type of intelligence. -the content of the message might teach us something. If we do send a reply and IF we maintain some kind of civilization with a 'corporate' memory, maybe, just maybe, someone will remember to keep a radio receiver working in 2*X years, when we expect a reply to our reply. We may have moved away from radio for our communications, but we might still be using some form(s) of radio receivers for astronomy. Is it a waste of time? Even if we never hear anything, it might help us set limits on how far or how rare other civilizations are, or what frequencies they do not use... (However, see al_moskowitz's answer: there are so many other ways that signals could be sent by other civilizations). At least, right now, a lot of the listening is done 'piggyback': while the radio-telescope is doing some other useful work, we add a line to listen in on our preferred frequencies, without interfering with the real work. So, little waste is involved compared to the large amount of signals being analyzed (thanks to all of you doing it at home).

Answer 3

well I guess that would depend on the strength of the signal, however we should consider that these signals traval at the speed of light so if we did get the signals they would be very old for example if we picked up a signal from a planat 50 light years aways (very close in terms of the universe) it would be atleast fifty year old tv show from there.

Answer 4

well we spam other planets u should know that. We have radio signals and satellites.