It appears Doc was lying to Marty and the audience the whole time in Back to the Future. The Back to the Future time travel rules can feel confusing, as it’s hard to tell whether there truly is just one timeline without causing some plot holes and headscratchers. Back to the Future seemingly operates under the ᴀssumption that there is just one timeline, but things are not so simple.

Both versions of how time travel works are very classic depictions of time travel in movies, and Back to the Future played a large part in making the time travel genre what it is today. Still, the established rules of time travel in Back to the Future create some plot holes and paradoxes, such as whether there’s another version of Marty wandering around and why 1985 Doc doesn’t remember meeting Marty in 1955.

1985 Doc Should Remember What Happened In 1955 If There’s Just One Timeline

Doc Brown Should Have Been Aware Of The Events Of The Films

One part of Back to the Future‘s time travel that feels confusing is the fact that Doc Brown doesn’t seem to remember everything he learns from Marty in 1955 during both Back to the Future and Back to the Future 3. If there’s only one timeline, Doc should remember it all, but the 1985 version of Doc doesn’t seem to know all of the information he should know.

If there were multiple alternate Back to the Future timelines, this issue would be solved because one Doc’s knowledge could be written off as not belonging to his counterpart, but Doc should remember if Back to the Future’s time travel takes place in the same timeline. Doc in 1985 should be aware of every important piece of information that he learned from spending time with Marty in 1955, as it’s unlikely that he’d just forget it even thirty years later. If it was important, it likely made an impact on him, especially since time travel is such a revolutionary occurrence.

Maybe Doc Did Remember What Happened In 1955 (He Just Didn’t Let Us Know)

Doc Brown Has Always Been Careful About The Timeline

It’s also possible that Doc Brown did remember everything in 1955 and just chose to be secretive about it to keep the timeline intact. Doc is very determined at first not to know his own future, so much so that Doc’s decision to wear a bulletproof vest at the end of Back to the Future was a surprise. Therefore, it is reasonable to ᴀssume that Doc remembered everything; he just couldn’t tell Marty or anyone else about it.

One issue with this possibility is that it would be impossible for Doc to perfectly replicate the original timeline now that he knows the truth, and he’s probably aware of that. Knowing the future has changed things enough already that he probably won’t make the exact same decisions, even if he tries to match the decisions he knows of as closely as possible. This calls back to the Ripple Effect of the pH๏τograph changing during the first Back to the Future film, as the changes to the timeline made by Doc’s knowledge continue to ripple after the moment the first changes occur.

What Else Could Explain 1985 Doc Not Having Memories Of Marty From 1955

Something Could Have Happened To Make Doc Forget

Other options could explain why Doc never acknowledged what he saw in 1955. The effect of time travel’s “Ripple Effect” that caused the pH๏τographs to change might not have any impact on a person’s memory. The effects of time travel on memories may be entirely different from the effects of time travel on the physical world. The Back to the Future films never quite confirm exactly what people are remembering after what point, when it comes to time travel changing their memories.

Another possibility is that Doc Brown doesn’t remember for some reason that has nothing to do with time travel. He could have lost his memories in a much more natural way, such as an accident. In the Back to the Future FAQ, Back to the Future writers Bob Gale and Robert Zemeckis joke that one possible cause for this is the drugs he took in the 60s, though they do not confirm anything beyond offering up these possibilities. They also mention the idea that the timeline is naturally inclined to prevent paradoxes and erase those memories.

These ideas each have merit. It’s possible that the Ripple Effect doesn’t change things for memory until the moment that the time traveler initially left. However, the Ripple Effect impacting memory immediately is also a possibility, and it’s one potential reason for Marty playing Johnny B. Goode in Back to the Future. Regardless of the truth behind Doc’s memory and what it means for Back to the Future’s time travel rules, the way the trilogy was structured ultimately worked.

Source: Back to the Future FAQ