= Future-pointing timelike four-vector
{title2=$P^0>|\mathbf P|$}
In <metric signature> $(+---)$, a future-pointing timelike <four-vector> has positive time component larger than its spatial magnitude. Two such vectors have strictly positive <Minkowski metric> scalar product, since $P\cdot Q=P^0Q^0-\mathbf P\cdot\mathbf Q\geq P^0Q^0-|\mathbf P||\mathbf Q|>0$. In the opposite signature the scalar-product sign reverses, while the causal condition is the same. The future timelike cone is closed under addition, which rules out a null <photon> momentum equalling a sum of two massive future <four-momenta>.
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