Why we might be alone in the Universe

[polymath257]

Yes, it only takes a few common compounds to get life started (as far as we understand). But what I am saying is that even once we have life, that doens't imply the existence of complex cells, let alone intelligent life.

And I definitely don't agree that we should expect life at our level wherever we go. As an example, if you look at the Earth over the last 3.8 billion years for which it has had life, there has been *agriculture* for only the last 10,000 years or so (well, except for ants) and the use of radio waves for only the last 100.

If we take the Earth as an example, it would be *very* unlikely to find a technological species over the time it has had life. Even as a percentage of the time Earth has had multicellular life, humans have been around a very small fraction of that time.

What I would *expect* in going to a planet similar to Earth orbiting a star similar to the sun and at the right distance, is to see *bacterial* life primarily with a chance of complex cells. Much less likely is some form of land 'animal' and even less likely is something technological.

Why should we not expect life that thinks at our level? Of course we have not found it, and likely we wont. But again, the universe is FUCKING HUGE.

You are far too focused on our modern science and saying, "Why haven't we yet". 

If you agree it is possible for microbial life to exist elsewhere, why couldn't the same compounds lead to a comparative evolution we see here?

Again, the universe is 13.8 billion years old with trillions of galaxies, and billions of stars each, which means trillions of planets.

DISTANCE for our life, and anything else that is out there is going to make it hard if not impossible to find. 

I don't know what other example I can give you.

Think of a single grain of sand on any beach, and know that there are more stars in the universe than sand just on our planet.

We are only alone in the context that we are remote. But to say that no other life at our level or more advanced does not exist, is absurd to me. 

Again, I'd bet, if I could live the entire future of the universe until it suffers heat death, and that we could visit every rocky planet in the universe, it would not surprise me one bit, if other life with our level of language existed. I simply would say, it is stuck where it is at, just like we are stuck where we are at.

Well, first of all, the universe would not have been capable of supporting life until the first couple generations of stars had gone through their cycles. They are what made the heavier elements from which life is made. That means, realistically, that we don't have that full 13.8 billion years to work with, but closer to 5-8 billion.

Next, and this is soely from using the Earth as a guide, bacterial life happened very quickly. Simply based on that, I owuld suspect that bacterial life is common. I would not even be overly surprised if it exists other places in our own solar system.

But, for the 3.8 billion years that there has been life on Earth, approximately half of that time there was *only* bacterial life. The step to complex cells seems to have been difficult. Given that it appears to be based on a symbiotic relationship developing between the different organelles, it isn't too surprising that this takes a while to show up.

Of the approximately 2 billion years that there have been complex cells, only about a third of that has had multicellular organisms. Again, this is a difficult, cooperative step that seems tricky to manage.

So, already, even if there is life somewhere, we expect about 5/6 of the places with life to only have single celled life. This is a bit of a wild guess, but I suspect that this is an underestimate and that complex cells and multicellularity are just unlikely.

Then, even once there is multicellular life, the development of *intelligent* life isn't so easy. I'm not convinced it is something that is actually selected for as opposed to being a byproduct of something useful, like sociability. So, sure, we get elephants and dolphins, and chimps. But, again, it isn't simply the distance that would prevent us from seeing these species if we were on a different planet. It is also the fact that these species simply don't put out any signal that could be detected off planet.

And, we didn't either until about 100 years ago.

So, out of the 3.8 billion years that there has been life on Earth, only 100 years even has the potential to be seen off planet.

So then comes the really interesting question. How long do we expect technological species to exist? Do we really expect, for example, humans to still exist in 10,000 years? how about 50,000 (the average duration of a mammal species)? let's be optimistic and say 1 million years. Then, there is about a 1 in 4000 chance that any particular time where there is life on Earth you would find technology.

So, now, how many technological civilizations do we expect in our galaxy? A good number of stars, even if they have planets, would not be suitable candidates for life. The more luminous stars don't last long enough with type A stars lasting around 10 million years. The dimmer stars don't have a very large habitable zone. Those too close to the galactic centers would have too much radiation to deal with.

Then comes the question of how many planets are actually in the habitable zones for their stars. This is harder to determine. Our methods for finding planets are biased towards those that orbit close to their stars and are big. Earth sized planets in the habitable zone are not so easy for us to detect, so we don't know how many there are. From what we have at this point, it *looks* like they are less common that the supergiant planets close to their stars.

Of course, the next filter is how many actually have enough water to give rise to life. Again, this is hard to determine. In our solar system, water does exist off of Earth, but it tends to be in the form of ice and sequestered. But that may be due to, for example, Mars not being large enough to keep a good atmosphere. A planet that is too small won't be able to maintain the type of environment necessary for life. So, even if there was once life on Mars, there certainly isn't a lot of multicellular life there now. How common this is is a good question.

So, no, I don't consider it all that likely that another technological civilization exists in our galaxy at the present moment. Of course, there are a lot of galaxies, so another *somewhere* in the observable universe may be likely, but our chances of finding it would be nil.