Nedda Gilbert, MSW, Educational Consultant, and Author
I agree with the answer provided by the other expert. Physics is not mandatory. But in talking about winning admission to Harvard, you should be focusing on how you can be most competitive, not just meeting requirements. Earth and Environmental Sciences - right or wrong - is often seen as the easier class over physics. So depending on your other qualifications, this may or may not hurt you. Harvard receives an insanely competitive pool of applicants all vying for a spot. If they all took AP Physics, and nailed an A, well you get the picture.
When it comes to academics, (apart from other aspects of your application like your essays, leadership, etc), you will be judged by the difficulty of your course load relative to what your school offers. For example, did you take the most demanding courses you possibly could at your school? This is in fact the benchmark Harvard will use. At many public schools, not all AP classes are offered. Further, with a limited number of AP classes offered, not all students can get into one. If this is the case, it's important to have a guidance counselor highlight this on your application so it can be reviewed in context. Framing your application is very important. Are there other issues to address? If you managed to take demanding classes and maintain a high GPA despite some adversity, this will be more meaningful to the admissions committee than one lone class.
At this point, I would not sweat the physics. Beef up all the other aspects of your application. Talk to a guidance counselor to determine if that physics class is a glaring omission. If it seems problematic, consider taking it as a summer class at a local community college or summer college program. In fact, take Physics 1 which is usually taught freshman year at Harvard. Only do this with the understanding that this by no means guarantees you admission there. Getting into Harvard is like winning the lottery. Few people do. But that summer class just might put you on a more even playing field.