Many scientific concepts developed in the last hundred years, especially those in modern physics and astronomy, are counter-intuitive and extremely complex. A few of them, such as the twin paradox, black holes or the event horizon, seem to have been assimilated by popular culture: they are immediately recognized in the media and provide convenient plot devices for literary and cinematic works. Even so, many of such popular notions have little to do with the original scientific concepts and function either as metaphors or as symbolic justifications of ideas that are often irrelevant from a scientific point of view. Most scientific concepts make sense only in the specific context in which they have been developed and are useless when removed from that context.
One example is the concept of time dilation which in public’s view has granted credibility to the idea of time travel. Wikipedia, an Internet resource created and sustained by the collaborative effort of hundreds of thousand of Internet users may provide a good estimate of the collective consciousness of contemporary society. The Wikipedia article related to time travel states: "one-way travel into the future is arguably possible given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox)". Despite this claim, the notion of time travel is meaningless in the context of the relativity theories in physics.
Relativity theories formed by Albert Einstein deal with inertial, i.e. constant in speed and direction movement of objects (special relativity) and with gravity (general relativity). A major postulate of the special relativity theory is the constancy of the speed of light for all observers. The consequence of this postulate is the elimination of the concept of absolute time from modern physics. Special relativity states that simultaneity is relative to a frame of coordinate axes (i.e., the position of the observer). Two observers in relative inertial motion to each other will always disagree regarding which events are simultaneous. Neither observer is wrong in his determination; their disagreement merely reflects the fact that simultaneity is an observer-dependent notion in special relativity. (The disagreement is negligibly small at typical speeds for macroscopic objects that we encounter in our every day life). Because a "present moment" is a set of events that are simultaneous from the point of view of a given observer, the relativity of simultaneity means that there is no absolute present (and therefore, no absolute past and future). This view is in sharp contradiction with the Newtonian physics, in which time is assumed to be absolute regardless of the frame of reference.
The time dilation referred to in the Wikipedia article is the “slowing down” of a clock as determined by an observer who is in relative motion with respect to that clock. Because of the relativity of simultaneity, when two observers are in relative inertial motion to each other, each of them has their own opinion regarding which events are simultaneous. In order to make a comparison of the rates of clocks carried by the two observers, we must assume a certain notion of simultaneity as a point of reference. If the first observer’s notion of simultaneity is used, it is found that the second observer’s clock runs slower than his by a certain factor. Similarly, using the second observer’s notion of simultaneity, it is found that the first observer’s clock runs slower by the same factor. Thus each inertial observer determines that all clocks in motion relative to him run slower.
The so-called "twin paradox" illustrates the phenomenon of time dilation. Suppose that one of two identical twin brothers flies off into space at nearly the speed of light. The special relativity theory states that each of the twins will observe slowing down of his brother's clocks with respect to his own clock. When the space-going twin returns, according to his clock, he will be older than his Earth-bound brother. The other twin, however, will find his brother younger then himself according to the Earth-bound clock. How can the space-going twin be both younger and older than his Earth-bound brother? The paradox, however, is only apparent because this situation is in fact not symmetrical for the two twins. To return to Earth, the spacecraft must change direction, which violates the condition of inertial motion central to special relativity. A full treatment requires general relativity and shows that the space-going and the Earth-bound twins’ estimates of time will agree after the return of the spacecraft: the space-going brother will appear "younger" than his Earth-bound twin.
The confirmation of special relativity and of the time dilation phenomenon has been obtained from the examination of subatomic bodies at high speeds and from the measurement of small changes by sensitive instruments. In particular, ultra-accurate clocks were placed on a variety of commercial airliners flying at one-millionth the speed of light. After two days of continuous flight, the time shown by the airborne clocks differed by fractions of a microsecond from that shown by a synchronized clock left on Earth, as predicted.
The time dilation is therefore a confirmed phenomenon of physical world. But what can we say about the possibility of time travel? Is the retarded aging of the space-going twin in the "twin paradox" thought experiment, a time travel? If so, is the time discrepancy between the airborn and Earth-bound clocks also a time travel? It must be, because the difference between the two cases is only in the amount of time elapsed. But if the clock disagreement is a form of time travel, what are the starting and final points of this time trip? Does the airborn clock travel to the future? Or is the Earth-bound clock transferred to the other clock's past? What are the future and the past then? These questions cannot be answered. The notion of travel in time as it is commonly understood presupposes the displacement of the time-traveling object along an absolute time axis, from a unique present moment to a uniquely-defined future. This is of course incompatible with the special relativity theory, in which the meaning of past, present and future is continuously redefined for every observer in a relative inertial motion.
Misconceptions such as the belief in time travel result from the collision of traditional Newtonian concepts of absolute time and space embedded in our understanding of reality with the apparent paradoxes of relativity of space and time in modern physics. Until the idea of the space-time continuum is assimilated by popular understanding such confusion is inevitable. On the other hand, under the conditions of our every-day existence the relativity effects are negligible. Applying such concepts to our daily experiences is then not unlike using a microscope for nut cracking.