One of my favorite movies from my childhood was the Back to the Future and its franchise, and I credit it for getting me interested in becoming a scientist. It was on television all the time, but because of a poor attention span on my part and the oddities of when I start or end watching it (starting watching half-way in, leaving early because of another engagement), many years went by until I actually watched one of the movies all the way through. When the trilogy came out on DVD last year, I bought that up, and now, being older, I can appreciate it both for the scientific thrill and for being a great drama.
But now that I’m older and know the sciences better, I can nit-pick its science. Oh, and I’m not talking about the whole time-travel bit. Instead, I’m going to look at the problems of Back to the Future based on the science and technology we do have. Basically, think back to Sheldon from The Big Bang Theory where he complained about the scientific inaccuracies of Superman but took a guy flying as axiomatic.
So, let’s accept that time travel is possible (something physicist Michio Kaku thinks is possible and says so in the extras to the Back to the Future DVD set). What else goes wrong? There are two things that stand out that make physical assumptions of the film most strange.
One of those points is that there seems to be a preferred reference frame. This concerns how the DeLorean has to reach a speed of 88 mph in order to travel through time. However, that is 88 miles per hour relative to what? Apparently the ground or road as that is what the speedometer reads for the car. But why 88 mph with respect to the road? Depending on your reference frame the car is moving much faster than 88 mph at any given moment. Since the DeLorean is on the Earth, and the Earth is moving about the Sun, the DeLorean is moving an thousands of miles per hour relative to the nearest star. Relative to Mars it’s another speed. On the other hand, if two DeLoreans are moving in the same direction and the same speed relative to the ground, they the DeLoreans are not moving relative to each other.
It seems that for this 88 mph to matter, the ground is the preferred reference frame. This runs contrary to the principle of relativity that Galileo figured out and, far more so, the special theory of relativity that Einstein figured out. There are no preferred reference frames in the universe.
This problem leads to another issue because of Noether’s theorem. Though it came in around the same time as Einstein’s general relativity and is very important to modern physics, it doesn’t have the same level of attention outside of physics folks. What the theory does is that it relates symmetries to conservation principles. A symmetry is where you change your perspective in some way yet do not change what is ultimately seen. For example, take an equilateral triangle; it has a symmetry if you flip it along any of its perpendicular bisectors and if you rotate it 60 degrees in either direction as many times as you want.
Now, one of the fascinating things about the use of Noether’s theorem is that it shows there is a relationship between the translational symmetry and conservation of momentum. The translational symmetry means that if you change your position it does not affect the laws of physics. Conservation of momentum is important as it describes things like collisions, and momentum is, like energy, something that cannot be created or destroyed. However, because there is a preferred reference frame, then we can’t expect translational symmetry, so we end up losing conservation of momentum. If there is also a special time frame as well as a special location in space, then we lose conservation of energy. Basically, having an absolute space and time frame is bad.
However, the preferred reference frame also relates to another problem that is perhaps the most obvious one, the one that is in contradiction to well-known astronomy: Back to the Future implies geocentricism. Like in the pre-modern era, people thought the Earth was the preferred location and the center of the universe of which all things go around it. Back to the Future also implies this via its methods of time travel because the DeLorean does not move to another location in space when it moves in time (compared this to the TARDIS from Doctor Who).
This leads to a problem of which I am not the first to realize and the second major scientific issue I point to. If the Earth moved about the Sun, then when the DeLorean traveled back in time the Earth would have moved in that time. The orbital speed of the Earth is measured to be about 30 kilometers a second. In the first time travel scene from the movie, Doc Brown’s dog, Einstein, is sent into the future by one minute; in that time the Earth should have moved about 1800 kilometers. The time of the experiment was around midnight local solar time, so that distance would be pretty much do tangential to the orbit of the Earth about the Sun, and Einstein would be well out in space. However, the DeLorean appears at the exact same location it left. The problem is only magnified when time travel happens at different times of the day and leaping through larger chunks of time.
The only way to avoid this is if the Earth doesn’t move; that is geocentricism, and it also relates to the absolute reference frame: all things move in relation to the Earth, not the reverse. But that’s a big astronomy fail.
Now, perhaps there is an out for this. Suppose that during time travel, the DeLorean is following Newton’s laws and continuing to travel with the same velocity as the Earth. If you have the same velocity as another object (both objects having the same reference frame to establish that velocity), then you don’t move relative to it. Two cars going down the highway at the same speed appear to not move relative to each other. If velocity remains the same after time travel, similar to what happens in Portal when using the portal gun,* then it may work. However, this won’t work for two reasons. One is that the time travel is instantaneous according to Doc Brown; if you keep going at the same speed but over no time, you don’t change position. Also, there is the issue that the Earth is in an accelerated reference frame; the Earth itself is turning so anything on the surface is rotating (and that needs centripetal force) and the Earth is in orbit about the Sun, which means there is acceleration due to gravity there. Again, these are forces that would be acting on the DeLorean over a zero-second time span, so the DeLorean cannot change its velocity while the spot on the Earth it was on initially was.
This all means that when it comes to time travel, we are going to be having some major issues if we follow Back to the Future as canon. If you want to travel to another time but remain in the same place on Earth, you need to have a machine that travels in both time and space. In other words, spend your money on a TARDIS. You won’t regret it.
Oh, and one more note: if time travel is possible, it’s going to take a whole lot more than 1.21 gigawatts of power to do it. (Perhaps an Eye of Harmony will do?)
* note that in Portal, it is only the magnitude of the momentum/velocity of the character that is conserved when going through a portal. However, momentum is a vector, so for momentum to be conserved one has to keep both the speed and the direction the same unless acted upon by an outside force.