Re: "Web-mediated SETI": Robert Bradbury Replies

Robin Hanson (
Wed, 03 Nov 1999 11:52:26 -0500

I've been buzy the last two weeks writing a new paper
( or .ps), and just
caught up with this long and interesting discussion about miracles, and ETIs all around us.

I share Hal's visceral horror of reverting to religion, but also share Eliezer's insistence that we entertain any hypothesis, no matter how horrifying.

The main problem with the "ETIs are all around" theories is that as a whole they fail to predict much of anything. The alternative class of theories, that the universe is mostly dead and uncaring, have in contrast led to many strong predictions, many of which have been dead on.

Postulating a mostly dead universe has led us to expect that relatively simple mechanisms underly phenomena, and that these mechanisms are similar across time and space. While predictions based on these expectations haven't always been dead on, on the whole they have been remarkably good.

In the space of all logically imaginable observations, the dead universe theories have, as a whole, assigned a relatively high likelihood to data we observed, relative to the data we did not see. In Bayesian terms, relative to theories which don't assign as high a likelihood to to observed data, our posterior that the right answer is in this theory class should rise.

In contrast, the class of living universe theories has not, as a whole, assigned a very high likelihood to the data we actually observe. For every more specific theory that predicted that, say, the allies would win World War II because God is on their side, another nearly equally plausible theory in this class predicted the opposite.

This lack of specific predictions seems to result from saying "they are all powerful and can get whatever they want", while also being willing to entertain a very wide range of ideas about what they could want.

I applaud Robert Bradbury's attempt to work some more robust and specific predictions out of the live universe class of theories. I'm not yet convinced he has done so, as it seems he is so far mostly taking the fact that the universe looks dead to constrain he theories of a live universe. But I look forward to his collecting his ideas into a coherent paper for us to evaluate.

On the topic of web invitations, I see that some think that making a web page inviting ETIs to visit might make such visits more likely. But I can equally entertain the idea that it might make them *less* likely to visit. So I can't support the effort.

Robin Hanson Asst. Prof. Economics, George Mason University MSN 1D3, Carow Hall, Fairfax VA 22030
703-993-2326 FAX: 703-993-2323