Time is the apparent progression of events from past to future. While it's impossible to completely define the nature of time, we all share many common experiences bound by time: Causes lead naturally to effects, we remember the past but not the future and the evolution of time appears to be continuous and irreversible.
Einstein's theory of special relativity revealed that the experience of the flow of time is relative to the observer and their situation. Previously, the work of Isaac Newton had assumed the existence of a "master clock" that kept synchronized time throughout the universe. This clock wasn't really thought to exist, but the concept allowed Newton's equations to work. The key idea was that all observers could agree on the exact same moment of time, according to the Internet Encyclopedia of Philosophy.
However, building on work before him, Einstein discovered that the passage of time is relative. In special relativity, moving clocks run slowly; the faster you move in space, the more slowly you progress through time. The closer you get to the speed of light, the greater this effect becomes.
In the decades since Einstein first proposed this concept, physicists have made multiple measurements that demonstrate this effect. An atomic clock aboard a jet airplane will tick at a slower rate than one on the ground. A subatomic particle called a muon doesn't exist long enough to travel from the atmosphere, where it's generated when cosmic rays strike air molecules, to the ground. But because muons travel at close to the speed of light, they seem to exist longer from our perspective, allowing them to complete their journey.
When Einstein developed his theory of general relativity, he extended this concept, known as "time dilation," to situations involving gravity. The presence of strong gravity also slows the passage of time, so a clock in a strong gravitational well (for example, on the surface of Earth or near a black hole) will tick at a slower rate than a clock in the middle of space, according to physicist Christopher S. Baird.
Time travel into the future isn't just permitted — it's mandatory. Indeed, with every passing second, all of us are moving forward into our own futures. The future is unavoidable, and it is impossible to escape. But the reality of relativity makes it clear that "jumping" forward in time is perfectly acceptable.
If a twin sets off in a rocket ship and spends a few years traveling near the speed of light, when they return to Earth, they will have aged less than their Earthbound twin. Although only a few years may have passed on the spaceship, decades or even centuries could have passed on Earth, depending on how quickly the rocket traveled, according to Cosmos magazine. In a real-life example, NASA astronaut Scott Kelly has experienced a few milliseconds less time than his twin Mark (Scott is also six minutes younger), thanks to spending a longer time in space, traveling at speeds of around 17,500 mph (28,100 km/h), according to Live Science sister site Space
But time travel into the past appears to be forbidden – at least in all experiments and observations ever done. For one, the possibility raises all sorts of uncomfortable issues, like the famous grandfather paradox which asks what would happen if you went back in time and killed your own grandfather: You wouldn't exist, so you wouldn't be able to travel back in time to commit the act
Second, there is no known mechanism in physics that permits traveling backward in time. While certain time-traveling situations can be constructed in general relativity, those situations require entities that don't seem to exist in our universe (like matter with negative mass, or infinitely long cylinders)
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