Why is Snot Green?

Extract

How big is the universe?

Big. Proper big. Crazy big. Billions of times bigger than the biggest thing you can imagine.
I don’t know about that – I can imagine some pretty big stuff . . .
OK, let’s give it a go. Let’s imagine the size of the universe.
Probably best to start small and work up – so let’s start with something fairly big – the Earth. The Earth is about 8,000 miles wide. If you drove a tunnelling car straight through the middle, you’d get to the other side in about 5½ days, going non-stop at an average motorway speed of 60mph.
That doesn’t sound so far.
Right – it’s not. So let’s try a longer journey. Say, from here to the Moon. The Moon doesn’t go round us in perfect circles – it gets closer and further away from us at different times of the month. But, on average, it’s about 240,000 miles away. It would take about 168 days to get there in a 60mph flying space-car. Even with rocket propulsion, the Apollo astronauts took about three days to get there (and it was a real squash in their spacecraft).
Similarly, the journey from the Earth to the Sun is about 93 million miles so would take about 176 years by spacecar.
To get right across our galaxy, the Milky Way, it would take about a million billion years (or 1,181,401,000,000,000 years to be more precise) to make the journey of 621 million billion (or 621,000,000,000,000,000) miles.
So what does that tell us?
That a space-car would be cool, but at 60mph it’d be pretty rubbish for getting about the place?
Yes. Quite.
That, and that the galaxy is pretty huge in itself – let alone the universe. I’m running out of space to put all the zeros after the numbers here.
All right – what if you had a space-car that could go at the speed of light?
Now we’re talking. The speed of light is about 670 million mph, so a car that fast could do about 6 thousand billion miles (or 6 trillion miles) if it kept driving, non-stop, for a whole year. We call this distance a light year, and it’s much more useful for measuring the huge distances – between stars and across galaxies – that we’ve been talking about.
For example, the Milky Way is about 100,000 light years across, so it’d take 100,000 years for our souped-up, superfast, light-speed car to cross it. Still way too long to manage, but easier to imagine, maybe.
Go on, then – how big is the whole universe?
Well, we can only measure the universe as far as we can see it. With the best telescopes we have, that’s about 15 billion light years (or 90 billion trillion miles – I won’t even bother trying to write that out with zeros) in every direction. So at the speed of light, it’d take at least 30 billion years to cross it. That’s about 2 billion years longer than the age of the universe itself.
Ah. So it’s big, then?
Like I said: crazy big. And that’s just the bit we can see. Beyond that, we know it extends even further, because the light from the stuff we can see at the ‘edge’ has taken 14 billion years to reach us, and the universe has expanded quite a bit since then! It might even curve back on itself, like the sea does as you sail round the globe. If that was the case, you could circle the universe and end up back where you started from.
Now that would be cool.
Yes, it would. But all your friends would be many billions of years older. So even if they were still around, they probably wouldn’t know what cool was any more. Bummer.

What is Space made of?

Well, it’s not just ‘nothing’. Space is, at the very least, filled with gases spread out very, very thinly. It also bends – and possibly rips – so it must be made of something . . .
But Space is, well, space, isn’t it? No air, no gravity . . .
Well, not exactly. Gravity is actually everywhere in Space.
Its pull becomes weaker the further you move away from one particular source – like a planet – but it’s still there.
And while it is true that there’s no air in Space, there are other things spread around it. It’s only because the stuff is spread out so thin, and Space is so big, that we can’t detect it very easily.
So what is this ‘stuff’?
Mostly hydrogen and interstellar dust left over from the Big Bang.
How much of it is out there?
Well, there’s billions of tonnes of it, but it’s spread so far and wide across the universe that you won’t find more than one atom per cubic centimetre of Space in most places. You’ve probably been told that gases spread out to fill their containers, right? Well, if there’s nothing else in the container, then they do. In this case, the container – the universe – was empty and is now at least 180 billion trillion miles wide. Spread over this distance, even billions of tones of material can look like virtually nothing. It just depends on how hard you’re looking for it. It’s a bit like Marmite on toast: spread it thinly enough and only the real Marmitehaters will detect it. (You can try that one for yourself.)
OK . . . so rather than say ‘there’s nothing in Space’, you could say ‘there’s almost nothing in Space’ instead?
Exactly. That will not only be more accurate, but it will also freak people out. Which is always fun.