and the possibility of a fourth dimension (and actually being able to reach it)
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I can tell from the question you are unclear what a dimension is. Briefly a dimension is just an argument in a point function, such as p(x1, x2, x3...xn) where this is a point function for n-dimensional space-time. Our universe is 4 dimensional, 3 spatial ones and 1 temporal one. So we are normally operating in four dimensions, we reach that fourth one, whatever you consider the fourth one to be, pretty much all the time.
And the four dimensions existed long before Al's theory of relativity was ever proposed in 1905.
There are actually two theories of relativity: the special and the general. The special is restricted to non-accelerating reference frames, like an airplane flying at a constant velocity between SFO and NYC. The general has no such restriction, so the reference frames are allowed to accelerate.
A fundamental tenet of both theories is that the speed of light in a vacuum remains the same no matter what the source and observer are, or what they are doing. For that to happen, space and time must make adjustments. Space contracts and time slows down on moving platforms like an airplane for example to keep the speed of light at C, the speed of light in a vacuum. And space curves and time slows down in the presence of mass, energy, and stress to keep light speed at C as well.
In other words, space and time are relative (get it?...relative), based on how fast a platform is moving or on how much mass, energy, or stress is present.
And the four dimensions existed long before Al's theory of relativity was ever proposed in 1905.
There are actually two theories of relativity: the special and the general. The special is restricted to non-accelerating reference frames, like an airplane flying at a constant velocity between SFO and NYC. The general has no such restriction, so the reference frames are allowed to accelerate.
A fundamental tenet of both theories is that the speed of light in a vacuum remains the same no matter what the source and observer are, or what they are doing. For that to happen, space and time must make adjustments. Space contracts and time slows down on moving platforms like an airplane for example to keep the speed of light at C, the speed of light in a vacuum. And space curves and time slows down in the presence of mass, energy, and stress to keep light speed at C as well.
In other words, space and time are relative (get it?...relative), based on how fast a platform is moving or on how much mass, energy, or stress is present.