The Time HoppersRobert Silverberg
They were disappearing, one at a time, in spite of the fact that in the crowded, hungry world of 2490 there was really nowhere worth going. Then they began to reappear, not in Moscow or Nairobi or Los Angeles – but in 1970, 1981, even the nostalgic days of the roaring 2100's. A way into the past had been found, and people were flocking through it for a better life – no matter what peril they might pose to the threatened present.
Genres: science fiction