July 26, 2004 11:36 AM

Statistics and Aliens

Several blogs have pointed out a claim from Seth Shostak at the SETI Institute that, given the Drake Equation and Moore's Law, we should expect to discover radio signals from another civilization within 20 years or so.

This seems like a good opportunity for me to mention that I think Drake Equation is flawed. It calculates the number of technological civilizations we should expect to find in our galaxy by multiplying a few estimated quantities and probabilities. Unfortunately, it makes the assumption that these figures are statistically independent. I see no reason to make that assumption, so I think the Drake Equation is incorrect.

If the Drake Equation's input values are statistically independent, the development of one technological civilization would have no impact on the development of other technological civilizations. That seems unreasonable to me.

I think it's likely that any technological civilization will build Von Neumann Machines, i.e. self replicating devices, that explore and colonize the universe around them in every possible direction. Very likely these constructs will travel at a large fraction of the speed of light. Our own civilization is extremely young, and yet it should be able to create such things within the next century.

I thus think it is likely that any species capable of technological civilization starts expanding out at near the speed of light within tens of thousands of years of evolving. We've been around for something less than 1/100,000th of the life of the universe, which is a blink of of an eye on cosmic time scales. You would therefore expect that if another civilization is in your light cone, it should already have traveled to where you are. (This is a variation of the Fermi Paradox.)

So, if there are other folks out there that we could hear, they should be where we are already. However, if they were here already, we probably wouldn't be. Intelligent life has a way of drastically interfering with its environment. We've seen this phenomenon on Earth, where we've spread over the entire planet in a miniscule time. It isn't likely that other intelligent technologically capable species are going to arise on Earth so long as we're here.

Thus, we hear no one because if we weren't the first out the starting gate in our light cone, we wouldn't be here in the first place.

Does this mean I think SETI is a waste of time? Far from it. Among other things, I have a mediocre record as a pundit, so I could be wrong, and it would be a great mistake not to find out if there is other intelligent life out there. However, I will not be shocked if we hear nothing.

By the way, we may hear nothing even if there are other technological civilizations out there. There is an assumption that with sufficiently sensitive equipment, we should be able to pick up the internal communications of other civilizations. Our own television and radio broadcasts are, after all, likely to be detectable many light years away. However, this will not continue.

Information theory tells us that more densely packed a signal is, the more it resembles random noise. We're getting better and better at using bandwidth, so pretty soon I'd expect everything we send to be pretty indistinguishable from random. This has already started to happen. We're also starting to adopt spread spectrum radio technology very widely, and that, too, has the effect of making your broadcasts look like random noise. Lastly, in order to maximize the available bandwidth, we'll start more and more narrowly focusing our communications using phased arrays and other active antenna technologies, so most of our communications may be inaudible outside of teeny chunks of space.

Although the specifics of how we communicate by radio in the future will change, the fact that new technologies will produce signals more and more like random noise and harder and harder to hear at a distance is likely permanent. It is a response to fundamental laws about information theory and radio communications. I assume that if there is anyone out there, they will be subject to the same fundamental laws, so their communications will follow the same trend.

Posted by Perry E. Metzger | Categories: Science & Technology