Why We Haven’t Met Any Aliens

As for why we haven't met aliens, I think there are two more fundamental causes.

(1) In 2011 we know that many stars have planets. This should be no big surprise based on considerations of angular momentum. The trouble is that Jupiter-sized planets tend to get sucked into the accretion disks of their stars, and in the process they tend to destroy Earth-sized planets that exist in the habitable zone. Planetary systems are common, but habitable terrestrial planets are rare.

(2) Interstellar travel and communication is highly difficult. It's possible that some civilization will manage an interstellar travel event among billions of civilizations and billions of years. However, the percolation threshold for a self-sustaining and growing interstellar civilization will never be reached. (Civilizations won't establish an outpost around a secondary star and create additional colonies)

A few years ago I did an analysis of interstellar war. The obvious mode of attack is to launch a deadly bombardment against a planet before any possible counterattack. One clear conclusion was that if you launched a missile that traveled at 10% of the speed of light, it wouldn't matter much if that missile were tipped with a hydrogen bomb or not -- you just can't get enough energy from either nuclear fusion or fission to propel a starship at a reasonable speed. (If you go slow, a 1000-year generation starship would need tons of antimatter simply to keep warm.) Note that interstellar hydrogen would impact such a starship at high velocities harder than radiation from a nuclear reactor.

The corollary is that neither fusion energy nor fission energy is sufficient for interstellar propulsion

On the other hand, recent studies apparently hint that things like electrical and gravitational constants may not be the same everywhere, so maybe they don't transmit their signals with light.

One more thing is that the argument that the age of the universe combined with its propensity to make life means there must be some intelligent life can also be applied to the probability that some intelligent life will wake up and save themselves from such a bad fate, as the author hints in the last paragraph. It doesn't mean all species get caught in that trap and never escape, it just means it's harder to become a species that saved itself from the perils of materialism, loss of social justice, and falsehood.

1) Fermi Paradox

Why haven't we seen any evidence in the observable universe of megascale activities and exponential growth comparable to those that we know that our descendants will be capable of, and fairly soon?

http://en.wikipedia.org/wiki/Fermi_paradox

2) The Outer Bounds of the Possible

Can we convert our entire future light cone into computronium? There seems to be no physical laws to prevent that outcome, and absent being stopped soon, the tiny fraction of our machine descendants to go all out for self-replication and expansion will set this program in motion.

3) The Great Filter

Assuming we're in base reality and everything out there is running much as our present physics understands it to run, are we past the barrier that stops intelligences from converting the observable universe into computronium? Or not?

http://en.wikipedia.org/wiki/Great_Filter

4) The Simulation Argument

Or are we simulations, alone in our virtual machine?

http://en.wikipedia.org/wiki/Simulation_hypothesis

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All these are linked via this rather depressing line of thought:

"A technologically mature “posthuman” civilization would have enormous computing power. Based on this empirical fact, the simulation argument shows that at least one of the following propositions is true:

- The fraction of human-level civilizations that reach a posthuman stage is very close to zero;

- The fraction of posthuman civilizations that are interested in running ancestor-simulations is very close to zero;

- The fraction of all people with our kind of experiences that are living in a simulation is very close to one.

If (1) is true, then we will almost certainly go extinct before reaching posthumanity. If (2) is true, then there must be a strong convergence among the courses of advanced civilizations so that virtually none contains any relatively wealthy individuals who desire to run ancestor-simulations and are free to do so. If (3) is true, then we almost certainly live in a simulation. In the dark forest of our current ignorance, it seems sensible to apportion one’s credence roughly evenly between (1), (2), and (3).

Unless we are now living in a simulation, our descendants will almost certainly never run an ancestor-simulation."

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The bottom line is that there appears to be a large gap in our understanding of the evolution of intelligence in the universe. By all counts, our galaxy should have many civilizations across its span, and by all counts if they're anything like us, their most aggressive descendant factions will turn the whole galaxy into computronium in a few tens of millions of years. But we don't see even the first glimmers of any such thing.