Concept:Pair annihilation occurs when an electron and a positron collide, mutually destroying their masses and converting them entirely into energy.
Formula:$$ \vec{p}_{initial} = \vec{p}_{final} $$
Solution:- Assuming the electron and positron annihilate at rest, the total initial momentum of the system is zero.
- To ensure the final state also has zero net momentum, at least two photons must be produced.
- These two photons must travel in exactly opposite directions so that their individual momentum vectors (\( \vec{p} \) and \( -\vec{p} \)) cancel out perfectly.
Why other options are incorrect:While mass-energy and charge are also conserved in this reaction, directionality specifically satisfies the
vector nature of momentum conservation. Energy is a scalar and has no directional constraint.
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