On your wavelength

Interactions: Conversation with Anil Ananthaswamy

After writing two successful books on neuroscience (“The man who wasn’t there”) and big instruments (“The edge of physics”), how did you choose quantum mechanics as the topic for your third book?

I’m drawn to such stories, whether there are about cosmology or the nature of the human self and consciousness, or in the case of the new book, the nature of quantum reality. While these topics seem disparate, I think they all address fundamental questions that deep down thrill us all: Who are we? What are we made of? What’s the fundamental stuff of the universe? Where do we come from? Where are we headed?

Given the scope of these questions, I feel lucky to have found ways to tackle them through stories that are grounded in everyday reality: in the telescopes and instruments we build in extreme places, or in the lived experience of people who are coping with their altered selves, or in the deceptively simple double-slit experiment and its many variants.

Why do you think physicists love the double-slit experiment to the extent that they voted it twice as the most beautiful experiment (both in Physics World and in Nature)? Could you try to define the aesthetic of the double-slit experiment?

Richard Feynman, in his paean to the experiment, said it best: he said that it contains the “central mystery” of quantum mechanics. Almost all of the conundrums and peculiarities of the quantum world can be understood using the double-slit experiment. In popular culture, the experiment is often used to highlight wave-particle duality and the apparent centrality of the observer in quantum mechanics: observe a photon going through the double-slit and it acts like a particle and it goes through one slit or the other; look the other way, and it behaves like a wave, seemingly going through both slits at once and interfering with itself. It’s obvious that such an account is enticing to a layperson—it opens a window to the mysterious world of the quantum, seemingly giving humans the power to create reality.

The actual story of the experiment is far more complex. Even hardened physicists are hard-pressed to explain just what is happening with the double-slit experiment. The simplicity of the experiment is obvious, an explanation feels palpable, but nonetheless, when physicists try and make intuitive, physical sense of it, they don’t quite succeed. There are mathematical ways of explaining the experiment, but when you try and interpret the mathematics to say something about physical reality, you stumble. Does a photon go through both slits at once? If yes, how can it do that? If not, does something else do so? If so, what is it? If nothing goes through both slits at once, how can you explain the interference pattern, which is a clear sign that something went through both slits at once? And so it goes, in circles.

And that’s the aesthetic allure of the double-slit experiment. How can something as simple as a photon going through two slits make us confront the very nature of reality? How can one experiment make us debate realism and anti-realism (whether or not reality exists independent of observers), locality and non-locality (whether or not there is instantaneous influence between two regions of space-time, defying special relativity), and determinism and non-determinism? And of course, the experiment raises questions about the role of measurement (observer) in collapsing quantum states to classical ones—and it’s nowhere near as settled an issue as lay accounts of quantum mechanics often suggest.

The experiment can even be used to explain the alternatives to the Copenhagen interpretation, including the Many Worlds interpretation, Bohmian mechanics (where reality is both wave and particle), and spontaneous collapse theories (where quantum systems in a superposition of states stochastically collapse to some classical state). The double-slit can thus be used to probe whether there is a divide between the quantum and classical worlds.

There is no other experiment quite like the double-slit in all of science.

Who is this book for? Do you worry that the general public may be intimidated by the experiment schematics throughout the book?

This book is for the interested lay-reader—someone who wants to engage with these profound questions about the nature of reality. I think the experiment and the schematics are important to get a physical intuition for the strangeness of the behaviour of the quantum world. Often, mere descriptions in words don’t get the point across. My illustrator and I worked really hard to make the drawings accessible and even alluring. I do hope that readers find them helpful.

Leaving the original double-slit experiment aside, which are the top two weirdest experiments in quantum optics?

Any experiment that demonstrates non-locality belongs in that list (keeping in mind that notions of non-locality are themselves debated, depending on the interpretation of quantum mechanics to which you hitch your wagon). While I describe in my book Alain Aspect’s pioneering 1982 experiment that demonstrated a violation of Bell’s inequality and showed that you cannot dismiss non-locality, there is a rich history that leads up to Aspect’s experiment that I did not get into, nor did I address the experiments that followed Aspect’s (such as the Bell’s inequality tests done across the mountaintops in La Palma and Tenerife in the Canary Islands), because they are not really double-slit experiments.

I’d also put experiments demonstrating entanglement-swapping in that list. Traditionally, when you do experiments with entangled photons (as in tests of Bell’s inequality), the photons are entangled because they have interacted. But entanglement swapping can cause two photons that have never interacted to become entangled. That’s pretty weird.

It is very nice that your book brings new actors to the stage, contemporary physicists, not only the fathers of quantum mechanics. How did you find the current generation of researchers working in the foundations of quantum mechanics?

I had interviewed some of these researchers over the years because of the stories I wrote as a journalist. It wasn’t very hard to find them. A few of them are rather well known in the community, including Markus Arndt and Aephraim Steinberg. One lead led to another and before long I had talked to many of them. I find experimentalists refreshing—they know what they are seeing in the labs, and are often less dogmatic about interpretations and more likely to have an open mind.

Credit: Rajesh Krishnan

Was there anything you left out of the book, but wished you did not?

If I had had the space—or the rather the luxury of explaining more thought experiments without overwhelming the reader—I would have liked to discuss Leonard Susskind’s take on the double-slit experiment as he explained it to me. It pertains to his ideas about ER=EPR (where ER refers to an Einstein-Rosen bridge, or a wormhole, and EPR refers to entanglement). But it would have required deviating considerably from the main thrust of the book, which is firmly grounded in the foundations of quantum mechanics.

Also, in the book, I focus on the Mach-Zehnder interferometer, as a special case of the double-slit experiment. I’d have liked to mention the Hong-Ou-Mandel interferometer, in which two unrelated photons enter the interferometer at the same time, and the setup is designed to demonstrate interference between the various states of the unrelated photons. It’s an exciting, but more complicated version of the double-slit experiment.

The book “Through two doors at once” will be released on August 7th 2018.

You can find out more about Anil Ananthaswamy on his website.

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