Physics in Spacetime
The equations of special relativity can be written in a manifestly covariant form. The position of an event in spacetime is given by a contravariant four vector with components:
We define x0 = ct so that the time coordinate has the same dimension of distance as the other spatial dimensions; so that space and time are treated equally. Superscripts are contravariant indices in this section rather than exponents except when they indicate a square (it should be clear from the context). Subscripts are covariant indices which also range from zero to three, as with the four-gradient of a scalar field φ:
Read more about this topic: Special Relativity
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