Is there intelligent extraterrestrial life in the Milky Way?

This resource is a wikidebate, a collaborative effort to gather and organize all arguments on a given issue. It is a tool of argument analysis or pro-and-con analysis. This is not a place to defend your preferred points of view, but original arguments are allowed and welcome. See the Wikidebate guidelines for more.

Subject classification: this is a science resource.

Subject classification: this is an astronomy resource.

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.

There is intelligent extraterrestrial life in the Milky Way

Arguments for

• Argument for We are discovering more planets and star systems all the time. It is almost inevitable that at least one of them will have intelligent life.
  • 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 for 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
  • Objection This usage of the Drake equation is wrong. 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. That means that a Drake equation with "highly conservative" estimates predicts millions of radio-capable planets ready to interact with us. 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.

Arguments against

• Argument against The conditions that allow life on Earth are special and quite possibly one in a billion, so there is almost none possibility for extraterrestrial life to exist.
  • 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 against Even if intelligent life ever arose elsewhere in our galaxy, it may be long-dead or otherwise inaccessible to us.
  • Objection The discovery of dead intelligent life would still be revolutionary. This discovery would still prove that intelligent life can and likely still will develop on other planets.
  • Objection This may be true, but it may also be false. No reasons are given to suppose it's more likely than not.
• Argument against 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.
  • Objection Absence of evidence is not evidence of absence.
• Argument against We have had radio astronomy programs for decades and have yet to detect any signs of intelligent extraterrestrial life.
  • Objection Decades are not a long amount of time in the larger scale of the galaxy. For example, the nearest solar system is 4.35 light years away, meaning that, since radio waves travel at the speed of light, it would take almost half a decade for them to reach it.
  • Objection It is possible that there are other more advanced forms of communication and that radio astronomy is only a brief phase in the technological evolution of the species.
    • Objection The most effective and fast communication, in addition to the one with the least interference, is the one obtained through radio waves.
• Argument against 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 therefore it is likely for each individual intelligent species 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.
    • Objection 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 a certain percentage 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.
    • Objection 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 against 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, per being the assumption with the least bias.
  • 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 absurd and impossible for them all to have a 0.5 probability.
  • Objection According to this argument agains, "God exists" has 0.5 probability being true.
    • Objection This would only invalidate the objection if we knew that God's existence has less than 0.5 probability. But we don't

There is no evidence for either side of the argument

• Argument for There is no way to prove that there is or is not intelligent life in the Milky Way, unless we directly observe intelligent life or something that they produced, which we have not. This does not prove nor disprove that there is intelligent life in the Milky way.

• Argument for So far, we have not observed any signs that there is intelligent life in the Milky Way. However, this could mean that we have not searched enough, or that there is none. Until we have more conclusive results, there is no hard evidence that is available for either side.

See also

• Should we colonize Mars?

External links

• Wikipedia:Colonization of Mars