it's a little silly, but they were able to communicate only because of their pre-established bond. in that situation, normal communication was impossible, but their "love"/bond/relationship enabled them to communicate in spite of the difficulties.
The idea that paternal love transcends space/time was excellent... It's great that Nolan portrayed it that way. Goes against the casual cynicism and litteral mindedness of so many lesser sci-fi movies and dramas...
She's talking... It's Coop's relationship with his daughter that actually materialises the ability to move forward and backward through time..
Metaphorically, this is true for us in real life... our bonds with our daughters or our fathers are what allow us to move through time in either way... That doesn't work for just a girlfriend...
Yes... which is metaphore of our paternal bond with our daughters and fathers... It's a movie, it's art... Not a problem to be solved... This is the human connection... It's a beautiful thing...
Even Coop doesn't LOVE his daughter, he still can use the watch to communicate to his daughter. "Hey, daughter, I know you hate me, but I have a formula can save you and human race."
He's love doesn't matter, without black hole, no matter how strong your paternal love is, you can't break through time and space.
He did love her and it did work in the movie... It's the central part of the movie... It's the central relationship of the movie... Interstellar revolves around Coop's bond with his daughter, Murph, nearly every scene is about this.. from car breakingdown in the cornfield where he explains her name before chasing the drone, to him chosing to support her vs the school, through to finding the airbase, him saying goodbye to her before he leaves and then the key blackhole & library scenes...
I'm talking about the movie world... Not reality of jumping into black holes (would be spaghettified based on our understanding of physics)... But even in that scenario, her love for her father and her idea of her father's love for her would survive...
It's such a sweat movie... a loveletter to daughters and fathers everywhere... Who knew Nolan was such a sensitive guy ;)