The most common answer in the post Albert Einstein era is obviously time, even though that might not have been so obvious pre-Einstein. The next most common and arguably more thoughtful answer might take the form of the question - do you mean spatial dimension or other dimension? By implication, the fourth dimension would need to be a spatial dimension otherwise there would be no reason to consider it the "fourth dimension", unless it did not exist, in which case time would arguably take its place. Most of us have heard the argument that 4D space is likely just a mathematical extension of 3D space, just as ants on a flat plane could not perceive anything above or below in the third dimension. Per Wikipedia 4D space could be just an extension of 3D space, thus the tesseract, the 4D equivalent of the cube. Alternatively stated, the cube is just a projection of the tesseract into 3D space. Similarly, higher dimensions project to the next lower. So that is about it unless you want to start talking about exotic dimensions under string theory.
With the above in mind, I was watching a Beatles video showing fractals and had an insight. What if the fourth spatial dimension was not a difficult to imagine extension of 3D space, but something entirely different that would allow humans to travel to different galaxies free of relativity's limitations? Could the 4th dimension free us of the prison of relativity? To do this, the 4th dimension would have to be reconciled with relativity. So, drum roll, here's the theory - the 4th spatial dimension is not a projection concept, but rather it is a zoom function or scale dimension. In other words, when you are in the 4th dimension, you can readily go from one scale to another, like viewing fractals. Relativity's asympotes like the speed of light would merely reflect the limits of a given scale. To travel to the stars, you would need to increase the scale and to travel to the atomic level reduce the scale. The scale would not be a continuum, but rather quantum, each with its own relativity within each scale level. I did a google search and to my surprise found this theory has already been posited and studied. See Scale Dimension, Wikiversity. Anybody out there have thoughts on all this?
Edited 3 time(s). Last edit at 20-Jul-18 01:15 by Lost_In_Place.