Dr. 'Doc' Gordon: The past, of course, still exists
Mr. Denning: Oh?
Dr. 'Doc' Gordon: Suppose a scientist on a planet some fifty light years from here were observing Earth, he would at this moment be looking at light rays that left this Earth fifty years ago. He might very well be watching the First World War. In effect, some full fifty years' of history would be contained in those beams traveling to that distant planet
Mr. Denning: That's quite an idea!
Dr. 'Doc' Gordon: It's more than an idea, Mr Denning, it's fact. And since space-time is a continuum, the present is only a point moving constantly along that continuum
Mr. Denning: When you put it like that, Doctor, even I can understand it
Dr. 'Doc' Gordon: Theoretically, with our equipment, we should be able to cut in anywhere along that continuum - actually journey to the center of time.