Why do people think that time is a spatial dimension?

[Pedro de San Patricio]

Dimension in this context is directionality. A point is a 0D space. Directionality isn't an applicable concept in its case. A line is 1D and includes two directions along its one axis. A square is 2D. You can move along both its axes and to any point between them. A cube is 3D. It's much like a square but includes the concepts of up and down and another axis of freedom. A tesseract is the next logical step. It is a 4D object and includes yet another axis enabling movement in directions we can't totally comprehend. This is why we find it impossible to visualize exactly what a 4D object, such as a tesseract or a dodecaplex, actually looks like.

 

[Moonglow]

I have a dodecahedron speaker system...

 

[Vikrant]

I do not think I have ever heard anyone on this forum state that time is a spatial dimension. Theory of Special Relativity talks in detail about spacetime continuum. It sates that our universe is embedded within four dimensions of spacetime. Three dimensional space and one dimensional time constitute this 4 dimensional spacetime. Special relativity unifies time and space but this does not mean time is a spatial dimension. It is just that neither space nor time can exist without the other. For example the existence of you as an object can be plotted on what is called world-line. This means that any given time, your location can be traced in space. So if you know the entire world-line of an object then you know the entire history of that object's existence. Now think of past light cone and slice it in different time intervals. This as you guessed it should give you a clue on whether the universe is expanding or not.

 

[Delta4Embassy]

Dimension in this context is directionality. A point is a 0D space. Directionality isn't an applicable concept in its case. A line is 1D and includes two directions along its one axis. A square is 2D. You can move along both its axes and to any point between them. A cube is 3D. It's much like a square but includes the concepts of up and down and another axis of freedom. A tesseract is the next logical step. It is a 4D object and includes yet another axis enabling movement in directions we can't totally comprehend. This is why we find it impossible to visualize exactly what a 4D object, such as a tesseract or a dodecaplex, actually looks like.

Maybe because time-space are one thing. Like energy-mass.