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Fermi Paradox and the expansion of technological civilizations
I've gotten into this discussion repeatedly, recently. So I've decided to expand further on my blog, here.



Certainly this is not the most exhaustive or airtight version of this argument, but it's one that attempts to anticipate most of the objections which I, personally, keep seeing raised in conversation with me. If you're interested in this topic, a google search on "fermi paradox" in conjuntion with replicators or "von Neumann" might be useful. I've included links that have supporting information at the bottom.

The core of the argument is this: any technological civilization (TC) will expand to make use of any resources near it. For the timescales involved in the lifetime of the universe, every star in our galaxy is nearby. Therefore, if there were any TC in our galaxy, our star would already be a managed star. Further, if we can see a galaxy that looks like ours (in the sense that there are obviously billions of stars wasting energy), we can deduce that there is no TC in that galaxy.

This means that either we're the first TC in our past light cone (minus a few thousand years) or something stops TCs from expanding.

Maybe expanding is just too hard?

Well, that doesn't seem to be the case, from what we know today. It's clear that unless our civilization collapses entirely, we'll have molecular manufacturing within the century, and with that, conversion of the entire solar system into finished product is only energy limited. That is, hundreds or thousands of years, not millions. Once a TC has expanded into its own solar system with molecular manufacturing, it seems a small leap to send machines to prep the nearest solar system for similar conversion. Average distance in the galaxy is not much higher than the distance between here and the nearest non-Sol star, and using laser boost, a few kilograms of smart replicator could be sent there quickly.

If everything moves slowly, then, we'll get full use of the solar system resources within, perhaps, a thousand years. Assuming very few resources are used to send replicators to other solar systems, within, say, 10 light years, and to make the numbers nice and round, we'll assume that replicators are sent to arrive at the new solar system just as the old one is fully utilized (a tiny, tiny fraction of the Sun's output would be required to boost a few hundred replicators to the nearest solar systems, and the payoff is immense, since each one opens up approximately the same amount of resources, on average, as our entire solar system).

So, in this just-so story, it takes a thousand years to develop each 10 light years of diameter in an expanding shell, corresponding to an expansion speed of 1% of light speed, which seems extremely reasonable. Remember, we're not even shipping anything large over interstellar distances: just replicators of a few kilograms each, and information, later.

The Milky Way is about 100,000 light years in diameter, which means that unless we're destroyed or something totally new crops up to stop us, our civilization will have completely utilized the galaxy in 10 million years. This is a conservative estimate, as it assumes that no one goes exploring across the galaxy by themselves. If that happens, or if any of this is done in a more parallel fashion, the galaxy could be completely utilized in only a million years.

Let's define "completely utilized". In order to capture most of the energy from the Sun (energy that is otherwise wasted into empty space), we'll need to put solar collectors in orbit around it. Since the light from the Sun will then be hitting them and converted into useful energy and waste heat, from the viewpoint of nearby stars, it would appear as though our Sun had suddenly begun emitting only in infrared. As our wavefront of processing expands across the galaxy, it would take a growing, visible bite out of the Milky Way as seen from well outside our galaxy. After the galaxy was completely utilized, the vast majority of the visible output of the stars in it would be absorbed, and the entire galaxy would be full of stars that emitted only in infrared.

If some other species had begun expanding in our galaxy at anytime before the last hundred thousand years, therefore, we would either not exist (since our planet would not have existed to evolve on), or we would be able to see an area of infrared-only stars.

Further, if any other species had begun expanding *anywhere* within our past light cone (minus a paltry few hundred thousand years), we should be able to see the area which they'd expanded into as an area of stars curiously dim, or only in the infrared. All the same logic applies to colonizing the nearest galaxies as well.

In hundreds of thousands of years of practice (assuming no new laws of physics), one would expect this process to become very efficient, such that completely utilizing a galaxy like our own would require only slightly more than travel time to it. Even remaining with the 1% of lightspeed estimate, however, the following things are clear:

There is no civilization in our galaxy older than a few tens of thousands of years, and since we're here, and since it is hard to imagine any catastrophe which could befall a species spread across hundreds or thousands of light years, we can further deduce that there *never has been* any such civilization.

There is no civilization anywhere in the past light cone of our location a hundred thousand years ago, because if there were, they would be clearly visible, having darkened large parts of the visible sky by now.

The first civilization to arise will naturally prevent others from arising in its own future light cone (except right on the edges of it), simply because there will be nowhere to evolve. The fact that we exist and the universe around us seems untouched, therefore, means that we're the first.

What if no one wants to do this?

It only takes a single individual in a sufficiently rich civilization (and we appear to be close to being sufficiently rich!), and there are already multiple individuals who want to do this kind of thing in ours.

What if FTL (faster than light travel) is possible?

FTL makes this argument *more* compelling, since it could all happen faster. That said, we don't need FTL to expand across the known universe very quickly (in astronomical time).

Surely there's more to physics than we now know; maybe there's something that would stop this.

So far, learning more about our world has always expanded our capabilities, rather than contracted them. Everything here is based on current-day physics, so even if there are major new discoveries to be made, it seems unlikely that they would make this process slower or less likely. Rather the opposite.

[more objections will go here as I hear them]

Miscellaneous links for further reading:

http://www.aeiveos.com:8080/~bradbury/MatrioshkaBrains/index.html
(all interesting, but especially http://www.aeiveos.com:8080/~bradbury/MatrioshkaBrains/PlntDssmbly.html )
http://en.wikipedia.org/wiki/Light_cone
http://www.sciam.com/article.cfm?chanID=sa004&articleID=000CC344-B043-1353-AF3383414B7FFE9F
http://en.wikipedia.org/wiki/Star#Distribution
http://en.wikipedia.org/wiki/Milky_Way
http://www.rfreitas.com/Astro/ReproJBISJuly1980.htm

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