Are infinite Minecraft worlds really infinite?

Answer 1

No. In order to store the positions of blocks, mods, your character, and even activities like collision detection, minecraft uses some sort numeric value, the "tradition" in computer games being the single-precision floating point number (float in Java; however, no matter if it is a float, double, or even int or long, the issue will remain the same). Numbers in computers are not infinite, and will eventually wrap around, in a process called "overflow" and "underflow."

The Minecraft community has made a discussion topic about the subject. The link can be found under "Sources and related links" below this answer.