Diadem wrote:But to go back to the main topic. I still maintain that Fermi's paradox is valid.
Humanity is currently expanding at a speed of about 0.005% of the speed of light.
Humanity hasn't managed to set up a Von Neumann machine off of Earth.
Over the first ~5 billion years, life on Earth has moved from one point, to covering the Earth. That generates a velocity of 3 * 10^-19 times the speed of light.
If we restrict ourselves to humanity and voyager, we have:
as our range. We could range this over industrial civilisation, human civilisation or the human race.
Another approach is to use Voyager's velocity itself -- ~17.089 km/s. I suspect that is what you did.
Except the problem is that, as Voyager doesn't self-reproduce, nor would we expect Voyager to be even a close approximation of what it would take to safely carry something self-reproducing, a civilisation emitting Voyagers won't be all that detectable.
Assume the launch a probe that moves 10 times faster than Voyager at a rate of 10 per year. These probes could then be spread over most of the galaxy by now. Over 1 billion years, there would be 40 billion of them.
There are ~1/2 a trillion stars -- which means 1 probe for every 10 stars.
But most of them won't be functional. And they sure won't be able to steer after floating in space for millions of years, nor will they be shiney, emitting, etc.
Assume they are ridiculous, and have a 0.01% chance of actually reaching a star, and can survive floating in interstellar space for 100,000 years (!). Then we have 1 million probes, of which ... 100 are currently at a star, looking around.
If there where 1 million such civilisations in the galaxy, we would fully expect that no such probe every reached or star in the entire history of human life on the planet.
The milky way has a volume of ~3e61 m^3. There are ~5e11 stars. Dividing volume by # of stars we get 6e49 m^3/star, or an average distance between stars of 4e16 m.
At 200 km/second, that is 6000 years between the closest pair of stars, on average. Which means that if our probes are star-hoppers, they are spending 7 months refueling (!) at a star before going onto the next one. Which is pretty ridiculously short period of time to fuel the ship for a 6000 year voyage. So I'm being rather generous with alien Voyager-like technology.
Or, to be brief, Voyager doesn't count.
In essence, in order for us to expect aliens to 'already be here', they need to be able to build a Von Neumann machine over interstellar distances. That means building an industrial civilisation. My theory is simple -- this is possible, but it requires [b]billions of years[b] to build a biosphere to fuel the civilisation, and the practical way to do it is to drop off the seeds of life at a star, and wait for intelligence to evolve.
Remember, not only do we have to build a complete ecology at the destination (be it industrial, or industrial-biological) from some seed, we need to preserve the seeds as they travel over interstellar distances, and then plant them on fertile ground when we get to the other side. Many of these involve engineering challenges of ridiculous magnitude, which may not turn out to be practical.