It all began in 1927 at the fifth Solvay Conference in Brussels when Niels Bohr presented his arguments which became basis of Quantum mechanics. Bohr believed that the particles (such as electrons) don’t have well defined properties (like position) until they are observed. The moment a particle is observed, it is forced to choose a position and that is the one which we observe. Before observation, the particle has no definite position but rather it has chance of being in all possible positions. This chance is not like the chance in classical objects. The chance in classical objects like toss of coin is due to lack of our knowledge.
If we know variables like initial force imparted to the coin, its rotations per second etc then we can predict its outcome with certainty. But the chance in quantum mechanics is not due to lack of knowledge or technology but the chance in quantum mechanics is fundamental. It is inherent. It was this basic point which perturbed Einstein the most. He argued that particles have definite properties irrespective of whether observed or not. For him it was completely absurd that individual particles are ruled by chance. Einstein along with Padolsky and Rosen wrote up a thought experiment to show this absurd outcome.
Consider a spin zero particle decays into two different particles, say particle A and particle B, having opposite spin orientations. So if we measure particle A’s spin to be spin up we instantly knew that particle B’s spin is spin down.It seems to be simple but it is not. EPR (Einstein, Padolsky and Rosen) gave two alternatives for this:
First, according to Einstein’s view, both the particles have definite spin values (say spin up for particle A and spin down for particle B) from the moment of decay. The measurement merely reveals this pre-existing state. The Quantum theory, being incomplete, needs certain hidden variables to yield these definite values.
Second, the Quantum theory is complete and true then particles do settle on a spin up or down only when measured. So the moment we measure the spin of particle A to be spin up, the particle B instantly knew it and settles on spin down. But how does the particle B knew it. Measurement of particle A’s spin has a definite effect on particle B’s spin.This was called by Einstein “spooky action at a distance” and was latter dubbed as “ Entanglement” by Erwin Schrodinger.
This thought experiment can be compared to a machine that throws out balls of opposite colours in opposite directions, in the direction of observer A and observer B. So when the observer A catches a ball and sees that it is white, he immediately knows that observer B has caught a black one. According to Einstein’s view, the colour of the balls is pre-existing and the observation just makes the colour visible. But according to Quantum mechanics, the balls were neither black nor white until someone looked at them. When the ball of observer A turned white then the ball of observer B instantly knew it and turned black. We say the balls are entangled.
The problem languished for three decades until in 1964, John Bell translated the two EPR alternatives into a mathematical relationship known as Bell inequality. If the predictions of Quantum mechanics are correct, the Bell inequality is violated and if hidden variable theory is correct, the inequality is not violated. Bell predicted this but the way he predicted it is not suitable for experimental tests. In 1969, John Clauser not only derived the Bell inequality in such a way that it could be tested but he designed and performed the critical experiment himself along with then Ph.D. student Stuart Freedman. In the experiment, pairs of photons were sent in opposite directions towards detectors that could measure their state of polarisation.
Clauser and Freedman showed that it violates Bell’s inequality. But the result was hardly conclusive because of various loopholes in the experiment. The most concerning was the locality loophole. If one of detectors could send a message to the other (at the speed of light) about their experimental results, so the detector settings must be quickly changed (in nanoseconds) while photons are on the fly. And it was Alain Aspect who proposed a way for doing this ultra speedy switch. His group’s experimental results, published in 1982, proved Quantum entanglement to be real. In 1998, Anton Zeilinger and his team improved on Aspect’s earlier work by conducting a Bell test over a then unprecedented distance of 400 metres and found Quantum mechanics as flawless as ever. John F. Clauser, Alain Aspect and Anton Zeilinger were awarded the Nobel Prize in Physics 2022 for their work on Quantum mechanics.
(Author teaches Physics at HSS Hawal Pulwama. He can be reached at Brotherjaveed@yahoo.com)