Final answer:
Momentum conservation applies when particles and antiparticles like an electron and a positron collide and annihilate, producing two photons. Photons, despite being massless, carry momentum, which is proportional to their energy and allows for the conservation of momentum in such annihilation processes.
Step-by-step explanation:
You're correct to note that momentum conservation is a fundamental principle in physics, and it applies to the collision and annihilation of particles and antiparticles as well. Despite photons being massless, they do indeed carry momentum. The concept of momentum conservation in the realm of quantum mechanics and also in the context of relativity changes the classical idea of momentum from simply p = mv to a more complex definition.
When an electron and a positron collide and annihilate, they produce two photons to conserve momentum. According to Einstein's special theory of relativity, momentum for a photon is not dependent on mass but is given by the relationship p = E/c, where p is the momentum, E is the energy of the photon, and c is the speed of light. Since photons move at the speed of light and carry energy, they also carry momentum.
Therefore, in an electron-positron annihilation, the total momentum before collision is equal to the total momentum after collision, with the produced photons moving in opposite directions to ensure that momentum is conserved. This has been verified experimentally by observations like Compton scattering, where photons scatter off electrons, visibly demonstrating the conservation of energy and momentum.