Det låter som om du beskriver den huvudsakliga " Med sina startremmar " av Robert A . Heinlein, ursprungligen skrivet under namnet Anson MacDonald
Per Wikipedia:
Bob Wilson locks himself in his room to finish his graduate thesis on a mathematical aspect of metaphysics, using the concept of time travel as a case in point. Someone says, "Don't bother, it's hogwash anyway." The interloper, who looks strangely familiar, calls himself "Joe" and explains that he has come from the future through a Time Gate, a circle about six feet in diameter in the air behind Joe. Joe tells Bob that great opportunities await him through the Gate and thousands of years in his future. By way of demonstration, Joe tosses Bob's hat into the Gate. It disappears.
Även om det inte publicerades i Analog på 1990-talet har historien blivit upprepade gånger antologiserad i olika tidskrifter, inklusive en gemensam Amazing / Analog -läsare 1972.
Du kan läsa en fullständig version online här .