Alien Invasion, The Likelihood Of
It's The End Of The World As We Know It, Just Not Very Probable
Invasion By Aliens, The Chance Of
Probability Of Being Invaded By Aliens, The
Alien Invasions, How To Fake
Alien Identification
Alien Elimination
Meat, Sentient
Earth Defence Shield
Opinions On UFOs
Faking UFOs
Area 51
Schroedinger's Cat

Mark Seaborn


On some occasions -- such as the ushering in of a new era with the most significant digit of the year being incremented -- a few people start to get a bit worried (a condition dubbed by some as PMT; pre-millennial tension). What they exercise their paranoia on is one of those oldest of science fiction chestnuts: that of alien invasion.

If you just sit down and think rationally for a few minutes (assuming that you are capable of this), you should come to the conclusion that it isn't very likely, now is it? [1]

Astronomer Dr Carl Sagan worked out from some rather dubious estimates that there ought to be around 10,000,000 advanced technological civilisations in our galaxy, the Milky Way [2]. From equally dubious assumptions we can also work out the likelihood of aliens invading, say, in the next century, in a worst-case scenario. If we assume:

  1. That the Earth hasn't been invaded by an alien race yet.
  2. That the Earth has been around for approximately 4.5 billion years (that figure may have to be updated as this article gets older).
  3. That the probability of aliens invading Earth has remained constant throughout the Earth's history.

Then, as a worst case scenario, presume that aliens invade tomorrow (or some time in the next century, if they're held up by bad weather, traffic congestion, or the wrong type of interstellar hydrogen). From a total of 45 million and one centuries, we will then only have been invaded in one century. Thus the probability of being invaded in the next century is vanishingly small, at only one in 45 million [3].

The chance of an alien invasion commencing in the very minute that the year 2000 starts [4] is therefore even smaller, at about one in 2,368,000,000,000,000. [5]

Sceptics might point out that the three assumptions I made earlier are wrong, which would make my estimate equally wrong. (What am I talking about? Real sceptics would be arguing the same as me!) However, I can justify these three premises.

For (1), there is no proof, as yet, that the Earth has ever, in its history, been invaded by outsiders (notwithstanding Nazca plain figures). That also means ignoring, for the time being, any theories that say life on Earth was of extraterrestrial origin (which might count as an invasion of sorts). However, it is probably impossible to prove that the Earth hasn't been invaded at all, only that is has (which it probably hasn't). Remember: absence of proof doesn't mean proof of absence.

As for (2), this is an estimate made by people with more geological, astronomical and cosmological knowledge than I (say that with your false teeth in). This can't be proven absolutely either, without being able to travel faster than light, which is impossible (although its impossibility cannot be proven, only extrapolated) [6]. Anyway, even if this estimate is out by several orders of magnitude, the chances of invasion are still slim.

You might say that point (3) is the weak point in my argument; that the probability of an alien invasion hasn't been constant throughout our planet's life. Firstly you might point out that as any hypothetical alien race becomes more technologically advanced, the chance of them invading increases. This is true... well, probably: the aliens might become more benevolent and wise over time, but since we don't know if there are any aliens out there, their existence and frame of mind are effectively random -- their quantum wave function hasn't collapsed yet, you might say.

Besides, aliens aren't really going to want to invade a planet just for us (what would give you that idea?). They wouldn't need to enslave us; if they had the necessary know-how to invade a planet, they could instead build a simple robot to make the tea and fetch their slippers for them. They wouldn't want the planet for its natural resources; if they've just travelled N light years to get here, expending <large-number> terajoules of energy in the process, they're not going to want Earth's <small-number> joules of fossil fuel energy.


For those high-powered executives among you who haven't had time to read this entire article and have instead skipped straight to the conclusion, here is the bulk of the article presented in one easy-to-digest sentence:

If the occurrence of alien invasions of Earth is random, Earth has not undergone alien invasion yet, and the Earth has been around for a long time, the probability of alien invasion over a small time scale is minute.

So, throw away your T-shirt emblazoned with the slogan "the End of the World is Nigh", and invest in some lottery tickets! That'll give you a better chance of getting it right.

[1] Conspiracy theorists might point out that this is precisely what the government wants us to think. But then, this is precisely what conspiracy theorists would want us to think. (Perhaps the conspiracy theorists are hatching a conspiracy to make us believe we are being conspired against?)
[2] In his book, Cosmos, the late Dr Sagan puts forward a formula for estimating the number of technologically advanced civilisations in our galaxy:
    N* x fp x ne x f1 x fi x fe x fL


  • N* is the number of stars in our galaxy (about 400 billion);
  • fp is the fraction of stars with planetary systems (about 1/3);
  • ne is the number of planets per system that are suitable for life (about 2);
  • f1 is the fraction of suitable planets on which life actually arises (about 1/3);
  • fi is the fraction of planets containing life on which intelligent life evolved;
  • fe is the fraction of planets with intelligent life on which technical civilisations developed (where fi x fe =~ 1/100); and
  • fL is the fraction of the planet's lifetime for which the technical civilisation existed (less than 0.0000001%).

This apparently comes out to 10,000,000 civilisations. (However, that's not what I get, which may be due to the fact that my source of this information is a small text file containing information regurgitated from the book. I make the number to be about 0.9 civilisations, which seems considerably more accurate, fitting in as it does with our current knowledge.)

[3] However, it has been found that million-to-one chances crop up nine times out of ten.
[4] Or the year 2001, if the alien race really gets it right.
[5] 45,000,000 x 100 x 365.25 x 24 x 60 = 2.367 x 10^15 (3 sf)
[6] The basic idea is that if you could travel faster than light, you could overtake the light rays that left the Earth all those billions of years ago, and when you looked back at the Earth, you would see what was going on down there all that time ago (so long as you had a really powerful telescope with you as well). It's not very easy though, folks.

See also

It's The End Of The World As We Know It, Just Not Very Probable
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