Is time travel possible?
- Time travel is both an ancient and modern concept about objects or people moving between different points in time and space. The first mention of time travel occurs in an ancient Sanskrit story compiled ~400 B.C. about King Kakudmi who takes his daughter to the home of the creator god Brahma to help him find her a suitor. While there, Brahma informs them one day equals billions of years, and when they return to their plane, the king’s entire kingdom and everything they knew had passed away.
- Einstein’s theory of relativity seeks to understand how gravity affects time-space matter. He was expanding upon a theory published ten years earlier, called special relativity, which did not acknowledge the existence of gravity and argued time and space were inseparably connected. Einstein’s theory argued how “particularly massive objects warp the fabric of space-time [which] manifests as gravity.”
- Popular time travel theories include wormholes, black holes, and string theory.
- A 2015 60 Minutes/Vanity Fair poll showed how 53% of people believed time travel is possible.
- Top time travel movies and TV shows include the Back to the Future trilogy, BBC’s Doctor Who, Star Trek: Voyager, Looper, and Interstellar.
There are theories suggesting certain geometries of spacetime or specific motions within space could allow time travel into the past and future. For example, we travel into the future one second at a time at a steady pace. However, we understand time behaves differently when applied to an accelerated object. At high speeds, time slows down for the object in motion. Time travel is possible, given our present understanding of physics. Backward time travel may be less likely due to the concept of causality, sometimes referred to as the 'Grandfather Paradox.' However, this paradox may be solved by understanding the universe as existing in a superposition of two states simultaneously. But let's get back to the future.
If you left Earth in a spaceship traveling at 99.9% the speed of light and traveled to a star ten light-years away, the trip would seem instantaneous when in fact, you would have aged 41 years. Yet, upon returning to Earth, you would find 70 years had passed. Your friends might long be dead, and your great-grandchildren could greet you. This is a plausible scenario based on the Lorentz transformation, which describes frames in spacetime that move at a constant velocity. In this case, the Lorentz transformation would apply to you in that spaceship for that ten light-years away journey. While a trip into the past may require rotating black holes or wormholes, travel into the future is possible given an ability to accelerate mass to 99.9C at a constant velocity, whereupon the traveler would experience time dilation and, in effect, be traveling into the future, an entirely plausible notion.
Einstein, the founder of modern perceptions of spacetime, determined time dilation to be the closest to travel time humans can get. In short, it's relativity—the faster one moves and the further from a source of gravity one gets, the slower time moves. For example, someone living in orbit for a year would come back imperceptibly younger than their twin sibling, being further from Earth's gravity well while traveling at 17,000 miles per hour. Einstein hypothesized that if something could move faster than the speed of light, it would perhaps experience time in reverse. However, as an object's relative speed increases, so does its mass. If a particle with any mass were to reach the speed of light, it would require infinite energy and obtain infinite mass, both of which are impossible. NASA hypothesized that wormholes (gates between specific places in spacetime) could facilitate time travel. However, these phenomena are only hypothetical.
Other proposed 'solutions' involve a massive object condensed into an infinitely long cylinder (which is impossible since mass and volume inherently have limitations), orbiting a black hole at nearly the speed of light (which would just slow time, not reverse it), and manipulating 'cosmic strings' of extreme mass (which are also hypothetical). On a simply logical level, every scenario for traveling backward in time creates a paradox, a conflict between two self-contradictory 'truths' that can therefore not exist. There is always the possibility science will evolve to allow traveling back in time (which has not yet been entirely disproven). But as of now, all of the evidence shows we can only move one way through time, and that is forward.