Update: also see my follow-on post about Sagan Syndrome
I wrote an earlier post supporting the view that Earth is a unique planet. It’s likely the only planet in our galaxy supporting complex life. I wanted to do an update after coming across an excellent post by Stephen Ashworth, who categorizes views on alien life into either “steady state” or “expansionist”.
Ashworth starts by noting that early attempts to estimate the amount of intelligent life in the galaxy were based on steady state models. His illustration is below:
In the steady state model, once the universe gets started the number of civilizations quickly plateaus. This is depicted as time “A” above. The steady state comes from assuming civilizations are born and die at a constant rate, and so over time the number stabilizes. This is the basis of the 1961 Drake Equation and also the model behind a lot of discussion of the Fermi Paradox. I love all the work and thought that went into this, but this approach is showing its 1960 roots. We should respectfully trash it.
Contrast this with what Ashworth characterizes as an expansionist model:
In this model, life is an invasive species. Once it reaches a point where it can cross star systems, it rapaciously expands to fill the galaxy. Now we’re talking. Darwinian life filling empty ecosystems to capacity. A nuance here is the expansionist model doesn’t require every civilization to be expansionist, though Darwinian logic makes this plausible. The real key is the very first expansionist civilization that comes along fills the galaxy. Game over. So why did Drake even consider the steady state model back in 1960? Well, back then it was not clear you could build a starship. And even if you could, how would people survive centuries of travel through radiation filled space? With the constraint that civilizations were restricted to their home sun, the steady state model made sense. Alien civilizations were born alone, trapped around their sun, and eventually died alone.
It was not until the invention of computers and robotics that sending self-replicating probes became conceivable. Now you have a technology that can survive in space for the centuries, replicating to fill the galaxy. Once you accept interstellar travel is possible, and understand robotics, the expansionist view becomes impossible to avoid. Likewise the steady state model becomes archaic and ridiculous, given it’s foundational premise is interstellar travel is impossible. Here’s a quote from Frank Tipler’s paper from 1981:
The basic idea of my argument is straightforward and indeed has led other authors such as Fermi (10), Dyson (11), Hart (12), Simpson(6), and Kuper & Morris (13), to conclude that extraterrestrial intelligent beings do not exist: if they did exist and possessed the technology for interstellar communication they would also have developed interstellar travel and thus would be present in our solar system. Since they are not here (14,15), it follows that they do not exist. Although this argument has been expressed before, its force does not seem to have been appreciated.
Exactly. It’s force was not appreciated in 1981. And is still not completely appreciated now, over 30 years later. The search for extraterrestrial life (SETI) muddles along stuck in the 1960’s, unable to get out. People are still publishing brand new books based on the Fermi paradox and the Drake equation. Please make it stop.
Of course the main reason the public believes simultaneously in interstellar travel and aliens is it makes for awesome popcorn movies. Well enough. I love good alien movies too. But even people who are aware of the argument above can struggle. I think the reason is the relative timescales involved. Let’s start with the age of the universe. It’s 13.8 billion years old as depicted below.
Our Sun and the Earth formed about 4.5 billion years ago. Microbial life evolved shortly after. Intelligent life evolved 3 billion years after the start of life. Repeat: billions of years to evolve intelligent life. In fact, life will end on Earth due to the expanding Sun in only another two billion years. So in some sense we barely evolved in time.
Now compare the billions of years for intelligent life to evolve to the time it takes to expand into the galaxy with self replicating probes. That’s only 20 million years. In the picture above on the far right is a green vertical line showing 20 million years. You may have to squint to find it. It’s an eyeblink.
Now step out even further in time. It turns out the universe will produce stars into the future for 100’s of billions of years. A typical figure is 10’s of trillions. Let’s just use 1 trillion for the scale below. On the far left in red is the 13 billion year age of the universe, which on the new scale looks pretty short. Each block is 100 billion years wide. We still have stars forming at 1 trillion years, and beyond. It’s still daybreak from the universe’s point of view.
We have three disparate time scales: millions, billions and trillions. For simplicity let’s round the numbers to 20, 20, 20:
- Time for intelligent life to fill galaxy: super short 20 million years
- Time for intelligent life to evolve: moderate 20 billion years
- Time of universe to keep having stars: long 20 trillion years
The first timescale is the expansionist view of populating the galaxy. It’s easy to reject emotionally but hard to reject logically. That’s because the expansionist view merely assumes interstellar travel is possible and Darwinian evolved life fills ecosystems. Some math here. The second number is based on the observation that aliens haven’t flooded the galaxy yet. Plus supporting data that intelligent life evolved very late in Earth’s history after billions of years. The last number is current astrophysics and seems very solid. Overall we have a spare, powerful and persuasive model for why there are no aliens. Life has barely had time to get started.
Unfortunately this model shows the chances of us meeting a second form of intelligent life are nil. It takes billions of years to evolve, and millions of years to flood the galaxy. First one gets it all. Also note there’s plenty more time for intelligent life to evolve elsewhere before the universe gets old. So if humans self-destruct before expanding, then other intelligent aliens will do it later. In say, another 10 billion years.