Quantum tunneling is a part of the theoretical branch of physics known as quantum mechanics. It states electrons can behave like both particles and waves, and can cancel the effects of an energy barrier if the energy barrier is thin enough. In other words, particles can travel through walls, doors, etc. if the door or wall is thin enough. The particle—usually a subatomic or quark particle such as an electron or photon—can become a wave to focus a great deal of energy on the barrier, ultimately negating it.