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Is time travel into the past impossible because it leads to logical paradoxes such as the Grandfather Paradox? If you traveled to the past you could kill your own grandfather. However, if you killed your own grandfather, you would not exist, and thus you could not have traveled into the past to kill him in the first place. Can these paradoxes associated with backward time travel be avoided?
Frank in Kansas
Dear Frank -
Time travel into the past is impossible because the past doesn't exist anymore - it turned into a series of present moments. You can access artifacts of the past, but you can't put the whole thing back together. (In quantum mechanics, certain particles can be seen as briefly traveling backwards in time, but these can also be seen as their anti-particles traveling forwards in time, and they don't give you any real leverage in accessing or changing the past.)
But that's the real world. Let's talk about time travel in movies and on TV shows. There's a simple solution to the Grandfather Paradox. (Most good SF writers are now familiar with this.) Every time you go into the past and mess around, you create a parallel world that splits off from our timeline beginning from the point you started messing with history.
For instance, let's go back in time and kill Hitler, which is really the main thing time travel is for. In the present day, a celebrity team is formed, sponsored by a large media corporation, which is making a movie about the expedition. Spielberg, Schwarzenegger, LeBron James, Dwayne Johnson, Dave Batista, Jennifer Lawrence, other celebs, athletes, Seal Team Six personnel, and their support staff travel back to 1936, where they add themselves to the U.S. contingent at the Berlin Olympics as athletes, coaches, and fans.
In and around the Olympics, they kill a bunch of Nazi big shots, humiliate Germany in athletics, and kill Hitler extra good. Or, acknowledging Hitler's leadership skills, they kidnap him, surgically implant explosives in his neck, and tell him to lead Germany in a non-evil manner. No world domination, no oppressing Jews and other people Hitler doesn't approve of, or they'll press a button and blow his head off. Maybe they give him a little tiny lobotomy while they're at it. Or put a behavior-modifying chip in his head. Hey, if they can time travel, they can certainly chip Hitler's head. Then they go to Japan, and get them to stop acting like assholes. Also, Mussolini.
So, World War Two and the Holocaust don't happen. Commerce and tourism are established between our present world and the world we messed with of the late 1930s. The large media corporation builds a couple of theme parks where people from our present can enjoy the peaceful world of 1938.
But our history hasn't changed. We still have our own miserable history. We just created a second world with a different history beginning in 1936. In this arrangement, we can visit that world as it moves forward from 1936, and people from that world can visit us as we move forward normally in time.
It's a pretty clean arrangement. And we can add dramatic touches. Maybe time travel tends to scramble your brain, so it's a squad of half-crazy people who have to kill Hitler. Or perhaps information leaks out of the heads of time travelers into the heads of anyone they come in contact with. So they have to complete the mission even as they're forgetting what their mission is. And what if some of that information leaks into Hitler's head?
So you don't need all that Marty McFly time-repair stuff. No matter what Marty does after arriving in 1955, he's created a second worldline. Why haven't we seen any second worldline stories on TV and in the movies? Probably because TV and movie writers have been slow to get the memo - they're TV and movie writers, with a unconscious conservative desire to have a time travel story end with few to no changes to the world, not hardcore SF and physics geeks who might appreciate a nice second worldline.
It's the same thing with stories of the future - they're mostly post-apocalypse, because that's the easy way to go, thinking-wise and production-wise. Just wipe out the world except for a few survivors in the desert a few miles off the road to Vegas - so easy.
Hope this helped. Now let's go kill Hitler.
Starting things off with one of the most common types of questions seen in advice columns - wrote a generic version myself.
Dear Dumbass -
I'm tempted to have an affair (at work, online, with someone I know from high school). I love my spouse, but he/she (has put on weight, is no longer interested in sex, is distracted by work or kids, had an affair of his/her own a couple of years ago). I could set boundaries, no one would have to know, etc., etc.
Hot to Trot in Pascagoula
Dear Hot to Trot -
First, get to couples' counseling. You seem to have some issues with your spouse, and that can be an advantage in the couples' counseling game - the person with the most gripes is often seen as the injured party, especially if the counselor is an idiot. Be sure to present your gripes in a considerate, loving tone, at least initially - you can get more worked-up as the session progresses. Let your partner get some gripes in, too - makes you seem reasonable. Your partner might appreciate that you're at least making the effort to go to counseling, and you could actually make some headway.
Second, if you're tempted to cheat or send dick pics to someone, don't. Instead, pleasure yourself to internet porn. It doesn't make you a loser - just about everybody does it. Stay away from stuff that's too creepy - it'll eat your brain. Do you really want to wreck your life and your family's and cost yourself your house and most of your savings and your career for some mediocre sex with an awful person? Lean to appreciate staying home and beating off after the fam is asleep. And no one wants to see pictures of your penis - no one except the guy who'll take your place in Congress after you're forced to resign.