It's a long, hard road I'm afraid.
You have to remember, that first off, the cohort of people who enter an undergraduate physics program are generally fairly high on the intelligence and industrious curves to begin with and just about all of them are at least considering academia as a career.
Along the way many discover (a) physics doesn't intereste them as much as they thought, (b) they aren't as smart as they thought they were, (c) they have other passions, or (d) they lose the ability to work hard either for internal or external reasons. As a result, maybe 1/2 who start end up graduating.
Then there's getting into graduate school. You go through another set of bottlenecks here just to get in. Some people simply decide to go and get a job in the real world. Others just aren't competative for the spots they apply for. I think it would be safe to assume another factor of 1/2 here, although it could be closer to 1/4.
Now you've got 4-7 years worth of graduate school. You live on a minimal stipend. You work long hours. Other interests develop. You have to pass a qualifying exam, cadidacy exam, publish papers and defend a thesis - the failure of anyone can mean that you don't graduate. Let's say again a factor of 1/2 are actually awarded the PhD.
If you're keeping track we're already an order of magnitude less than the number of people who chose to embark on this journey and the bottlenecks tend to select in favour of the intelligent and industrious and dedicated ones.
From this point there are dozens of posts already on these forums that attempt to guess the chances of landing a permanent, academic position at a research university once you have you're PhD. This is generally believed to be another factor of 1/10 for a number of reasons. These include the fact that your typical professor will graduate approximately 10 PhDs, but will only need one to replace him or her.
All of these are gross estimates, but I don't think it would be too far off to assume you're looking at decrease of about two orders of magnitude from the initial cohort that starts out intending to become professors.
Restricting yourself to theory further reduces your chances.