system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] Can superheroes who fly reach lightspeed by continuesly adding to their momentum in space?
gpt
Some can, some can't. 

As you get closer to *c*, you require exponentially more energy to accelerate the same amount. For some heroes with near infinite amounts of energy (superman, the flash if he could fly, Martian Manhunter). For others with less stamina or what not, the strain would either impossible, or kill them trying.