Time travel is a staple in science fiction stories. Marty McFly traveled into the future and back into the past by means of a flux capacitor designed by the eminent Dr. Emmett Brown. Until he surrendered it to Thanos, Dr. Strange used the power of the time stone to control time. And Dr. Who cavorts merrily through time and space in his TARDIS with the simple flip of a lever. It all seems so easy. Humans have invented all manner of dazzling wonders, from pottery to ships to steel to microchips to orbiting telescopes. Surely it is just a matter of time before some genius working in a garage builds a variant of Dr. Brown’s flux capacitor and is thereby able to zap himself into the future, whether with the flip of a lever or by racing through a mall parking lot at 88 miles per hour.
But is time travel actually possible? Well, certainly it is. With no energy expenditure at all everything and everyone in the universe moves inexorably forward into the future. And it is certainly possible to move into the future at a faster rate than other observers. Special relativity says that two observers moving relative to one another experience the flow of time at different rates, and that difference depends on their relative velocity. This prediction of relativity has been confirmed in experiment many times. For relative velocities that are a small fraction of the speed of light, the difference is quite small. But even so, the Global Positioning Satellite System is so time dependent and so accurate that it had to be designed to account for this and other relativistic phenomena.
Just how extreme can the difference between the clocks of two observers get? Well, the most extreme case concerns one observer at rest and another moving at the speed of light (in their mutual reference frame). In this case the moving observer’s clock actually stops while the clock of the at rest observer continues at its usual pace. If the moving observer travels at the speed of light for a million years, then returns to the physical position of the at rest observer, the at rest observer would be long since dead though the traveling observer would not have aged a single second.
Okay, so travel into the future is certainly possible. What about travel into the past? For an answer to this question we must turn to an astounding result due to Kurt Godel. In 1949 he constructed a solution to the field equations of Einstein’s General Theory of General Relativity that allows an observer to travel to any point in space and time– present, future, or past! This particular solution of Einstein’s theory is a fascinating and instructive study in its own right, but it is decidedly not a solution that corresponds to our own universe. The universe of the Godel solution has an intrinsic rotation about an axis. Our universe has no such rotation. The following video demonstrates how the closed time-like paths of Godel’s solution would enable one to travel into one’s own past: https://www.youtube.com/watch?v=078jOiaevAQ
Let us assume for the moment that such minor difficulties can be overcome in the grand cathedral of future human knowledge. Time travel still presents many practical difficulties that must be considered. Imagine that you are sitting in the driver’s seat of Dr. Brown’s DeLorean, and that you set the time control device for six months in the future. Now you stomp on the accelerator, get the car up to 88 miles per hour, and fzap! You reappear six months in the future, in precisely the same physical location where you disappeared.
But the Earth moves. The Earth is currently revolving around the Sun. In six months the Earth will be on the other side of the Sun. So the DeLorean cannot simply move in a straight line through time and space to reach the point where the future Earth will be in six months. It must move along an arc that exactly follows the path that the Earth will take.
And more than that, the Sun itself is moving. The entire solar system is revolving around the center of our galaxy at the rate of one complete revolution about every 225 million years. So six months in the future, the solar system would have moved a considerable distance around the galactic center from its present location. Dr. Brown had better make corrections for that, or the DeLorean will reappear in interstellar space.
There are other motions to consider as well. The Earth rotates on its axis, and the axis itself has a precession– that is, a wobble. Earth’s axis makes one complete revolution about every 26,000 years. So the position from which the DeLorean departed will have moved, irrespective of the other motions we have discussed.
There are other influences as well. Johannes Kepler showed that the paths of the planets are ellipses, not circles. But that is only to a first approximation. The moon and the other planets exert gravitational forces on the Earth. Those forces distort Earth’s orbit from that of a perfect ellipse. So to ensure that the DeLorean returns to the exact point from which it departed, every detail of Earth’s orbit will have to be considered– including all of the influences due to other gravitational objects in the solar system.
And there are more mundane considerations as well. What if someone builds a cement wall just a few feet beyond the point from which the DeLorean disappeared. When it reappears, the DeLorean will travel just a few feet before smashing into a cement wall. Not good. 😦
An earthquake might thrust up a chunk of the Earth’s crust right into the DeLorean’s path on return. A river might change course, causing the DeLorean to plunge into a torrent of water. Someone could park a car right in the DeLorean’s future path. Ouch.
Time travel as a literary device is pretty ridiculous. If its purpose is to get the reader to think about the possible future course of events, it may have some value. But I have never encountered any science fiction story that makes a full accounting of all of the many considerations we have discussed. There is in fact little or no “science” involved in the way time travel is generally portrayed. And therefore time travel will have to remain fully in the province of fantasy, rather than science fiction. Wave a wand, utter magical incantations, discover an ancient artifact that will open a doorway to a time portal. But please don’t pretend that time travel has any basis in science. It’s just not possible.
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