Matter and energy are two forms of one thing (E=mc²). Give a photon enough energy near a nucleus and it turns into a PAIR — an electron and its antimatter twin, a positron. Let them meet again and they annihilate back into two photons. But there’s a toll: you need at least the mass-energy of both particles, or nothing happens. Slide the photon energy past the threshold and watch the pair wink into being.
Mass is frozen energy: E=mc². A photon can convert into a particle–antiparticle pair (near a nucleus, to conserve momentum), but only if its energy meets the rest mass of BOTH: for an electron–positron pair the threshold is 2mₑc² = 1.022 MeV. Below it, nothing; above, a pair appears with the surplus as kinetic energy. Run it backwards and an electron meeting a positron annihilates into two 511 keV photons (the basis of PET scans). A fail-loud self-check throws unless a sub-threshold photon makes no pair and an above-threshold one does. ◆ real particle physics, node-verified.
The exact rest-mass threshold and E=mc² bookkeeping (electron–positron); a full treatment needs the nucleus for momentum and gives energy-dependent cross-sections — the 1.022 MeV threshold and matter↔energy conversion are exact.