The idea behind the Drake equation is to put the vastness of the universe beside the probability of contact with aliens.
The idea is deceptive -- the universe is large, so it has so many planets, so the possibilities become probabilities and then become absolutes, right?
The communication bubble ∞
First, considering the idea of a species communicating.
There is no way for communication to occur unless we can receive it.
Our technology is based on the physical laws we know. Even at the speed of light -- the fastest thing we can use -- there is a basic limitation to the sphere of listening and talking. Light has a speed, it takes time to travel. The distance between ourselves and a second party becomes significant.
So it doesn't matter if the universe were infinite in size, the limitations of light force a size-bubble.
What is the size of the sphere of communication?
One limit is the age of the universe. We can only detect light out to a certain distance, because light takes time to travel and so the further we look is the further back in time we see. At some point the image becomes misty from the aftermath of the Big Bang. Roughly fourteen billion years ago / fourteen billion light years in distance.
That sphere shrinks in size when we consider the time needed for the first generation of suns to form, for planets to form and for communicative life to form. At this moment we can throw our hands up and declare "Magic!" and say communicative life arose.
It would be magic, since the elements we know to form life here are not borne in the early universe but from second-generation suns. Those first suns must age and die, go super nova and throw out their enriched elements -- to be formed into new suns and planets with a composition that may then support life.
So the sphere of communication shrinks from the age of the universe to the age of the earliest second-generation suns. This is also a knoable non-infinite value. Again I stress this is not infinite.
The Drake Equation does a lot of hand-waiving to reduce the number of contactable civilizations. How many stars have planets? How many with life? Intelligent life? Technologically-capable life?
Technology vs extinction ∞
This is where I want to interrupt that string of wishful thinking.
So far I have written on the basics, but let me now explore the rise of technology. I want to keep in mind the problem of technology as understood with evolution and resource availability.
For a species to survive, it must strike a balance with its environment. If it overpopulates and over-exploits, it must inevitably find balance or go extinct. There are examples of this throughout our archaeological record.
On the other hand, all examples of balance maintain a very low-profile and low-technology lifestyle. First nations, native, indigenous people all demonstrate this. Most fauna and flora also. Again, examples to the contrary will and must go extinct.
So with respect to the Drake Equation, a civilization must rise to a technological height while faced with an inevitable self-destruction. This is the case when and if the technology demanded by the equation is only attainable by risking this extinction.
It seems obvious that one level of early technology cannot give rise to the high technology that would allow interstellar communication. It also seems obvious that a high technology that aggressively exploits will not give rise to that technology.
This leaves two scenarios:
- One is the exploitation of the planet just enough to bootstrap into a non-exploitative sustainable balance.
The other is the continued exploitation, abandonment and flight from extinction... supported by still more exploitation.
Exploring this second "locust"-style technological development, it must be learned if it is possible at all.
The limitations of a home world may not give the resources to find another exploitable world before extinction. Further, some sort of equilibrium may be required to survive getting to another world.
At any moment, an exploitative culture could extinguish some of itself and leave the remaining populace with enough resources to persist, and perhaps to find balance. This limits the sphere of possibilities to not just our distance from an alien culture, but the distance between exploitable worlds. It may well be that no home world has the resources to permit getting to a second appropriately-exploitable world. The distance or rarity may be too great.
Next is the problem of the relationship between a home world's exploitability and technology. Precisely what kinds of technology are required to fulfil the role in Drake's Equation? How much impact, and how much risk for extinction, must such technology have? We can imagine all sorts of science fiction technology, and we can suppose there is technology simply outside our imagining. Fair enough, but for any level of technology to exist, it seems necessary to build it with the tools from previous technologies.
The necessity of fossil fuels [updated] ∞
Update: NASA has been working on green propellants, and they've had rather extraordinary success!
Even giving the most minimal impact, and speedy transition from lower to higher technology, there must be a need to exploit a world. Can we imagine a careful, low-population, alien culture which only pursues exactly what is needed to bootstrap themselves into the highest level of technology necessary while avoiding the problem of exploitation-extinction... all on one home world? Sure.
What resources must exist in an easily-exploitable form for primitive technologies to craft advanced technologies. This is something we can understand very well.
Of all of the exploitable resources, the one I am most interested in is fossil fuel. Is it possible to give rise to the necessary high technology if a world has little or no fossil fuels? I doubt it.
Not only does a world have to be born from the death of a first-generation star, but life itself must be sacrificed and reborn. A vast deposit of fossil fuels on our world is largely thanks to a meteor impact. How much, of what type and what availability of fossil fuels could exist without such a major event?
The components of life are trivial. The chemistry of proto-life is mechanical. Life seems inevitable with reasonable circumstances and ample time. Perhaps a fossil fuel-creating global event is also frequent enough. Then the problem becomes if a culture can rise up after one event and before the next.
That culture would need to develop communicative capability before the next event, or develop high technology to prevent, survive or escape it.
Concluding thoughts ∞
I've given a number of ideas which can help turn the discussion from hand-waiving into something which can have more concrete information attached. Actual distances, times and values can be applied.
With reasonable information, it's possible to do a back-of-the-envelope calculation that can at least say if it's possible at all to receive or send a message to an alien culture.
All of this revolves around assumptions of the laws of physics being stable over time, applying to another location as they apply to us here, and for communications technology being limited to the physics we know. None of these are unreasonable, just like we assume that we're actually thinking our own thoughts and that other people are other people and not just characters in our dream.
As soon as we start to hand-wave and grant magical powers and technologies, we remove this from something reasonably intellectual which can have concrete calculations into metaphysics with whatever bullshit information people make up.
I'm not being pessimistic, just realistic.
I could have also mentioned the need for either an asteroid field (failed planet?) or large gas giant planets which can help protect the inner planets from strikes, as well as the need for a moon for that same reason, and probably also a moon for tides.