There has been much discussion of time travel, but I haven't yet found an answer to a question I have. That is, what are the specific theories or factors that allow for the possibility of time travel? The issue has been discussed and debated many times on this forum and I'm not so much interested in rehashing that here. I would like to know what are the basic scientific facts or principles that allow for this topic to even be considered as a possibility. Can someone provide a succinct answer?