Time travel

Time travel is the theoretical process whereby a person travels through time into the past or the future. In theory, a person generally arrives in the exact same place they traveled from but at a different point in time. Speculative time travel often involves a time machine, a device that enables travel through time. Few scientists believe in the possibility of time travel. Those who do consider it possible have generally contended that such a journey would likely result in the traveler's death. Regardless, the concept has remained popular in pseudoscience and conspiracy theories, and has also been a mainstay of science fiction.

Time Travel Theories

The scientific theory of time travel begins with Albert Einstein's theory of special relativity (1905), which states that time speeds up or slows down depending on how fast an object moves through space. If a person traveled near the speed of light in a spaceship, that person would age more slowly than a person traveling at a regular speed on Earth would. In his general theory of relativity (1916), Einstein considered time a fourth dimension outside of space's characteristic three dimensions of length, width, and height. Time is an individual dimension defined as "direction," and Einstein believed it could only move forward, though some experts have disagreed with this assumption.

Unifying time and space creates space-time, a four dimensional fabric-like model that includes space's three dimensions and the fourth dimension of time. Einstein's theory of general relativity holds that space-time can be bent. The resulting curvature is known as gravity. Space-time makes a number of physics theories simpler, including time travel. The theory can be illustrated by envisioning the fabric of space-time, which is dented when anything with mass sits on it. When space-time is bent, the object traveling through it travels on the resultant curved path. Bending space-time far enough creates a shifted sense of time.

An astronaut in a spaceship travels through the space-time fabric at a great speed. Combined with the force of gravity, the astronaut experiences time more slowly than someone on Earth does. This effect is called time dilation. The experience technically means that astronauts are capable of time travel. A space traveler would age slower than a person born on the same date and time who remained on Earth, but the space traveler would only be microseconds younger.

Another possible time travel scenario involves traveling faster than the speed of light (186,282 miles per second in a vacuum). This theory has been considered impossible per Einstein's theory, since an object's mass increases and its length decreases the faster it moves. An object moving at the speed of light would have infinite mass and no length, which is physically impossible. Some theorists have continued to hypothesize about the possibility, however. Other scientists believe the creation of wormholes, or bridges in space-time, around certain points in space would allow travel through time. Wormholes are theoretical in nature, and the technology to create them does not yet exist. Wormholes are also thought to be microscopic and easily collapsible, making travel through them very unlikely.

An alternate time travel theory is the black hole theory. This scenario involves a spaceship traveling around the length of a spinning black hole at the speed of light. According to this theory, expanded upon by British physicist Stephen Hawking, the spaceship would experience half the time of the people on Earth. Five years to the astronauts would be ten years to the people on Earth. In addition to the obstacles that traveling at the speed of light might pose, scientists remained unsure if a spacecraft would be able to withstand the force of a black hole. The craft would be at great risk of coming apart. Despite these challenges, scientists continued to pursue possible methods of time travel.

Time Machines

Many time travel theories have remained complicated by physical limitations. Some theorists believe a protective time traveling vehicle would be needed for humans to travel back in time. A time machine is a device that would have special properties that allow it to bend space-time so much that a timeline bends back onto itself to form a loop, or closed time-like curve. Some time travel theorists suggest that a time machine would require an exterior made of negative energy density, theoretical matter that moves in the opposite direction of normal matter, to accomplish this feat.

Other time machine researchers have suggested manipulating gravity within a doughnut-shaped vacuum surrounded by normal matter. Scientists would then create a focused gravitational field to bend space-time and form a closed time-like curve. Within this loop, a traveler would be able to pass further through time with each lap taken. Though time travel is, therefore, considered by some to be technically possible, scientists understand that the human body is not physically equipped to make such a journey unharmed. Only the invention of a machine that could protect the traveler or a breakthrough in the field of physics would lead to actual time travel.

Time Travelers, Paradoxes, and Skepticism

Some scientists and philosophers have suggested that the absence of visitors from other times is, in itself, proof that time travel is impossible. Hawking, among others, noted that if time travel were possible, there should surely be some sign of time travelers. Others, however, have claimed that there could be any number of reasons why no time travelers have ever been discovered. Noted scientist Carl Sagan, for instance, listed possibilities such as the potential that only time travel into the future is possible; that time travel is only invented in the far future and is limited in range; that travel to the past is only possible to the point at which time travel is invented; or that time travelers do exist among but either have the technology to conceal themselves or are not perceived in ways currently understood (as ghosts or UFOs, for example).

Some efforts have been made to attract potential time travelers or otherwise document their existence. In 1982, a group self-styled as the Krononauts made headlines by holding a party advertised as a welcome for travelers from the future; no time travelers appeared. In 2005, MIT students held a conference on time travel and made an effort to promote it with documentation that would last well into the future, so that any future time travelers could learn of the event and attend. Again, no one from the future was known to have appeared. A more directly humorous attempt to host time travelers came several years later, when Hawking arranged a reception but only spread word of the event after it happened, so that only time travelers would be able to show up. Other efforts to establish evidence of time travelers took a more academic form, such as a study published in 2014 that searched the internet for examples of anachronistic word usage, to no avail.

Skeptics of time travel have also pointed to the many paradoxes apparently inherent to the concept. Most famously, the idea that one could travel back in time and prevent oneself from ever going back in time—or even existing at all—presents a problem. Still, some scientists have suggested ways around this paradox, generally involving theories that may seem out of science fiction yet have earned real scientific consideration. Most prominent are variations of the so-called multiverse theory, in which infinite parallel universes, alternate realities, or timelines exist. This could theoretically mean an action such as time traveling to prevent oneself from ever being born could take place in one history and not in another.

Time Travel in Popular Culture

The concept of time travel found great popularity in works of fiction during the late nineteenth century. Mark Twain's A Connecticut Yankee in King Arthur's Court (1889) tells of a nineteenth-century man who is transported back in time to the medieval court of King Arthur. However, it was H. G. Wells's best-selling classic The Time Machine (1895) that cemented time travel's significance in science-fiction literature. Wells coined the term time machine, and his ideas inspired additional time travel stories over the next century. Acclaimed authors such as Ray Bradbury, Isaac Asimov, Kurt Vonnegut, and Douglas Adams wrote about time travel as well. Writers continued to experiment with, and audiences continued to respond to, time travel as a fictional plot in itself or subplot into the twenty-first century.

Film and television have also long made use of the concept of time travel, including in the twenty-first century. Twain's and Wells's classic works were eventually adapted to film, and other adapted or original time travel movies—such as Planet of the Apes (1968), Time Bandits (1981), and The Terminator (1984)—also found success in theaters. The wildly popular Back to the Future and Bill & Ted franchises continued the time machine trend, adding the DeLorean sports car and the classic phone booth to the list of conceptual time machines. Television programs such as The Twilight Zone, Star Trek, and Doctor Who further familiarized audiences with the concept of time travel, bringing the fantastical possibilities into the modern era.

Bibliography

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