I understand your points to be these?
1 - the galaxy is old - there should be time for civilisations to appear
2 - the idea that humans would not recognise alien signals is flawed because aliens would use similar physical phenomena to us to communicate
3 - there are galactic limitations for habitable planets which limit their numbers
4 - dyson spheres have not been found by seti
Umm. Pause for breath...
1 - the galaxy may be old, but humanity has been searching for a very short time, and has not even looked properly yet. I submit that even in an area teeming with intelligent life, a short untutored glance may notice nothing.
2 - this seems to be a misconception of what I said. I said nothing about technology being unable to be recognised because it was alien. I said it might not be recognised because it was beyond our current technical capabilities. Clarke's third law applies, as does my 'jungle drums' illustration. A human tribe which did not understand electromagnetic wave propagation could not detect radio which other humans understand - we do not understand, and cannot detect, for example, the instantaneous transmission of data through quantum spin manipulation. In fact, since I just made that up, we have no idea whether it makes sense or not, but if it did exist it would be a far superior data transmission method for long distance use, and would be the obvious way of passing inter-system information - radio or light would be thought useless and not used at all...
3 - I know of no suggestion that there are major restrictions on human-habitable areas of the local galaxy. Obviously places where there are strong radiant clusters would be unsuitable - the galactic centre, for instance, but most of the areas in the arms would be fine. Our system is primarily influenced by the heliosphere; the sun is the main driver for habitability. Of course, if there were such galactic limitations, that would go some way towards explaining why we have found little evidence of alien intelligence so far...
4 - This is an odd point to make. Seti is not looking for phenomena associated with dyson spheres, so of course it would never see any. In fact, I can only think of one project which has done any work on looking for these - the 2004 Fermilab IRAS filter-based search - which reported in 2009 that it had found a few candidates but remarked that it was hard to distinguish between natural and artificial shading. The general feeling seems to be that we are right at the start of this kind of investigation, so it is odd for you to say "We have had the technology to search the sky for dyson sphere constructs for at least 100 years" (which I think is an exaggeration - CCDs have been around for less than 30 years). This statement suggest that we have been looking for 100 years, which is certainly not true.
Given the small amount of work that has been done on Dysons, I am surprised that you claim that "The rotation and orbit of the earth around the Sun means all the sky is covered and the supposed lack of suitable telescopes in the southern hemisphere doesnt really hold up to scrutiny.". Any individual telescope can only see a maximum of 1/2 of the sky (more like 1/3 when horizon limitations are taken into account). The earth's rotation and orbit are essentially in the same plane, so northern telescopes cannot ever see southern star systems. And the Arecibo telescope that SETI uses is fixed, and can only scan a 40-degree band of the sky anyway.
As well as these limitations, few telescopes are ever tasked to search for 'extraterrestrial intelligence'. They are too busy with basic research. We may have had the technology to look for a fair time, but the actual history of SETI investigations is best described as 'symbolic', and certainly not comprehensive. Occasionally small projects to examine a restricted set of stars at one radio wavelength have been undertaken - it is unsurprising that these have found little. We are only just now moving into an age of readily-availible computing power which would make large-scale studies practical. Which takes me back to my original point - the most obvious reason for not finding anything is that we have not seriously looked so far....
Oh, and a point for those who see other animals as a 'different' form of life. As far as we can tell, there is only one form of life on this planet (though research in deep sea trenches might throw up something unusual). It's protoplasmic, protein-based and uses DNA to store, transmit, and duplicate genetic information. (let us leave RNA and viruses for another thread). Sometimes this life exists as a single cell, sometimes cells come together to form a symbiotic relationship. This can result in strange shapes. But they're all the same single lifeform.
Point 1, the galaxy is old. Latest findings put it at over 13 billion years. You use the premise that since we have not been looking for long, this means we have less chance of findin them. But Fermi's Paradox is that if the galaxy is teeming with intelligent civilisations, they should be
easily spotted by use because of colonisation, their artefacts and super sized constructs such as dyson spheres and their variants etc etc.
In a very old galaxy teeming with intelligent life, ie it is plentiful, 40 years of looking should be rather irrelevant. In fact, when Fermi first proposed his paradox, the 'space age' had barely started if at all. Fermi's Paradox is based on the fact that with plentiful intelligence, it should easily be spotted. It is not. It can be concluded therefore that the galaxy isnt teeming with intelligent life. If it is not teeming with intelligent life, it must by definition be quite rare.
Point 2, again in a galaxy teeming with intelligent civilisations, it is very unlikely that
all of these civilisations are using a communications medium we do not know off or understand. Some may be using communication mediums we do not understand or have but it would not be the case that they all have this special medium or even a majority of them would have it. Ergo many would be using radio \ elctromagnetic or light \ lasers.
Of course if the galaxy is not teeming with intelligent civilisations as per Fermi's Paradox, then it is quite possible that the one or two intelligences that exist may have this unknown communications medium and therefore they and us would never be able to communicate. But then again, this would go a long way to proving Fermi's Paradox.
I would point out that the proposition I mentioned regarding aliens being so alien each would not recognise the other was in answer to a general comment by others who support that proposition.
Point 3. The Rare Earth Hypothesis would give you more information. I'd also suggest having a magnetar, nutron star or super nova within a 1000 LY's could make your region of space in the galaxy a rather unsavioury place to be.
Point 4, I have never suggestted SETI is looking for Dyson Spheres. I have said Dyson himself stated such a construct around a star would make it ideal to search for and be a sign of an intelligent civilisation. There are others who are looking beside SETI o signs for intelligent civilisations. Amongst their means are searching for these civilisation's arefacts. These arefacts include von Neuman and Bracewell probes and Dyson Spheres and their variants. It was Dyson who suggested searching for such constructs many years ago and he didnt seem to think it was beyond our technology now so Im surprised you claim the technology isnt there. Dyson proposed in 1959 such constructs could be detected in the infra red. Spectral analysis of light by using telescopes could also be used. Telescopes have been available for far more than a hundred years so it isnt beyond realism that even quite straightforward observations of the galaxy would not reveal such items.
I'd also point out that while FERMILAB may state it has a couple of candidates, SETI said the same thing about Wow! and SHGb02+14a so nothing new there.
You are correct regarding the amount of sky an individual telescope can see however, we arent talking about individual telescopes. We are talking telescopes all over the globe. Even if we were not specifically looking for such constructs as a Dyson Sphere or its variation, the infra red signature of such would literally stand out like a beacon. In the observable light range, given the size of such a construct, this should also be noticable to the observer if any existed.
Once again, perhaps not unnaturally, the subject is turned to one of 'we havent been looking long enough.' However, this ignores a fundemental of Fermi's Paradox. If the galaxy has ever been or is teeming with intelligent civilisations, we
should not have to look very hard or very long because the galaxy would be so populous, we should be falling over them.
Fermi's Paradox cannot simply be dismissed because we havent looked long enough since in a galaxy teeming with intelligent civilisations, they would not be difficult to find.