At the weekend, I was trying to show some people where the Andromeda Galaxy is in the night sky. I failed fairly miserably (here's a good way), but it did spark a good discussion on the possibility of extraterrestrial life in the universe. And this led on to looking at the size our galaxy (the Milky Way); the distance from us to the andromeda galaxy; and then the size of the universe. And once we started looking at these distances, I began to struggle to comprehend these distances. Our galaxy is approximately 100,000 light years in diameter, and the distance from us to the Andromeda Galaxy - one of closest galactic neighbours - is approximately 2.5 million light years away.
Now, whilst I hear that and just think "So, basically a very, very, very, very long way away", it feels important to really look at that. And to do this I find that it helps to get some sense of scale. For this, we could use many units of measurement, but I'll use light distance as the measure of scale, as the experts do! A light year is the distance that light travels in a vacuum. To put it another way:
One light year is equal to 9,461,000,000,000 kilometres
Or written another way: 9.461 trillion kilometres
And another way: 9.461 million million kilometres
I watched a very interesting BBC programme on travelling to our neighbouring solar system, called Alpha Centauri. It is 4.2 light years away. If we could travel at the speed of light, it would take us 4.2 years to get there. Doesn't sound so far, eh?! But, if we were to use our current space-travelling technology (ie. that which has already been used, as opposed to what is currently being developed), it would take us 100,000 years to get there. 100,000 years!
So, coming closer to home, the distance from Earth to the Sun is 8.3 light minutes, or approximately 150 million kilometres. This means that the light which leaves the sun takes 8.3 minutes (or 498 seconds) to reach our planet (well, our retinas to be precise). So, when we view the sun, we are seeing it as it was 8.3 minutes in the past.
To apply this scale to our galaxy, if we could stand on one side of it and look at the far edge of it, the light coming from there would take 100,000 years to reach us. We would be viewing the far side of the galaxy as it was 100,000 years ago. And so on with the Andromeda galaxy, which we are seeing as it was 2.5 million years ago. Humans did not even exist in anything close our present form back then.
The rough estimate from Space.com is that there are 100 to 200 billion galaxies in the Universe, and there are roughly 200 million stars (and hence solar systems) in our galaxy alone, that leaves a lot of solar systems where life could evolve. If we take a conservative estimate and say that there is only one planet in each galaxy where life has spontaneously arisen, that is still 100-200 billion planets with life on them in the entire universe.
The maths, for me, is insurmountable: there is extraterrestrial life.