What can give physical access to CPU?
If you have unlimited physical access to the CPU, then you can send your own instructions into it, including instructions to read from arbitrary memory addresses. Looking at the voltages on the data bus then shows you exactly what's in that word of memory.

Remember, any data the computer is able to use, has to be accessible by at least one CPU. That's just how it works. So access to the CPU gives you theoretical access to everything else.

(Note: this is an answer to the question in the body, not the question in the title.)