Time Travel

I think that if humanity ever travels in time, it will be a concept entirely of perception, and not of physical being in time.
In a sense, we travel in time already. The stars that we see in the sky put off light that we see years after the light is actually put off by that star. We're seeing things as they were a hundred or a thousand years ago, not as they are now.
So, if at some future point, humanity happens to build a telescope well enough to find an inhabited planet elsewhere in the universe, we will be seeing the inhabitants as they were tens or even hundreds of years before the present moment.
Following those same lines, if humanity were to figure out how to travel faster than the speed of light, we could theoretically catch up to the light that reflected off of the Earth in years past, and literally watch history unfold.
It is also possible that there is more sophisticated life in the universe that already is watching the Earth, and may even have recorded our existence. Pretty cool, huh? :D
To go even further, if humanity could move faster than light, we also theoretically could catch up to the end of light, and watch the creation of the universe happen.

So, how did the Egyptians build the pyramids? 

I don't know. Ask the aliens.