First of all, I apologize for the title of this article to anyone who has already picked up on these things. I usually cringe when I see titles like this one, mostly because they are usually things that everyone already knows. It can be infuriating. That said, most people really honestly don't get the things I am about to discuss. It isn't really because they aren't paying attention or don't notice the details, but this isn't the usual "Things You Didn't Notice" article. We aren't going to be talking about plot holes, or movie mistakes. We're going to be talking about physics. Most people aren't really up on the practical physics of time travel, mostly because it is very obscure theory and the vast majority of the populace are only familiar with time travel theory from movies, and movies mostly get it completely wrong. To the credit of Back to the Future, they get the science more right than almost any other movie. Also to the movie's credit, I don't think the things they got wrong were oversights, I believe they were concessions they deliberately made for the sake of plot, which is absolutely acceptable, and the movie does a great job of telling you the story so sincerely that don't even think about where they get it wrong.
First off, as I already mentioned, Back to the Future gets the physics of time travel more right than pretty much any movie. Yes, ladies and gentlemen, for the most part Back to the Future is hard science fiction, believe it or not. I think it is because of everything they get just so right, especially in the first film, that when they do get something wrong it just really annoys me. Not enough that I can't enjoy the movies, I love them, and as I said, I understand why they overlooked some things, even to the point of violating the internal logic set up in the first film. They got things so right some times that they had to fudge those same things later or they couldn't tell the stories they wanted to tell. Unfortunately, hard science can be downright inconvenient when trying to write science fiction. But since BTTF did get so much right, we should acknowledge what they did get correct before completely shredding them on the annoying stuff.
The biggest thing the movies got right was in knowing that it would take a tremendous amount of energy to time travel. An almost unattainable amount of energy. Unfortunately, they were wrong on the amount of energy (1.21 Gigawatts ain't gonna get 'er done), but mad props for knowing that the key was energy. Interestingly, in the original script, they had Marty drive into the heart of a nuclear explosion, which would have been sufficient energy. They had to change it for budgetary reasons. Another thing they got extremely right was in that time travel is most probably achieved through wormholes. Look real closely at the DeLorean when it hits 88 MPH. If you notice, some lightning bolts shoot from the car and create an opening in front of the car. The car is creating a wormhole! It is never explained, for obvious reasons, but the car has some sort of wormhole generator that allows the car to travel through time. In 1985, and especially for a movie, that was a huge thing for them to get right and is such a small thing, most people don't even realize exactly what is going on. For those in the know, however, it gives the movie so much credibility.
One of the earliest things I noticed, and one of the most prominent, is that the entire plot of Back to the Future Part II shouldn't exist at all. What do I mean, you ask? Well, to explain, we must look at something the first movie got right. In the mall parking lot, when Doc conducts what is presumably his first time travel experiment, he sends his dog one minute into the future. They take the time to establish that his watch as well as a watch he sends with the dog are in perfect sync before the experiment and that the dog's watch is one minute behind Doc's after the experiment. Doc even explains, "He skipped over that minute to instantly arrive at this moment in time." Are you with me yet? In BTTF2, how can Marty go with Doc to the future and see his kids or even himself as an old man? If you have ever thought of going to the future and seeing how you turn out, I hate to break it to you, but you can't do that. When the dog went one minute into the future, where was he? He was gone, right? Because he skipped over that time to arrive in the future. Marty wouldn't exist in 2015. History would record that Marty and Doc disappeared without a trace on October 21, 1985 (the day they left for the future). Doc and Marty (and Jennifer, for that matter) would have skipped over those 30 years and would not have been there to live their lives, get married and have children. When the police scan Jennifer's thumb print in the alley, they should have been like, "Holy crap! This woman has been missing for 30 years! We finally found her!"
Why does Marty's brother and sister disappear from the photograph? For that matter, why does Marty begin to fade from existence while he is on stage playing "Earth Angel"? Well, the answer to that is to give some sense of consequence and to give a visual representation of the changing of the timeline. Unfortunately, that isn't how it works. When a time traveler travels through time, he exists outside the flow of time. Essentially, the time traveler, as well as anything the time traveler brought with him (such as a photograph of his brother and sister) would be immune to any changes in the timeline. In plain English, if you change the past and prevent your parents from falling in love, yes you will return to a timeline where you don't exist, however you will still exist. Marty is from a timeline where he does exist and he would continue to exist as that person from that (now alternate) reality. And if Marty does fade from existence when he changes the timeline, why doesn't Marty's memories change so that he remembers what is going on in the new alternate 1985 he created? Or in BTTF2, when Old Biff changes the timeline to make himself rich and powerful, why don't Marty and Doc's memories conform to that timeline? Why does Doc have to look stuff up at the library and then explain it to Marty? It's one or the other, movie!
People often get the physics of time travel wrong. That's not surprising, time travel physics is confusing sometimes, and it's made even more confusing by people who don't quite understand it. Even The Big Bang Theory, which has a very smart physicist who does very well at making sure the science is usually right, got some major things wrong when trying to explain a BTTF paradox. First of all, you have to understand that the idea of a time paradox really doesn't exist. The 2002 remake of The Time Machine tried to address the idea of a self-correcting timeline by stating that if you build a time machine specifically to go back in time and save your fiancee from being murdered, you will be unsuccessful because she has to die for you to be motivated to invent a time machine. If she doesn't die, you don't build a time machine to save her, so she still dies. This is WAY more complicated than the reality. As I stated before, if you change the timeline, you are still the you from your original timeline, where she dies and you invent a time machine to save her. So yes, you can save her and no you have not created a paradox. So back to The Big Bang Theory, our heroes are arguing the paradox of Old Biff giving himself a book that foretells the future to make himself rich. They are basically arguing that any reality where Biff needs to steal the time machine is a reality where Biff never got the sports almanac and never became rich, therefor Biff never stole the time machine and went back in time. Ugh! No! Again, this is actually more complicated than the actual truth. Now, they were right that Old Biff would not have been able to return to the original 2015 and return the time machine from where he stole it, because from his perspective that 2015 would no longer exist, however there was a precedent for there being a delay in the changes of the timeline (at least withing the internal logic of the BTTF movies), because Marty didn't disappear the instant he prevented his dad from being hit by his Grandpa's car. It's wrong, but it is consistent with the other wrongness of the film. The guys of TBBT weren't wrong to question this scene, but their analysis was completely wrong (and Sheldon should have known better).
By Back to the Future Part III, they were pretty locked in to the internal logic of the paradox thing, so this next thing really needs to be forgiven, but it's still wrong scientifically. Why does Marty go back the day after Doc writes him the letter, giving himself only a few days to save Doc from Mad Dog Tannen? Obviously, the answer the movie was going for was because if they went back before Doc wrote the letter, then Doc would never have had the need to write the letter, Marty would never have known what happened to Doc and he would have been stuck in 1955 forever. Of course, this fails on a couple of different levels. First of all, Marty is from a reality where Doc did write the letter, so whether Doc writes the letter or not is irrelevant. Marty should go back to January 1885 and then they would have eight full months to get Doc back home, and he could even have told Doc about the horse shoe incident and Doc would have either made sure not to shoe Tannen's horse (which in itself could have caused Tannen to murder Doc) or he could have made sure the horse wouldn't have thrown its shoe. But even if you take into account that Doc not writing Marty the letter would created a paradox, why couldn't Marty have gone back earlier and told Doc to make sure he writes the letter so that past/future/alternate Marty would still be able to know where Doc went? Of course, the idea of Marty and Doc having only a few days to get back home creates suspense and drama, and without these things we don't have a movie.
I love the BTTF movies. They are classic cinema (yes, all three of them). As I stated, BTTF gets the physics of time travel more right than any other movie ever, and the things they got wrong really do seem to be deliberate things changed to fit the narrative they wanted to tell, and that really does have to be forgivable. But I thought it would be interesting to have a discussion on film vs science of time travel anyone. I hope you were thoroughly entertained and maybe even learned a little bit.