John Bell conducted an experiment where he tried to understand EPR’s paradox of “spooky action at a distance”. In his experiment he considered a pair of entangled particles that travel away from each other to different distant locations. Measurements were then performed on them by two different SPOT detectors (A and B) in which the spin of the particles was measured as being either spin up (+) or spin down (-). The detector could be set to measure in either direction, either testing for spin up and getting a positive, testing for spin down and getting a positive, or testing for spin up and getting a negative or testing for spin down and getting a negative, thus resulting in results of spin up or down. We can imagine the detectors being at two different locations, say detector A, let’s name it Alex, is located in Abbotsford, BC, and B, named Boris, is located in Bannockburn, Ontario (and yes, that is a real town). Then the light source was located in perhaps Whitehorse. The results of the experiment did not depend on the placement of the light source. The light source in Whitehorse emitted a pair of entangled photons, one sent towards Abbotsford and the other sent to Bannockburn. The theory of entanglement was that even over a great distance, the two photons should still remain entangled.
Then, in Abbotsford, Alex used the SPOT detector to measure the direction of spin of the photons that arrived from Whitehorse. In the experiment, it was discovered that it did not matter how the detector was set, it always found half of the particles to be spin up, and half to be spin down, which would mean that the stream of photons heading towards Abbotsford was unpolarized.
The same thing happened in Bannockburn to Boris. When the photons were measured at this detector, it also found that the photons were half spinning up and half spinning down.
Therefore, no matter how Boris set his detector, Alex’s results were still 50/50. Thanks to quantum physics, theory predicts that Alex’s results will always be 50/50 no matter how Boris set his detector. In fact, no matter what Alice or Boris do with their detector, nothing changes and the sequence is always random 50/50 spin up and spin down.
HOWEVER….. in the experiment, Bell found that Alice’s and Boris’ detectors, when oriented in the same direction, detected that the order of photon spin for each sequence was IDENTICAL! Whoa…. WEIRD! When the two detectors measured in the same direction, they both measured the identical random sequence! And when the angle between the two detectors was 90* from each other, there were opposite results for the two random sequences! Or are they so random…
As results for the experiment were gathered, it was found that when the two detectors, Alex and Boris, were aligned with each other, the sequence of spins for the photons matched 100% of the time (exactly the same). When they were aligned at 90*, the spins matched 0% of the time (exactly opposite). Then, when the detectors were adjusted to being 30* and 60* apart, it was found that there was cosine squared the angle that matched, so 30* angle was 75% matching and 60* was 25% matching.
As the analysis of the experiment continued, the assumption had to be made by Bell that reality is local. This means that the factor that determines the results at Alex’s detector, can play no role in Boris’ results. Simply, Alex’s results from rotating the detector are caused only by his actions and are in no way impacted by Boris’. The same applies in the reverse. This is the locality assumption.
Following that assumption, we can look at the two different sequences found. Alex’s results, when the detector was turned to 30*, produced a 25% mismatch. Boris’ 30* turn resulted in a 25% mismatch as well. Thus, the total mismatch, with both turned, would be at most 50%. Continuing in the same way we can expect that with both detectors turned to 60*, the mismatch would be again at most 50%.
However, the results showed differently. When Alice and Boris’ detectors were turned to 60*, the mismatch was 75%! This was 25% greater than expected, even beyond error in the detectors. This demonstrated that the assumption made previously, that reality is local, was false.
And that is where the question became… either the two particles were communicating with each other or they have hidden information in them that designate the spin orientation.
Bell thus proposed two choices that would shape reality. The first is that “reality is irreducibly random, meaning that there are no hidden variables that determine the results of individual measurements” or that “reality is ‘non-local’, meaning that the setting of one measuring device can influence the reading of another instrument, however remote”.
In a non-local reality, this means that the results at Alex’s detector did not depend on causes existing only in Abbotsford, but also somehow depended on Boris’ detector in Bannockburn.
Bell surmised that reality is non-local and his theorem was that “no physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics” It is here that Bell’s experiment extended beyond entangled particles to a question about reality itself, as to whether reality is local or non local. This is a debate that still arises amongst physicists.
Bell said “…there must be a mecahnism whereby the setting of one measuring device can influence the reading of another instrument, however remote. Moreover, the signal involved must propagate instantaneously, so that such a theory could not be Lorentz invariant.” Bell, John (1964). “On the Einstein Podolsky Rosen Paradox” (PDF). Physics. 1 (3): 195–200.
And in a radio interview, he said “There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behaviour, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the ‘decision’ by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster-than-light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already ‘knows’ what that measurement, and its outcome, will be.” The Ghost in the Atom: A Discussion of the Mysteries of Quantum Physics, by Paul C. W. Davies and Julian R. Brown, 1986/1993, pp. 45-46
Bell’s theorem summarized is that “either causal influences are not limited to the speed of light, or events can be correlated for no reason”. This allows the two choices – either “hidden meaning” or “non local reality” to come to a compromise and agree with at least a single Bell’s theorem. Meaning, that for those who are non-localist can conclude that causal effects can go faster than light, and for localists, they can conclude that some events are related for no reason.
This concept of entanglement has led to much research, leading into new ways of working with entangled particles, including cryptography and quantum computing. The idea of quantum teleportation also stems from this research. However, while Bell’s theorem does lead to new practical research and technology, even moreso it challenges us to question how we view the world and reality.