Time travel is a topic that has fascinated both the lay folk and scientists, fringe and real alike. Apparently it has also engaged Randall Munroe from xkcd to the point that he has drawn his best comic in a while.
Time travel is generally believed to be impossible. Indeed, the hypothetical tachyons may well exist, allowing - in theory at least - information to be sent back in time - but that's nowhere near the plotlines that the concept has allowed Holywood to explore. The implications of time travel, if possible, are however so bizarre, such a rife source of paradoxes, that it has captivated science fiction writers, phylosophers and theoretical physicists to the point that the topic seems almost exhausted. Randall managed to squeeze some very fresh and witty humour out of it anyway and also the discussion that this sparked was another source of fresh insights.
The obvious problem with time travel is that if you go back in time you can kill your father before he gets to screw your mother (sorry to remind about this, but your parents have fucked over a thousand times, no matter how hard it is for you to come to terms with the fact). If time travel is possible, then there are only a limited number of ways we can resolve this situation.
Everything you did to you parents has already happened by the time were born - so obviously you haven't managed to kill your father, even if you go back in time specifically to do that, if only to commit an extravagant suicide, flipping the bird on spacetime in the process. Maybe the doctors end up saving him but he's never told you the story? Maybe he had a twin? If this hypothesis is true, then maybe some sort of 'preemptive temporal natural selection' never lets people with such a tendency come about, as they have already killed themselves 'hyper-already', backwards in some higher dimension?
There is also a solution to this conundrum that will allow you to kill your father. Maybe you just jumped into (or even created!) another spacetime when you pulled the trigger on your dad. It is conceivable that all possible universes exist, with your father dying in countless ways and you time machine just jumped into another one, where he happens to get killed by his time-travelling brat. You might as well stay and live in his time and have a fulfilling life there. You'll be an alian from another universe but no one will know it, and hey - this will be an universe where your parents have never fucked!
Hell, you can even date your own mother without the nasty age difference that bugged Oedipus and you are pretty much guaranteed to be able to get laid with her, no matter how hot she's been in her time.
Back to the comic
Looks like the multiverse where the comic is set obeys to the first rule. The way I interpret it, it's not like the Black Hat Guy was dumb enough to kill Hitler in 1949, just before he commits suicide. It's more like Hitler was killed by BHG in the first place but he was cunning enough to make it look like suicide.
Popular culture seems to associate time travel with killing Hitler quite often. A jewish conspiracy, some will be quick to exclaim, but Hitler is indeed easy to take for an epitome evil, having perpetrated the most meticulously organized mass murder. He can quickly be identified as the most easy target to make this a better universe in one go.
Or can he? What could Stalin have done if Hitler wasn't around to oppose him? The soviet crimes against humanity account for tens of millions of lives - but that's only in the soviet-dominated areas. What if they have managed to spread their rule of murder across a much bigger part of Europe?
Also, the wikipedia list of mass murders reveals some shocking facts. The observation that there are cities in China that you've never heard of with more than 10 million ihabitants is curious - but bloody conflicts that took the lives of more than 30 million people - that's mind-boggling. Why havent' we heard anything in school about this? I mean, WTF, the An Lushan rebellion wiped out something like 15% of the world population at the time! And what about the Taiping Rebellion, which was as recent as the 19th century and the upper estimate of the death toll goes to 100 fucking million! This simply boggles the mind.
And now on international summits them whiteys keep talking about this 'human rights' thing. At no time in Chinese history, it seems, were millions of lives too high a price to pay to stay in power - and now G8 are bitching about thousands being killed in Syria. Of course Beijing is not going to object - this would mean that they agree that such a thing is wrong, as if they won't kill thousands of theirs any time of the day. And Moscow seems to be on the same bandwagon.
The Friesian School on Facebook
Now, away from mass murder, back to time travel - this fertile ground for paradoxes (like pretty much anything else) has been eloquently addressed by my guru Ross Kelley. As you can see from the last link his website is pretty decrepit, although it contains priceless analysis and opinion on pretty much any issue imaginable. He's never published a book, he's seldom posted in journals - he's agregated his life's work in this dodgy website, which actually contains tons of pure gold! It is, indeed, poorly presented and poorly organized but in terms of content it offers a lifetime of occasional reading. As there's pretty much no straightforward way to approach it, I just created a facebook page to occasionally post the gems I've spotted there over the years. Like it, and you'll get regular pieces of enlightenment on this confusing and overly complex world.
If you're eager to start exploring, you might as well pick a topic of interest and delve into it right now! Some essays may be quite long and dry, with countless facts and graphs, but every once in a while you'll be struck by the sheer amount of sense this guy makes!
Ethics, Philosophy and Epistemology