The Dual World of Bright Memory

Traveling back in time is impossible because when the sign of time changes, the main term in the equation becomes infinite. It's clear that time travel back in time would definitely change the course of things there, at least slightly. But no matter how small the impact time travelers have on the past, they will also bring changes to the present, and they are likely to be major changes. Since the past appears to be unchangeable, it makes sense that a return to the past is impossible. But the future is not immutable, so time travel to the future should be possible. ——Excerpt from Asimov's "Robot Fantasy"