Time travel has for many years been the subject of speculation by philosophers, scientists, and the imagination of those interested in the subject. And yet even with this speculation there are several road blocks, or paradoxes that must be worked out in advance of a working model of time travel to be realized and put to practice. And the predecessor to most paradox theories is the so-called “Grandfather Paradox.” The question it raises is, “If you went back in time and killed one of your ancestors, would you ever be born?”
Of course if time travel became commonplace and accessible to a great number of people there’s no telling how many people would travel back in time and ultimately either by accident or design create paradoxes by doing nothing more than what tourists do, such as accidentally step on butterflies (and the grandfathers of future butterflies).
Seth Lloyd, outlines his theory at arXiv.org where he has proposed that he may have solved the Grandfather paradox by suggesting that “Closed Time Circuits” or CTCs could employ quantum entanglement to “fix” paradoxes before they occur so that the future that will be snaps back to where it should be. In this scenario, a man angry at the future (or the present) could go back in time and reach the past with a gun and attempt to shoot at his grandfather, but his grandfather would through some connection with the “future that must be” would be able to survive anything that comes his way. No matter how much ammunition the killer time traveler would expend, something would through the unknown forces stop that person from reaching his target. Bullets would inexplicably misfire, knives would suddenly disappear or be dropped in incredibly unlikely ways.
This theory has been credited as circumventing paradoxes, but largely seems to focus on larger paradoxes dealing specifically with humans. In reality, however, there are smaller effects that a person could have on reality. The very act of observing an environment often has far reaching changes. Even the photons emitted from the sun that the time traveler blocked with his or her shadow could have major effects in the future. Standing in the way of another person’s line of sight could result in them never meeting and starting a company later that ultimately results in one’s own birth. And paradoxes don’t need to deal merely with the act of procreation, but must also extend to the ultimate universe remaining exactly the same or else any change could create an effect or “paradox” in the future even millions of years hence.
One possible reality of time travel makes it seem as though the process is possible only by tracing the atoms of previous occurrences and moving them purposefully all back to where they were at a previous time along with all the expended energy with them. Such would be the only observable way of traveling through time as we understand it today.
But there are other methods of time travel already being considered that do not directly effect the traveler’s point of origin. If different parallel realities existed that were facsimile’s of Earth but were different only in the date currently taking place on the planet, then one could go “back” to an alternate reality where it was 1910 and kill one of their ancestors and then return to their point of origin only to learn that it happened solely in an alternate dimension. There would be massive differences in these universes in the future when they happened, but there would be no effect on this universe.