Not in our lifetime vs. never in a million years

The great physicist Enrico Fermi once asked "Where is everybody?", by which he meant "It seems quite likely that there are other civilizations in the universe, so why haven't we seen convincing evidence?"

Without going into detail, I agree with Fermi that there's no convincing evidence that there are other civilizations in the universe.  However, despite the lack of smoking-gun evidence, I'm pretty well convinced there is life elsewhere in the universe, even in our own galaxy.  It seems reasonably likely that there is life within our neighborhood, and not out of the question that there is some form of life elsewhere in our solar system.

I also think it's pretty likely that there are intelligent civilizations (leaving aside exactly what that means) in our galaxy, and almost inevitable that there are such civilizations somewhere in the universe besides here.  So again, why haven't we heard from them?

When we use terms like "in our neighborhood", it's easy to forget that, when talking about astronomy, "neighborhood" is a very relative concept.  Here, I'll use "in our neighborhood" to mean "within 50 light-years, give or take a few percent".  That's close enough that we could send a signal and get a response in something on the order of a human lifetime.  It's also vastly farther than we have ever travelled, or could hope to travel with any kind of technology we know.  Within this 50 light-year radius there are about 2,000 stars.

Compared to our galaxy, this is a pretty cozy little corner.  Our galaxy is much bigger, on the order of 100,000 light-years with hundreds of billions of stars.  The observable universe is much, much bigger still, with some hundreds of billions of galaxies, depending on how you define "galaxy" and "the universe", each with a huge number of stars.

Summarizing: If you talk about "the galaxy", you are talking about on the order of a hundred million times more stars than our neighborhood, and if you're talking about "the universe" you're talking about on the order of a hundred billion times more stars than the galaxy, or on the order of ten quintillion times more stars than our neighborhood.  If only one in a million stars harbors an intelligent civilization, then there are almost certainly no others in our neighborhood, but some hundreds in the galaxy, and tens of trillions in the universe.

That one in a million figure is just for the sake of illustration.  At this point, we really don't know how likely or unlikely life is, if only because we don't have a lot of data points to go on.  We know for sure there's life on Earth.  It looks pretty unlikely that there's life on the Moon, or Mercury.  If there's life on the surface of Venus, it's got to be pretty bad-ass, but the best guess is probably not.  The jury's still out on Mars; we're pretty sure it had liquid water, but not at all sure either way about the life part.  Quite possibly there used to be but isn't any more.

There are a couple of other possibilities.  Jupiter's moon Europa probably has considerably more liquid water than we do, Saturn's moon Enceladus appears to have a large subsurface ocean, and Saturn's moon Titan has a dense atmosphere and pools of liquid.  Methane and ethane, that is, at a temperature of about -180C (-292F).  Could life develop in either of those environments?  We really don't know, but, maybe.  Certainly not a definite "no way", particularly since we've discovered life on earth in all kinds of extremely harsh environments where we used to think life had no business being.

It's also possible that there is some form of life in the clouds of the gas giants or floating in Venus's thick atmosphere, or on some less likely-looking moon than Enceladus, Europa or Titan, but at this point, Mars, Enceladus, Europa and Titan look like the best bets.

By that reckoning we have, in our solar system, one place that definitely has life and four others that plausibly might have, or might have had.  From what we know, our sun is not a particularly unusual star for our purposes here.  There are plenty of other main sequence stars of similar mass and age, and from recent discoveries, it looks like there are plenty of planets outside our solar system.  There are also plenty of stars not like our sun, but still with planets that might plausibly hold life.

Again, we don't know what the real odds are, but we can try to break things down more finely.  We might consider that any planet or moon with a large amount of liquid water is "favorable to life".  In our solar system, that would mean us, Europa or Enceladus (so far ... the jury is still out on Jupiter's moons Ganymede and Callisto).  Before too long we might have a good guess at how common such situations are.  For the sake of the argument, let's say that one in ten stars has such places.  Likewise, we could guess that there's a 50% chance that a place favorable to life actually develops life.  So that's one star in 50, or about 40 in our neighborhood.   And so forth.

This exercise of taking wild guesses at probabilities and multiplying them together goes by the formal name of the Drake equation (though it probably originates with Fermi).  Writing a formal equation doesn't reduce the wide error bars on our guesses about, say, how likely life is to develop or what portion of planets have favorable conditions, but it does give a well-defined framework for talking about such things.   That's helpful, but if you hear a statement like "According to the Drake equation there are N other technological civilizations in our galaxy," whether N is zero or a million ... um, no.  All that means is that under someone's particular set of guesses, there would be N technological civilizations.

You could make a reasonable argument that we're not just guessing at the numbers to plug into the Drake equation, we're guessing about what some of the terms even mean.  What is life, after all?  Does "technology" mean essentially the same thing for all possible kinds of life?

I suppose at some point I should explain the title of this post.  Why not start now?

Suppose that there's a planet fifty light-years away orbiting a star identical to the sun and with the exact same history and technology as us.  Could we detect signs of intelligent life from it?

The Earth (and so, therefore, Twin Earth) has been pumping out radio signals for about a century.  This means it's at least physically possible that we could pick up Twin Earth's broadcast signals from fifty light-years away.  Right now we would be picking up Twin Earth radio and TV shows from the early 1960s.

Before getting too excited about that, keep in mind that Twin Earth's radio signal is going to be very, very faint at that distance and right next to a much brighter radio source, namely Twin Sun (which is still pretty faint compared to most things we can pick up with radio telescopes).  Radio telescopes, even the really big lots-of-dishes-hooked-together kind, have considerably lower resolution than optical telescopes, and as far as I know we're not even close to being able to distinguish Twin Earth from Twin Sun in the radio frequencies, even if they were similarly bright.

Maybe we could, with a few more advances in technology and after careful observations, figure out that something unusual was going on around Twin Sun, but we're not just going to point a radio dish at Twin Earth and tune in to The Beverly Hillbillies.  Which may be just as well.

At this stage in our development, we are at the beginning of being able to contemplate detecting something like a civilization similar to ours orbiting a star in our immediate neighborhood.  Our telescopes (optical and radio) will doubtless improve, and we'll figure out ways of squeezing more and more information of of the signals they provide, but at the same time something else is going on: Earth, and therefore Twin Earth, is liable to go dark.

I'm not talking about civilization ending in the near future, or humanity morphing into some sort of cyber-species with no need for physical bodies.  Whatever you think the odds of those things may be, we're probably not going to spend too much more time spewing radio waves into empty space simply because it's wasteful.  Ultimately, it reduces bandwidth.  Even now, we can listen to the radio and watch TV over land-based connections.  That's probably just going to get more and more prevalent.  There's a good chance that through sheer technological progress we'll stop sending out whatever faint signal we've been sending.

So say that we, and thus Twin Earth, spend about 200 years sending out a radio signal indicating intelligence.  There are other ways we might detect Twin Earth and deduce that it has life, but only through a structured signal like our radio transmissions would it be clear that it was intelligent life -- OK, we could also look for Dyson spheres and such, but let's not go there just now.  It's also possible that Twin Earth could decide to deliberately send out a signal, permanently, to every star in its neighborhood, after it stops using high-powered radio broadcasts.  But "permanently" to creatures such as us is "momentarily" on planetary time scales.

To get a feel for what that last statement means, suppose that Twin Earth is like us in every way, except that it formed just a little bit earlier or later.  Say a tenth of a percent earlier or later.  Earth is about 4.54 billion years old.  A tenth of a percent of that is 4.54 million years.  If Twin Earth formed a tenth of a percent earlier than us, then we're several million years too late to pick up the brief flash of detectable signals of intelligent life it put out.  If it formed a tenth of a percent later, we won't have a chance to detect its hairless, tool-making social primates for millions of years yet.

This is why Drake's equation has a factor for how long we guess that an intelligent civilization would put out a detectable signal.  Obviously, the Twin Earth scenario is a gross simplification compared to the possibilities of life in the universe, but I think it sheds some light on the Fermi paradox.  There's a decent chance that everybody's out there, or will be at some point in the future, but we just plain missed them or they're not even here yet.



Electromagnetic radiation, which is all we currently know how to detect from other star systems, follows the inverse square law.  Twice as far away, a signal is four times fainter, three times farther away, nine times fainter, and so on.  That means that a star system 20 light-years away would have to produce a signal four times as strong as one 10 light-years away in order to be equally detectable.

Earth has has broadcast some sort of radio signal into space for the past hundred years or so, but at first that signal was very weak -- just a single small transmitter.  Eventually it grew, and quite likely it will eventually fade out.  The amount of time we've spent transmitting our brightest signal is shorter than the time we've spent broadcasting half that signal, and so forth.

Just so, the amount of time that a given civilization spends putting out a signal that we could detect decreases with distance.  At some point, probably well within our galaxy, it becomes effectively zero.  There could be civilizations 1,000 light-years away (again, the Milky Way is about 100,000 light-years across) that never have and never will put out a signal bright enough for us to detect.

In short, the search for extraterrestrial intelligent life probably boils down to watching a few thousand star systems in our immediate neighborhood for signatures of intelligence.  It seems quite plausible that some number of those planets harbor life, and that of those, a not-too-much smaller number of those have developed or will develop intelligent life and at some point in their histories, and for a brief time, put out something we could detect.

If that's the case, then some portion of them have already passed their detectable phase.  Maybe there are a dozen out there that have yet to announce themselves.  As always, a wild guess.  There might be none at all.  There might be hundreds, but probably not much more -- there are only so many stars close enough.  If there are such a dozen, and we can keep listening, we'll eventually spot one.  But "eventually" here likely means millions of years, not decades.

Are we likely to detect signals from civilizations around other stars?  Not in our lifetime, I'd say.  But some time in the next million years?  Maybe.



Postscript: I forget where I saw this mentioned, but another problem is that as our radio communications get more and more efficient, the signal we put out gets hard and harder to tell from noise.  Faint as it may be, a morse code radiotelegraph signal is clearly non-random and statistically unlike anything known to be naturally produced.  A compressed digital television signal looks much like random noise, which can come from any number of sources.  Mix together all the digital television signals currently broadcast on earth and you get something even more like noise.  Probably the only way we could detect a signal from Twin Earth and tell that it was a signal from something intelligent, would be for them to be sending a signal directly at us.  But if they're like us, they're only beaming signals to a few stars, and for relatively short periods of time.

Post-postscript: Randall Munroe's What If makes most of the same points as here, with a lot fewer words (and a couple of pictures).