Why there are no time travellers

• In the history, we may have met time travellers, but not recognized them as such, or not believed them.
• We don't believe them even if they claim to be so. (See John Titor.)
• They blend in and don't get exposed.
• They're too evolved so we cannot even communicate with them. (Interpreting them as aliens instead.)
• Time travel to past can only send signals, but not material, and we do not discover those signals as originating from future.
• Evidence of time travellers is retroactively erased.

• Time travel is discovered, but a strict rule system inhibits anyone from using it to travel to our time, and the system is never breached until the ultimate demise of time travellers.
• Time travel is discovered and used to travel to past, but a strict rule system inhibits anyone from exposing themselves as time travellers.
• When a time travel method is created, it can only go back until the moment of its creation, not earlier, and that moment is not yet.
• Time travellers to the past end up in an alternate reality (paraller universe) not connected causally to ours.
• Time travel is invented so far in the future that our time period is relatively uninteresting to them.

• They can only observe the past, but they cannot leave a mark in it, such as someone observing them. (Perhaps they don't even fully materialize.)

• It is physically impossible to time travel to the past at all. (In the traditional sense. Quantum-mechanical antiparticles may be travelling backwards in time even normally.)
• The mankind (or simply the memory of human civilization achievements) expires before time travel is discovered, and no future civilization ever discovers modern day Earth as a potential time tourism target.
• We cannot receive time travellers from the future, because the future does not exist yet. We are creating the future by living at the lead of the universe's existing time. Time travel is only possible towards the past.

Contributions by

• Warp