If time travel existed...

If there were people that could travel back in time, we'd probably have met them by now.

The theory goes, if going back in time was possible, there isn't any possible way you could change the present from where you came from. That would cause a paradox. Imagine if you went back in time to stop your father from ever meeting your mother. You can never completely stop your father from ever meeting your mother, because if you had been successful at doing so, how would you have been able to go back in the first place if you had prevented the entire meeting from occurring at all? You see it cannot work. The theory is that nature would correct itself from allowing the paradox to occur. You may delay the meeting, but in the end, your father will always meet your mother, thus creating your existence. Once an event in time occurs, it can never be changed. The only exception to this is if you were able to go back and upon doing so, you created an entirely different universe/ separate time stream. If this is how it works, it would kind of suck since you would never be able to return to your original time/universe since you merely arriving in the past changes/creates the new universe. Now it, is possible to travel to the future. If you were to travel just under the speed of light to say.....the next star closest to ours, and let's say it took you like a week to get there. Because you are traveling so fast, time around you speeds up which means your trip there and back for you, would only be about a week or whatever length, but here on Earth, 50 + years would have passed.

One of the more feasible methods of time travel is actually based off of Einstein's concept that gravity is a space-time geometry, so basically that time and gravity are related (i.e. gravity is a distortion in space-time). You know, the man who spent the most time in space (I forgot his name) has (technically speaking) traveled in time, because since gravity is obviously significantly less in space, time also (according to the theory of Special Relativity) is actually different in space (basically it slows down), and since this astronaut spent so much time in space, technically speaking he is somewhere around 0.20 milliseconds younger than everyone else. Here's the link [url]http://pillownaut.blogspot.ca/2012/05/worlds-first-time-traveler.html[/url] Stephen Hawking used an analogy for this. Imagine two identical twins are born on Earth and say when they're both 20, one of them is put on a spaceship that travels close to the speed of light, while the other stays on Earth. Now, let's leap ahead 50 years later, when the twin on Earth is 70 and the other twin slows down his ship and decides to go back to Earth. Since the astronaut twin that was travelling close to the speed of light (~3.0*10^8 m/s), , time slowed exponentially for him, and technically speaking, he wouldn't have aged at all or aged very little (i.e. he'll still look like he's in his early 20s while his twin brother who stayed on Earth looks his grandfather)! A black hole is like a massive distortion in space time (the fourth dimension), so basically time slows down there as well because the gravity is so immense (ya know, not even light can escape the power of a black hole). Stephen Hawking, although not a supporter of the notion of building time machines and going into the past, does believe in this type of time travel. One theory that he supports on how the method of "time travel" I described earlier is to get a bunch of astronauts on a space ship and fly [b]really[/b] close to a black hole as long as it does not cross into the black hole's event horizon (the region where it is impossible for any type of matter to escape the gravitation force of a black hole). Basically, since space-time is altered in the area around the black hole, time slows down for the astronauts in the space ship around the black hole, and compared to Earth time would be much less....depending on how fast the spaceship is traveling. But feasibly, using this method, decades could be mere years for people in the spaceship around the blackhole. Another really cool idea I saw on the show "Universe" on H2 the other day was some astrophysicists were saying--*theoretically*--it could be possible to build a train that moves at the speed of light (but of course, the train would circle the earth, and probably would need lots of isolated land mass---and tracks that cross over all the oceans) but basically, the "passengers" of the train would also experience the effects of distorted space time (the incredibly fast speeds would affect gravity, which would affect time, essentially slowing it down) so people in this train could, theoretically speaking, be in this train for a century, and be the same age when they get off the train a century later. Another interesting aspect of space time is the Alcubierre device. So basically, it's a possible solution to how NASA could travel to distant stars---cause current space crafts travel at 0.0004 (?) percent the speed of light, or something like that, so a trip to alpha centauri would take tens of thousands of years for our current space craft to take----which is sad considering the stars in the centauri system (alpha centauri, beta centauri, and proxima centuari) are the closest to our sun. But the Alcubierre device ([url]https://en.wikipedia.org/wiki/Alcubierre_drive[/url]) basically, in a nutshell, also distorts space time so, hypothetically speaking, a spacecraft could achieve faster than light speed (NASA is actually getting to work on this as we speak to try to make this theoretical work possible---but it'll probably take a really really long time before anything tangible comes from this research-----[url]https://en.wikipedia.org/wiki/White%E2%80%93Juday_warp-field_interferometer[/url]). But the problem here is that, since you're traveling faster than the speed of light, space time (the fourth dimension) gets altered too in this "hyperspace" of sorts, so people would experience alterations in time because gravity is significantly affected when you go at really really fast speeds (basically the whole time slowing down stuff), so a trip to a really far place like a star system in the Andromeda galaxy (the closest galaxy to the Milky Way Galaxy) and back could be (feasibly) only a few years to people traveling in the altered space-time hyperspace--but could be centuries, or even millena (!) for people on Earth. This is one of the more feasible methods of "time travel" a lot of current physicists (even Stephen Hawking--who is a really big critic of time machines that could allow people to go back in time) believe in. Maybe, a few centuries down the road, we could actually try out this nifty experiment and get some astronauts to circle around a black hole (but not close enough to get consumed by its ferocious force) and measure how much time has passed for them. But to the time machine thing, where you can go back to time and alter the past, there's a lot of debate on whether or not this is possible. Stephen Hawking as I mentioned before, really hates this idea, and even kind of did a pretty stupid experiment to prove his point. He basically held a "time traveler's party" and sent out invitations all over the place, and asserted that people in the future would hear about his party and, if these people in the future had built time machines, would travel back in time to his party. No one showed up, and then Stephen Hawking was all like "SEE? told ya time machines are a bunch of horse shit" (I'm paraphrasing to a significant degree) but yeah basically that's "a lot of evidence to him" ( I mean, hypothetically speaking, there could be a chance that there are time machines in the future, except no one gave enough -blam!-s to go Stephen Hawking's party or, more likely, wouldn't want to alter human history by exposing themselves or future technology). Hawking didn't ever really mention that in his "I'm probably right" speech. But a lot of brilliant minds sometimes do stuff like that. Newton, one of the founders of calculus and physicist extraordinaire, actually spent very little time doing mathematics and physics comapred to predicting the next doomsday---or ingesting copious amounts of mercury during his little alchemy "experiments" (mercury is a really poisonous chemical, that can potentially kill or cause serious health problems like psychoses and paranoia). Though, one of Hawking's more rational arguments against travel into the past is that, the way to achieve this actually through black holes, or (theoretical structures) called wormholes---and both involve getting through A LOT of radiation (even supporters of time travel acknowledge the radiation bit), I mean black holes emit a really large amount of gamma radiation, and going through one of these, even if you had the power to not be ripped to shreds by the black hole would involved going through unimaginable amounts of deadly radiation that, according to Stephen Hawking, would be impossible to survive through. [url=http://bigthink.com/dr-kakus-universe/is-time-travel-possible]Michio Kaku wrote a nice summary of it on his blog [/url] (cont.)

I think the biggest thing I would do is prevent 9/11 froim happening.. Hopefully that would prevent our country from going so much in the shit hole.

I like the alternate timeline approach. Time is like a line that is forever growing in one direction. When you go back in time, you branch off from the 'line' and into a reality where you had already gone back in time. So it would normally be like this >>>>>>>>>>>>>>>>>>>>>>>>>>>> Everyone and everything is going 'forwards'. By going back in time however, you're actually doing this >>>>>>>>>_>>>>>>>>>>>>>>> _____________>>>>>>>>>>>>>>>> Where everyone else who didn't go back in time is living in the original timeline, but you're in an alternate one where you had always gone back in time. Obviously you can never go back to your original reality because you'll get a paradox (if you go back in time to kill Hitler, you'll only be killing him in an alternate reality. The reality that you came from would still know of Hitler... otherwise you wouldn't have killed him in the first place)

You talked about two types of time travel. There's the type where every change you make changes everything immediately. You step on a butterfly, you've just erased yourself from existence. This type is confusing and stupid. The second type was strict-timeline time travel, where everything is set in stone. You kill baby Hitler, and the maid switches him with a different baby who turns out to be the real Hitler. Nothing can change because everything has already happened. This type makes more sense, but it's boring. Both of these theories have something in common - they each have only one timeline. In the former, it's fluid and malleable; in the latter, it's rigid - you can't change it. Now take the idea of a single, main timeline, and wrap it around itself like a big ball of wool. Throw in a bunch of other timelines and wrap them around it as well. Then add some yarn for good measure. You now have a ball of [i]time.[/i] If you fish around in the ball of wool, you'll find the little strand that you're currently on. Look, there it is. Say hello. With your time machine (you have a time machine, by the way) you can travel backwards or forwards along the strand of wool. Normally, being a rookie, you'd go back in time to kill Hitler, but you've misplaced your English-to-German dictionary so you decide to put that off. Instead, you decide to travel to, say, the Jurassic, so you can ride a T-Rex (which, by the way, weren't actually around back then). If this were strict-timeline time travel, you'd accidentally land in the late-Cretaceous, becoming the meteor that wiped out the dinosaurs, wrapping up that string nicely. Instead, you land right on target in the Jurassic, as a time machine that wipes out all the dinosaurs early. Fortunately for you, your time machine is shielded on the inside, saving you from the planet-wide destruction that you just caused (you absolute [i]jerk[/i]). Deciding to cut your losses, you head back to the present (or the future, or whatever). Now, if this were fluid single-timeline time travel (I really need a better name for that), you would have erased yourself and every other human from existence with your premature dinosaur nuking (which, on that note, again you [i]jerk.[/i] What did the dinos ever do to you?). But this isn't singular fluid timeline time travel, this is time as a ball of yarn time travel, so you don't start to fade away or poof away or whatever. Instead, you return safely to the present. Of course, it's not [i]your[/i] present any more. Because you see, when you annihilated the dinosaurs (jerk!), you dropped off the thread you were on, and onto a different, stranger thread. Things have indeed changed. Instead of humans, the dominant lifeform is, uh... chipmunks. Giant, intelligent chipmunks. With wings. And, uh, monocles. Because, although as far as you're concerned (due to your personal timeline) the year 2013 was not inhabited by giant, flying, classy chipmunks, you've now changed timelines, [i]so it is.[/i] Also, they're flying swastikas from their chipmunk-y buildings. Because, as Godwin's Law of Time travel clearly states: [i]any and all changes made to the timeline will result in the Axis winning World War II.[/i] Way to go. You just nuked the dinosaurs [i]and[/i] put national socialist chipmunks in charge. You absolute jerk! ... I hope that cleared things up.

I think if/when time travel is created/discovered/harnessed that it would only go back in time, and I think that we would either just have time reversed or be transported as our current selves back in time but our past self would still exist.