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We are trying to understand Special Relativity through internet, and we would like to ask a couple of questions about the subject. We plan to post more questions as more doubts arise.

Both doubts concern the Minkowski space-time diagram.

**First doubt**: in the temporal dimension, why is it necessary to choose

*ct*instead of

*t*? Most pages say it's for the time coordinate to be in the same units of the space coordinates (since metre/second*second = metre). If the only problem was that the time coordinate should be at the same units as x, y and z, then why c, and not any other velocity?

**Second doubt**: consider, for instance, a diagram with 3 spatial dimensions and 1 temporal dimension. The space-time interval between the

*origin*and a point

*(x, y, z, ct)*would be:

[tex]s^{2}=x^{2}+y^{2}+z^{2}-c^{2}t^{2}[/tex]

What is the reason for the minus sign, instead of a plus sign, in

*c²t²*?

Thank you in advance for your patience.