Half a moment's hesitation. In that moment, Luke realizes Darth Vader is his father without having to be told. It makes all the difference.
He ends up encased in carbon freeze.
They say the whole body, including the mind, is put to sleep in this state, but they lie. The mind is constantly thinking. Luke's thinks the whole time about what it means for Vader and Anakin Skywalker to, after all, been one and the same. It means, for instance, that Old Ben lied to him.
He loses track of time, certainly. It could be hours or days or weeks or years. His friends could be dead any number of ways. The vision he'd had back on Dagobah, the one that scared him so much, seemed like it strongly suggested them hurtling toward defeat in the fight against the Empire. Growing up on a remote world like Tatooine, until the fateful day he chased a rogue droid across the desert, Luke had never considered which side should win.
When he is awake again, when he is struggling to regain his senses, he hears someone laugh. It's a sound he's never heard before, or at least from a person he's never heard before. But it feels familiar. He also hears the distinctive breathing of his father.
The laughter goes away for a moment. Luke stands up, turns in the direction that is now radiating silence. "Good," a voice says. "Now you are ready to take your place by my side."
"I'll never join you," he tries to say, but like everything else about him at the moment, his voice feels like it is still lagging behind. For the first time in a long time he doesn't even feel the Force.
There's a loud clanging. The disembodied voice is saying, "The Force is a gateway to many possibilities," almost as if the speaker had read Luke's thoughts. He feels it surge within him again. He doesn't know what to think.
"Unlimited power," the voice says, and suddenly Luke can see again, and it is the Emperor standing before him. Beside them is the lifeless body of Darth Vader.
"Master Yoda told you many things, didn't he? He wanted you to think there was nothing good about what I've done," the Emperor says.
"He told me enough," Luke says. "He told me trying is not good enough."
"Not for a Sith lord."