Are you telling me that this sucker is nuclear?

The last time I watched Back to the Future, I made an exciting discovery. "That's your mug!" my roommate exclaimed as the camera panned across Doc Brown's untidy garage. A white Pyrex mug sat on top a stack of papers just above Einstein's overloaded dish of canned dog food. She was right; I have two mugs and four teacups in the "butterfly gold" pattern Pyrex manufactured in the 60's and 70's. I bought them at a garage sale for 10 cents each. If I had any idea of getting rid of them, it was gone now. "I have Doc's mug!" I thought gleefully.

In my spare time I like to meditate on the complexities of the space-time continuum as presented in the Back to the Future trilogy. So did Derek Webb when he was my age, judging from this journal of his from 1998. Here Derek exposes what may be called a “fatal flaw” in the logic of BTTF. He begins with a recap of the opening scene in Part II, in which Marty, Doc and Jennifer travel to 2015. While there, Marty buys a sports almanac with the idea of taking the statistics back with him to 1985. When Doc discovers this, he warns Marty that the time machine was not invented for the purpose of gambling and throws away the almanac. An aged Biff retrieves it from the trash and uses the Delorean to deliver the almanac to himself in 1955. He then returns to 2015 and slinks away. When Doc, Marty and Jennifer return to 1985, they find all the changes that have taken place since 1955 Biff was empowered with the almanac (i.e. rampant wickedness). Derek continues:

doc puts together what happened...now, this is important: marty suggests that they simply go into the future and make sure old biff doesn't get the book, so that way he can't go back and give it to himself. doc then correctly points out to marty that if they go into the future of that point and time that they will be in the future of THAT POINT AND TIME. in other words, there's no point in going into the future from where they are, because in reality biff is already powerful and possesses the almanac. doc shows with a diagram how when old biff gave himself the sports almanac he altered future reality, and it skewed off into a tangent reality which he labeled "1985 alt." rather, doc and marty would have to find out when exactly young biff (from 1955) received the sports almanac, go back in time to that point, and keep him from getting it. interestingly, doc then says they'll have to let old biff (from the future) think that he succeeded in giving himself the almanac so that he'll leave with the time machine and return the vehicle to 2015.

all the pins are now in place; if i may, allow me to knock them down.

…if the rules that doc brown set up in the above scene are true, then doc and marty would have been stuck in 2015 trying to get the young jennifer and get back to 1985, and none of the rest of this could have happened…here's the problem: ...once old biff got to 1955 and successfully gave himself the almanac, there's absolutely no way that he could have gotten back to where (or i should say WHEN) he started. he would be trying to do exactly what doc brown just told marty that they couldn't do. at the point where the biff from 1955 possessed the almanac, old biff had altered future history. he created an alternate reality that would at that moment be in operation. if he tried to get back in the delorean and go back to the year 2015 where he was a poor defeated old man, he would've be able to do it. he could only go into the future of the alternate reality that he had created the moment he handed himself that book.

i hope you are beginning to see the problem... doc, marty, and jennifer would either be stranded there in the future with no transportation home, or disappear from existence altogether (a threat that we saw in the first movie with the picture that marty had of himself and his brother and sister on vacation).

It's an excellent point: once a change is made, there is no going forward to the circumstances you left. We see a positive example of this in the first movie, when Marty returns to 1985 to find his father's annoying laugh erased...from existence. Yet the point raises more questions on the nature of time travel:

Could the Delorean be classified as a sort of cloning device? Example: Marty watches himself outrace terrorists at the Twin Pines Mall. Has Marty's DNA been replicated?

And the converse: how does the process of being erased from existence occur? We see Marty's siblings slowly fade from a photograph and Marty himself begin to disappear while playing the guitar. If Marty had been erased, would not the traumatic experience of watching that happen have had further life-altering consequences on the people at the Enchantment Under the Sea dance?

And finally, why does Doc Brown look the same no matter how old or where in time he is? Has he discovered a process for halting or slowing down the aging process, which he keeps from Marty? Did he share it with Principal Strickland, who also seems not to age?

If you have any insight or ruminations on these or other time-travel questions, by all means—do share.