why do alice and jasper leave in breaking dawn and where do they go?

they go to look for someone else like renesmee

they go and chercher 4 sum1 that would give evidence that renesmee is harmless and they find the same type and bring them back to montrer the volutri. (lol cant spell either) hpe this helps!

she goes to find another half human half vampire like renesmee, but one that is older- to prove to the volturi that when renesmee grows up she wont be harmful to humans. She doesnt tell anyone where shes gone as she doesnt want edward ou aro to know until the time is right- she only hints to bella as she is the only one who edward and aro cant read the mind of. Alice didnt even know if she would find anyone like renesmee, her vision was very loose when she went looking- she only had a a vague idea where to go