Welcome to my first blog article in the Science and Technology section, titled ‘Does God play Dice?’. It is about the mystery of quantum mechanics, and introduce the concept behind the so-called ‘hidden variable theories’ of quantum physics. I wanted to write about it because it is related to the topic of my next book which is called ‘The Hidden Variable Theory’, a Sci-Fi thriller set in China and England. I expect to complete it at the end of 2016.
‘God does not play dice’
Einstein once famously said the above quote about quantum mechanics – the most successful physical theory in physics that has proven itself in numerous experiments and applications. Although Einstein himself was a founding father of quantum theory, he grew increasingly disillusioned with the theory of quantum mechanics because physical properties (e.g. position and momentum of a particle) according to quantum mechanics can only be predicted via a probability interpretation. Furthermore, according to Heisenberg’s uncertainty principle, an object cannot simultaneously have certain properties, e.g. it is impossible to know both the position and momentum of a particle exactly.
For Einstein, the world of quantum theory was simply not right. It led him to develop several ingenious thought experiments to disprove the validity of quantum mechanics. Most famously his titanic struggles with Niels Bohr, another pioneer of quantum theory, were well documented. Every time Einstein proposed an ingenious experiment attacking the integrity of quantum mechanics; every time he eventually lost the argument to Bohr, including one time when Bohr applied Einstein’s own General Relativity theory to defeat Einstein’s argument. But Einstein did not give up easily.
In the spring term of 1989 I was very fortunate for my final year project to have the opportunity to do my first ever physics research project about Bell’s theorem which delved into the nature of quantum mechanics. Bell’s theorem was inspired from the famous 1935 scientific paper by Einstein, Podolsky and Rosen (or the ‘EPR’ paper for short) entitled “Can Quantum Mechanical Description of Physical Reality Be Considered Complete?” Having failed to prove the inconsistency of quantum mechanics, Einstein and colleagues were able to formulate a problem which apparently demonstrated a negative answer to the above question. This time, even the great Bohr was struck; “This (the EPR paper) onslaught came down upon us as a bolt from the blue” remarked Rosenfield, a colleague of Bohr.
However, Bohr was eventually able to refute EPR’s argument using arguments based on a quasi-philosophical interpretation of quantum mechanics – the so-called Copenhagen interpretation which roughly says that it is impossible to ignore the role of the observer and the measuring apparatus when uncovering the nature of any (elementary) object. Such was the success of quantum theory and the authority of Bohr, Einstein’s concern was pretty much marginalised and ignored until 1951 when the arguments of EPR can be appreciated more readily from a thought experiment proposed by David Bohm. He considered a pair of spatially separated particles originally prepared in the so called spin singlet state, i.e. before the particles were separated they were in a combined system with zero total spin. According to quantum mechanics, each of these particles when separated possesses a quantum mechanical property called spin, where the spin components of each of these particles can be measured independently along any direction. Hence, if one measures the spin component of either particle, then one can predict the spin state of the other particle without disturbing it, e.g. if particle 1 was measured to have spin-up (say) then without measuring particle 2 we can predict with certainty that it will have an opposite spin (or spin down) because quantum theory predicts it. Quantum mechanics cannot predict if particle 1 is in the spin up or down state before the measurement, but once the measurement is made and the experimenter knows its spin state, then as if by magic, another experimenter with the same setting of the measuring apparatus (i.e. measuring the spin about the same axis) would always measure the opposite spin result for particle 2.
This ‘entanglement’ property of the two particles is a fundamental nature of quantum mechanics, but it is extremely strange that such entanglement should persist or exist at all once the particles have separated- indeed the particles could be many thousands of miles apart when such measurements are made. How can one particle be instantly correlated to another at all times and distances?
Consider the last question above; according to quantum mechanics, whatever spin component is first measured in particle 1, the measurement of particle 2 always produces the opposite spin when measured along the same axis. Could there be some form of communication between the particles, or as Einstein put it: some ‘spooky action at a distance’ happening? Another view to consider is that quantum mechanics is incomplete. That is, these particles were actually created with definite opposite spin about every axis. Hence, there follows the inference there are some hidden variables associated with the properties of the particles that are missing in the current formulation of quantum theory.
We can examine these problems another way by considering the following; what if the experimenter who was making the measurement for particle 2 decided to measure its spin component along an axis different to particle 1? According to quantum mechanics, particle 2’s spin state is completely random, with a 50% chance of spin up or spin down. But how does particle 2 know that particle 1’s spin state was measured along a different axis for it to give rise to a random result, and also to give a definite opposite result had particle 1 been measured along the same axis. Hence there must be some form of action at a distance (or ‘non-local’ action) between the particles, or that particle 2 knows more information than quantum mechanics allows via additional hidden variables.
Subsequently, a number of hidden variable theories were formulated, although none of them were very compelling. However, world attention to the ‘EPR paradox’ returned with vengeance in 1964 when Northern Irish theoretical physicist John Bell showed in a series of brilliant papers that no hidden variable of any kind can reproduce all the experimental predictions of quantum mechanics. This is the famous Bell’s theorem which was very significant because it led the way to practical experiments that would verify the nature of quantum theory. Subsequently these experiments have been shown to agree completely with quantum mechanics while part of the predictions made by the hidden variable theories differs from experimental observations.
What does this mean? As the predictions of quantum mechanics is correct then we must accept that 1) somehow ‘spooky’ action at a distance (non-locality) is happening in the quantum world, and/or that 2) the concept of physical reality as commonly perceived doesn’t really hold. Personally, I don’t like ‘action at a distance’ as a plausible explanation. Could one really believe that two elementary particles separated by any distance could communicate and then conspire to act with each other? Hence, abandoning conventional understanding of realism is the option that I have chosen – which is tantamount to seriously questioning some apparently strongly held views of the world, such as “Is the moon really there, when nobody looks?”
With much gratitude to my supervisor Dr Tony Sudbery, I felt extremely privileged to have been given a taste of the excitement and challenges of this area of research. I hope some readers have also felt that sense of wonder too. Do you agree that one needs to abandon realism? Or perhaps you wish to remain highly sceptical like Einstein – who once wrote “Quantum mechanics is most awe-inspiring. But an inner voice tells me that this is not the real thing after all”.
 Via his famous 1905 paper on the photoelectric effect where he postulated the existence of light as a particle object (i.e. photons).
 A reasonable and precise definition of physical reality was proposed in the EPR paper; “If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity”.
 Realism is the philosophical doctrine that the world is made up of objects whose existence is independent of human experience. Realism is therefore the criterion in EPR’s definition of a ‘physical reality’.
 Don’t worry if you don’t totally understand this section – it took me many hours of re-reading and re-reflecting on the EPR problems before I understood it again! The brilliant American physicist Richard Feynman once said” I think it is safe to say that no one understands quantum mechanics”.