I beg to differ. Obviously, they have not had the luxury of a starship and either a black star or just a regular old star to help them around that little Principle. There were two Star Trek: The Original Series (as opposed to Star Trek: The Next Generation, Deep Space Nine, Voyager or Enterprise) episodes that used the "slingshot effect" (aka the "breakaway effect") to travel through time: "Tomorrow is Yesterday" and "Assignment: Earth" (http://memory-alpha.org/wiki/Slingshot_effect). Not to mention Harlan Ellison's brilliant, but mangled for television, episode "The City on the Edge of Forever," but that used a time portal, which hasn't been addressed by Einstein (I don't think). Time travel in Star Trek (using only the original characters) was also featured in Star Trek IV: The Voyage Home (remember the whales?) and J.J. Abrams re-booting of the original characters in his Star Trek movie.
I know, I know. You're shaking your head and saying, "Missy, that's Star Trek, and Star Trek is not real." I know that. Most of the time. But, my point, really, is that by proving that time travel isn't real, those scientists have destroyed a favorite plot device in science fiction. I guess it would still work in fantasy and romantic fiction (Outlander, anyone?), but unless you're writing humorous science fiction, you have to give up your time travel plot.
Lots of books deal with time travel: the aforementioned Outlander series by Diana Gabaldon. The Restaurant at the End of the Universe by Douglas Adams (in which Arthur Dent and Ford Prefect find themselves marooned on pre-historic Earth with Telephone Sanitizers--among others.), Guns of the South), by Harry Turtledove and Timeline, by Michael Crichton. Of course, plenty of stories warn of the dangers of time travel: change things far enough back and you might not exist (also known as the Time Travelers's Dilemma) in your era.
We like to think of how we would travel back in time and fix the mistakes we've made--big and little ones. (I, for one, would have organized my house better and kept up with the organization so that cleaning it would just mean running the vacuum cleaner once in a while.) Of course, fixing those "mistakes" could have a profound impact on your life.
For example, what if I hadn't moved up to Maryland with my then-boyfriend/fiancé? What if I had stayed in Tallahassee, got my Master's in International Affairs? Well, presumably, I would've gotten a job either with the CIA, or with the DIA (Defense Intelligence Agency) or with Military Intelligence. I might only have moved as far north as Northern Virginia and might never have met my husband (yes, funny story, my ex-fiancé actually introduced me to my husband. Not that any of us had an inkling that life would turn out the way it did...), or had my two little boys. I used to wonder who I would be if my mother hadn't married my father. The answer is beyond my imagination. So, would I change that? Nope, not on your life. Although, maybe a letter from my future self telling me that the fiancé wasn't worth the tears would've been nice.
But what if you could travel back in time and just observe? You couldn't change anything, because you don't really exist in that time. Of course, you couldn't interact with any of the people you meet, so, no thinking, "Oh, I'd love to sit down and have a chat with......". When would you go? Would you visit a battle and see how it really unfolded? (Bloody and messy and not heroic--there are individual acts of heroism, of course, but the battle itself? Chaos personified.) See one of Shakespeare's plays in the Globe? See if Shakespeare really wrote his plays, instead of Christopher Marlowe or Francis Bacon? Watch your parents on their first date? (Ewww... a little too Back to the Future, ya know?) Watch Rome establish it's Empire (only if you have a fast-forward button--it took a while. Same with the Fall of the Empire.) See the Beatles' either on the Sullivan Show or at Shea Stadium?
I go to the Maryland Renaissance Festival every year in costume. It's the closest I can come to time travel. And every year, I'm thankful that I don't have to wear clothes like that every day (and I wear a "tavern wench" costume, not a Noblewoman's costume--comparatively cooler, but not a lot), and that I can get home in my car (as opposed to my feet, or a wagon), turn on the A/C (did I mention that those costumes are hot?), plug in my iPod, come home to my computer and watch my t.v.
Time to go aMuse myself and contemplate where and when I'd like to go.....
(This is actually the second time I had to write this. The first time, I was using spell-check and some how managed to delete this. My friend Leigh Ann's husband Joel suggests that I go back in time to just before I accidentally deleted this. He's a funny guy.....NOT!)