[phizy]

It's not a powerful argument for 'we must be alone', because it's an equally powerful argument for 'space colonization is nearly impossible.' You're saying things are "certainly possible" but you don't have a shred of evidence to support that belief. It's more likely that you're vastly underestimating the cost of maintaining a life-supporting ecosystem independent of the benefits of a billions of years of evolutionary adaptation.

[JoeAltmaier]

That thinking is about a decade out of date. The number of expolanets that are in the habitable zone grows geometrically. Some think every star has some planets. Simple chance makes it possible to find agreeable destinations nearly everywhere.

And 'evolutionary adaptation' is irrelevant. We can change ourselves (are changing ourselves) genetically already. That will only increase. The result will be a species more adaptable than any in a billion years. If it has the will to colonize other planets, it certainly can.

'A shred of evidence' is really not hard to find, if we aren't needlessly negative and contrary.

[phizy]

A decade out of date? It'll take you 30,000 years to reach the nearest star at the rate of the fastest moving object ever constructed by man. If anything any of us are talking about are able to be out-of-date in a mere decade, the whole premise of this discussion is flawed beyond measure.

All of the resiliency that you've seen in life is due to the fact that the environments to which we're adapted are uniform. Oceans everywhere are nontoxic and water. Air everywhere is mostly clean and breathable. There is no example of life being resilient under conditions as varied as other planets. We are only able to adapt within the confines of our clean-room Earth. We can't even adapt to the bottom of the ocean or Antarctica.

Optimism isn't going to overcome 6 orders of magnitude of error.

[JoeAltmaier]

Not a single correct thing in that, I believe! Life on Earth has evolved to live in hundreds of dissimilar environments. Heavy salt is toxic, yet fish manage the ocean. Yellowstone has life in mud pots at hundreds of degrees. Air doesn't exist underground yet bacteria thrive there. Hell, there are living things on the surface of the space station.

To get people to tolerate more salt, we'd have to have a fish's ability to regulate it. Ok, genes for that. Air issues? Filters, microstructures to migrate particulates out, baffles (like many creates already have). Deep-ocean pressure? Only a couple of issues there, and certainly life thrives at the bottom of the ocean.

It's not optimism; its simple observation of what we have already on Earth in existing life forms.

And 30,000 years changes not a thing. The log of 1 Trillion base 2 is 39. In 40 generations of space colonization, that's 40 times 30,000 times however long it takes to rebuild the capability on each colony. In a dozen million years the galaxy would be covered. Its 20B years old. So, nobody's tried it yet.

[phizy]

>In 40 generations of space colonization, that's 40 times 30,000 times however long it takes to rebuild the capability on each colony.

Assuming each generation survives. Stop making that assumption, and see how many ways there are for you to reach a similarly empty universe.

>It's not optimism; its simple observation of what we have already on Earth in existing life forms.

I wish you'd observe that none of Earth's lifeforms have colonies on other planets. Goldfish didn't colonize other worlds, is that then a strong argument that Goldfish don't exist?

What you're saying makes no sense: just because animals can colonize a single planet over many generations does not mean they will be able to colonize an entire galaxy in even less time.

[m4rtink]

>I wish you'd observe that none of Earth's lifeforms have colonies on other planets. Goldfish didn't colonize other worlds, is that then a strong argument that Goldfish don't exist?

I would not be so sure about dolphins. ;-)

[JoeAltmaier]

Again, not true. Geometric growth can't be dismissed. Even if 90% didn't survive, it just takes (a little) longer.

[phizy]

How the hell is a 10% survival rate "not optimism?" Try a trillionth of a percent. It makes an astronomical time difference.

[jcranmer]

You're ignoring what I believe to be the most important point: how do we know that is actually possible to build an interstellar colonial vessel?

Or, to bring a concrete element to the question: all known rocket fuels require some nonrenewable Earth resource. How much fuel does it take to launch a vessel built with currently-known technology that would be capable of landing a living human being on a hypothetical planet around Alpha Centauri? And what does that fuel number look like, when expressed in terms of "years of current resource extraction" (or, somewhat more audaciously, percentage of known proven reserves)?

To my knowledge, interstellar colonization is flat-out impossible with current technology. (I'd argue that even intrastellar colonization is impossible, rather than impractical). Arguing that we should discount this non-knowledge of how to do it when explaining why it hasn't happened to our observation is irresponsible to me.

[m4rtink]

Well, the ideal chemical rocket fuel is hydrogen + oxygen - eq. the two things that make up water. Thats perfectly renewable once you pump some energy into it to split water into the two.

Methane + oxygen popular in the new generation of rockets is not much different, you basically add some carbon to the process & Methane can be made on Mars from local resources.

Best fuel for nuclear thermal rockets is again hydrogen, abundant everywhere.

Current Ion Engines run on noble gases, but supply does not seem to be a problem so far.

Sure, the uranium needed for Orion and Nuclear Salt Water rocket indeed is not really renewable but given the performance &opening the way to grab more of it from space make it a non-issue as well.

[sigstoat]

> Some think every star has some planets.

20 years ago, before we had direct evidence of exoplanets, we had evidence from the spectrum of stars that about 80% of the ones which had grown large enough to swallow any of their planets had previously had gas giants.