Can physics allow us to know the reality of nature or does it merely tell us how nature appears? Or for that matter, what are the limits of knowledge in physics, constrained as it is to giving responsible proof for the claims it makes? The enquiry into what we can know can be traced back to the philosophy of Ancient Greece. In pre-Socratic times there was a fundamental disagreement between Protagoras and Democritus on the issue of realism. Protagoras held that all we can know are the sensations that we receive. He maintained that we can know nothing of what is out there causing these sensations. Democritus, on the other hand, insisted that “things” exist separately from our perception of them.

Fast forward to the mid-1920s and the jury is still out on this issue. Based on developments in quantum physics, Niels Bohr, an indisputable giant of early 20th century physics, decreed that “… it is wrong to think that the task of physics is to find out how nature is. Physics concern what we can say about nature.” For Bohr our observations create the reality that we are observing. Albert Einstein, on the other hand, maintained that “Physics is an attempt conceptually to grasp reality as it is thought independently of its being observed.” This dispute became what is now called the Bohr-Einstein debate.

In 1935, in his bid to hold on to his concept of reality, Einstein (with collaborators Podolsky and Rosen) launched his most scathing attack on quantum theory by declaring that it was incomplete. Using a property of quantum systems, called entanglement, he demonstrated that there were elements of reality that were not captured by quantum theory. Bohr fairly quickly produced what he claimed to be rebuttals of Einstein’s claims. Although many accepted that Bohr had prevailed, the argument by Einstein and his collaborators (EPR) was relegated as a paradox in quantum mechanics.

It was not until the mid-60s that a seminal paper by John Bell provided a first hint that the deadlock between the two viewpoints could be broken. He showed that if reality was assumed in the way that Einstein did, one could, in principle, subject this notion to an experimental test. Writing down, what is now called Bell’s inequalities, he showed that if these inequalities are violated then one has to abandon Einstein’s brand of realism. The results of this test only emerged in the early 80s when Alain Aspect and collaborators showed that the inequalities are indeed violated.

But as one starts to draw the curtains on this debate, other interpretations emerge that attempt to salvage realism by reinterpreting quantum theory. So the saga continues…. In this module students will be taken through a journey that showcases the developments and the key ideas that have shaped our current views on reality (or lack thereof).