[RadioAstronomer]

If we'd found it by now, it would mean that the galaxy is bursting at the seams with civilizations

The problem here is the distances involved. I love the motto by Physicist; "The universe is the way it is, and not how we would wish it to be".

It appears that interstellar travel is a feat that is incredibly difficult to undertake. I read about Bussard Ramjets or generation ships and I think of the vast expenditures that would be required with zero return for the society building such. Not withstanding the engineering hurdles that would be required. We last walked on the moon over 30 years ago now and it’s only approximately 240,000 miles away orbiting our own planet. The nearest star is 4.3 light years away giving it a whopping distance of about 25 trillion miles. We haven’t even solved the entire engineering problem for a trip to Saturn, much less star flight.

Even if you take into account the Lorenz Transformation and you were able to build a ship that would travel at near "c" (the speed of light). Would anyone want to undertake a journey where they would know for a fact that not only everyone they knew back home would be dead, but their entire civilization may be also? Even if the journey took only 20 years "ship time", would you do such if you knew 20,000 years would pass back on you home planet? (These numbers I am pulling out of the air. They would be different depending on how close you approached c and how far you traveled.)

Thusly we are left with (at out current level of technology and understanding of physics) radio. SETI is just that, looking for a civilization emitting radio wave into space not unlike what we are doing ourselves. The rub is that it is terribly difficult to detect those emissions at the vast distances of interstellar space. To further complicate the picture is what frequency do we look for, where do we look, and for how long at each location. Let us surmise a civilization is looking in our direction at the frequency of one of our BMEWS sites. As our earth rotates, the beam into space will also move. They would have to be looking for possibly 20 to 24 hours to "see" us if they happened to look at just the wrong time. This is why our current SETI program is looking at multiple frequencies and the same locations over and over. The big question is where, what frequency, and for how long at each location. The other problem is radio still travels at c. So even our emanations have encompassed an area of about 120 light years in diameter. If a species happened to be looking in our direction whose planet was more than 60 light years away, they would not even be able to detect us yet.

Do I think SETI is a waste of time? Since I "do" SETI with a radio telescope the obvious answer is no. However, I cannot even predict if or when a signal may be discovered. The WOW signal is still intriguing, but it never repeated. IMHO, SETI is a win win science. If we are alone or find that we do not "see" anyone else out there; that in itself is profound. If we do see someone there, this too is profound.

So if we do find that extremely narrowband signal heralding the fact we aren't the only intelligence in this vast universe, I think it would further our understanding of our relationship with this universe. With C being the fundamental limit (speed) of communication, "talking" with another race would be quite impracticable. Not much of a conversation if you have to wait 3000 years for a reply to your hello. Still we would know as a species that we were not the only organisms contemplating the existence of the universe. Finding another intelligent race would definitely redefine our place in the universe.

[PatrickHenry]

Finding another intelligent race would definitely redefine our place in the universe.

As long as we're number one, of course.

[js1138]

Suppose we find a signal from 3000 or 10,000 light-years away. That would mean, absolutely, that there are other civilizations having at least some correspondence to out level of technology. That would also imply -- considering the rather freakish history of biology on our planet, influenced by our unusual moon and well-timed asteroid collisions -- that there are many habitable planets with no technological civilizations. That would be an invitation to explore and colonize.

Now assume that the civilization at the other end of the signal thinks the same way. Where are they. If habitable worlds are only hundres of light-years apart, then travel to them is a workable engineering task. One could easily envision an empire expanding at a reasonable fraction of the speed of light -- say one percent, or 0.1 percent. (You could reasonably assume that technology continues to advance, making energy and manufacturing available and easy.)

At any reasonable rate of expansion, one could assume we would have been visited, colonized or contacted. So where are they? Unless we are they?

[Physicist]

This is a very interesting graphic that shows the extent to which we've searched the universe for alien civilizations. Earth-like radio emissions represent the bottom edge of the graph; as you can see, we haven't even begun to search for them. As for civilizations that have transmitters the size of our largest dish (Arecibo) transmitting directly at us, we've only searched thoroughly out to about 100 light years. The galaxy is 100,000 light years across, and we live in a portion of it that is somewhat more rarefied than average.

[For reference, a "Type I" civilization would be roughly capable of transmitting a signal equivalent to all of the sunlight striking the Earth, 2x10¹⁷ Watts. A "Type II" civilization would be roughly capable of sending a signal equivalent to the power of the sun, 4x10²⁶ Watts. (I believe the canonical signal limits for these civilizations are given as 10¹⁶ Watts and 10²⁶ Watts.)]

[longshadow]

With C being the fundamental limit (speed) of communication, "talking" with another race would be quite impracticable. Not much of a conversation if you have to wait 3000 years for a reply to your hello.

Can you imagine the first interstellar post card we get back from the space aliens after we contact them.... we wait 3000 years for the response, and finally it arrives. We hurridly translate it, only to discover that it reads: "Having a good time; wish you were her."

What a bummer, man!

[Aracelis]

Wonderful post as always, RA. Thank you for taking the time to explain the situation so clearly.

[A.J.Armitage]

Even if you take into account the Lorenz Transformation and you were able to build a ship that would travel at near "c" (the speed of light). Would anyone want to undertake a journey where they would know for a fact that not only everyone they knew back home would be dead, but their entire civilization may be also? Even if the journey took only 20 years "ship time", would you do such if you knew 20,000 years would pass back on you home planet? (These numbers I am pulling out of the air. They would be different depending on how close you approached c and how far you traveled.)

Why would any of that matter if you were never planning a return trip? If you're going to colonize, not explore and return, you won't see anybody back home anyway. Or maybe, everybody from home you care about is coming with you.

[steve-b]

Would anyone want to undertake a journey where they would know for a fact that not only everyone they knew back home would be dead, but their entire civilization may be also?

It wouldn't matter to people who didn't intend to go home.

My favorite answer to the Fermi Paradox is that, yes, they're out there, but it's impossible to break the lightspeed barrier and unreasonably difficult to travel at relativistic speeds. Any interstellar travel is by way of closed-system slowboats (a few percent of lightspeed). The inhabitants are adapted to that way of life, and so regard planets as items of purely intellectual interest (which is why they haven't colonized or left visible artifacts).