Hopefully this doesnt come across as being arrogant, just seems for whatever reason people are seeking me specifically to answer time travel related questions so i figured I'd put the common ones on one thread
1. What is the 'ripple effect'
The ripple effect is simply put changes the the timeline caused by events during time travel. Usually the ripple effect goes forward; meaning events caused by time travel change the future but the ripple effect can go backwards and has happened at this point in the timeline; changes made during 1931 time travels, prevented clara from being saved in 1885 hence the ravine being named "clayton ravine" in the FCB timeline
2. Does the ripple effect affect human memory?
It does for everyone EXCEPT those traveling through time when the ripple is triggered; so marty is the lone individual who remembers the very first timeline of biff being his fathers supervisor and crashing Georges car as he was the lone time traveler when that timeline was changed. This is also why 1985 Doc (in 1885) seemingly has no memory of his second visit from marty in 1955 including sending marty back to 1885, putting the silly pink clothes on marty, and finding his own tombstomb. So at the end of the trilogy, doc would have remembered marty's first visit to him in 1955 from part I as doc was not time traveling at this time but he would not remember the visit from the beginning of part III
3. How can Marty and Jennifer go to the future in part II and see their future selves? Why doesn't this happen to einstein on his maiden time travel?
First off there are 3 cases of time travels to the future.
i) Einstein goes 1 minute into the future and does not encounter another version of himself as he never goes back to relive this minute
ii) Marty, Jennifer, and Doc go to 2015 where future versions of the former two are present. The reason the future versions of Marty and Jennifer exist is that the future is always the most likely future from the time you left. Doc had every intention of returning Marty and Jennifer to 1985 and this is never in jeopardy so this is why they exist in the future.
iii) Marty and Jennifer spend 3 hours in 2015 and return 10 hours after they left and are thus technically still in the future but other versions of them do not exist as it is never intended for them to relive this time
4. Why did Biff return the time machine to doc and marty in 2015?
Most of what we know about biff is his 1955 self not being too bright. He does seem to get somewhat wiser by 1985 so it's possible by 2015 he is fairly wise. Biff does figure out that it is indeed doc brown and marty from the 1980s riding in that delorean. Biff also notices Marty Sr. dressing like Marty Jr. and may clue into the fact that Doc brought Marty Sr. to the future for the direct purpose of impostering his future son. But if Marty Sr. were stranded in 2015, then he'd have no son to imposter at that time and thus doc would never bring the delorean there for biff to steal. You could also argue that the chest pains Biff has when returning were happening to him in 1955 so he decided to go back to the future due to his illness.
5. What is the significance of the ravine names?
The ravine in which the train goes over at the end of part III has had 3 names: i) Clayton ravine in the original timeline prior to time travel to 1885 in which Clara dies going over the ravine ii) Shonash ravine which is the ravines original name and would have remained the name in the timeline in which doc is killed in 1885 without marty being there iii) Eastwood ravine in which Marty (under the alias Clint Eastwood) is presumed to have died in the train going over the ravine. The name of the ravine at any given time would indicate which timeline occured in 1885
6. What happens if there is a paradox?
It is unknown as there has either never been a paradox or there has and the time line survives; one theory is that every time the ripple effect occurs, a new timeline is created and any time travelers would come from prior timelines. So for instance had marty failed to get his parents back together in part I, he would have faded out but the timeline would still show Marty show up for a week in 1955. This would be the only point in this timeline he exists. Another theory is that the timeline essentially has a mind of its own and would not allow itself to be compromised to that extent. We have seen examples of similar events happening in different timelines (Joey ending in jail, marty burning the rug, jennifer falling asleep on her porch).
7. Explain exactly what happens at the end of episode 2;
The delorean crashes into the billboard while flying. In this time line, the delorean has not been given a hover conversion which may have been why it crashed and it has not been converted to a time machine hence why the time circuits and flux capacitor are blinking. These previous two facts are up for debate as some feel marty crashed it and the fact that the tire still hovers. It is possible the parts are still there as they would still exist (as would the delorean) but not to the extent of being put together.
8. Why do doc and einstein disappear and not marty?
Some people feel that doc disappears so he can become FCB but this is false. The reason is that in this timeline doc, marty, and einstein do not travel through time. For marty this isn't a big deal since he only spent a few weeks of his life away from his natural timeline but doc spent at least 20 years away from his timeline and einstein spent at least 10. Let's assume doc spent 25 years on his time travels. His disappearance would imply he dies by 2011 in this timeline (25 years after 1986) hence why would disappear since he does not live to the age he was in the previous timeline.
9. Can there be two versions of the same person in one timeline?
This is never confirmed nor denied. We dont see it but it is possible there is another marty in swizerland in the biffhoric timeline, one out of town in the brief alternate timeline in episode 2 and one at math camp in episode 3.
10. Did the previous time travels get erased in the alternate timelines?
Biffhoric timeline; No. We do see Martys time travel from part I still happen and old Biff in 1955 meaning the 2015 time travels still happened
episode 2 alternate timeline: Probably not. The delorean still flies so that would imply the 2015 time travel still happened. Biff and his brothers do reference george punching biff out in 1955 (which only happens when marty travels there) but curiously indicate that Marty causes Biff to crash into A manure truck even though in the trilogy, Biff crashes twice. This could mean the second trip did not occur, biff hides the second crash, or that they mean Biff crashed into the same truck twice. Also it is apparent when Doc and Marty go back to 1931 that their previous trips to 1931 still happened.
FCB timeline: yes and no. The ravine is called 'clayton ravine' so that would imply the 1885 trip did not happen. There are no references to the trips to 1955 so those may have been erased (especially since FCB would remember them). We do not see the delorean flying (though FCB would not know how to fly it or the hover conversions were damaged with the bill board crash and marty removing one of the tires) so it's possible the 2015 time travels did not happen. But there is ample evidence the previous 1931 travels DID happen; Old doc and marty are still in FCB's photo and there are plenty of references to the events of episodes 1 and 2 during episode 4. The likely explanation is that the First Citizen Brown timeline is only possible through Marty and Old doc ending up in 1931 so that time travel HAD to stay intact.
11. Which timeline is considered the current one at the end of episode 4 and begging of episode 5?
Most likely the FCB timeline but this is unconfirmed. Let's review the facts we know
-Emmet and Edna are apart NOW but may or may not get back together
-Doc is still first citizen brown. Normally I would say that this does not imply anything since the ripple effect does not effect time travelers but keep in mind that at FCB's current age (which is a year and a week older than he would have been at the beginning of part I), Doc either would have been in his time travels at the end of part I or in 1885 (or possibly the day part II takes place) but clearly would NOT be in 1931 so if the timeline does change, FCB should likely fade out due to not being in 1931 at his age in the changed timeline.
-in the brief preview of episode 5, 1986 is shown under the FCB timeline- First citizen edna is shown with the citizen plus chair. This means one of three things; this is a dream, the timeline has been changed but FCB changes it back to his timeline, or the FCB timeline is still intact
12. Assuming it is still the FCB timeline, does the Old Doc exist at any point in this timeline?
He would at the points where his time travels still exist; 1931 we know he would. 1955 and 2015 he may or may not. 2025 he would have had to as this is where this delorean came from
13. What paths have the deloreans taken?
the original delorean went from 1985 to 1955, then back to 1985. Then to 2015. Then it may or may not have gone to other places. It did return to 1985 and then back to 2015, then 1955 and back to 2015 again. Then 1985 followed by 1955 when it was struck by lightning and split in two
first delorean; sent to 1885 by the lightning bolt, and then back to 1885 after sitting for 70 years, then 1985 before being destroyed
second delorean; sent to 2025 where its whenabouts remained unknown until arriving in 1986 where it has made several trips back and forth from 1931 to 1986.
14. Was doc duplicated with the delorean?
This has never been confirmed or denied in the first 4 episodes.
15. why did one delorean lose the hover conversions and the other one retain them?
This is up to imagination; you could argue that the one sent to 1885 had a harder crash than the one sent to 2025. Or both lost the hover conversions but the second one arrived at a time when hover conversions did exist so doc had the flying circuits restored.
16. who can we rule out as the speakeasy arsonists?
Marty and first citizen brown could not have done it as neither had been to that point in 1931 at the start of episode 1.
Old doc could not have been the original arsonist (assuming he's telling the truth and the arson happened prior to him time travelling to 1931). It is possible that old doc went to the scene of the crime and committed the arson prior to the original arsonist.