Telescopes

[cjackson]

How large would a radio telescope have to be to pick up artificial signals from beyond the solar system?

[ShinAce]

Do Voyager and Pioneer count? These things transmitted on a few watts of power.

Now if you consider how far our strongest ever radar signals could be received with an Arecibo sized dish, I'm not sure, but I would guess it's at least 5 light years.

[Romanus]

I think a better question is: what strength of narrow-band signal from a given distance could be picked up by extant equipment?

[cjackson]

That isn’t news to many, of course, but maybe this is: These signals are not hard to find. If there are any aliens within a few hundred light-years, these clues to our existence could be found with an antenna the size of Chicago. For any society able to threaten us across such distances, that’s a pretty easy construction project.

Is it true you would need something that big to detect signals from light years outside the solar system?

[Grey]

Here's a nice discussion. To summarize, if you assume a receiving telescope about the size of the Arecibo dish, you won't be able to detect anything beyond a fraction of a light year, if you're talking about signals like typical radio, television, etc. But if the signals are narrowband high intensity emissions from something like the Arecibo dish, we could detect them from hundreds of light years away. We could have a conversation, but we'd have to be very patient when waiting for replies.

[Amber Robot]

How large would a radio telescope have to be to pick up artificial signals from beyond the solar system?

Depends on how far beyond, how strong the signal is, and the spectral nature of the signal.