There are many theories among scientists about how the universe is shaped. Part of the reason for this is space is difficult for us to visualize. The standard human experience of space can be described in terms of three dimensions: length, width and height. All three of these dimensions can be easily engaged by humans: You can walk down a road, zigzagging back and forth as you travel, and then you can climb a tree at the end of your journey. Congratulations -- you have navigated all three dimensions of space. According to the laws of physics, however, real space needs at least four dimensions to be described accurately. Cosmologists have determined that the primary four dimensions are length, width, depth and time -- a combination referred to as space-time.
According to string theory, there are actually even more dimensions than this: At least 10 dimensions must exist for the working equations to be considered valid. According to the space-time model that helped us picture Einstein's theory of general relativity, the fourth dimension is commonly defined as time. It's directional, just as length, width and height. In the case of time, however, it's more than just 60 minutes totaling an hour, or the time of day. According to Einstein's theories of relativity, time is bound up in the concept of space; the two are inseparable, and thus any event can be described as occurring on a space-time continuum.
Considering time a fourth dimension helps us understand the ways that space and time affect one another. For example, a gravitational field -- which is, according to Einstein, just a depression in the topography of space -- can alter the rate at which time passes for objects in the field. Satellites with highly accurate clocks can measure this, in fact: When they return to Earth, they show that the rate at which time passed in space was slightly different than the rate at which time passes on the surface on the Earth. Such experiments illustrate that even though humans experience time in a much different way than they experience space, these concepts are really part of the same system.
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