My new advisor didn't reply to my email, is it a sign that she doesn't care? I am a 2nd year graduate student in physics, at one of the top 20 universities in the US. This semester, I am supposed to start a research project with a professor who agreed already to be my thesis advisor.  

So, I emailed her at the beginning of the semester -10 days ago- but I didn't get a response yet. Nothing. I emailed her again yesterday and I hope for a response. I tried to go to her office, but I couldn't find her. She comes to the university only to teach and then she disappears.

I am really worried. Is this normal or is this a sign that the advisor doesn't care at all? Should I try and change advisor? She is a top scientist but I am very disappointed...
Flippant version: It is normal AND it is a sign that the advisor doesn't care at all.

Less flippant version: I would wait a couple more days. Then, if still no response, I would ask around to see if the professor is sick/having a major life crisis/etc. If not, then I would conclude that they do not meet a reasonable standard of availability, and I would choose to work with someone else. There are few things more depressing in academia than colleagues who drop the ball and ignore you, especially when they are senior to you and you rely on them to make progress on your work. Life is too short to be working with someone who is not interested in working with you.