Your OP was pretty intense, and in my opinion, both it and the subject are GD-worthy. Perhaps Tom will disagree and move it back.
By the way, I’m almost certain that both opinion and conjecture are allowed in this forum.
First of all, if UFOs are real, we’d know it. No one is going to travel those distances and play games with jets and go popping out of the woods going boo. You don’t need any of these calculations to refute them - you just have to assume that space traveling alien races have IQs higher than a turnip. (Despite what we see in the standard '50s scifi movie.)
You’re assuming, though, that the alien race stays at home and sends out probes. That is almost certainly not the way I’d do it. How much exploring of the West would there have been if the British started from London each time? No, a race would transform likely planets, or establish large space habitats in systems with the needed resources, and slowly expand. The life time you are assuming seems laughably short - we’ve had real medicine for about 150 years, and already we are beginning to understand aging. Increasing lifespans is one scientific project that just about everyone would get behind.
I don’t think you can explain the absence of aliens through technical limitations like these.
Frank- I understand and yield to your superior judgement. 
Voyager- I’m not certain but I hope you understood I was NOT supporting the case for UFO’s or using this discussion as a basis for my disbelief. My point was that I would love to think of a universe with a city building race on every other star system. I would love to believe we are on the brink of a FTL means of travel. I was saying that sometimes your reasoning’s yield an outcome that conflicts with the world you’d like to see.
Such is the status of my reasoning’s on extraterrestrial life at this time. I just have had these pieces of information in somewhat of a fragmented arrangement and sought to bring some order to the process and at the same time, put them before the smartest think-tank I have access to in order to see perspectives or flaws I cannot see myself.
The input about the gas giants is a good one for example. I can see this as a possible place for life to begin but considering the gravitational forces and the possible time-in-shadow combined with extreme ranges in temperatures from such an orbit moving closer to the star and then much further on a regular basis it seems complex life would be less likely.
The life time I was referring to was that of current day humans.
Squid can solve problems, such as opening screw top bottles. I don’t know what tests have been done on self awareness.
Red dwarves are neither; they are main sequence stars like our own yellow dwarf, and are the majority of such stars. They also live much longer than a star like ours.
Cite?
The technology we possess may be pretty neanderthal compared to our neighbors.In a hundred years we have changed our understanding of scientific knowledge to a tremendous degree. Do we really know anything. Speed of light as fast as we can go. If so, we have no neighbors. They all live too far away. Then we are alone. If the future finds a way around that all bets are offf.
Two changes over the last years of import. One is the realization that life is a more formidable force than we had realized. We had operated under the assumption that life was very delicate, requiring very exacting conditions, very Goldilocks - not too hot, not too cold, not too much methane, etc.
Of late we find differently. Life will squeeze out an existence in the most unpromising situations, like the volcanic vents in the ocean. This coincides with pushing back the earliest forms of life farther and farther back.
So did it start here? I think not, I don’t think the time frame allows for such complex development from “scratch”. I lean very heavily towards notions of panspermia, the theory of extraterrestial origin and “contamination”. Which would imply that DNA is the basis of life in Known Space.
The good news is when we meet them, we can eat them. Bad news, the opposite also applies.
White dwarfs aren’t nova remains, but rather remains of a main sequence star.
And red dwarfs are very very common, about 10 times as common as “main sequence” stars. And they last orders of magnitude longer. The more massive a star is, the hotter it burns, and the hotter it burns the faster it burns. Blue-white stars burn very fast and have life spans in the range of millions of years. Red dwarfs are the exact opposite, they burn very cool and will last for hundreds of billions of years. Our sun is in the middle, it will last for another couple of billion years before it swells up to become a red giant. The red giant stage only lasts for a short while before most of it’s material is ejected.
No worries - just giving a reason I agreed with you.
We’re nowhere near FTL travel - but if we are as close as the Greeks were to space travel, we’ll be in good shape. I’ve constructed an ftl travel method for a book. The secret is that it can’t violate anything we do know - we just can’t go faster than light in this or any other universe. I think I can show that causality won’t be violated - no wormholes. But as far as we can tell now, we’re alone, and if anyone is out there, they’re not talking to us.
We’ll give them some of our women. Preferably in go-go outfits.
Have you considered doing that with the autistic people?
-Joe
Where do you get that figure?
If we simplify a bit and imagine that each star is in the center of its own cube 6 ly on a side, that assigns a volume of 216 ly**3 to each. That reduces your subsequent figures by a factor of 24.
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What size dresses do these autistic people wear?
It may well be that advanced life is very rare. perhaps the conditions that gave rise to human life are pretty unique, so if the universe is mostly populated by blue-green algae, no surpize. I am surprised that we haven’t picked up any kind of radio signals by now…or maybe we are in the wrong neighborhood.
I recall an episode of Cosmos in which Carl Sagan was speaking of Mars and the fact that, to all appearances at this time, it is a lifeless world. And paraphrasing, he said that if so, we should use it as best fits the needs of mankind. However, what if there is life. If we find that life exists, even in it’s most simple form, should we take steps to change this place for our needs? He said he did not think we should.
What are your feelings on this? If there is a basic life form, like the blue algae mentioned above, do you think we should proceed with colonizing and terraforming if it meant the extinction of this native life form? Do we ignore the one world most like our own and perhaps the only one we will have access to for centuries to come for the sake of a bit of slime at the edge of one of the poles?
If yes, then is there a point were we would not intrude?
When I consider the effects western civilization has had on the native species in Australia of the Hawaiian Islands it gives me pause. Even with the precautions used by NASA, one of the Apollo missions brought back a piece of an early unmanned mission on which was found bacteria that continued to thrive once returned to a warmer atmosphere environment.
I think that if Mars becomes crucial to our survival and future expansion colonization of other systems, then yes, we should change it if we can.
It is the natural order of things. When the price of not doing so is extinction, it will be done.
Look, the process of terraforming Mars would take centuries, if it is possible at all. Surely we have no need to make split-second decisions about whether exterminate any native Martian life. Terraforming of Mars will never be critical to human survival, because surviving on Mars is going to be 100 times harder and 1000 times more expensive than surviving on Antarctica. If we can’t build human habitats in Antarctica, the Sahara, Baffin Island, and the middle of the Pacific Ocean, then there’s no way we can build self-sufficient habitats on Mars. If we can’t survive on a nuclear-winterized or post-asteroid strike Earth then how can we do so on Mars?
First, what makes you think aliens could even interpret our signals as anything other than interesting noise patterns?
Second, I don’t know why people think aliens are somehow more noble or altruistic or moral than we humans.
And finally, people always seem to give anthropomorphic properties to aliens. Aliens could be so unlike us that we might not even recognize them. They might not even operate on human scales or timelines.
Think of it like this. A forest is a complex ecosystem. It consists of many components - trees, soil, animals, etc that all interact and depend on each other. A forest grows and recedes based on a myriad of conditions. It doesn’t decide to, it just does.
Urban sprawl is also a complex ecosystem of sorts. Viewed from space over a long period of time, it appears like a metal, glass and concrete fungus growing all over the planet tended by various smaller suborganisms.
Now imagine an alien coming into contact with Earth. They might see various competing organisms that appear like alternating green and gray illuminated fungus slowly competing for space. In their quest for getting at whatever it is they are interested in, they might demolish some of our cities with about as much regard as we might demolish an ant colony while digging the foundation to a new skyscraper.
Lemur866
Oh, I think we can afford to give the Dopers here a little more time than that to consider the question ::Jeopardy music::
The Earth areas mentioned are far easier to resupply on a regular basis making a self-sustaining habitat not cost-efficient. Not so with Mars.
msmith537- I realize your reply was not directed to me specifically but, if I may:
If we are discussing a city building and possible space faring race there must be some form of technology involved and as such an ability to intercept radio signals. There must also be some degree of mathematical knowledge and therefore the ability to distinguish between naturally occurring noise in common frequencies and a sustained signal of a type seemingly artificial. And if so, they could recognize mathematical progressions like 1,1,2,3,5,8 or 1,2,3,5,7,11 type of stuff.
Judging by what I see on the news each day, I can hardly imagine that they would not be.
I agree whole-heartily with this. The mostly-human bipeds of sci-fi is entirely unlikely. However, I do think there are survival strategies that work here and may be very common in other world species. To wit: Eyes that work in the visible spectrum at the least. A method to detect vibrations in the surrounding air, like ears. A method to sample free-floating molecules in the air, like a nose or a snakes tongue. I think these items would be grouped closely together and close to the brain to minimize signal travel time and signal degradation. They should be on a head that allows increased elevation when needed to survey an area of possible danger and must swivel ( as on a neck) to afford the greatest panning of an area. I recall reading of a prehistoric sea creature that looked almost exactly like a dolphin but obviously was not a mammal. Why? Because that form works very well in that environment so the unconnected recurrence seems like a logical outcome.
I think this is likely the most we can expect as to recognizably.
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:smack: 27 ly3 of course. I had done the first calculation with a 4 ly average distance (which comes out to an 8 ly3 sphere, and got stuck there. :smack: :smack:
Radio waves are somewhat simple to intercept. The regularity of them in terms of speech and music, would stand right out. I was joking about Amos and Andy, of course, and the aliens might never be able to translate our language (though receiving TV signals should make it easier) but they’d know it was intelligently generated.
I don’t. But an advanced alien species would have managed to make it through a number of crisis points, including the development of nuclear weapons. If the are too nasty they won’t survive in the long run, though we might get unlucky.
I’m quite familiar with this argument, and I don’t buy it. First of all, our thought rate is determined chemically, and even if aliens move into computers, where the thought rate will be faster, you’d expect them to be able to interface with the natural world which moves more or less at our rate.
I agree that aliens would plan over much wider horizons than we do (or even more than the cathedral builders did 1,000 years ago) but they’d still be able to recognize short term events. Actually, that is why I think spreading through the galaxy is more probable than the OP does.
The first problem with this is that intelligence is very evident. I don’t buy that intelligent aliens wouldn’t recognize it - at least after a bit of study.
Second, anyone coming that far won’t need anything we have. We’re far more likely to be the bad guys than they are - not that we’d get very far.
The far greater danger when confronted with an advanced alien race is to give up when we find that that they know everything worth discovering for the next thousand years, and we move from discoverers to not very advanced school children.