Even under the best of circumstance, going through a PhD program in physics can be a challenge. While doing most of your work online can save you numerous trips to campus, there are also some drawbacks: lab work can be problematic if you don't have access to a college physics lab with all of its expensive equipment. This is why many online physics PhD programs focus on combining on-campus and online instruction in one program. Before undertaking to spend the next four to six years of your life studying physics, consider all of your options, an if you are at all able to do so, choose a program that combines online and on-campus elements.
Some schools that offer this degree exclusively online include Stanford University and Michigan State University, though keep in mind that these programs are offered through these schools' extension divisions, and will not include some of the stellar faculty that teach at the physical locations. On-campus/online combination programs are offered by Oklahoma State University, SUNY Stony Brook and Howard University, among many others (see Resources below).
Be prepared to study even harder through distance education than through regular schooling. Without extremely strong organizational and self-motivational skills, pursuing a physics PhD degree online may prove to be a challenge. There won't be anyone pressing you to complete assignments on time, and even though you'll be able to interact with professors online through emails and chats, you won't be able to simply pop into a professor's office to ask a question and receive an immediate answer. And though you will be able to devise your own study schedule and work through summers and school vacations, you won't be able to cut down the time that it takes to get a PhD in any effective way.
Many physics PhD students earn some money towards their tuition through TA work, acting as professors' helpers and actually teaching freshman physics courses. This option is not available in many online programs, though some online and on-campus program combinations may give you the opportunity to work as a TA. Be prepared to spend as much, if not more, money on earning your PhD in physics online.
Also, be aware that even though some prestigious institutions offer distance education coursework, many programs are offered through these schools' extension programs, which may not be taught by the same faculty members as their regular, on-campus programs.
Distnace education has come a long way from the days of correspondence courses through "snail mail." Today, online classes include sophisticated student-teacher interaction through video conferencing and whole-class participation through such software packages as Blackboard. With gains in technology occurring on a regular basis, it is expected that online learning will continue to flourish well into the next few decades, with virtual classrooms that feel as if you're sitting in a campus classroom, the ability to conduct real experiments in virtual reality applications and reading entire digitized textbooks online.