How Likely is Radio Contact with Extraterrestrial Intelligences?

## NIC = RIC x LIC where

### To calculate RIC, we have to know some Astronomy, some Biology, and some Sociology. The astronomical part is in the best shape, the biology is coming along (but we need to know a lot more), and the sociology is pure guesswork at this time. That means that any final answer we give is going to be at best an educated guess (and not all that educated). But it is instructive to see what would have to be true in order that lots of messages are out there to be heard.

We can break RIC down into the rate at which stars appear, times a bunch of probabilities that the other conditions necessary to the transmission of radio messages will occur, if you start with a star. I supply my own optimistic estimates for each factor. They are all arguable, but I don’t think one can make it much more favorable than I do here, with strong justification. We can do it something like this:

RIC = Rstar x Pplanets x Phabitability (Astronomy)

x Psimple life x Pcomplex life (Biology)

x Pradio transmissions (Sociology)

Astronomy

## Biology

Sociology

### That’s it, we are done! Let’s see what we get. To recap, we have:

NIC = RIC x LIC = LIC x

Rstar x Pplanets x Phabitability x Psimple life x Pcomplex life x Pradio transmissions

= LIC (years)x 20/year x 0.5 x 0.1 x 1 x 0.1 x 0.01 = LIC (years) / 1000

### What does this mean??? You have to supply your own guess about the total time any given planet harbors complex creatures which are transmitting radio signals LIC ). What is clear is that if that lifetime is 1000 years or less, then we are likely to be the only civilization currently doing so in the Galaxy. Note that this is true even if thousands of such civilizations have arisen over time, because the Galaxy is 10 billion years old. It is easy for them to be spread out in time enough that they tend to miss each other, unless they last for timescales that are interesting by Galactic standards. The problem is even worse if we would like our nearest current neighbor to be within, say, a thousand light years of us. In order to have a reasonable chance of that, there would have to be roughly 10,000 such civilizations currently broadcasting, because the Galaxy has a diameter of 100,000 light years! Using our solution above, you can easily show that the average lifetime of radio broadcasts has to be around 10 million years!! We don’t have to decide how likely that is – we can just say that it’s what is required in order that there is someone “nearby” to talk to. The same thing applies if you are talking about spacecraft visitations instead of radio messages. If you are even more wildly optimistic than I have been, it is very hard to reduce it to fewer than a million years. A clear implication of this is that if we do hear from someone, they are likely to be far more advanced than us (we have been technological for a tiny fraction of millions of years). On the other hand, it is quite easy to be less optimistic than I have been, and easily reach the conclusion that we are the only civilization currently broadcasting artificial radio signals in the Milky Way Galaxy. There is great room for pessimism on the sociological front, and the biology is really unclear in the absence of examples of life other than on the Earth.

How Likely Is Life in the Galaxy?

### If you want instead to ask only about how many planets have life on them, the situation changes drastically. Now we have

NSL = RSL x LSL = LSL xRstar x Pplanets x Phabitability x Psimple life

### We can use the numbers from before, except that LSL should be a much longer number if life tends to hang onto planets once it starts (which seems to be the case with Earth). Then a reasonable number is LSL= 5 billion years (half the lifetime of the Galaxy), and we find that NSL = LSL = 5 billion! There should be billions of planets in our Galaxy with simple life on them at all times. If complex life typically lasts 1 billion years (and only appears on 0.1 planets with simple life), the number of planets with interesting complex alien organisms right now could be something like 100 million.

This makes it clear why most astronomers believe the Galaxy is teeming with life. Our estimate could be 1000 times too high and that would still be true.It is important to end by saying that I don’t think a valid conclusion of all this is that we should abandon SETI: the search for extraterrestrial intelligence. Firstly, there was a lot of guesswork above – maybe we are missing something. For example, it is probably possible for civilizations to set up automated, self-repairing broadcast stations that could last essentially forever (if they chose to do so). Secondly, it doesn’t cost much for us to listen, and the payoff is extraordinarily important if we hear something. Nonetheless, it makes sense to be realistic about the chances of hearing something, so that we don’t misinterpret silence (if that is the eventual result of our efforts). One conclusion seems to be that we should consider ourselves to be currently Galactically important; that makes it well worth taking good care of ourselves!