- Doc:"Marty! You're not thinking fourth-dimensionally!"
- Marty:"Yeah, right, I have a real problem with that."
- -Marty and Doc discussing how to get back to 1985 from 1885.
The fourth dimension is the portion of the space-time continuum which the DeLorean time machine was designed to utilize when travelling to the past or the future. According to spacetime theory, and Albert Einstein's special theory of relativity, space consists of three dimensions, height, width and depth; the sensation of the normal passage of time is a result of traveling at a constant speed in a fourth direction that is at right angles to the other three.
The Delorean is able break the time barrier and travel instantaneously along the fourth dimension to the past or the future, but only while it is traveling at least 88 miles per hour in the three dimensions of space.
Marty was noticeably worried whenever Doc recklessly drove the DeLorean at an obstacle when he neared 88 miles per hour, assuming in his normal experience that the car would either hit them or crash. However, Doc Brown would calmly reassure him that his actions were perfectly sane as long as he remembered the four-dimensional motion of the car (and assuming his calculations were correct), since the obstacle in the path of the DeLorean would not be there when the car reappeared in another time period or something that didn't exist in one time period would exist decades from the time they left (e.g. a bridge that's safe and still in use).
- At the Drive-In, Marty said that if he traveled through time headed straight for the movie screen, he'd crash into the Indians that were painted under the screen. Doc explained that he wasn't thinking fourth-dimensionally. "Marty, once you hit 88 MPH headed directly at that screen those Indians won't even be there." The statement was true enough, since although the painting disappeared, a group of real Indians had appeared in its place.
- When Doc and Marty were trapped in 1885, they decided to push the DeLorean up to 88 MPH using a train over Shonash ravine, but the bridge was not completed yet. Doc explained that the bridge would exist in 1985.
Behind the scenes
- Marty's problem with "thinking fourth-dimensionally" was understandable. Although Doc usually managed to break the time-barrier without incident (barring nearly hitting a vehicle on the skyway in 2015), Marty seldom avoided a mishap. While behind the wheel, he (1) smashed through a barn wall (2) crashed into the Assembly of God Church (3) found himself in the middle of an oncoming army of Indians and (4) found himself in the path of an oncoming train.
- In 27 episodes of the animated series, fourth-dimensional thinking was displayed only once, in the episode "A Friend In Deed". Marty was trying to untie a damsel in distress from a Hill Valley railroad track in 1875. A locomotive that was about to strike Marty disappeared, and reappeared one second later, further down the track. This would have required two separate jumps by the Jules Verne train.