The only way you might be unaware that today -- October 21, 2015 -- is the date that Marty McFly and Doc Brown time-travel to in "Back to the Future II" is if you literally go out of your way to avoid anything remotely related to pop culture. Hell, even "Good Morning, America" had an extended piece on it this morning, complete with a Delorean and
life preserver vest.
Thing is, some of us (read: tremendous dorks) have been looking forward to this day for years. We are the ones who, when someone posted photoshopped images of the time interface from the Delorean in years past, claiming that whatever day was shown in the altered image was THE day, immediately left scathing comments about the poster's genetic makeup.
As luck would have it, one of the aforementioned tremendous dorks (UprootedTexan) wrote to another of said dorks (me) a couple weeks ago with a question for the Bag. And, both because I knew this date was coming soon and because his question is wonderfully long and detailed, I decided to save it for today and use it for an entire Bag.
So, without further ado, lets jam some garbage in the Mr. Fusion and get on our way.
I recently watched the movie "Bill and Ted's Excellent Adventure" because I hadn't seen it in over a decade and I missed seeing Genghis Khan destroying a sporting goods store.
Anyway, this got me to wondering [two] things:
1) Which movie has the more plausible time travel physics: Back to the Future (1, 2, and/or 3) or Bill and Ted? Please provide examples of your work and reasoning.
Certainly, the answer is BTTF, if only because it attempts to give a mechanism (flux capacitor, requirement for a certain speed (roads optional after BTTF 1), tethered travel through time and relative dimension in space, etc.). Bill S. Preston, Esq., and Ted "Theodore" Logan travel through time in a magic phone booth, which doesn't even attempt to explain the "how" that allows for the travel, relying entirely on Rufus ex Machina. That's cheating, unless we assume that Rufus was the predecessor of Morpheus and Ted Logan grows up to become Neo, which makes the phone booth make sense, but changes the entire meaning of the "time" travel.
2) Which is the better time travel movie: Back to the Future or Bill and Ted's Excellent Adventure?
BTTF. Hands down. Specifically, BTTF 1 & 2, though I still prefer 3 over Bill & Ted's (Excellent Adventure or Bogus Journey, which was terrible). There's a reason that people care about today's date, and it's because those movies were actually good. The only people who care about Bill & Ted at this point are folks who have occasion to make jokes about San Dimas High School Football (and how it may or may not rock).
This also got me to thinking about the problems with infinite loops in the BTTF movies, specifically a point you made to me about how an infinite loop can't be self-correcting.
Every so often, I would ponder this quandary that you mentioned in that post; partly as work on my novel, and partly just as mental gymnastics (usually when the power goes out).
First to your point about timelines not sorta-existing, I think they can, actually. Think of the famous Schrodinger's cat experiment. He posited that, since we couldn't see inside the box, two separate states of existence occurred within--the cat was both alive and dead--until the box was opened and the two states of existence collapsed into a single certainty. The same can be said for the timelines in BTTF. Once Marty leaves 1985, the timeline he came from could exist in two different states: the original 1985 he left, or an altered 1985, and he would not know which was the case until he returned.
A somewhat superficial problem I see with this theory is that, in Schrodinger's experiment, no one could know whether the cat was alive or dead until the box was opened. That is, the box was literally sealed off from observation or interaction by anyone.
In the BTTF scenario, however, while Marty (and Doc and Jennifer and anyone else traveling through time) might not know which timeline existed in the place/time they'd left behind, the people in that timeline certainly would. To apply Schrodinger's theory would require that everyone who did not travel through time with Marty be left in a sort of suspended animation, unable to observe or interact with time until Marty returned. I think this goes well beyond the scope of time travel and puts Marty, et al., into a weird superpower realm beyond what the movie implied or intended.
To think about it in a slightly different way, consider this example: Marty goes back to 1955, saves George, saves the day, inspires Chuck Berry, etc. Everything he is doing in 1955 has a ripple effect on 1985, right? But there are billions of people in 1985 living in the future that Marty's 1955 actions have created. For Schrodinger's experiment to be analogous, we have to assume that Marty's return to 1985 causes the two timelines to collapse into one observable timeline; except we know that there's been a single, continuously observable timeline for everyone but Marty from 1955 to 1985. Marty's return to 1985 doesn't bring that timeline into being any more than my waking up each morning brings that day into being for everyone else.
A more concrete problem with the Schrodinger analogy, however, is that it sort of misapplies what Schrodinger was getting at. He was dealing with quantum superposition (e.g., how, on a quantum level, certain particles could be in two states at once and only collapsed into a single state when observed), and he was attempting to point out a problem with superposition by demonstrating that a large-scale (e.g., non-quantum) event could be dependent on a quantum outcome, such that, until you opened the box, you wouldn't know the quantum outcome. By extension, until you knew the quantum outcome, you couldn't know if the cat was alive or dead.
His 1935 article in Naturwissenschaften, entitled "The present situation in quantum mechanics," explained that he was not suggesting that a cat being both alive and dead was a serious possibility, but that the cat paradox was designed to show the absurdity of superposition as a theory.
Which, I suppose, is to say that even Erwin Schrodinger would probably disagree with the application of the superposition theory of quantum mechanics as being applicable to multiple timelines.
To your point about infinity loops not being self-correcting, I disagree that it's an infinity loop. If you accept that in the world of the movie the general theory of relativity exists, then the events of the first two BTTF movies would fall under the Novikov Self-Consistency Principle.
The principle asserts that if an event exists that would give rise to a paradox, or to any "change" to the past, then the probability of that event occurring is zero, making it impossible to create the paradox in the first place.
In the first movie, the probability of the paradox of George and Lorraine not meeting and marrying is reduced to zero when Marty goes back to 1955 and changes events so that George and Lorraine do what they're supposed to do at the Dance. Once Marty fixed the paradox, he returns to 1985; the superposition of all possible timelines then collapse into a single timeline where the original course of events between 1955 and 1985 happened (because what Marty did in 1955 is what was supposed to happen and how it was supposed to happen). Because Marty would come back from 1955 to the exact moment he left, no time would actually pass between when he left and when he returned in 1985, so nobody in the original 1985 timeline would be aware that anything was different. Since Marty returned to the exact moment which he left, it closes not an infinity loop but a closed timelike curve, which, according to Novikov, is guaranteed to be self-consistent.
Same thing in the second movie; the probability of the paradox of Biff giving his younger self the almanac and creating 1985-A is reduced to zero when Marty and Doc go back to 1955 and destroy the almanac. While Doc and Marty are in 2015, Biff goes back to 1955 and gives his younger self the almanac. Theoretically, it should alter time from that point forward. But since Biff's temporal transgression would be a paradox, the Self-Consistency Principle states that the probability of the paradox occurring is zero. So between that moment in 1955 and where Doc and Marty are in 2015, something happens to nullify the paradox (namely Doc and Marty go back to 1955 and destroy the almanac). Granted, if the paradox is stopped in 1955, it kind of throws a wrench into how they wind up in 1985-A. The only thing I can figure is that since they are the ones who are supposed to fix the paradox in 1955, if they come back to 1985 and see everything was status quo, they wouldn't go back to 1955 and actually fix the paradox. Once they fix the paradox, they return to 1985, and since the paradox was corrected, and they arrive at the exact moment in time they left, they close the closed timelike curve and time progresses as normal.
The theory does essentially argue that events are predestined but not totally predestined; in that you can go back and change things, but because of the paradox, something else will occur in time which will nullify the paradox and revert time to the way things originally were.
I think if BTTF did any sort of research into theoretical temporal physics, they were banking heavily on Novikov's Principle.
Anyway, that's all I've got.
This is interesting, and, broadly, I think you're right that any research into temporal physics in the making of the BTTF movies relied on a general application of Novikov's Principle. I'm just not sure that their application of it was correct, so I'm not convinced that it answers all of the questions.
Novikov's Principle, which was expounded upon by other theoretical physicists and similar uber-geeks over the years, holds that if an event exists that would give rise to a paradox or would enact any change to the past whatsoever, then the probability of that event is zero. The classic example of this, of course, is that you can't travel back in time and kill your past self, since that would have made it impossible for you to go back in time in the first place.
BTTF 1 violates this almost immediately upon arrival in 1955, however. When Marty pushes George out of the way of Lorraine's dad's car (Three fun facts: (1) Lorraine's dad was named Sam; (2) his car was a 1953 Chevy in Surf Green; (3) I knew facts 1 and 2 without having to look them up), he has already created an event that would give rise to a paradox. That's why his siblings start disappearing from the picture. If it were guaranteed under Novikov that Marty was going to save the day and get George and Lorraine together, Dave and Linda would not have started to disappear, because it would have been impossible for Marty NOT to fix the problem in 1955, meaning Dave and Linda would have been born on schedule in the future.
Now, if we allow for making the story-telling more interesting, one could argue (as you do) that the film makers took a little creative license with the application of Novikov's Principle, solely for the purpose of drama. That's plausible, but it's still a problem. Marty saved George from getting Chevy'd in the face on November 5, 1955. He did not go see Doc Brown until November 6, 1955. And it's not until November 12, 1955, a full week later, that the Enchantment Under The Sea Dance takes place. Meaning, for a full week, the future was altered because of Marty's interference on November 5. (We can see that the future was in fact altered by Marty's own hand starting to disappear while playing guitar at the dance.)
(An additional problem is the fact that the changed future was allowed to hang in limbo for a week, with Dave, Linda, and Marty slowly disappearing. Even from a story-telling perspective, that's a little silly; either George and Lorraine got married and had those kids, or they did not. Once Marty changed the timeline, if Dave was going to disappear, all three kids should have disappeared immediately, including Marty. You cannot sort-of exist in the future -- you either do or you don't.)
But the larger point here is that Novikov says that an event that would change the past cannot happen (i.e., its probability is zero). Marty saving George was an event that absolutely changed the past, at least for a week. Meaning, (a) that the probability of Marty saving George was greater than zero and (b) focusing on the ultimate outcome (George and Lorraine's kiss/marriage/farting out babies) only gives the illusion that Novikov's Principle was adhered to. In reality, even after George and Lorraine got together, Marty's actions had changed the past, which we can see when he returns to 1985 and Biff is subservient to George, Dave and Linda aren't depressing a-holes, George is a famous author, etc. So, even with the seemingly Novikovian corrections (George punching Biff, George and Lorraine still getting together on 11/12 instead of 11/5, etc.), we did not wind up right where we started. We ended up in a better place than we started, which is all well and good, but it also allows for the possibility that you could wind up in a worse place than you started, both of which mean that you can go back and change the past and thereby alter the future.
This same sort of analysis applies in BTTF 2 as well, though that movie is more faithful to the dramatic-film version of Novikov, since they wind up more or less at the same spot they left from (though it's still the better 1985 from the end of the first movie). The thing is, much like with the discussion of Schrodinger's cat above, Novikov was only satisfied if we assume that the perspective, reality, and timelines of everyone else in the world are suspended as Marty travels through time. Otherwise, you've got a hypothetical guy--who we'll call "Scott"--in 1985 whose life at the beginning of BTTF 1 is, say, as George McFly's boss. By the end of BTTF 1, Scott has never been George McFly's boss. During BTTF 2, in the "dark" 1985, it's entirely plausible that Scott is now working for Biff (though he theoretically could have been George's boss at some point prior to George's death). By the end of BTTF 2, Scott is probably not working for Biff in 1985, but he has also never been George's boss, but George is now alive. So, from Scott's perspective, his past has been altered multiple times (though he would be unaware of the changes), and Novikov's Principle was violated.
I suppose that's my biggest problem with apply Novikov to the BTTF world, actually. The Novikov Principle treats the paradoxes in a vacuum, where I am not allowed to go back and kill my earlier self, but it says nothing about going back and killing someone else's earlier self. (Unless the purpose of going back was specifically to kill that person, since it would be impossible to know that person existed in the future if he were killed prior to doing something that made him need to be killed.) Yet, while going back and killing Peyton Manning when he was just a 4 year old with the forehead of a 12 year old would not technically change my past, it would change the past for many, many other people (including Peyton). In theory, unless all you did was go back and observe, without interacting in any way with anything in the past, the mere act of traveling back in time changes the past and would appear to violate the Novikov Principle, wouldn't it?
ANYWAY...I'm reasonably sure that only about 3 or 4 people actually read this far, so I suppose I'll make like a tree and get out of here. I will say, however, that these are fascinating questions, and I enjoyed thinking about them and writing these answers far more than a normal person should.
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