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Originally Posted by Knalus
And that's an upper limit, with a lower limit of 1.
Even with 160,000, the amount of empty space that would be left in the galaxy is astounding. That's 160,000 out of 480,000,000,000 planets. Even with travel at the speed of light, and communications at the speed of light, we would likely never know they exist. They would be too far away for us to ever reach.
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The distance is more a problem for communication, and less so for detection. I will say that ignoring some more exotic solutions and with the exception of the immediate neighborhood, interstellar travel for purposes of
colonization is less of a problem than for anything that would require you to report back to your home planet for. A trip to Kepler-22b (thought to be a candidate for life) at a constant acceleration of 1G would take us about 12 and a half years ship time, but just over 1000 years would have passed on Earth by the time you got there. To send a radio message back to Earth would take another 1000 years, which would be impractical. A similar trip to the Andromeda Galaxy would take just 29 years for the travelers, but more than 2.5 million years would have passed on Earth; by then, our planet might be very different.
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Originally Posted by Knalus
Factor in the fact that the center bulge of the galaxy is where the majority of stars exist, and the likelihood that the entire region is so bathed in radiation as to make it uninhabitable - the galactic habitable zone. Add to this the fact that we know almost nothing as to the cause of the Cambrian Explosion - the rise of true multi-cellular life, and that 160,000 becomes a really, really high estimate. I am very skeptical about this, and am glad that someone out there that is respected is throwing cold water on the idea that our galaxy is teeming with life. There is zero scientific evidence that there is any life of any discernible type outside of earth, and to suggest not just otherwise, but that the universe is teeming with life - especially intelligent life - is not very scientific.
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Huh? The only possible ways* for us to discern the existence of extraterrestrial life at present are:
1) Analog signals from an intelligent civilization.
2) Discovery of extraterrestrial life in our solar system, say for example on Europe or Encyladus.
3) It comes to us.
We're attempting #1 and #2, and the arguments for #3 being a present reality are unconvincing in my opinion. Your argument is in effect:
We've not yet discovered extraterrestrial intelligence, therefore to merely
suggest that there might be life outside our solar system, not even intelligent life, somewhere else is unscientific. However, we can use what we do know to make a guess. Based on the amount of time life took to arise on Earth,
Lineweaver and Davis estimate at least 13% of inhabitable planets go on to develop life. In other words, the pro-ET conjecture is a better one than the anti-ET conjecture based on what we know so far.
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Originally Posted by Knalus
My guess - there's only one life form that has any kind of intelligence whatsoever. Unless you count Dolphins. Even with these weirdly optimistic calculations, there had to be a time when there was only one. What if that time is now?
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The problem with this is, of course, that other locales have had an enormous head start on us - billions of years worth.
*If I've missed anything else we could currently do, please correct me.