photons

• quanta of electromagnetic radiation, from radio waves to gamma rays
• the boson that carries the electromagnetic force: spin 1, no mass, no charge
• energy $E=h\nu$ ($h$: Planck constant, $\nu$: frequency)
• travel at the speed of light, so the electromagnetic force has infinite range
• mass limit: < 10⁻¹⁸ eV/c²
• barely interact with each other (unlike gluons)
• wave-particle duality: detected one at a time at single points, distributed like a wave (two-slit interference)