Time travel is possible - we are all travelling forward into the future right now. The only question is at what rate are we travelling into the future? That question is answered by special relativity.

Time travel into the past? The fact is we've already been there. Right now, this instant, is actually the past, and we will visit this moment just once before moving forward in time.

Travelling further into the past, before we're born? Those moments may be experienced at a level that we don't yet understand. But just because we weren't born yet just means we weren't sentient. Non-sentient existence could mean anything. It could mean that we existed but weren't aware of it.