It’s been written about in hundreds of books, the subject of fantasy for everyone at one time or another, and the government has actually devoted research at one time or another on the subject. If you do a search on the Internet for time travel you will find millions of entries on it, and hundreds of websites fully devoted to talking about it. Wouldn’t it be cool if you could travel back in time? You could correct mistakes you’ve made in your life, study any period of time that interests you, not to mention build a financial empire on your foreknowledge of events. Beginning with H.G. Wells’ The Time Machine, the concept of time travel has been one of the main staples of science fiction. Some of my favorite reads are David Gerrold’s The Man Who Folded Himself and The Light of Other Days by Stephen Baxter and Arthur C. Clarke.

So is it really possible to travel in time?

First of all we are all already time travelers in the sense that time moves forward, and at the same apparent rate of speed, for all of us. There seem to be no obstacles in physics to accelerating the forward momentum of time in one way or another. Cryogenics is a concept much written about as one method of “forward” time travel; lowering the body temperature to a little above absolute zero to nearly stop the metabolism as a way to sleep away millennia. The practical hurdles to this put any possibility of this far into the future. Although simpler organisms have been successfully frozen and returned, the human body is too complex to yet survive the process because of water crystallization and other factors. Another method of accelerating time is time differentials due to the relativistic effects of high velocity.

According to Einstein’s Theory of Relativity, as an object approaches the speed of light one of the effects is time dilation. As a relativistic object’s speed increases the passage of time slows for it in relation to a non-moving object. Take for example a spacecraft traveling at 10% of the speed of light, or 18,628 miles per second. If this imaginary spacecraft maintained this speed consistently for 24 hours (according to our clocks back home), then at the end of that 24 hours only about 23 hours, 53 minutes would have passed onboard the spacecraft. Much higher speeds to within a fraction of a percent of the speed of light have to be achieved to get a really noticeable effect. Take that same spacecraft and accelerate it to .999999 light speed (or to 186,281.81 miles every second) and something really bizarre happens, achieving something more like time travel. If you take that spacecraft out on a joyride at that speed for 24 hours of your traveler’s time and return home, you will find that almost two years have past on Earth.

In actuality, this has been observed in experiments done when atomic clocks were sent on jetliners to observe the effects of time dilation. The difference was observed as predicted, helping to support Einstein’s theories. Naturally the difference was small, measured in nanoseconds. Unfortunately for any aspiring time travelers, the kind of speeds needed for relativistic effects are still well outside our technology. The fastest spacecraft yet launched were the Helios spacecraft sent to study the sun in the 70s. They achieved speeds of about 158,000mph, or about 44 miles per second; this is about .02% light-speed, still not close to relativistic speeds.

And what about the possibility of travel back in time?

This makes great material for science fiction, but the data here doesn’t seem promising. Physicists have been able to envision certain circumstances under which time travel MAY be allowable under the laws of physics, but the energy levels and exotic matter requirements seem to be well beyond anything we are likely to achieve anytime soon. Some have suggested that wormholes may be bridges to other universes, distant parts of this universe, or other times. Wormholes remain a theoretical concept, neither proven nor dis-proven to exist. It seems that for all practical purposes the universe has (at least temporarily) denied us the opportunity to revisit our past directly. So let us turn to a discussion of what the possibilities would be if time travel did exist.

First of all we must look at the fact put forward by modern physics that space and time are related aspects of the topology of our universe. In other words, our universe consists of the three observable dimensions of space and one of time. Putting together a theory that explains the existence of our universe required combining time and space into one continuum. Assuming this to be true, it follows that there should be a parallel measurement in space equivalent to measurements in time. It may seem nonsensical to talk of measuring space in seconds or time in miles, but the two are tied together through the speed of light. Therefore it follows that to convert one second of time into distance, we simply look at how far light travels in one second. That would be approximately 186,282 miles or three quarters the distance to the moon. This means that traveling one second back in time would be equivalent to traveling nearly the distance to the moon. Then there is the fact that a change in temporal position would mean having to adjust for the motion of the earth, sun and galaxy as they rotate and revolve. A lot harder than it looked, huh? Ok, let’s pretend we overcome this obstacle and achieve real, meaningful time travel. Could you go back in time and kill your grandfather early in his life, assuring that you will never be born? Time travel is full of paradoxes such as this. For the most part this can be overcome by incorporating quantum mechanics into the concept of time travel, and branching realities.

Quantum mechanics is a field of theory which developed in the first quarter of the twentieth century through the work of Niels Bohr, Pauli, Planck, Heisenberg, and Schrodinger. It’s basic tenets are that at a fundamental level matter exists as a cloud of uncertainty and probability. Heisenberg’s Uncertainty Principle states that one cannot measure both the position and momentum of an elementary particle because the act of observation changes the outcome. In this branch of physics cause and effect is said to break down and one can only state the probability of something being true. The most famous example of what quantum mechanics means in the real world was given as a thought experiment by Erwin Schrodinger and is known as Schrodinger’s Cat. Here it follows:

A cat is placed in a sealed box. Attached to the box is an apparatus containing a radioactive nucleus and a canister of poison gas. This apparatus is separated from the cat in such a way that the cat can in no way interfere with it. The experiment is set up so that there is exactly a 50% chance of the nucleus decaying in one hour. If the nucleus decays, it will emit a particle that triggers the apparatus, which opens the canister and kills the cat. If the nucleus does not decay, then the cat remains alive. According to quantum mechanics the unobserved nucleus is described as a superposition (meaning it exists partly as each simultaneously) of “decayed nucleus” and “undecayed nucleus”. However, when the box is opened the experimenter sees only a “decayed nucleus/dead cat” or an “undecayed nucleus/living cat”.

The paradox of this experiment is that the cat is said to be both dead and alive until someone opens the box. (*No cats or animals of any kind were harmed in the writing of this article). This paradox can be resolved if we say that instead of both being true in one reality, that reality actually branches into two. In one universe the cat is alive and in the parallel universe it is dead. In this way our universe is constantly splitting into alternate universes in which every possibility is encompassed. This also solves the paradoxes of time travel. When our time traveler returns and makes changes in the past he would be creating an alternate universe without destroying the other. In this way, as he or she continued to make changes, our time traveler would never be able to return to their original timeline, although he could create one similar to it with the right changes. All of the possibilities and repercussions of a scenario such as this are spectacularly presented in the science fiction novel The Man Who Folded Himself, by David Gerrold.

In summary, time travel is a highly entertaining concept for science fiction, and actually holds some plausibility in certain concepts of modern physics. But as a practical application, it is not likely to become a part of our lives anytime soon. Of course, not being a time traveler myself, I cannot speak with certainty.