Hi,
they are very different. Can't you hear the difference between these two? Click on them:
/ae/ => /caet/ ---
CAT/a:/ => /ca:t/ ---
COTYou can check this out too:
American VowelsThat comes from this webpage
http://www.americanaccent.com/pronunciation.html , which talks about some important feature of American English (it's Ann Cook's website). I don't know if you are interested in American English or British English, but I'll give those links anyway... someone might be interested.