Oops, should of course have read: epsilon < 0

How is that different to command.com?

Interestingly, this URL already takes me to Google.

No, in the story they will just be hackers.

Oops, I just notice I miscalculated (I mixed up meters and kilometers): 60 Hz corresponds to just 5000 kilometers, i.e. 1/8 of earths circumference. So a long enough power line on earth to efficiently send is possible. However the point of the atmosphere still remains: I doubt that you could pick up the power line signal even from a satellite circling earth.

But I assume no matter how advanced we as a society get, we'll continue using electricity, and the same could be presumed for other intelligent life. Transmitting power across power lines should generate SOME level of EM-spectrum signal, no? Could we detect that?

Note that a 60Hz electromagnetic wave has a wavelength of 5 million kilometers, which is more than 10 times the distance between earth and moon. That is, even the longest power lines are far too short to be efficient antennas for electromagnetic radiation at this frequency. Not that you'd want them to be; all EM radiation generated by power lines means an energy transmission loss. Also, who tells you that the aliens don't use directed current transmission (which causes no EM radiation at all, apart from a negligible amount when switching on and off)?

Anyway, I don't think the little low-frequency electromagnetic radiation which is generated by power lines even leaves the earth. There are only a few frequency windows where the atmosphere is transparent.

I don't think even humans were meaningfully intelligent until the time we started broadcasting radio.

So you say the radio waves have somehow modified our brains to get more intelligent?

Also I don't think the old inventions needed less intelligence than the modern ones. The only difference is that back then, much fewer people were thinking about new inventions. But that's not related to radio waves at all, but more to common education (during most of the history, the vast majority of people couldn't even read and write) as well as there simply being more people on earth today.

Actually, the cosmic microwave background is not really the afterglow of a big bang. Rather, the aliens have found out the ideal encoding for long-range space communications, which happens to look like 2.7 Kelvin thermal radiation if you don't know how to decode it. :-)

However that was a one-time signal. We also have received a one-time signal (the so-called wow signal). We have waited for a repetition, to confirm it's really an alien signal. Since we never got a repetition, we concluded it was probably not a real alien signal. Anyway, we never answered.

If any aliens have received the Arecibo signal, and those aliens are somewhat like us, they also have waited for a repetition of that signal, and since that repetition never came, they'll not answer.

Well, just assume that epsilon 0.