Is there intelligent extraterrestrial life in the Milky Way?
Space may be infinite and filled with all manner of distant cultures and ways of life. Are any close enough to us that we could communicate with them? The insights of technology such as the Hubble Telescope have lead to an explosion of new planets discovered in the past few decades. If intelligent life exists, it could change everything about how we understand ourselves and our place in the cosmos.
We define intelligent as being able to communicate through radio astronomy. We define extraterrestrial as of extraterrestrial origin, not residence, which means that forms of life originated on Earth but currently residing out of it (such as astronauts) don't count, and forms of life originated outside Earth but currently residing on Earth do count.
There is intelligent extraterrestrial life in the Milky Way
- Argument We are discovering more planets and star systems all the time. It's inevitable that one of them will have life which has evolved for millennia into having sentience and intelligence.
- Objection This assumes what must be proven. Yes, there are innumerable habitable planets and moons. But it's still possible that intelligent life forms have not arisen, at least in this galaxy. Microbial life, yes, intelligent life, who knows?
- Argument By the classical Drake equation, if we replace the variables by some of the best current estimates we get that there should be at least 2 intelligent extraterrestrial civilizations. And this is only by the most conservative current estimates. By less conservative estimates, we get many thousands, even millions of civilizations. But in any case we get at least one civilization, which is all that is needed to answer the question.
- Objection You are misrepresenting the Drake equation. The output, i.e. between 2 and billions, estimates the number of currently radio-capable civilizations that we should be able to observe, not the number of intelligent lifeforms out there. Furthermore, it makes the assumption that civilizations stay confined to a single solar system, which is frankly idiotic from a long term perspective (see my argument in the against-section). That means that a Drake equation with "highly conservative" estimates predicts millions of radio-capable planets ready to communicate with us or invade our Solar System. Therefore this usage of the Drake equation is wrong. There is a reason why we don't see anyone, and there is no known reason why the error would not be before intelligent life is reached. Therefore the standard Drake equation is not a sound argument.
There is no intelligent extraterrestrial life in the Milky Way
- Argument The conditions that allow for life on Earth are special and quite possibly one in a billion.
- Objection The Milky Way galaxy is estimated to contain 100-400 billion stars, and at least 200 billion planets, so if life on Earth is one in a billion then there should be approximately (at least) 200 planets with Earth-like life. And that's without counting moons.
- Argument Even if intelligent life ever arose elsewhere in our galaxy, it may be long-dead or otherwise inaccessible to us.
- Objection This may be true, but it may also be false. No reasons are given to suppose it's more likely than not.
- Argument If advanced civilizations in the Milky Way existed, we should have clear evidence of their existence already. But clear evidence hasn't be found. Therefore, we can already infer that there aren't any advanced civilizations out there.
- Objection The Fermi paradox does not argue that we should have clear evidence of civilizations if they exist. It asks why we don't have that evidence even though it seems our galaxy contains many solar systems like ours. There are many ways to answer this question, and not all of them preclude such civilizations from existing.
- Argument The time it will have taken humanity to evolve from "intelligence" to interstellar travel (5 million years plus change) is less than a thousandth of the time there have been four billion year old habitable planets in the Universe. Von Neumann probes are physically possible, and since the Universe is made of stuff that can be used to make stuff intelligent beings want, it is likely for each individual intelligent species that it will want to build Von Neumann machines. The four-volume of five million years of the Milky Way is less than a trillionth of the four-volume of our entire past light cone. Unless the jump from radio to Von Neumann machines has a lower than one-in-a-billion survival rate, somehow, it is statistically extremely unlikely for there to be intelligent life in the Milky Way, other than our own.
- Objection Von Neumann machines replicate exponentially. Therefore, given enough time, they will replicate beyond control, consuming significant amounts of useful mass in the galaxy, and becoming a problem to other species and maybe even their creators (something like the grey goo). So it's possible, even likely, that any intelligent life forms will actually AVOID creating Von Neumann machines, and destroy any they encounter. Furthermore, if the replication rate were somehow controlled, then it's possible, even likely, that there will be relatively few Von Neumann machines out there, which would explain why we haven't found any.
- Objection Civilizations with the potential to build Von Neumann machines (such as ours) may inevitably self-destroy before doing so.
- Given that we don't know the fate of intelligent civilizations, it's more cautious to think in probabilities (like in the Drake equation), and say that something like 99% of intelligent civilizations will self-destroy before building Von Neumann machines.
- Objection Saying that at least one in a hundred intelligent species will build Von Neumann machines requires proof.
- The Universe is filled with usable energy and potential risks. Due to the principles of natural selection, all intelligent life will value self-preservation. Von Neumann machines are therefore desirable to build for most intelligent life. The numerical estimate is, of course, an educated guess.
- Argument There is no hard evidence supporting the existence of intelligent extraterrestrial life in the Milky Way. When there is no evidence for or against a proposition, one ought to assign it a 0.5 probability of being true.
- Objection Some sets of propositions have multiple mutually-exclusive possibilities (e.g. 'n is zero; n is positive, n is negative'). It is a general law of probabilities that the probabilities of mutually-exclusive possibilities sum to no more than one. In the case of more than three mutually-exclusive possibilities, it is inappropriate and impossible for them all to have a 0.5 probability.
- Objection There is no evidence supporting the assumption that one ought to assign propositions without evidence a 0.5 probability.
- An equiprobability assumption is one with the least bias.
- By your logic, all faces of a dice have a 0.5 probability, which is impossible.
- An equiprobability assumption is one with the least bias.
- Objection According to your objection, "God exists" has 0.5 probability being true.
- This would only invalidate the objection if we knew that God's existence has less than 0.5 probability. But we don't.
- Argument There is no hard evidence supporting the existence of intelligent extraterrestrial life in the Milky Way. The burden of proof always falls on he who claims the existence of something, and not on he who denies it.
- Objection The burden of proof falls on he who makes the initial assertion. For example, if someone declared that this Universe does not exist, he would certainly have the burden of proof, even though he is not claiming the existence of something.