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The 4th dimension in physics, or spacetime, is any mathematical model that combines space and time into a single contiuum. Spacetime is usually interpreted with space being three dimensional and time playing the role of a 4th dimension that is of a different sort than the spatial dimensions. According to certain Euclidean space perceptions, the universe has three dimensions of space and one dimension of time. By combining space and time into a single perception, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels. In classic mechanics, the use of Euclidean space instead of spacetime is appropriate, as time is treated as universal and constant, being independent of the state of motion of an observer. In relative contexts, however, time cannot be separated from the three dimensions of space, because the rate at which time passes depends on an object’s velocity relative to the speed of light and also on the strength of intense gravitational fields which can slow the passage of time.
In the past, from experiments at low speeds, time was believed to be independent of motion, progressing at a fixed rate within all frames of reference; however, later high-speed experiments revealed that time slowed down at higher speeds. This slowing is called time dilation. Many experiments have confirmed time dilation, such as the atomic clocks onboard space shuttles running slower than synchronized Earth-bound inertial clocks and the relativistic decay of muons from cosmic ray showers. The duration of time can therefore vary for various events and various reference frames. When dimensions are understood as mere components of the grid system, rather than physical attributes of space, it is easier to understand the alternate dimensional views as being simply the result of coordinate transformations.
There is a 4th dimension. There is no doubting that, but understanding it is the very difficult. There are two great, in depth articles that you should checkout to get a better understanding of the concept. Start your research here and here. Here on Universe Today there is a great article on finding the 4th dimension. Astronomy Cast offers a good episode on time travel and other dimensions.