Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the need for the traveler to experience the intervening period (at least not at the normal rate). Any technological device – whether fictional or hypothetical – that would be used to achieve time travel is commonly known as a time machine.

Although time travel has been a common plot device in science fiction since the late 19th century, and the theories of special and general relativity allow methods for forms of one-way travel into the future via time dilation, it is currently unknown whether the laws of physics would allow time travel into the past. Such backward time travel would have the potential to introduce paradoxes related to causality, and a variety of hypotheses have been proposed to resolve them, as discussed in the sections Paradoxes and Rules of time travel below.

"The Man" is a slang phrase that may refer to the government or to some other authority in a position of power. In addition to this derogatory connotation, it may also serve as a term of respect and praise.

The phrase "the Man is keeping me down" is commonly used to describe oppression. The phrase "stick it to the Man" encourages resistance to authority, and essentially means "fight back" or "resist", either openly or via sabotage.

The earliest recorded use^{[citation needed]} of the term "the Man" in the American sense dates back to a letter written by a young Alexander Hamilton in September 1772, when he was 15. In a letter to his father James Hamilton, published in the Royal Dutch-American Gazette, he described the response of the Dutch governor of St. Croix to a hurricane that raked that island on August 31, 1772. "Our General has issued several very salutary and humane regulations and both in his publick and private measures, has shewn himself the Man." ^{[dubious – discuss]} In the Southern U.S. states, the phrase came to be applied to any man or any group in a position of authority, or to authority in the abstract. From about the 1950s the phrase was also an underworld code word for police, the warden of a prison or other law enforcement or penal authorities.

Stephen William Hawking, CH, CBE, FRS, FRSA (born 8 January 1942) is a British theoretical physicist, cosmologist, and author. His key scientific works to date have included providing, with Roger Penrose, theorems regarding gravitational singularities in the framework of general relativity, and the theoretical prediction that black holes should emit radiation, which is today known as Hawking radiation (or sometimes as Bekenstein–Hawking radiation).

He is an Honorary Fellow of the Royal Society of Arts, a lifetime member of the Pontifical Academy of Sciences, and in 2009 was awarded the Presidential Medal of Freedom, the highest civilian award in the United States. Hawking was the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009. Subsequently, he became research director at the university's Centre for Theoretical Cosmology.

Hawking has a motor neurone disease related to amyotrophic lateral sclerosis, a condition that has progressed over the years. He is now almost completely paralysed and communicates through a speech generating device. He has been married twice and has three children. Hawking has achieved success with works of popular science in which he discusses his own theories and cosmology in general; these include A Brief History of Time, which stayed on the British Sunday Times best-sellers list for a record-breaking 237 weeks.