Right now it's looking as though every star has planetary systems. And there are billions of stars... just within our own galaxy.... and then you take into account the billions of other galaxies out there... And of course the reality that there is an absolute limit to how much of the universe we can see, based on the speed of light, meaning there is certainly huge swaths of the universe we CAN'T SEE...

It'd be vastly more surprising if we were the only world with life.

Probability that extraterrestrials exist: 99.9%
Probability that we've received messages from them (not directed at us specifically): 0.01%
Probability that they have identified us as another life form: 0.001%
Probability that they've already visited Earth: 0.00000001%

When you consider that the nearest star is 4 light years away, and that the laws of physics make it extremely difficult to even approach the speed of light for any reasonably sized object, it just seems unlikely that any other civilization could (or would) have made the trek from wherever to our pale blue dot. In the latest Cosmos episode, NdGT mentioned that Voyager, moving faster than any spacecraft we've ever made, would still take 80,000 years to reach Alpha Centauri. It's hard to fathom these distances.

VWolf,

If you were to travel back in time 500 years, or 5000, and ask an intelligent person of that era to do a probability rating of the likelihood of the internet ever existing in the future, what percent do you think they'd give it?

My point being, you come up with your estimates based on what you know now. You don't know what the future development of technology and physics holds.

Times like 100,000 years are blinks of an eye on the scale of universal history, and there is no reason to believe that other races aren't that far ahead of us and more.

Are you absolutely positive that the speed of light will never be approached? Or that there will never be a shortcut like, say, a wormhole that can be exploited?

We can always hope for some shortcut. Those are my subjective probabilities, and I'd love to be proven wrong.

As for wormholes, a stationary black hole has been ruled out as a candidate, but a rotating one holds some promise. The trouble is actually getting to one would take an extraordinary amount of time. Again, this is all based on what we know now, not what we might know in the future.

I would guess that the odds of our planet being visited by others is certainly greater than the odds of our species travelling to visit them.

Dr. Michio Kaku: ""We could be in the middle of an intergalactic conversation...and we wouldn't even know"

There are a number of points you raised that I don't necessarily agree with; but, I'm going to wait to develop my arguments for some original post that I've been planning.

for technological aliens in our galaxy. The more we learn about biology the more I think getting past the bacteria level is difficult, getting to primitive tool use is difficult, and finally getting to technology is very difficult. To put in perspective dinosaurs were dominant for 150 million years without evidence of movement towards even primitive toolmaking. Our ancestors and our species went through several keyhole events to get to this point. Now we are at this point it appears that maintaining technology is not necessarily a stable condition. In addition lots of dangerous celestial events are possible that could wipe out us out or reset or technological development.

Only one species developed on this planet in 4B years. Granted once we took off we locked out any other likely contenders, but, other than primates, what else would have been a contender?

We have a 100 billion or so chances but we are talking about some pretty unlikely events (formation of single cell bacteria using RNA as both a catalyst and information storage), development of DNA as a stable code, multicellular life resulting from absorption of mitochondria and chloroplast bacteria - not to mention the development of a cell nucleus to protect the genetic code).

The best arguments of course are 100 billion chances and the fact that bacterial life developed so early in our planets existence. Here is a paper that talks about how the early development of bacteria may not be an indicator of a high probability of this happening.

http://www.technologyreview.com/view/424795/probability-of-et-life-arbitrarily-small-say-astrobiologists/

He is just a joy to watch and listen to.