The movie shows other cases where it doesn't follow its own rules: they have spaceplanes that can get to orbit from a planet with 1.8 times earth gravity, but not from earth without huge boosters?
Normal fuel is just less expensive.
Normal fuel is just less expensive.
But here's another one: They set up this nice system with pings to know what planets might be suitable, but they can't detect when a ping is being repeated instead of actually sent by a person? They couldn't put a timestamp in it, or some minor variation? They couldn't figure out that it's impossible to receive a ping every hour if the relativistic effects mean an hour takes 7 years?
There are other issues like this, I'm sure I could come up with a load more if I had the time :)
Wouldn't that be yet another reason to avoid that planet like the plague? Even if they set up a colony there, at any time the planet could run into an orbiting black hole or neutron star, or get slingshotted by one of them into an unfavorable orbit or into Gargantua itself.