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## Main Question or Discussion Point

I'm looking for an explanation as to why a lot of results of Special Relativity are described using [tex]ct[/tex] as a fourth dimension instead of just [tex]t[/tex]. Now, I understand that using [tex]ct[/tex] in a Minkowski diagram with identically scaled axes will cause worldlines of light to angle at a nice 45 degrees, and I've seen how [tex]x-ct[/tex], if I remember correctly, is Lorentz invariant, as well as some other cases where it does seem useful to use ct, but the general nature of this usefulness eludes me. Could anyone tell me more about this?