Time Travel can be achieved my many means, one more weird than the other.
Time Travel Theory
Three distinct, and irreconcilable, theories regarding time travel have long co-existed, in the DC Universes:
Fixed Time: The past cannot be changed. Events will conspire to prevent any meaningful changes to history. In some cases a time traveler is forced to be part of the very event they were to observe and make sure it happens as recorded. In others a person will become a phantom unable to interact with events.
Plastic Time: History can be changed, but there can be consequences from doing so.
- The concept of hypertime can be viewed as a strange version of the Plastic Time theory.
- An extreme version of Plastic Time is Chaotic Time where the smallest of changes can have mammoth effects on the timeline.
Parallel Universes: Time travel to your own actual past or future is impossible, and all time travel does is either shift you into a parallel universe, or into an alternate timeline. In some cases travel to "the" future was actually travel to an parallel Earth with a similar but also different history.