 What does the older man know the younger man will do?
There is one fact no sane man can quarrel with ... You will, you know, so why quibble about it? At least, you always have ... or do ... or will. You'll get used to it, of course, but it will take about thirty years. It's a real wrench when a man meets himself for the first time. So let's begin when you get up in half an hour and come out with me. 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. You won't exactly believe it, but you'll want to go along. Doesn't hurt, and when you pull your arm back, you're still sound and uninjured. If you don't, it's a good way of going crazy. I figured out once that such a universe would need some seven or eight time and space dimensions. Maybe there is no machine, and it's just easier for us to imagine it. 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