I just read this article, Xenopsychology, by Robert Freitas Jr. It asks the questions: What are alien minds like? What predictions can we make about alien goals, behaviors, and brains? What universals might exist in the psychology of creatures across the universe? What metrics do we have to compare different minds? "So far we have very little direct knowledge of alien minds -- but we have some fascinating bases for speculation."
What methods can we use to begin answering these questions? We can look at the diversity of brains on earth, studying their overall architectures and the evolutionary reasons those architectures exist. For example, one major division is that between ganglionic (decentralized network of independent sub-brains; less scalable) and chordate (central brain with many peripherals; very scalable) nervous systems. What are the benefits and drawbacks of these different types of brain design? Why did they emerge in the first place? Should we expect alien brains to fit one of these two patterns? Are there other options?
Concerning metrics, the author introduces the "sentience quotient" (SQ). "Generally the more information a brain can process in a shorter length of time, the more intelligent it can be." This is basically a measure of a brain's information processing efficiency. The exact definition is given as log(I / M), where I is the information processing rate in bits/second, and M is the mass of the machine. (Because I and M can cover very large ranges, the logarithm is used to focus on the ratio's order of magnitude.) So a small machine that can process quickly will have a higher SQ. The SQ ranges from -70 to +50. (Note that this definition only deals with processing speed, not solving problems/achieving goals. I like the more general definition of intelligence given by Shane Legg and Marcus Hutter: "Intelligence measures an agent’s ability to achieve goals in a wide range of environments."). The author suggests that there may be a "minimum SQ 'communication gap,' an intellectual distance beyond which no two entities can meaningfully converse." Just as rocks and trees are barely aware of our existence, let alone being able to communicate with us or understand our goals, we might find it difficult to communicate with or even sense the presence of an alien intelligence with an SQ 10 points above ours.
Here are some excerpts from the article:
"Will ETs be more or less emotionally motivated than humans? Will they have emotions foreign to us, or are there any universal emotions?"
"Extraterrestrial logicians may find many of our most enduring paradoxes to be trivially solvable, just as we may be able to resolve some of theirs equally effortlessly."
"Consciousness may be an emergent property of intelligence, a fortuitous feature of a terrestrial animal brain architecture originally designed for other jobs. Is it possible that there could exist yet higher-order emergents beyond consciousness?"