How Far Can We Exclude Alien EM Broadcasts in Our Light Cone?

[BWV]

out to what distance in our light cone can we exclude the existence of an alien civilization broadcasting EM signals at about our current levels?

 

[rootone]

I'm pretty sure that elsewhere in the solar system can be excluded.
Beyond that, it is sometimes argued that a technologically advancing civilisation is likely to have only a brief period of radio broadcasting before it develops a more effective communication technology for their own purposes, as we have in fact done.
.. unless of course they wanted to intentionally broadcast radio signals with the hope somebody 'out there' would pick them up.

 

[Chronos]

The detection window for radio broadcasts is probably a few centuries, the blink of an eye on cosmic scales. The detection range for radio signals is probably only a few hundred light years. Again a trifling distance on the grand scale of things. IOW, the odds of picking up a SETI signal from Earth in radio frequencies are fairly slim assuming tech capable societies are relatively uncommon in our galaxy.

 

[newjerseyrunner]

Based on pure odds, any species out there is most likely really far behind us, or really far ahead of us. Behind us, they are still using stones and not listening, but if they are in front of us, even a few hundred years is an incredible amount of technology, and more realistically they could be hundreds of thousands or millions of years ahead of us. Something like that would be patient, and curious and start just sending out probes to every planet that it thinks can harbor life. Then program the probes to alert them upon detecting certain things, radio waves may be the lowest of them. If they've detected our radio waves, they've probably also detected our nuclear weapons, they may be waiting to see how we progress, if we make it past certain filters: like the threat of nuclear war. We may have several technological steps that they are watching and waiting for. We may not want to know what happens upon us inventing new technology. To move to science fiction for a moment: The Vulcans and the Borg both waited to detect a warp drive before initiating any contact.

 

[BWV]

Based on pure odds, the aliens should have us in a zoo otherwise prof Fermi would not have himself a Paradox

I am aware of all the possible reasons why we may not hear anything, just curious at what radius in space-time we could positively exclude the existence of a civilization with a level of technology that was broadcasting a similar amount of EM signals to us.

 

[rootone]

I think there are too many variables involved to come up with any definite answer.
An important one has nothing to do with whatever aliens may be broadcasting, but limitations of our own technology.
At some point any signal will be lost in noise, however modern state of the art receivers and signal analysis software would do a lot better than 20 year old technology, and similarly, the improved technology available in 20 years time could well find a signal which we can't detect today.

Another important variable is that the Universe has some very radio noisy places which makes signal detection more difficult than in quieter places.

 

[BWV]

So nobody knows for sure if all the SETI equipment was set up on, say, Tau Ceti (12 LY away) any signals could be heard from Earth?

 

[rootone]

Ah, well the way SETI mainly works at the moment is to analyse data from all available sources looking for signs of what could be an intelligent signal.
This as opposed to choosing a specific target, since so far there is no particular reason to suspect intelligent life at any particular target.
If however we discovered an exoplanet with an oxygen rich atmosphere, that would be evidence of life even if not intelligent life, so a focused search in that case might be appropriate.
Otherwise it would be a pointless exercise. Concentrating on one specific target for no reason amounts to the same thing as ignoring billions of other possible sources of a signal.

 

[newjerseyrunner]

Imagine yourself watching a scary movie: a person is alone in the dark, not sure where he is, but completely defenseless. As loudly as he can he yells "Hello?" into the dark. Then you the viewer roll your eyes and say he's an idiot.

That's why SETI mostly just listens.

 

[Chronos]

The SETI message directed towards M13 in 1974 would by discernable by SETI class detectors in M13 - http://www.seti.org/seti-institute/project/details/arecibo-message. M13 is over 20,000 light years distant. So the potential range of an intentional radio broadcast is on that order. Detection of mundane [unintentional] radio signals employed by a technologically Earth equivalent alien civilization would obviously only be detectable over much shorter ranges - say perhaps a hundred or so light years.

 

[russ_watters]

So nobody knows for sure if all the SETI equipment was set up on, say, Tau Ceti (12 LY away) any signals could be heard from Earth?

They could, but that's not what you asked. You asked, to what distance could we "positively exclude" the possibility of aliens transmitting like we are. We're not continuously listening in every direction, at every frequency at once and depending on the type of signal and strength, we would miss a lot. So I'd say that f someone were transmitting something like what we transmit, from 12 LY, we PROBABLY, would have heard it, but even that close I would not guarantee it.

If you want, we can get into some nuts and bolts about the different types of signals and how far, with what equipment, we might detect them.

But that still both assumes that their level of development aligns with ours in time and type. What if they don't watch tv?