Time travel has been a plot device in fiction since the 19th century. Travelling backwards or forwards in time has never been verified as a process, and doing so presents many theoretical problems and contradictive logic which to date have not been overcome. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine. A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of a temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between branch points, parallel realities, or universes. Another solution to the problem of causality-based temporal paradoxes is that such paradoxes cannot arise simply because they have not arisen. As illustrated in numerous works of fiction, free will either ceases to exist in the past or the outcomes of such decisions are predetermined. As such, it would not be possible to enact the grandfather paradox because it is a historical fact that ones grandfather was not killed before his child (ones parent) was conceived. This view does not simply hold that history is an unchangeable constant, but that any change made by a hypothetical future time traveller would already have happened in his or her past, resulting in the reality that the traveller moves from. More elaboration on this view can be found in the Novikov self-consistency principle.
A father is the male parent of a child. Besides the paternal bonds of a father to his children, the father may have a parental, legal, and social relationship with the child that carries with it certain rights and obligations. An adoptive father is a male who has become the childs parent through the legal process of adoption. A biological father is the male genetic contributor to the creation of the infant, through sexual intercourse or sperm donation. A biological father may have legal obligations to a child not raised by him, such as an obligation of monetary support. A putative father is a man whose biological relationship to a child is alleged but has not been established. A stepfather is a male who is the husband of a childs mother and they may form a family unit, but who generally does not have the legal rights and responsibilities of a parent in relation to the child. The adjective "paternal" refers to a father and comparatively to "maternal" for a mother. The verb "to father" means to procreate or to sire a child from which also derives the noun "fathering". Biological fathers determine the sex of their child through a sperm cell which either contains an X chromosome (female), or Y chromosome (male). Related terms of endearment are dad (dada, daddy), baba, papa, pappa, papasita, (pa, pap) and pop. A male role model that children can look up to is sometimes referred to as a father-figure.