Back to the Future: Part 3- No, You're Not Thinking Fourth Dimensionally

One of the basic tenants of the Back to the Future series is that the DeLorean does not travel through space as it travels through time. This is a major plot point in Back to the Future: Part 3 as Doc Brown must pick places to travel through time which he knows will be safe to travel to in the destination time. But, an effective time machine must also be capable of moving instantaneously through space as well. (And as space and time are actually different ways of perceiving the same thing known as space-time, a device capable of instantaneously moving any direction and amount through time should be capable of doing the same in space.)

The general problem is clearly illustrated in this comic.

It's easy to forget, but that thing you're standing on is always moving. The Earth orbits the sun at about 66,600 mph. It rotates at about 1,040 mph. There is also the matter of eccentricities in the Earth's orbit and rotation. And plate tectonics are not a trivial consideration, if you want to see some dinosaurs.

If you want to safely return to the Earth after a journey though time, you must hit a target that is a thin roughly spherical shell between the surface of the planet and about 20 feet above it. (Too low- materialize within the planet and get crushed; Too high- fall and then get crushed) And as you probably wish to land over a solid surface in one of the habitable zones of the planet, your target is even smaller. The idea that you could simply return to the same point relative to the surface of the planet without moving through space, as suggested in the movie is ridiculous, no matter how short the jump. To safely travel though time on the surface of the Earth, you must move relative to the surface of the planet based on a fixed reference at the center of the Universe. And if you can do that, you don't have to worry if a bridge will be finished by 1985 because there's no reason you can't just land on the other side of the canyon. The calculations for changing the destination a few feet would be very simple in comparison.

At this point, you are probably redesigning your time machine to function of part of a spacecraft, but that only solves part of the problem. The Sun orbits the center of the Milky Way at 486,000 mph and the Milky Way is moving way from the center of the Universe at 1,234,789 mph. In other words, unless you move relative to a fixed reference frame, you will be lost in space and be in danger, Will Robinson.

One final thing to consider: momentum. We are unaware of all this motion, but if we carry any momentum into the journey, as the DeLorean clearly does, any accelerations (exiting one time or entering another) would likely be fatal, or more accurately, liquifying. And it's unclear how time behaves during the trip, which would have a significant effect on the safety of time travel.

For further reading on the subject, consult the works of Professor Eric Idle.