What? No, absolutely no we would not just see them with sensitive telescopes. Perhaps, if you're thinking of some sort of interstellar space travel, maybe we'd see the engines? But no, even that seems unlikely.

Any space habitat of any plausible size will be completely undetectable, or at least indistinguishable from a planet. And I would think they'd be much smaller than that.

And structured radio? Lots of possible reasons not to see it. One, possibly not using radio. Two, encryption. Communication will likely look like noise. But even so, the distances involved means long range transmissions will need to be very directed and narrow beam. And even then, it's only a few light years before the signal is undetectable. I would suspect any really long distance communication would be bounced through relays. Also, why? You have to wait years for messages to go around. All of this means no, we're not likely to easily find stray radio signals.

I think you are completely misunderstanding the point. He have to vent heat somehow in the most efficient ways of doing so are not natural in their spectra. An encrypted radio transmission is still perfectly detectable -- you may not know what it says but you know that it is there. Artificial signals are blindingly obvious.

And this is without getting into Dyson spheres and other efficient but detectable technology.

"We have to vent heat somehow..."

Yes, of course. And the heat of a star will make that heat completely invisible in comparison.

The "spectra" that heat would be vented at is infrared, which is infrared no matter the source.

Say you had some gargantuan fusion power harnessing space station in deep space. lots of assumptions here about alien cultures, but this is likely the easiest to detect. They will do whatever they do with their energy, and then vent the waste heat. That heat will be infrared. What does a dim infrared source look like (if even visible at all)? It looks like a brown dwarf. But this is assuming a lot. It assumes some alien would even build brown dwarf sized space stations. We don't know if or even why they would do that.

Encrypted signals are not blindingly obvious. An encrypted radio transmission will appear as pure random noise and no different from all the rest of the noise that we pick up.

If an astronomer picks up a radio signal it is extremely obvious whether it is an artificial source or not. Natural sources have very inefficient spectra of very specific types. Artificial sources have very clear frequency cutoffs, having come from an antenna. Even frequency-hopping sources may jump around in a way that makes eavesdropping difficult, but if you stick around long enough on a single channel (as radio astronomers do) you'll get little blips now and then as the signal hops through your channel and is gone again.

How do we know this? Because astronomers see these every single night and have developed sophisticated filters to remove them. But by checking how the source is moving it is quite easy to tell if source is local or not, and sure enough all these artificial sources come from satellites, the troposphere, or an unshielded microwave.

It is actually remarkably difficult and a challenging problem in itself to create a communication mechanism which looks like a natural source. It defies imagination to think that every single intelligent species out there has purposefully masked their existence in such a way, including giving up useful technology like ground control radar or mobile communications.

The reason they have to filter out artificial sources, and the only reason they're a problem, is we are literally right on top of the source. These aren't even as far away as, say, Mars. New Horizons, for example, can't communicate with Earth unless it points its transmitter / receiver directly at us. And that's in our own solar system! Only a few light hours away.

Just think about it. The signals are going to be so weak by the time the photons get here they will be indistinguishable from the CMB.

> We have sensitive telescopes. If there was life out there, we would have noticed.

I'm sorry, but that's way off.

Our current telescopes can't even see the majority of the exoplanets in our galaxy; we are made aware of their existence by observing the behavior of their host star (wobbling/dimming). Unless something is extremely close, unfathomably enormous, or emitting/reflecting large amounts of electromagnetic radiation (light, radio, etc.) that isn't bent/filtered/reduced to noise, then we are completely unaware of it.

They can see stars. There wouldn't be stars in a type 2 civilization. There would be amorphous blackbody spectra which we do not see.

That assumes that civilizations derive power by encasing their host star in a Dyson sphere, which is not a foregone conclusion; that prediction was based on our current understanding if technology. Think of Huygens who imagined that Venusians sailed the seas on ships held together with hempen rope.

An "amorphous blackbody" source, from anything reasonably distant, just looks like a smaller star. There are a bunch of if's and unknowns involved, but it could be maybe possible to detect a Dyson sphere. This[1] article talks about it. The upshot is that it is nowhere near blindingly obvious, and we don't currently have the ability to differentiate between a natural or artificial infrared source.

https://en.wikipedia.org/wiki/Dyson_sphere#Search_for_extra-...

We just found the first planet that is somewhat similar 1400 light years away... and you think our 'sensitive telescopes' would have definitely discovered all possible life in the universe if it existed???

"SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light years, less than 1/10 the distance to the nearest star."

https://en.wikipedia.org/wiki/Fermi_paradox