Annihilation and pair production

Everyone has seen this equation. It's not strictly on the syllabus this year: ie you don't need to do calculations with it. However, it explains a lot. It tells us that pure energy (E) can be changed into particles of mass m. Or particles of mass m can be destroyed and turned into energy E.
This is annihilation. A positron and an electron are destroyed and two gamma ray photons are made.

• Positive and negative because charge has to add up to zero. The energy that is made has no charge.
• Equal and opposite motion so that momentum before is zero.
• Two gamma ray photons so that momentum is zero after the annihilation.
  

In reverse, two gamma ray photons can come together to make a positron and an electron. This is pair production.