I had an idea about time travel possibilities, and have come to a convoluted theory on the subject. I'll try to write it down in the off-chance I'm not a loon after all. ;o) Keep in mind that I came to this while watching a "Back To The Future" marathon.
I think time travel is possible, but it's somewhat limited. Think of it as portrayed in the afore mentioned "Back To The Future" films.
In the film series, scientist, Dr. Emmett Brown, invents a time machine on November 5, 1985. He talks of going to the past to see events like Edison inventing the electric light; I believe this part is impossible, in my opinion. I don't think you can travel through time by using a device to a time when that device didn't exist. Likewise, even though the DeLorean first went forward in time earlier with the dog, Marty can never physically go back to a time before he left. This is probably wrong on several astro-physical levels, but this seems to be a logical reasoning to me. Or at least a good philosophical reasoning. I think there's a possible exception to this, but I'll expand on that later.
So in my version of the films, Marty & Doc could go to the future with no problem. However, even if set for Nov 5, 1955, the car would not go there upon reaching 88mph. However, if the DeLorean had been set to go to the future before the "Lybian terrorists" showed up at the "Twin Pines Mall", and the chase had ensued otherwise unchanged, Marty would have been transported into the future when the car reached 88mph. Now I just changed the film so that Marty made his now "forward" leap in time on Nov 5, 1985 @ 1:15:000...am exactly. I think that when he tries to come back early to "save Doc," in this new version, he won't be able do that. He can only go back to Nov 5, 1985 @ 1:15:00.000...am; the exact time that the DeLorean made its first jump through time.
Now for the explanation of my theoretical exception to my overall theory. It could be possible for Marty to go back to Nov 5, 1955, but similarly to the second movie in the series, it would be an alternate Nov 5, 1955, which would exist after time travel had been invented in that... and I'm not sure how to describe this... either an alternate Astral Plain, or alternate Physical Dimension. So, given this exception theory, Marty & Doc could go back to July 4, 1776, but that July 4, 1776 might not have the same events of our (or the time traveler's) past. Therefore, it's possible that if Marty had gone back to same date as he did in 1955, Doc Brown would've been the same as the film, with the exception that he actually invented the time machine with a '56 Chevy and first used it before Marty arrived from 1985. However, if Doc Brown of 1955 were to take Marty from 1985 to 1985 in a time machine Doc Brown built in 1955, the 1985 they travel to would be a future alien to both, because it's an alternate to Marty's 1985, and unknown to the 1955 version of Doc Brown. It starts to get kind of complicated from here, but I hope you get where I'm going with the exception. You can go to a previous date, but it won't be the same as the date you know/remember.
There's another part to this overall theory. Marty goes to a version of 1955, spends a week there and comes back to his version of 1985 just a nanosecond after he left, just as in the film. Then later in the day, after his return, he goes into the future with Doc Brown, as in the film. Marty and Doc can now return no earlier than when they first left together on *this* time-trip. Even if Marty made his second trip alone, he can return to no earlier a time than an instant after he left on the current time-trip.
I dunno. ;o)
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