The item in this week's Bang Goes the Theory, about the size of the Solar System, shows a good way of getting your head around cosmic distances in our local neighbourhood, the size of the planets, and how far apart they are. However, that's just back-yard stuff when it comes to the wider reaches of space. In the programme, Dallas explains that the edge of the Solar System (where we find the Oort cloud of comets) is about one light-year away, and that the nearest star, Proxima Centauri, is about four-light-years distant.
Our galaxy, the Milky Way, is about 100,000 light-years from side to side, and the nearest other big galaxy, the Andromeda Galaxy, is about 2 million light-years away. Now, it's easy to say that light would take 2 million years to get from us to Andromeda, but can we get any idea of how far that really is?
Imagine that you could travel at a speed of 1,000 kilometres per second. That's roughly equivalent to going in a straight line from Land's End at the tip of Cornwall to John O'Groats in the north of Scotland, in the tick of a clock. This is a speed we might just be able to imagine and it sounds pretty fast, but it's still only one-third of one per cent of the speed of light.
Now imagine travelling at that constant speed of 1,000 km/s for 30 million years. That's how long it would take to cross from one side of our galaxy to the other. If you can, imagine travelling at that same speed for 600 million years. That's how long it would take to get from our galaxy to our nearest neighbour in space, the Andromeda Galaxy. Jump between Land's End and John O'Groats once every second for 600 million years, and you'd still only travel a distance as far as the nearest galaxy.
These distances might set you thinking about whether it will ever be possible for humans to travel to other stars in our galaxy, just like they do in Star Trek. What do you think? Can you see a way in which the human race might travel to other stars, or even colonise the galaxy? If we could do it, might creatures from other planets be able to do so too? 30 million years to cross the galaxy might seem a long time, but the galaxy has existed for several thousand million years, so there's been plenty of time for someone to make such a trip. Why aren't they here yet?