A thread on “stuff people get wrong in movies” engendered an enthusiastic discussion of time travel, which didn’t belong there, but seemed like it might want to continue. So I’m moving it here.
I’ve decided that the only way time travel to the past can possibly exist, whether in fiction or reality, is that when someone or something is sent into the past, an alternate timeline immediately comes into existence.
Let’s say some scientists are working on a time travel device. To test it, they decide to send a blue widget 5 minutes into the past. With no alternate timeline, they would see a blue widget suddenly appear 5 minutes before they start the test. Success! But now they have a dilemma similar to the grandfather paradox. Either they go through with the test 5 minutes later, and create a closed loop in which the scientists at the T-5 mark would actually have gotten infinite blue widgets, or they don’t perform the test, and create a time paradox.
As opposed to an alternate timeline scenario. At T-5, the scientists see a blue widget appear. They know the machine works. They can choose to go through with the test, in which case they create another alternate timeline in which the scientists got two blue widgets, or choose to not perform the test, knowing that it works and they created the alternate timeline they are now part of.
Or at T-5, they see the blue widget appear, then 5 minutes later, send that blue widget back in time - no proliferation of widgets takes place, but one blue widget is created out of nothing for the 5 minute duration.
Except the problem with this one is that, by handling the blue widget during that 5 minutes, they cause a tiny amount of wear to it, so the widget they send back is 5 minutes older and more worn out than the one they received, which I think means it wears down to nothing instantly because it’s a loop
Wouldn’t there be 2 blue widgets existing during that 5 minute time period? The one that is 5 minutes older that instantly appeared, and the one they had all along they were planning on sending back? And once they send the one they were planning on sending back, back, they would be down to just the one widget again, which would have only one cycle of an extra 5 minutes of wear on it, not an infinite loop of cycles.
ETA: Or instead of widgets, let’s say they are watches, the kind that don’t sync their time with the internet. They have a watch that they plan to send back from 1: 05 to 1:00. At 1:00, a watch appears that says 1:05. At 1:05, the watch that appeared now reads 1:10. The other watch, the one that reads 1:05, is placed in the time machine and sent back. Then going forward, there is only one watch again, which reads a time that is 5 minutes ahead of the actual time.
Yes, you’re right that the “Somewhere in Time” pocket watch paradox doesn’t really apply in that scenario, since the scientists would have already had the blue widget ready to be sent at T-5, when they would have gotten another blue widget.
But the closed loop scenario would still apply- they now have two blue widgets. 5 minutes goes by and they have to send one of the widgets 5 minutes back in time, in which case a closed loop of infinite widgets is created, or not send either of the two blue widgets back and cause a time paradox.
My ETA to your ETA: But how would the second watch disappear? There’s really a paradox either way, because a starting point is assumed in which there’s a first experiment where there is only one watch to send back. But if the test had worked, (assuming no alternate timeline), there’s always a second watch appearing at T-5. And since a closed loop would be created, thre would actually be infinite watches proliferating. I don’t understand how the second watch would disappear. But then, time travel is complicated 
No, just one, the one they sent back. Why would there be infinite?
No, they get the blue widget out of the closet where they’d been storing it, and send that back. So for those five minutes, there are two instances of the blue widget in the lab, one slightly older and more worn than the other.
You do realize, I hope, that the trope of the Holy Grail dates back long before there was ever any such thing as any movie?
There’s two blue widgets for five minutes. When the one is sent back, it disappears into the past, leaving the one that appeared five minutes ago. This leaves you with a total of one widget, balancing the equation.
BUT
For five minutes, there are two widgets. There is one widgets worth of matter that appears to have been created anew in the universe.
I explained when I added an ETA in my previous post, but sending the blue widget back in time creates a paradox either way. They already had the blue widget all ready to send at T-5. They send it back in time 5 minutes later, and there’s now 2 blue widgets at T-5. Then they either send it again, and there are 3 widgets at T-5, and so on, creating a closed loop paradox; or they don’t perform the test and there’s another kind of paradox.
Is my logic wrong? You’re Chronos, so if you see a flaw in it, I’ll take your word for it 
I think you’re counting arrival of a widget as addition without counting sending of a widget as subtraction.
If they send it back an infinite number of times, then an infinite number of widgets arrive. But why would they do that? That sounds like an awful lot of work, and I don’t think my grant has funding for an infinite number of time-travels. What if they only send it back once?
But back to the thread, another one: In the Duel of Wits in The Princess Bride, Vizzini’s “logic” of “Clearly I cannot choose the glass in front of you” is meaningless. He’s not trying to reason out where the poison is. Rather, as he presents his “logic”, he’s watching Wesley’s reactions, and indeed, his reactions are different when Vizzini is concluding it’s in one cup or the other. From there, Vizzini just has to figure out whether Wesley is bluffing or not (or more precisely, whether he’s bluffing an even or odd number of times), and he’s very good at that.
Of course, Wesley is a very tough nut to crack, and he can tell that, too, so he does the cup-swap just to be sure.
I think there would still be only 2 during that 5 minute period. If it does get tricky (assuming we ignore the conservation laws), it would be if they get mixed up and instead of sending the original widget, they send the one that appeared at T -5.
Again, they get a blue widget sent back at T-5. Now they have two. 5 minutes later, they have a decision to make-- either they don’t do the time travel test, in which case, how did they get the second widget, or they go through with the test, in which case there are now 3 widgets at T-5, then they perform the test again 5 minutes later and there are 4 widgets, and so on, in an infinite loop.
But if at any point they choose not to do the test, how did any of the blue widgets get sent back in time? As I see it, it’s either a closed loop paradox or a grandfather-style paradox.
No, two, the one they had in the closet, and the one they sent back.
Take a piece of string, hold in it your hands, and make a loop in it. How many segments of string are in the loop?
But they have the two widgets, the one in the closet, and the one that appeared 5 minutes ago. Their past has been altered to have two widgets at the T-5 point in their recent past, the one in the closet, and the one sent back. At test time, they can either go through with the test, which to them is the first time they performed the test, and then alter their past again to have 3 widgets at T-5, the original and now two sent back, and so on. Or they stop the test and create a grandfather paradox.
It’s discussions like these that effectively prove the arrow of time is one directional in real life.
That’s why I said upthread, when this discussion started, was that the only way time travel to the past could possibly work is for there to be an alternate branching timeline created at the moment someone or something is sent back into time.
It’s interesting to me that Neil DeGrasse Tyson says that the only movie that even comes anywhere close to an accurate portrayal of what time travel would involve is “Back to the Future”.
The BttF movies were fun, but I’m surprised to hear Neil DeGrasse Tyson say that. BttF is full of logical paradoxes that make no sense at all if you think about it for a minute or two. For example, Marty keeps his parents together, but changes them so radically that they are very different people-- happier, cooler, more successful. Yet Marty still exists in the alternate future. The exact chain of events that led to the one sperm and egg combo that led to Marty still happened, despite how different his parents’ lives became?
BTW, I’m starting to come around on @FlikTheBlue and @Chronos’s arguments in my ‘blue widget’ scenario. In their view, the past in the first iteration is altered so that there are now two blue widgets at T-5, the one in the closet and the one sent back, but in subsequent test loops the blue widget that is sent back becomes the one that had already appeared in the past, it doesn’t proliferate into infinite widgets generated. I still think my scenario makes sense as well, as much as time travel does at all, but I get their viewpoint. In any case, a closed loop paradox is still created-- the second blue widget has still been created out of nothing, just like the ‘Somewhere in Time’ pocketwatch.
He should watch the film again.
The biggest unsolved mystery is, what happened to “other Marty”? The one that grew up with cool parents, had a Toyota in his garage, and probably knew how to drive? He probably didn’t wreck the Delorean by driving into a barn. So, where (when?) is he?* And can he return? Did Doc have extra plutonium in the car? He knew what was coming.
And if he does return, I assume he’s going to want his life and girlfriend back!
Marty, the one we follow through the film, is a ghost person. He has no connection to the world at the end of the film. He was not born from that particular Lorraine. He never lived in that particular house. There are probably a million things he doesn’t know about the world. Forget the Toyota, can he even be sure his locker is the same?
And as for blue time traveling widgets, in Superman, there are two Supes existing at the same time. The “original” one out stopping the missile and the time traveling one at the same time saving Lois. Eventually, the first is going to show up trying to save Lois, and be totally surprised that…he already did. But, he’s not the same Supes! The time traveling one remembers her dying, the “original” does not. They are different people.
And now what happens? We have two Supes. There’s no time travel path that gets rid of the dupe supe. There’s no loop to close. Now, like the case of Other Marty, we have two Supermen (Supermans?). We do we do with him? Them?
*my fun theory? He is “George”. He accidentally killed George and took his place (in this reality, they look much more alike.) He’s his own father! Talk about zombies…