Almost every civilisation has myths about people disappearing for a while, then returning home as young as the day they left (or unfathomably aged), and telling stories of strange, exotic, or forbidden places.
But this is not the idea of time travel that grips the popular imagination today. The idea now is inextricably tied to mechanical devices, operated by the conscious actions of a traveller, that allow him to go back and forth in time at will.
The concept was popularised by H.G. Wells in his science-fiction novel The Time Machine, and has become a mainstream popular culture theme thanks to films such as Back to the Future and its sequels, not to mention Doctor Who and his police box.
Wildest dreams: Films such as Back to the Future have made time travel a theme of popular culture.
However, physics is not always prone to be bent to our wishful thinking. We are now aware, thanks to Albert Einstein and his theory of general relativity, that time does not flow at the same pace everywhere in the universe. It slows down under the influence of gravitational fields, and the stronger the field, the slower the flow of time.
Therefore, a space traveller who crosses strong gravitational fields – Interstellar is a movie that depicts this type of time travel – sees his personal time pace slowing down with respect to the time pace of the universe farther from those gravitational fields. And he can say, with good reason, that he travelled forth in time, making true those ancient myths.
But travelling back and forth in time, while staying in the same place and using some sort of device, will never happen. The desire to glimpse the outcomes of our decisions, or to go back in time to change our actions, are banished to the world of science-fiction.
The very absence of time travellers from the future is a good indicator that physics does indeed have a metaphorical Chronology Protection Agency, as the late Stephen Hawking conjectured. "It's easy to be wise after the event", says an old proverb, and the only way to avoid any regrets about wrong decisions and thoughtless actions is to "science out" our world right now, and act accordingly.
