"Aldo,” Regan said, “what’s the ether?”
“It’s what people used to believe the universe was filled with,” he said. “They believed light needed to pass through something, only Einstein proved light can be particles, which don’t need a medium to travel through. And before that,” he added, “ether was what they called the air in the realm of the gods. A shining, fluid substance.”
“So when people say we’re alone in the ether . . . ?”
“Alone in everything. In time and space, in existence, in religion.”
“But,” she said, and stopped. “But the bees.” She felt certain she could feel him smile. “Yeah,” he said, “the bees,” and she felt the weight in her chest ease a little, the sea that had risen to her ankles fading away with the tide.

This book was incredible! I have never read a book like that! A love story between two outcast people - one of them a theoretical mathmatecian, the other a former criminal.

Your future self will always see what your present self is blind to. This is the problem with mortality, which is in fact a problem of time.

It made me question a lot of things and made me realize even more ...

"Because if you don’t have something to figure out, then you have no reason to keep going?” Or maybe it wasn’t that difficult to explain. “Yeah,” he said. “Basically.”
[...]
“No, it totally is. You gave yourself an impossible problem so you’d never be able to stop thinking about it. It’s brilliant, actually.” She sounded close to impressed. “Other people probably think it’s crazy, don’t they?”

The characters were so intersting and deep, with their own struggles and talents ...

There was a difference between craving and compulsion. 
[...]
Cravings were wishes that could be satisfied, but compulsions were needs that must be met.