Thinking outside the linguistic box

The Universe is not small. You won't believe how unbelievably not small it is. When most people hear how old the Universe is they still think it is smaller than it actually is*.

As well as being big, the Universe contains a lot of stuff. Rather embarassingly, we only know what a tiny fraction of that stuff is. The small part that we can see we usually divide up into neat little categories. Making distinctions between different objects can be useful; it is helpful to know the difference between kittens and pythons when you go to a pet shop.

Sometimes our simple division of the world into one box or another doesn't work too well. Take a stone. It is mostly made of silica. It has pretty much the same composition as both sand and mountains. Obviously mountains don't often pile up on beaches so we make a distinction between them and sand.

We invent terms like gravel, pebbles, stones, rocks, and boulders to try to make more distinctions. But lumps of silica don't care if they fit into our neat, linguistic boxes and they can be any size. If we are sensible, we allow our linguistic boundaries to be a bit fuzzy to reflect reality. I might call your stone (the one you took earlier) a rock or a large pebble. The label isn't that important. The fact that it'll break my window is.

We have similar problems with linguistics in astronomy. A small cluster of stars is different to a group of 100 thousand million stars sitting in a big spiral structure. As time goes on we're finding the stellar-group equivalent of pebbles and boulders making a clear dividing line trickier.

On smaller scales we've found objects at pretty much every scale from being just big enough to start hydgrogen fusion at their centres (which would make them a star) down to tiny grains of dust. We can try to draw distinctions based on things such as mass, roundness, composition or location but these are only slightly better than arbitrary choices. There will always be objects slightly less or more massive/round/rocky/gassy/distant than our definition but you'd be hard pressed to tell apart two, one on each side of a boundary, at a glance. We need to accept fuzziness.

Is Pluto a planet, a dwarf planet, or a large asteroid? My answer is that the category does not matter** and will probably change anyway as our language and ideas evolve. Those labels are only for convenience and should not be thought of as too important near fuzzy boundaries.

Pluto hasn't been "demoted", "killed", or destroyed to make way for an interplanetary bypass. Pluto is still there. It is interesting, has a bunch of close friends, and will soon be getting a visit from the resourceful residents of Earth. Please don't put it in a box.

* The Universe is measured to be about 13.7 billion years old. That leads people to assume that light can only have travelled 13.7 billion light years since the start and hence the furthest we are able to see - the extent of the observable Universe - is 13.7 billion light years. Although no thing can go faster than light in a vacuum (or hoover if you are British), space is exempt from that rule. It turns out that space got big really quickly at the beginning. The result is that (depending on which model and definition of distance you use) the observable Universe is actually a few times bigger.

† About 4% of the stuff in the Universe consists of baryons - what you, I, this screen, and even hot dogs (however awesome) are made of. About 23% is matter that shuns interaction with light even more than a moody teenager. We also don't know what the remaining 73% of stuff in the Universe is but it seems to be making space bigger. Presumably that is to provide somewhere for all the lost socks to hide.

‡ Well they do but on continental time and size scales.

** Don't say that it does. It doesn't.

Posted in astro blog by Stuart on Wednesday 27th Jul 2011 (13:08 BST) | Permalink
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