# E² = p²c² + (mc²)²

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dextercioby
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Who's ita_{0,x,y,z) ??

Daniel.

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As it often happens I found the answer to my question while typing it :tongue2:

$\eta$ was for the proper speed... i.e. $\eta_{0,x,y,z}$ are the components of the four-velocity. What symbol do you use?

dextercioby
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Ita is for the metric...$u_{\mu}$ is the four-velocity...

Daniel.

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What's a 'metric' ?

dextercioby
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A distance,in mathematics (a metric space is a topological space with a metric)...Here,in physics jargon,it stands for the metric tensor $g_{\mu\nu}$ for spacetime and for the flat Minkowski spactime $\eta_{\mu\nu}$

Daniel.

krab
quasar: $\eta$ is pronounced like something between etta and ayta; not eeta.