The Vulcan Science Academy/Directorate has determined that time travel is impossible. ;-) I doubt that time traveling humans (or any other species) would spend that much time buzzing our airspace. While its true we have some great working theories on how to accomplish time travel, in each and every case the amount of energy requires is so excessive to make such a journey impractical. This is like the flying barn scenerio - where it is possible to fly a barn if you strap a large enough engine to it, however it is highly impractical thus we build airplanes instead. When it comes to time travel the practicalities are such that one would not send anything physical forward or backward in time. Instead one would 'open a window' to watch say the past. We are speaking about energies as one would find within a black hole, or gravity wells of black hole strengths. The greater the mass you desire to send through time the greater the mass/energies required. If we humans ever reach that level of technology, interest in the 20th and 21st centuries would be merely anthropological - humanity would have accomplished far much more of interest than just harness fission nuclear power. What I am saying is that while we may feel that we are the peak of human greatness we are insignificant in the scheme of things. As a civilization as weighed on the scale of galactic meaningfulness we are nearly as primitive as spear chucking hunter gathers of the Pleistocene. We are so technologically backwards - as a potential real civilization, that we haven't even become a first stage civilization - one that can control weather perfectly is a first stage civilization. Besides all indicators suggest that we stand a very high probability of not surviving for very much longer. It is theorized that the reason why interstellar space is so quiet is that intelligent civilizations self destruct. All we have to do is open a newspaper to see just how easily it is for our species to vanish, we have created a world that is extremely dangerous and have several methods at our disposal to completely wipe ourselves out. Nuclear, DNA engineering, viral experimentation, pollution, our sheer numbers (over population is the biggest problem we face in the coming century) and many other ways. What we have discovered is that technological gains are relatively easy to attain - While technology has far outstripped our wildest imaginations in the past 300 years, in that same time we have gone a very short distance on social issues like equality, tolerance, poverty. We have yet to even see and treat criminals as mentally ill (as they are) instead we punish people for being sick of mind and throw them into holding pens. A very unenlightened way to deal with the problem. It is thus concluded that any species can attain a sophisticated technology, one that is dangerous and in the wrong hands ends up wiping out whole species or even the whole planet. What may not be achieved easily is the maturity and sophistication of social and emotional values in order to wield such power to achieve such things as interstellar space travel and time travel.

Yes... either that or a much, much more advanced version of humans from another planet. We exist so... why can't they? Some people who work for certain gov contractors would agree, I'm sure.

Bingo. ;D

Someone I know has a theory that the aliens are humans who are from the future from a time when we've screwed up the planet so bad that it caused humans physical appearance to change drastically and that's why everyone who sees them describes them looking basically the same way! They have learned time travel and are returning to the past to find out how they can prevent whatever tragic event that happened in the future!

Nope - humans originated on earth, aliens originated on other planets & probably have bodies very unlike ours. Since we originated in different places, in different environments, I have no doubt that we'll always be different.

Me and my friends used to discuss this when we were kids. It was an interesting subject then, and one that has not completely been answered by anyone as yet and of which still facinates me. So, yes I have considered it. I think that maybe the aliens are future school children, learning history? How much more would you remember about the past if you were there to witness it? This may go some way to explaining why mysterious shapes appear in the skies on tapestry's depicting medieval battles etc!! If they are from the future, then I'm confident that they are not coming back to try to change anything. If they changed things, then it may very well mean that they do not develop the technology to come back!