Lee Smolin is a theoretical physicist and cosmologist who is best known as one of the contributors to the theory of loop quantum gravity, the major alternative to string theory, which Smolin criticized in his 2006 book The Trouble with Physics.

Lee Smolin was a founding faculty member of the Perimeter Institute in Waterloo, Canada.

In his latest book, Einstein's Unfinished Revolution, Smolin says he accepts Einstein's idea that quantum mechanics is "incomplete," that it can be completed by adding "hidden variables," and that this would restore a "realistic" picture of nature.

Einstein and other realists believe that quantum mechanics gives us an incomplete description of nature, which is missing features necessary for a full understanding of the world. Einstein sometimes imagined that there were “hidden variables’’ which would complete the description of the world given by quantum theory. He believed that the full description, including those missing features, would be consistent with realism.

Thus, if you are a realist and a physicist, there is one overriding imperative, which is to go beyond quantum mechanics to discover those missing features and use that knowledge to construct a true theory of the atoms. This was Einstein’s unfinished mission, and it is mine.

Einstein's Unfinished Revolution, The Secret for What Lies Beyond the Quantum, p.xxi

Smolin describes his opponents, the anti-realists...

Some anti-realists believe that the properties we ascribe to atoms and elementary particles are not inherent in those objects, but are created only bv our interactions with them, and exist only at the time when we measure them. We can call these radical anti-realists. The most influential of these was Niels Bohr. He was the first to apply quantum theory to the atom, after which he became the leader and mentor to the next generation of quantum revolutionaries. His radical anti-realism colored much of how quantum theory came to be understood.

Another group of anti-realists believes that science, as a whole, does not deal in or talk about what is real in nature, but rather only ever talks about our knowledge of the world. In their view, the properties physics ascribes to an atom are not about that atom; they are instead only about the knowledge we have of the atom. These scientists can be called quantum epistemologists.

And then there are the operationalists, a group of anti-realists who are agnostic about whether there is a fundamental reality independent of us or not. Quantum mechanics, they argue, is not in any case about reality; it is rather a set of procedures for interrogating atoms. It is not about the atoms themselves; it is about what happens when atoms come into contact with the big devices we use to measure them. Heisenberg, the best of Bohr’s protégés, who invented the equations of quantum mechanics, was, at least partly, an operationalist.

In contrast to the disputes between radical anti-realists, quantum epistemologists, and operationalists, all realists share a similar perspective—we agree about the answer to both questions I posed above. But we differ on how we answer a third question: Does the natural world consist mainly of the kinds of objects that we see when we look around ourselves, and the things that constitute them? In other words, is what we see when we look around typical of the universe as a whole?

Those of us who say yes to this question can call ourselves simple or naive realists. I should alert the reader that I use the adjective “naive” to mean strong, fresh, uncomplicated. For me, a view is naive if it is not in need of sophisticated arguments or convoluted justifications. I would argue that a naive realist is, whenever possible, to be preferred.