Editing Mandelstam variables (section)

==Relativistic limit==
In the relativistic limit, the momentum (speed) is large, so using the [[relativistic energy-momentum equation]], the energy becomes essentially the momentum norm (e.g. <math>E^2= \mathbf{p} \cdot \mathbf{p} + {m_0}^2</math> becomes <math>E^2 \approx \mathbf{p} \cdot \mathbf{p}</math> ).  The rest mass can also be neglected.

So for example,
::<math>s/c^2=(p_1+p_2)^2=p_1^2+p_2^2+2 p_1 \cdot p_2 \approx 2 p_1 \cdot p_2</math>
because <math>p_1^2 = \left(m_1c\right)^2</math> and <math>p_2^2 = \left(m_2c\right)^2</math>.

Thus,
::{|
|align="right"|<math>s/c^2 \approx</math>
|align="right"|<math>2 p_1 \cdot p_2 \approx</math>
|align="right"|<math>2 p_3 \cdot p_4</math>
|-
|align="right"|<math>t/c^2 \approx</math>
|<math>-2 p_1 \cdot p_3 \approx</math>
|<math>-2 p_2 \cdot p_4</math>
|-
|align="right"|<math>u/c^2 \approx</math>
|<math>-2 p_1 \cdot p_4 \approx</math>
|<math>-2 p_3 \cdot p_2</math>
|}