In 1935, physicist Albert Einstein published the Einstein-Podolsky-Rosen (EPR) experiment with colleagues Boris Podolsky and Nathan Rosen in a paper titled, “Can the Quantum Mechanical Description of Reality Be Considered Complete?” In the experiment two quantum particles located at the same source move a great distance away from each other in opposite directions; one particle flies toward an observer named Alice, and the other particle flies toward an observer named Bob. Note: a quantum particle is just an object that quantum mechanics describes. It’s not necessarily a particle, as it can demonstrate wave-particle duality. In fact, a more accurate word would be a quantum object.

          At this point in the experiment, the spins of the two particles are unknown. As a result, the particles are said to be in a pure state; the spins of both particles are indeterminate as each particle is in a superposition of up and down (each have a blend of both states). 

          Each particle is equally likely to have a spin up or down when a measurement of spin is made; the probability is 50:50. Unfortunately, whether or not a particle will have a spin up or down can’t be determined by quantum mechanics because quantum mechanics can only offer the aforementioned 50:50 probability; it can’t provide a more specific answer. This is where quantum mechanics falls short in the eyes of Einstein and his colleagues as will be discussed.

          Once the observers make a spin measurement, the spins become known. Additionally, if both particles are measured in the same direction (e.g. in the X, Y, or Z direction), measuring the spin of one particle is enough to determine the spin of the second without the second particle’s spin ever being measured. For every pair of quantum particles whose spins are measured in the same direction, each particle within the pair has a spin opposite to that of the other—this has been experimentally proven. Therefore, if both particles were initially in a pure state, and particle 1 was measured to have a spin down, particle 2 will have a spin up. 

          Considering the great distance between the particles, Einstein and his colleagues raised the following question: how did measuring particle 1’s spin cause particle 2’s spin to instantly change from being superpositioned to having the opposite spin? 

          Einstein and his two colleagues came to two conclusions in an attempt to answer the question above, which was: how did measuring particle 1’s spin cause particle 2’s spin to instantly change from being superpositioned to having the opposite spin? 

          The first conclusion states that the two particles in the experiment communicated with each other; they kept in touch with one another to make sure that whatever spin particle 1 took upon measurement, particle 2 would have the opposite. This communication can happen regardless of how far apart the two particles are; they can even communicate when they’re on opposite sides of the universe! Einstein called this long distance communication “spooky actions at a distance.” This idea of physical processes (such as a particle’s spin selection) occurring at one place and having an instantaneous effect on the elements of reality (such as another particle’s spin selection) at another location is called the non-locality principle. However, in order for “spooky actions at a distance” to occur, the communication occurring between the two particles would have to be both instantaneous and capable of occurring over a long distance. Einstein stated that the communication would have to travel faster than the speed of light. Due to this reason, he did not advocate for the first conclusion that would go directly against his special theory of relativity.

          The second conclusion states that the two particles had a mixed state all along; particle 1 always had a physical property of, for instance, spin down, and particle 2 always had a physical property of spin up. Therefore, in contrast with the principle of non-locality, the principle of locality would hold true. Einstein and his colleagues supported the latter. This led them to state that because quantum mechanics was unable to determine particles’ mixed states prior to a measurement of spin being made and was only able to predict that the probability of each particle having a spin down or a spin up was 50:50: it was incomplete.  

          Until the work of John Bell, the true state of quantum particles remained unknown. The quantum mechanics’ community was left with two options. First, particles exist in a pure state with their spins known only after the spin of at least one paired quantum particle is measured. Second, particles always exist in a mixed state; one particle always has a spin up due to unknown, intrinsic information (termed hidden variables) that gives it a physical property of spin up, and the second particle always has a spin down due to its unknown, intrinsic information.