True, time travel must involve the fourth dimension because science has shown that in many physical equations a fourth dimennsion must be taken account of and is always a function of time. Strange to say, the constant that appears in equations involving time is the same constant that appears in the new equations of wave mechanics. That constant is i. square root of minus one. The fourth dimension exists so far as physical science is concerned. It must be used and interpreted-however that is the physicist's fourth dimension. The geometrical fourth dimension has no proof of existence. Time travel stories involve the fourth dimension as having reality. Most time travel stories involving the fourth dimension speak of revolving a body into the dimension and then moving along that dimension. Thus, one may have two-way travel and one-way travel. In two-way travel, the subject can go into the future or past and return to his own time. Sometimes authors say onlt the future and return as the past is fixed. In one-way time travel the person travels into the future only and cannot return. The third and most interesting form is a geometrical projection of the subject's body into the future or past without a corresponding physical projection. Thus the three dimensional shadow travels, and not the matter within the body.
That has been used many times, and Palmer in "The Time-Ray of Jandra" gives an excellent description of this theory. Manning's "Voice of Atlantis" describes a projection in which the thoughts of the operators go into time and not the body. It is mental time travelling. "The Time Projector" by Keller and Lasser reaches the acme of perfection in regard to projection. In this story the operators are able to catch future events only.
The means of being revolved into the fourth dimension are carefully scouted by most authors. Williamson, in "The Moon Era," develops an anti-gravity force that moves the operator into time as well as space. The scientific explanations may be found in a few stories. Leinster's "Fifth Dimension Projector" describes means of rotating a body at four right angles-the hypothetical metal ammonium being used, and then magnetic fields at right angles. E. E. Smith's "Skylark of Valeron" gives a logical explanation of rotating magnetic fields that push a body into the higher dimension.
It is true that the fourth dimension is the most probable means of travel in time, in the true meaning of the phrase. However the proof such travel is nonexistent. The most interesting and often amusing incidents occur as authors attempt to explain their fourth dimensional projectors. After reading a description one is reminded of a junk shop.