Time travel

Time travel is the hypothetical idea of moving between different points in time. This concept is explored in philosophy, space-time physics, and science fiction. In fiction, characters often use a device called a time machine to travel.

While traveling to the future is a well-understood phenomenon, traveling to the past is much more uncertain. Scientists study these ideas to understand the very nature of our universe.

Forward time travel is an extensively observed phenomenon. It is explained by the theories of special relativity and general relativity. This occurs through a process called time dilation. Time dilation means that time passes at different rates depending on speed or gravity.

Currently, technology cannot make a body advance or delay more than a few milliseconds compared to another body. However, the physics behind it is considered a real part of how our universe works.

Backward time travel is much more difficult to prove. Some solutions in general relativity suggest it might be possible. For example, a rotating black hole might allow for such travel. Another idea involves wormholes, which are hypothetical warped paths through spacetime.

To use a wormhole as a machine, one end would need to be accelerated to a large fraction of the speed of light. This would cause that end to age less than the stationary end. Because time connects differently through the wormhole, the two ends would remain synchronized for anyone passing through.

Many ancient religious stories feature characters who leap forward in time. In Hindu mythology, Raivata Kakudmi visits the god Brahma and returns to find many ages have passed. The Japanese legend of Urashima Tarō tells of a fisherman who visits an undersea palace for three days. When he returns, centuries have passed.

Similarly, the Quran narrates the story of the Seven Sleepers. These young men slept in a cave for centuries and awoke to a world that had changed.

Science fiction has developed several ways to categorize time travel. These include an immutable timeline, a mutable timeline, or alternate histories. An immutable timeline is one that cannot be changed. A mutable timeline allows for changes, which might create a new timeline. Some stories use prolonged sleep as a way to move forward. Washington Irving's 1819 story, Rip Van Winkle, is a famous example of this.

Charles Dickens also used mystical travel in 1843 when Ebenezer Scrooge saw Christmases past and future.

Early literature also introduced the idea of mechanical time machines. Edward Page Mitchell wrote about a backward-running clock in 1881. Enrique Gaspar y Rimbau described a time-travel vessel in 1887. However, H. G. Wells' 1895 novel, The Time Machine, truly popularized the mechanical device. These stories helped move the concept from magic into the realm of imagined technology.

In physics, scientists discuss "closed timelike curves." These are paths in spacetime that form loops, allowing an object to return to its own past. Kurt Gödel proposed a mathematical solution called the Gödel metric that includes these curves. However, this requires the universe to rotate, which it does not appear to do. This makes the physical possibility of such loops uncertain.

Backward time travel also creates problems with causality, or cause and effect. The most famous example is the "grandfather paradox." This is the idea that a traveler could go to the past and prevent their own ancestors from being born. This would mean the traveler could never have existed to make the trip. Some physicists, like Novikov, suggest that these paradoxes might be avoided through specific principles or the many-worlds interpretation. Stephen Hawking also suggested the "chronology protection conjecture," which argues that the laws of nature prevent time travel to the past.