We Are Not Alone

Fact: The aliens should have been in touch by now.

There is absolutely no convincing evidence of intelligent – or indeed any – life on other planets to date. However, taking into account the huge size and age of the universe it is also extremely unlikely that we are the only life-forms in the universe.

We call this apparent contradiction The Fermi Paradox, a term coined by the physicist Enrico Fermi in the first half of the 20th century. The logic goes like this:

  • There are between 100-400 billion stars in our galaxy – The Milky Way.
  • Of these, we estimate that around 5-20% will be like our own sun, which gives us ~5 billion ‘sun-like’ stars.
  • Around 1/4 of these stars will be orbited by Earth-like planets, giving us roughly 1 billion earth-like planets

Up to this point, these estimates are based on astronomical data. Going any further means getting a bit speculative, as we can’t know how likely life, and then intellegent life, was to develop on Earth, but for the sake of argument Fermi assumed that:

  • 1% of these Earth-like planets develops life.
  • On 1% of those planets, that life develops into intelligent life, giving us around 100,000 intelligent civilisations in The Milky Way alone.

This line of logic led Fermi to famously ask “where is everybody?” and you can see why. According to the maths there are many other advanced civilisations out there in our galaxy alone.

Furthermore, given that The Milky Way is about 13.2 billion years old, but Earth is only 4.54 billion years old, many of these planets will be much older than ours, meaning that some civilisations would likely have had billions of years longer to develop. Given we have already cracked getting to the moon and back, shouldn’t civilisations with an extra few billion years under their belt have developed interstellar travel, or at least a way of communicating with us?

Fermi came up with 3 explanations for his paradox to explain the lack of evidence for aliens:

  1. We’re the first. We the most advanced civilisation in the galaxy, so there has been no-one out there to contact us. This is incredibly unlikely, given the age of The Milky Way compared to the age of the Earth.
  2. We’re special. There is some kind of obstacle that prevents most life from developing as far as we have. This might be the origin of life itself, or the transition from unintelligent to intelligent life, but whatever it is, we got past it. This is also pretty unlikely – scientists generally agree that there is probably nothing special about Earth.
  3. We’re doomed. There is some major obstacle that stops life developing, but instead of being behind us it is ahead of us. All civilisations older than us have reached this point and are alive no longer. For example, this could be because advanced civilisations inevitably develop powerful technology and end up destroying themselves, which seems quite likely in our current world.

So the most likely explanation is that, like all other advanced civillisations in the galaxy, the human race is destined to over-develop and destroy itself. Of course, all is not lost – another explanation is that the galaxy is run according to Star Trek, and all the advanced interstellar civillisations are hiding themselves from us. Again that seems unlikely, but god I hope its true.

Waitbutwhy do an excellent analysis of the Fermi Paradox if you have unanswered questions!

‘Another one of Mr Shaunak’s Little Bites of Science’
Image under Creative Commons license



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