It wasn't unusual to find Edward slumped over, fallen asleep, with his head pillowed on a pile of books and notes. Whenever Hohenheim found his son like that, he would carefully move anything that might get ruined (Edward drooled in his sleep sometimes, when he slept like this, with his mouth slightly parted, and a pen still clutched between his fingers), and put the ink out of the way.
If it was cold, Hohenheim would drape a blanket over his son's shoulders; sometimes he would do that even if it wasn't. Eventually, of course, he found his way into his own bed. Edward he left where he was. Hohenheim understood him well enough - too well, sometimes, some ways - to know that he wouldn't appreciate being moved.
Most of the time, the names on the books and essays were unfamiliar - or only marginally familiar; Goddard, Newton, Kepler. Copernicus he knew, and DaVinci. It was when the names were too familiar, that Hohenheim's chest clenched, and his throat closed up. Flamel, Kelly, Hollandus.
What hurt the worst was the one night Hohenheim had come home, to find the works of Paracelsus strewn about the table.
"It was your fault, wasn't it?" Hohenheim almost jumped; Edward had seemed asleep, and he hadn't been expecting the words (perhaps tomorrow; he knew that they would come). "This world as well as our own... Only things turned different..." Edward's voice trailed off in a mutter, and he turned his head slightly, to send his accusing golden glare back at his father.
"I don't know what to say to you, Edward," Hohenheim replied.
"I don't know what I want you to say," Edward confessed, in a mutter. He turned back to his notes, and books. "But... There's a clue in here, somewhere. It's the same - all the same - up to a point. I was waiting for you to get back, you know," he added. "After all, it's your work." He glanced back, and Hohenheim wondered if he had wanted a reaction; if so, he got one. Hohenheim flinched, a little.
"...I'll look," he said after a little while. Without another word, Edward moved a little to the side, leaving space for Hohenheim to come beside him. It was... strange, looking over the writing that was his own, and yet not. The man who wrote this has been dead for nearly half a millennium... And yet here I stand. He searched in silence, aware of his son's eyes on him, watching him and waiting.
Hohenheim wasn't certain how long it took - several hours, at least, that he spent absorbed in words so familiar. Sometime in the process, he had gotten himself a chair, or Edward had gotten one for him, because he was sitting when he found it. The point of divergence--
"Here," he said, his voice cracking roughly. He cleared his throat. "This one." He could almost feel Edward's eyes turning down to the table. "I never wrote this-- Or, I imagine, anything after." Edward reached over, and slowly ran his fingers down the paper, leaving them, oddly, at rest next to Hohenheim's hand.
"...Dad... Thanks." Hohenheim looked down at his son; those had been about the last words he expected to hear.
"I hope it helps," Hohenheim replied.
"Me too," Edward replied.