On the other hand, there may be another kind of time travel that actually fits in with modern physics -- time dilation. In the theory of relativity, time dilation is an actual difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. Here's an excerpt of the Wikipedia entry on the subject:
An accurate clock at rest with respect to one observer may be measured to tick at a different rate when compared to a second observer's own equally accurate clocks. This effect arises neither from technical aspects of the clocks nor from the fact that signals need time to propagate, but from the nature of spacetime itself.
It goes on to provide a more specific example:
A case of time dilation in action is that astronauts return from missions on the International Space Station (ISS) having aged less than the mission control crew that remained on Earth. Such time dilation has been repeatedly demonstrated (see experimental confirmation below), for instance by small disparities in atomic clocks on Earth and in space, even though both clocks work perfectly (it is not a mechanical malfunction). The laws of nature are such that time itself (i.e. spacetime) will bend due to differences in either gravity or velocity—each of which affects time in different ways.
In theory, and to make a clearer example, time dilation could affect planned meetings for astronauts with advanced technologies and greater travel speeds. The astronauts would have to set their clocks to count exactly 80 years, whereas mission control—back on Earth—might need to count 81 years. The astronauts would return to Earth, after their mission, having aged one year less than the people staying on Earth. What is more, the local experience of time passing never actually changes for anyone. In other words, the astronauts on the ship as well as the mission control crew on Earth each feel normal, despite the effects of time dilation (ie. to the traveling party, those stationary are living "faster"; whilst to those stood still, their counterparts in motion live "slower" at any given moment).
With technology limiting the velocities of astronauts, these differences are minuscule: after 6 months on the ISS, the astronaut crew has indeed aged less than those on Earth, but only by about 0.007 seconds (nowhere near the 1 year disparity from the theoretical example). The effects would be greater if the astronauts were traveling nearer to the speed of light (approximately 300,000 km/s), instead of their actual speed—which is the speed of the orbiting ISS, about 7.7 km/s.
Time dilation is caused by differences in either gravity or relative velocity. Both factors are at play in the case of ISS astronauts (and are actually opposing one another).
So in other words, if astronauts left earth and traveled far enough away at a fast enough speed, when they returned, more time would have elapsed on Earth than had elapsed for them on their spacecraft...although it wouldn't seem like more time had passed to them.
Now let's move on here.
If you've read my blog for a while, you probably know that I'm interested in the topics of lost civilization and the possibility of lost advanced prehistoric civilizations. I've written about or mentioned the topics in various posts from the past. Here's links to some of those posts if you haven't read them before and would like to:
Forgotten Civilization: The Science of Questioning or the Questioning of Science?
Did the 'gods' forget how to be 'gods'?
Civilization Lost Documentary
So to sum up here, I like to consider the possibility that there were once advanced civilizations on Earth that have long since been gone...possibly highly scientifically advanced civilizations.
Now let's put these two topics together.
Time Dilation plus Lost Civilization
There's a theory in the UFO research community that has been tossed around before that suggests that (some) UFOs may not be aliens from another planet, but time travelers from the future. But what if they were actually "time travelers" from the past? Lets say for a moment that there was once a highly advanced scientific civilization that existed in the distant prehistoric past of the Earth. Let's say they developed space travel and the ability to fly near, maybe even exceed, the speed of light. Taking time dilation into consideration, if prehistoric astronauts left Earth sometime in the distant past and traveled far enough away at a certain speed, is it possible that they would return to Earth at a much later time than they left? I'm not a physicist and math is not my best subject, but lets say some prehistoric astronauts left Earth to embark on a mission that would take them away for several years. Is there a distance they can travel and speed (theoretical or not) that they could travel at that would have them returning to Earth several hundred or even several thousand or several 10s of thousands of years later on Earth? I don't know what that speed or distance would have to be, but if the theory of relativity is accurate, then I would assume it must at least be theoretically possible.
So if prehistoric astronauts left Earth for a few years and when they returned a few thousand years had passed on Earth, they might find the civilization they left was gone and new civilization had developed that may have seemed both foreign and primitive to them. The question then would be did they know they would be returning to Earth thousands of years after they left? Did they understand time dilation? For that matter, do we really even understand time dilation? If they did understand time dilation and our understanding of it is accurate, then why would they leave knowing they wouldn't return for thousands of years? Since I'm only speculating here, I can think of a few possibilities.
-They knew they wouldn't return for thousands of years and knew they would be leaving their old lives behind, but chose to do so anyway either because they wanted the adventure or they wanted to learn more about the universe.
-They were expecting some catastrophic event and wanted to avoid it by leaving Earth and returning after the event had long been over.
-They didn't actually plan to be gone that long, but ran into a problem that delayed their return.
Maybe you can think of some other possibilities.
So what would the world they returned to be like? That all depends on what year it is when they returned. Maybe there were multiple teams of astronauts leaving on multiple missions that would have them returning at different times. If so, then various UFO sightings throughout history may have been prehistoric astronauts returning to Earth. If they are human, then it would be easy enough for them to blend in with the populace. Except the world they returned to would probably seem foreign to them and they wouldn't understand the language. So even though they may be able to fit in by their appearance, they might not understand the customs or know how to communicate effectively. Because of that, they may avoid making direct contact. On the other hand, maybe they would contact the people and set themselves up as 'gods'. It could be that prehistoric astronauts returning at different time periods may handle their situation differently. Even if two teams left around the same time, if their missions were different lengths, one team may end up returning many years after the other team returned. So with no way to contact others from their time period anymore, they may have no choice but to do whatever they see fit. Perhaps some would choose to avoid contact, perhaps others would choose to rule over the more primitive people they returned to find. Maybe others would just say screw this and leave again.
While time dilation may not be quite as cool as driving a Delorian at 88 miles to travel through time, it is something that is based in actual science and not science-fiction. The idea that some UFOs may be time travelers from the past is, of course, based on the possibility that advanced scientific civilizations may have existed in the distant prehistoric past, which is a debatable topic. Mainstream academia doesn't typically consider such a possibility, and even though I do feel that advanced prehistoric civilizations probably once existed, even I doubt they had advanced space travel. But I do like to consider possibilities. So what if time travelers actually came from the past instead of the future?