What does ‘big’ mean? How big does something have to be in order to reasonably carry that adjective? The answer, of course, is ‘it depends’.
For example, I am pretty tall. But after standing next to someone much taller than me on a tram last week, I realise that maybe I am not so tall after all. I got to see what it was like for many people looking at me.
The realm of physics, in terms of sizes, is vast. The diameter of a nucleus of an atom is measured in femtometres (one femtometre, or fm, is 10 to the power of minus 15 metres, that is 0.000 000 000 000 001 metres.) Big nuclei are several fm across. Galaxies, on the other hand, are measured on rather larger scales. The milky way is about 100 thousand light years in diamter (One light year about 10 to the power 16 metres, that is 10 000 000 000 000 000. But the Milky Way barely makes much impression on the universe as a whole, which has a ‘size’ of about 50 billion light years. (N.B. ‘Size’ is not a straightforward term for the universe, for reasons I won’t go into just now.)
So physics covers an impressive range of length scales. Interestingly, contrast this with the case of chemistry, which more or less confines itself to talking about atoms and molecules. These are of the order Angstroms (10 to the power of minus 10 metres) or nanometres (10 to the pwer of minus 9 metres) in size. It misses the very very small and of course the very big. (Biology has a pretty large length range though – going from molecular biology all the way up to to trees, and, if you count things like ecology, possible entire continents or oceans.)
[Correction 19 October 09 – I said in the original post ‘Big atoms are several fm across’. Big atoms are much much bigger than big nuclei].