Not only is immortality theoretically possible, there are already immortal beings on this earth right now.
Or maybe more accurately, under the earth. The turritopsis dohrnii, or the immortal jellyfish as it is more commonly known, is biologically immortal.
It starts life when a group of larvae called planula, settle on the ocean floor. They come together to form a colony of polyps. The immortal jellyfish eventually grows from this colony.
If the jellyfish is attacked, or feels death is at the door, it reverts to it's polyps form. Then it spends the next few months growing back into a full jellyfish. And so the biological cycle is complete, and the immortal jellyfish remains immortal.
However it would be a mistake to confuse immortality with invincibility. It can still die due to disease, or being eaten in the polyps stage.
Although not the conventional image of immortality, it's still pretty interesting, eh?
Dude I want to be a Jelly Fish now...
More seriously, interesting fact i learnt something! So is it like, he grows back from the same colony from which he was born? Or does he have to "wait" for another one to form?