What Is Quantum Entanglement?

Written on 07/10/2025
Amanda Hicok


Quantum entanglement is one of the most mind-bending concepts in physics, a phenomenon so strange that Albert Einstein famously dismissed it as “spooky action at a distance.” At its core, entanglement refers to a quantum connection between two or more particles such that the state of one instantly affects the state of the other, no matter how far apart they are. Unlike classical objects that behave independently, entangled particles behave like a single system—even across light-years.

To understand why this is so bizarre, consider this: imagine creating a pair of entangled photons, and then sending one to Tokyo and the other to Toronto. If you measure the polarization (a quantum property) of the photon in Tokyo, you immediately know the polarization of its twin in Toronto, even before you measure it. This happens faster than the speed of light, violating what we intuitively understand about causality and distance. Yet, despite its weirdness, this phenomenon has been repeatedly demonstrated in experiments.

Entanglement emerges naturally from the mathematics of quantum mechanics, especially from the Schrödinger equation and the principle of superposition. A pair of entangled particles exists in a joint state—neither particle has a definite identity until one is measured. That act of measurement “collapses” the wave function, instantaneously determining the state of both particles. This idea doesn’t sit well with our Newtonian instincts, which expect physical properties to exist whether or not we look at them.



Einstein, along with Boris Podolsky and Nathan Rosen, challenged the completeness of quantum mechanics in a famous 1935 paper (the “EPR paradox”), arguing that if entanglement were real, then either information travels faster than light, or quantum mechanics is missing something—a “hidden variable.” However, later work, especially John Bell’s theorem in the 1960s, and subsequent experiments, showed that no hidden variables can explain away entanglement. The universe really is nonlocal in this quantum sense.

Entanglement isn’t just a curiosity—it’s a foundation of emerging technologies. Quantum computers rely on entangled qubits to perform calculations at speeds unimaginable to classical systems. Quantum cryptography uses entangled particles to ensure secure communication; any attempt at eavesdropping breaks the entanglement, alerting the sender and receiver. Scientists are even developing “quantum teleportation,” where information about a quantum state is transmitted via entanglement.

Still, no one fully understands why entanglement works the way it does. It challenges deeply held assumptions about separability and locality. In a way, it rewrites what it means for two things to be “different” or “apart.” Entangled particles are not two objects with correlated behaviors—they are one system, stretched across space. It’s not that information travels between them faster than light—it’s that they were never truly separate to begin with.



This philosophical discomfort is part of what makes quantum entanglement so compelling. It forces physicists, philosophers, and even theologians to ask foundational questions about the nature of reality. Are we looking at an incomplete model of the universe? Or are our classical ideas of space and causality simply inadequate in the quantum realm?

In the end, entanglement is a reminder that the universe is far stranger and more wondrous than we ever imagined. What Einstein once called “spooky” has become not only real, but essential—a hauntingly beautiful thread connecting particles, places, and possibilities. It’s not magic, but it’s as close as science gets.