- When particles (like photons or electrons) interact and become entangled, their properties—such as spin, polarization, or position—become correlated. For example: • If one entangled particle is measured and found to have “up” spin, its partner instantly collapses into “down” spin (and vice versa), regardless of distance. • This correlation is instantaneous and appears to defy the speed of light, though no usable information travels faster than light (preserving relativity).