system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: If time travel was truly humanly possible to achieve, we would have visitors by now.

Pretty much self-explainatory..If time travel was humanly possible to achieve during our existence, chances are,  the future "us" (where we are supposedly super technologically and knowdlegably advanced), would have visited us from the future by now. 

Even for study and exploration purposes alone, just like we have studied the ancient civilizations at all points in time as well.

Ultimately, that leaves us with 2 possible predicaments:

1. Time travel back in time is not humanly possible to achieve during our existence as species.

2. Time travel back in time was never done to any point of our modern human history (about 12000 years), which is extremely unlikely, not make much reasonable sense as to "why not?", and there would be some evidence/knowledge/effects of that occurring.

3. Time travel back in time is not possible outside of a human's travelling back in time life span. As he cannot travel back to a time when he did not exist.

Thus, time travel (at least back in time) is not humanly possible to achieve.
gpt
Several possible answers:

1. Yes, we do study primitive cultures, but our policies regarding research are mostly designed not to interfere with the primitive cultures being studied. That’s not from a Star Trek/Prime Directive type of moral imperative, it’s because we get better data when we only observe and not interfere. Not being seen is likely very important to getting good data. That policy of noninterference may be why, if visitors did come back in time, they have not been seen or identified as time travelers. 

2. Until quite recently (the last few hundred years), communication between groups of humans was limited/impossible. It’s quite possible that contact did happen, but the people affected by it were not able to tell the rest of the world.

3. Have you ever seen any of the “Ancient Aliens” programs on History Channel? Yes, plenty of the wild claims are laughable, but they are correct that there are a number of historical drawings and other records of things that look like aliens, astronauts, etc. It’s quite possible that these records don’t refer to aliens, but to time travelers. If this is the case, then time travelers HAVE been back to visit. 

4. Time travel doesn’t necessarily mean that the past can be interacted with. Since the physics of it are not currently known/understood, it could be that visitors can view events and look around, but not actually touch, speak, or interact with the past. 

But most likely:

5. Visitors from the future haven’t come back in time for fear of being killed. Not by us, but by the planet. The Earth is moving insanely fast - it rotates at about 1,000 mph, and in addition to that it is orbiting the sun at about 67,000 mph. If you were to travel back in time even a second, the location of your time machine at the time of departure would be vastly different from its location at the intended time of arrival. Thus you would “arrive” in the past either somewhere out in space or inside the Earth itself. 

It should be possible, for any sufficiently advanced civilization, to calculate the change in position of the Earth for short hops in time. However, the further back in time you go, the harder it gets, because the speed of the Earth, the tilt of the axis, and the orbital path are constantly changing in very minor but non-standard ways. Over the short term that shouldn’t be a huge problem, but going back very far at all would cause the amount of unknowable changes to stack up and make the Earth’s actual position almost impossible to accurately calculate, let alone calculate with enough accuracy to arrive on the surface. 

Speaking of the surface, the surface of the Earth is constantly changing. The tectonic plates move slowly - 1 to 10 cm a year - but they move at different speeds at different locations. Their movement causes various parts of the Earth to rise or fall, depending on how the plates collide. Add to this the cumulative effects of volcanoes, erosion, sedimentation, and other natural effects, and it is impossible to determine with any specificity the actual height of a given piece of the Earth (relative to the center of the Earth) at any time. So even if you correctly calculated the location of the Earth at a given point in time, and matched the speed, your estimate of the height of the location you want to travel to could be wildly off, again resulting in you either showing up quite high off the ground, or under it.