I think 5, 7, & 8 are the most likely

Honestly if they are smart enough to travel all the way to the earth they are likely smart enough to know that we would not be able to handle the idea of there existence very well.

The universe is enormous, the planets that can bear life are few, and those that bring forth life may not bring forth advanced civilisations, let alone advanced civilisations which don't destroy themselves.

It's hugely unlikely that there's extraterrestrial intelligence near enough to us to detect, let alone communicate with, and that's very likely going to be the situation long past the extinction of homo sapiens.

It's not much of a paradox, really.

I'd say that we might just be very early to the party. A spacefairing species takes time to appear, after all. The starsystem needs to be formed, the planet has to be formed (and end up in a favourable spot in the system), cool and harden, become biologically viable and then the miracle of life and evolution begins, which will also take alot of time. And it's all really up to chance.

Humanity might simply be one of the first samples of biological creatures that's popped up so far. There really ought to be someone else out there, but there's no telling if it's somewhere near us in the Milky Way or way out in some other galaxy we can't even see yet. Or if they're still ambling around on the sea floor somewhere.

Or, even worse (but hopefully unlikely)... We were too late to the party.

Of course, my little theory (speculation is magic) hinges on our own biological evolution. There could be other ways by which some form of sentient life could appear, which we don't know of. Perhaps they are less sensitive to eco-disasters wiping them out and less sensitive to the cruel twists of chance ruining everything.

My responses to each of your points:

1. Horseshit I say. We've found bacteria on Mars. There IS other life in the universe. I refuse to accept the alternative.

2. Plausable. If humanity is a case-study, it's not far beyond the realm of possibility that any existing intelligence has already destroyed itself (although I certainly HOPE this isn't the case).

3. I'd actually believe this. Again, if past human actions are any indication, we'd pose just as much a threat to them as they would to us, regardless of how advanced they are.

4. Eh, unlikely. Unless you count "a planet's sun eventually being extinguished", I don't see why natural disasters would destroy EVERY civilization. Isn't the case here, that's for sure.

5. Maybe.

6. Highly possible, hell, even likely. Without the ability to teleport to wherever they want, I could see Star-Trek-like space travel being impractical.

7. Perhaps. Patience is a virtue, after all. Radio signals we broadcast into space probably HAVEN'T reached any intelligent life yet. It could be a very, very long way to the next technologically advanced race.

8. Eh, not sure on this one. If aliens WERE trying to communicate with us, I'm sure it'd be something obvious enough for us to pick up on.

9. Doesn't quite make sense to me. Why would you be listening for transmissions, but not attempt to send any yourself?

10. Hmm... no real way to say either way. We've never encountered anything to even suggest that this could happen, but then again, we haven't encountered anything to DISPROVE this either. Just seems like wild speculation.

11. Could be. With no basis for comparison, we couldn't really say HOW life would evolve in space. Basically, the same response as the last one.

12. Another case of "highly plausable, even likely". Earth may simply be a fertile ground when it comes to technological advances. Hell, without certain types of gases and metals, we wouldn't have light bulbs. I could totally see this.

13. No. Just no. Don't buy it for a second.

14. I could see this. Perhaps "zoo animals" isn't the right description, but aliens purposefully keeping their distance to study us? That's my personal theory.

15. Again, no. Don't buy it.

"I think the strongest evidence to suggest intelligent life on other planets is the fact that they haven't tried to contact us yet." ~ Bill Watterson (paraphrasing)

life is out there if you look at just the extremophiles we have here on earth it shows just how creative and adaptable life really is. as for intelligent life. the odd of finding an intelligent civilisation near by us and who developed at the same time as us is astronomically low. even 100 years would be enough to miss each other

Daystar Clarion:
I think the answer is pretty easy.

Distance.

Aliens exist, it's a mathematical certainty

Being close enough for them to ever contact us is a whole other matter.

This says it all

I think Bill Watterson had the correct answer:

The odds of us being alone are extremely low.

Unfortunately, the odds of other species being in a position to actually make contact are also extremely low.

For example, there is a decent possibility of life forming or having already formed on the interior of one of Jupiter's moons, known as Europa. But the environment is such that it is extremely unlikely that it could have or ever would progress any further than the equivalent of our bacterial stage.

Then there is the problem of time, there's no guarantee that any intelligent alien species within listening range are currently advanced enough to have heard us or be transmitting back. It took us billions of years to evolve, and millions to establish civilization, yet we've only been seriously looking to the stars for a mere century. There's even a possibility that they are listening to us right now, or rather to radio transmissions beamed decades ago, and it will take just as long to send a return reply, if they ever do. (scarily enough for the sake of interstellar politics, some of Hitler's propaganda is among the first transmissions strong enough to possibly be picked up outside of the solar system.)

And then of course, there is sheer distance. If we were to assume that advanced life can only form on terrestrial planets at least marginally comparable to Earth (not necessarily a safe assumption, but its the only baseline we have), then possible locations for our ET counterparts are fairly rare on a galactic scale. Don't get me wrong, there are plenty of Earth-like planets out there, but their numbers are dwarfed by the vast distances in between.

The universe is over 20 billion years old. We have the technology to look up there and see more than the planets of our solar system for a few decades or so.

I don't find it that surprising.

Obvious answer: The Prime Directive states that there is to be no contact with alien species before they achieve warp travel on their own. They may observe us, but until we can get past that hurdle, we're not ready to meet them yet.

Because We are the aliens
Dun duh daaaaaaaaahhhh....

aba1:
Honestly if they are smart enough to travel all the way to the earth they are likely smart enough to know that we would not be able to handle the idea of there existence very well.

This and the fact the the universe is frikkin huge. Seriously the chance of running into intelligent life just isn't that high. Either that or they too are looking for intelligent life as well, and they saw the worst of us.

captcha: That's it. How Apt.

Distance is the only reason that counts. Unless our physicists are drastically mistaken regarding the maximum speeds at which mass can be made to travel, we will never contact any alien race, nor they us.

Also cultural immaturity. We're basically teenagers: greedy, short-sighted, paranoid, self-conscious, self-destructive idiots who think we know everything there is to know. "The surest sign of intelligent life in the universe is that they haven't attempted to contact us." -Bill Watterson

I think we have, but try to imagine us from their POV. It's like us trying to communicate with chimps or dolphins, sure they're cute, but it's not like you can talk philosophical with them or anything significant like that.

Daystar Clarion:
I think the answer is pretty easy.

Distance.

Aliens exist, it's a mathematical certainty

Being close enough for them to ever contact us is a whole other matter.

An important thing this is missing is:

Time.

Recorded human history is an incredibly tiny snapshot of the history of the universe. Also, what's with people assuming we're the dumb ones?

As long as something REALLY stupid doesn't happen within the next couple of centuries, humanity will live for thousands and thousands of years, possibly millions or more.

Getting to other star systems won't be a problem if we make trips designed to take several hundred years and several generations.

Then we'll be able to see, thousands of years from now, firsthand, just how alone we really are.

I'm of the opinion that life is fairly common in this galaxy, with intelligent life being somewhat-common. As for life intelligent enough to make it to space? We may well be the first species to do it.

Dags90:

Daystar Clarion:
I think the answer is pretty easy.

Distance.

Aliens exist, it's a mathematical certainty

Being close enough for them to ever contact us is a whole other matter.

An important thing this is missing is:

Time.

Recorded human history is an incredibly tiny snapshot of the history of the universe. Also, what's with people assuming we're the dumb ones?

I think I remember someone saying that if you set the age of the universe on a 24 hour clock, the entire human species has only been around for about the last two seconds until midnight.

OT:
Yup, it's just logical that aliens exist.

The answer is distance. Other planets are very, very, very, very far away. And space is very, very dangerous.

Big Universe, lots of space to cover, not much more to say. I find it incredibly hard to believe that we are alone, but I'm not holding my breath for first contact.

Number 5 seems the most likely to me. Time is infinite. Space is infinite. It would be full of life but not all at the same time and not all in the same place. Everyone thinks of aliens as being the more advance race but I've always thought of us as the advance race. The previous one wiped itself out and the new race is still at single cell stage. I would be extreamily surprised if we found a living alien race.

I think that the universe is just to big for only one intelligent species

SaneAmongInsane:

Hallow'sEve:
I think we have, but try to imagine us from their POV. It's like us trying to communicate with chimps or dolphins, sure they're cute, but it's not like you can talk philosophical with them or anything significant like that.

But that said, we have humans that enjoy working with dolphins and chimps. Hell some of them we've even taught sign language just for the fuck of it.

You'd imagine if there is an alien race that knows of us, and has the capability to get to us at very least one of us would make contact. Study us. Maybe try and teach us shit just for the pure science of us.

plus, if your a friendly intelligent alien race, wouldn't you want to make contact if only to not feel so alone in the universe? I mean thats a chunk of the reason we have pets right?

Yeah, but I think an alien race would be able to guess that we wouldn't take kindly to being "pets"
I'm sure whatever contact we've made (which I think is very limited, I don't think the iPod came from aliens) has been restricted from public knowledge for obvious reasons.