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Comment by nhaehnle | Hacker News Reader

I often wonder this as well. The speed of light isn't really a physical limit to expansion - generation ships are theoretically possible. It may just be that the Great Filter takes the form of relatively benign[1] economic and/or sociological obstacles.

At least, if you take a diff between typical beliefs at the time the Fermi Paradox was formulated vs. typical beliefs now, I would say the experience of the shutting down of Apollo and overall reduction of human space exploration is probably the most relevant new observation we have made since then.

[1] In the sense of: You don't need nuclear armageddon to stop humans from spreading to other solar systems; the price tag of interstellar travel is sufficient.

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It may not even require a high price tag. I present as a hypothesis that exploration is a form of competition, i.e. the most competitive societies explore the most.

As an example, the European Conquest of the Americas was driven by competition between European nation-states, it might not have occurred so swiftly or at all without some kind of competitive pressure.

Said another way, imagine yourself waking up in the court of King James of England or Queen Isabella of Spain. You establish rapport and they believe you come from the future. You communicate faithfully what you know about history. What conclusion do you think they are more likely to reach?

1) They have done the right thing by colonizing the Americas.

2) They have created what will eventually become new competitors, of whom at least one (the USA) will have subjugated them under the disguise of an alliance called NATO.

In the crucible of 15th-century European politics, neither England nor Spain may have had much choice in their desire to acquire more resources and open new markets.

And maybe the 21st Century will have cheap space travel and good reasons to take the trip. But we shouldn't ignore the possibility that Earth's governments may just decide to not let anyone leave and blast anyone who tries. Let's call it the "Nitrogen Curtain" in honor of Winston Churchill.

If you consider the declining birth rates of modern nations as the standard of living goes up, then it stands to reason that once a civilization reaches the point where it might be able to engage in large scale space exploration, the population that can do so has become relatively stable.

So you simply never (or rarely) get exponential cosmic expansion. There might be a few billions of that race roaming the stars, but their numbers do not increase.

Evolution takes care of that obstacle: whatever individuals keep replicate themselves - win, and keep pushing expansion forward.

But declining birth rates don't lead to stable populations, they lead to shrinking populations. So if we don't manage to restore birth rates, this would be an example of 'Great Filter ahead of us'.

Declining birth rates only lead to shrinking populations when the birth rate was stable to begin with.

Date 1: Every two people produce four offspring.

Date 2: Every two people produce three offspring.

Between Date 1 and Date 2 the birthrate declined by 25%, but the total rate is still positive.

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