Spooky Action at a Distance

In 1935, Albert Einstein coined the phrase “spooky action at a distance”—not to describe Halloween, but to express his discomfort with the implications of quantum mechanics. Quantum mechanics is a theory in physics that describes the interactions of matter and energy at the smallest scale, from atoms to photons. Though everything relies on quantum mechanics, it seems strange to us because it contradicts the physics of the everyday world–classical mechanics. Unlike classical mechanics, which deals with visible objects and relies on set laws, quantum mechanics' relies on probabilistic principles, which exhibit behaviors that contradict our everyday experiences. The perfect example of such behavior is the phenomenon of quantum entanglement, specifically what Einstein called “spooky.”

Quantum entanglement is when two particles have a special relationship to each other. They don’t act independently; instead, they become linked, such that the quantum state of one particle is directly related to the state of the other, no matter how far apart they are. For a particle to have a quantum state means that the particle's physical properties are described by a wave function—one of the key probabilistic principles of quantum mechanics. Unlike classical physics, where objects are either particles with well-defined positions or waves that spread out over space, quantum mechanics unifies these concepts by stating that electrons exhibit particle-like and wave-like behavior. Entangled particles share a combined wave function that describes the entire system as a whole rather than each particle individually. Therefore, a change in the state of one entangled particle will instantly affect the state of the other, even if they are separated by vast distances, potentially across the universe. This is “spooky,” because in classical mechanics, interactions are local, meaning that they occur at a single point in space, and according to relativity, no information can travel faster than the speed of light, only less than or equal to. However, quantum entanglement suggests that entangled particles are connected so that a change in one particle's state affects the other instantaneously, regardless of the distance between them. This seems to violate the principle of locality, a cornerstone of classical physics and relativity.

Therefore, the “spookiness” of quantum entanglement does not come from the fact that it contradicts quantum mechanics; instead, it does the opposite by representing it entirely, but it comes from the fact that Einstein refused to believe that the phenomenon could go against classical mechanics. Specifically, in 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen, all famous physicists at the time, proposed the Einstein-Podolsky-Rosen (EPR) paradox, arguing that the theory of quantum mechanics was somewhat incomplete, lacking "hidden variables" that would account for the observed correlations in entanglement while preserving locality and relativity. Adding on to their work, in 1964, physicist John Bell developed a mathematical inequality known as Bell's Theorem to test whether the predictions of quantum mechanics (allowing for entanglement and nonlocality) or those of hidden variable theories (preserving locality) were correct. Experiments conducted since the 1970s using the inequality have consistently supported quantum mechanical predictions, demonstrating that nature does indeed go against classical mechanics that any hidden variable theories cannot explain.

Aside from the “spookiness” of quantum entanglement, it can drive advancements in cutting-edge technologies like quantum computing and cryptography. In January of 2023 actually, new observations of quantum entanglement were made. Before this, quantum entanglement only occurred between pairs of photons or electrons that were identical in nature. But now, for the first time, a team of physicists at Brookhaven National Laboratory (BNL) have detected pairs of dissimilar particles undergoing quantum entanglements. They did this through high-energy particle collider experiments that not only helped observe this new form of quantum entanglement but also allowed them to peer inside atomic nuclei with unprecedented detail for the first time. This discovery expands our understanding of quantum entanglement and provides deeper insights into the internal structure of atomic nuclei. It opens new avenues for research into the fundamental forces and particles that govern our universe.