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Does reality need realism?

If this sounds like a strange question to you, consider the fact that it is most pressing for physicists and for the most successful theory about the physical world. This theory is called quantum mechanics – and every digital electronic device you have ever used owes its existence to understanding the physics of the atomic scale that accompanies it.

But for all its success, quantum mechanics has one small problem: you do not understand.

To be more exact, even a century after its birth, no one really understands what quantum mechanics is telling us about the nature of reality itself. This open and uncertain territory is the focus of Lee Smolin's new book *Einstein's unfinished revolution: the quest for what is beyond quantum*.

Smolin is an extremely creative thinker who has been a leader in theoretical physics for many years. He is also a talented writer who can translate his own ideas about how science works in engaging language and compelling stories. For the full disclosure, I had the pleasure of meeting Smolin several times at science events and admire his originality and iconoclasm. Both are in full *Einstein's unfinished revolution*. The fact that I disagreed much with him and his view of quantum mechanics made me appreciate the book even more.

Disagreement practically defines the debate over the meaning of quantum mechanics, and Smolin's book is the latest addition to a number of excellent works on the subject, including *Through two doors at the same time: the elegant experiment that captures the puzzle of our quantum reality* by Anil Ananthaswamy and *What is real? The unfinished quest for the meaning of quantum physics* by Adam Becker. As you can tell from the titles, 100 years after its foundation, quantum mechanics and reality remain a hot topic.

And *is* the problem with quantum physics and reality? If all you want are calculations to build a laser or a computer chip, the answer is *anything*. Quantum mechanics gives physicists a hyper-precise mathematical machinery to manipulate the atomic world with hyper-precise specifications.

The problem comes when you ask a simple question like, "What is an atom?" Before quantum mechanics, you could imagine atoms as little billiard balls. They were small "things" that, like the great things we encounter in our daily lives, had definite properties like their position or the speed with which they are moving. They were, in other words, *real* Likewise, we find that tables and chairs are real. What that means is that we think that the tables and chairs (and the other things we find on a day-to-day basis) exist independently of us. This makes us, in the words of Smolin, *Realistic* on tables and chairs. Smolin thinks this kind of realism is what it is to be a physicist:

"We realists believe there is a world out there whose properties in no way depend on our knowledge or perception of it … We also believe that the world can be understood and described accurately enough to explain how any system in the natural world behaves. "

Unfortunately, the history of quantum mechanics made it difficult to be Smolin's realistic version of atoms. It is a field full of strangeness. For example, in the standard mode of dealing with quantum physics, if you put an atom in a closed box with a partition, the atom will exist on both sides of the box. *at the same time*. It is just by opening the box (making a measurement) that the atom "collapses" on one side of the box or the other. Unpacking the experimental basis for this type of claim is the Ananthaswamy point *Two doors at the same time.*

How can the atom be in two places at the same time? And why would the act of looking at it force the atom to choose one side or the other? Let's face it, tables and chairs do not behave this way. So the question becomes: how to interpret quantum mechanics? It is the role of measurement, necessary to give the defined properties of the atom (like position), which really bothers the realists. Measurements and observations are an essential part of Copenhagen's standard interpretation of quantum mechanics established by the Danish physicist Niels Bohr. Realists hate the Copenhagen interpretation. So how then can the realism we have on tables and chairs still be maintained for atomic-scale quantum phenomena?

Smolin's answer is that the strangeness that undermined the realism of the atomic world is a fundamental problem of quantum theory, as it is now. He sees the strangeness as "gaps and failures" that "support the fact that we can only partially solve the central problems of science before they seem lost." For him, there must be a deeper theory waiting to be discovered.

The task that Smolin set for himself is to show readers that already existing paths can lead to a new theory that goes beyond quantum mechanics. To do this, Smolin begins the book with a lucid explanation of the basic rules of quantum theory and where exactly what he calls "anti-realist" focus on measurements and observations appears. He then takes readers through a compelling tour of history, including a reappraisal of the famous arguments between the anti-realist Niels Bohr and the unrepentant realist Albert Einstein. Smolin's particular focus then turns to American physicist David Bohm and his "pilot wave theory," which has never required observers to concede reality to tables and chairs. The story of Bohm, as well as of other quantum rebels against Copenhagen, was also approached cleverly in Becker's book. *What is real?*

*Einstein's unfinished revolution* is at its best when it is laying these foundations for realistic vs. anti-realism. Smolin's description of how quantum mechanics works is both stylish and accessible using real-world examples, such as the order in which you put your clothes – first underwear and pants second against the other – to demonstrate a fundamental principle of weirdness quantum theory. His description of Bohm's alternative to Bohr's Copenhagen interpretation is also clear and will be useful to non-scientists who try to understand how these different interpretations are resolved and why they are important. If you want to understand the basic issues at hand in these long-running debates, as well as at least one alternate line, you will enjoy this book. The last third of the book, where Smolin articulates the directions to proceed now, may be a more difficult climb for some people – but it is still worth seeing where Smolin believes the promise is.

I liked very much *Einstein's Unfinished Revolution,* just as I did with the books of Ananthaswamy and Becker. Together, they demonstrate how this debate remains vibrant. Part of the fun of Smolin's book, to me, is that I'm just not your kind of realist. For me, even the use of the terms "realism" versus "anti-realism" prevents us from seeing the exaggerated price that every interpretation of quantum mechanics forces us to pay in describing the atomic world. There is no way to go back to simple pictures of simple polka dots jumping in space. When it comes to the atomic realm, there will be some kind of weirdness. Then, until we have experiences that separate one interpretation from the other (if we ever do), the real question becomes: What strangeness can we accept?

But Smolin so clearly and cleverly articulates his position that *Einstein's unfinished revolution* it becomes useful if you agree, disagree or just want to learn some really cool ideas about physics and reality.

*Adam Frank is professor of astrophysics at the University of Rochester and author of *Starlight: Alien Worlds and the Destiny of the Earth*. You can find more of Adam here: *@ adamfrank4.

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