 What does the older man know the younger man will do?
You will, you know, so why quibble about it? At least, you always have ... or do ... or will. Yes, it'll be pretty obvious it must be a time machine. You'll be puzzling over what I'll tell you, and you'll be getting used to the idea that you are the man who makes atomic power practical. If you don't, it's a good way of going crazy. When you spend thirty years thinking about it, as I did—and you will—you get further and further from an answer. That means you either carry a small time field with you, or you are catching a small increment of time from the main field.
 exactly as he's been told
 exactly as he's been told  invent the next great invention  change the future  fight with him and try not to go