h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hi Spike,

You have my entry on Olbers' Paradox - I guess you might have read on the subed's list that there were changes to come, anyway, I'm sorry that they've taken so long (I was ), and the up-to-date version is at A753004. The changes are minor, but important.

I'm so sorry once again that it's taken me so long to update this!



J'au-æmne

No worries!

Thanks for the update.

BTW, what does the (BSc) mean?

-Spike A.

BSc = Bachelor of science

I just graduated (last Friday) so I'm now entitled to letters after my name! I now have a degree in physics and astronomy... in fact, the entry that you have is as a result of my revision for my finals...

Congratulations!

About the entry: what do homogenous and isotropic each mean? They're sort of smooshed together in the entry. (Plus I'm curious for myself; I should know this...)

-Spike A.

isotropic means identical in all directions; homogeneous means (in this context) uniform in composition - the same structure throughout.

I hope this makes sense. those are both dictionary definitions, pretty much. There isn't really much point in considering something isotropic but not homogeneous, but we don't seem to have a combined word for the two properties... if that makes sense...

But...how can space be homogeneous but not isotropic? If it's the same throughout then isn't it the same in every direction?

-Spike A.

I think it maybe could if it wasn't infinite- if you were near one edge, you could say that space was homogeneous, but there was less in one direction than another so it wouldn't be isotropic.

Conventionally one always uses both terms, I guess somewhere someone has worked out a version of space which is homogeneous but not isotropic, or something.

I think that in this context the 'isotropic' means that we're allowed to have a symmetrical integral over all space, because it's the same in all directions, and 'homogeneous' means that we're allowed to have a constant number density of stars.

However, I'm not entirely sure. I know that one always does assume both things, and I'm sure that if one didn't have one of them, it wouldn't work... but this is kind of my gut feeling, unfortunately... ...there's reasons why I've given up physics. There's some things that seem simple that I just cannot get my head round.

I'm sorry, I looked at this entry again the other day and I spotted another mistake I made, this time in the maths section.

All you need to do this time is copy the Guideml from the following entry between the <!--second bit of latex--> ... <!-- end -->: A794946

into the <!--second bit of latex--> ... <!-- end --> part in the entry you're editing.

This doesn't represent any deep or meaningful change to the entry; basically there's an error in one line of the maths, but it doesn't affect the next line, and thus doesn't affect the overall conclusion.

Thank you!

J'au-æmne xxx

I knew fate had a reason for making me so late with this one. Sorry, RL has been catching up with me. I've been a bad sub-ed. Pity eris has closed her dungeon...

-Spike A.

I don't mind 'cuz I know how long I am sometimes