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Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

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One of Smithsonian's Favorite Books of 2018
One of Forbes's 2018 Best Books About Astronomy, Physics and Mathematics
One of Kirkus's Best Books of 2018

The intellectual adventure story of the "double-slit" experiment, showing how a sunbeam split into two paths first challenged our understanding of light and then the nature of reality itself--and continues to almost 200 years later.

Many of science's greatest minds have grappled with the simple yet elusive "double-slit" experiment. Thomas Young devised it in the early 1800s to show that light behaves like a wave, and in doing so opposed Isaac Newton. Nearly a century later, Albert Einstein showed that light comes in quanta, or particles, and the experiment became key to a fierce debate between Einstein and Niels Bohr over the nature of reality. Richard Feynman held that the double slit embodies the central mystery of the quantum world. Decade after decade, hypothesis after hypothesis, scientists have returned to this ingenious experiment to help them answer deeper and deeper questions about the fabric of the universe.

How can a single particle behave both like a particle and a wave? Does a particle exist before we look at it, or does the very act of looking create reality? Are there hidden aspects to reality missing from the orthodox view of quantum physics? Is there a place where the quantum world ends and the familiar classical world of our daily lives begins, and if so, can we find it? And if there's no such place, then does the universe split into two each time a particle goes through the double slit?

With his extraordinarily gifted eloquence, Anil Ananthaswamy travels around the world and through history, down to the smallest scales of physical reality we have yet fathomed. Through Two Doors at Once is the most fantastic voyage you can take.

304 pages, Hardcover

Published August 7, 2018

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About the author

Anil Ananthaswamy

9 books76 followers
ANIL ANANTHASWAMY is former deputy news editor and current consultant for New Scientist. He is a guest editor at UC Santa Cruz’s renowned science-writing program and teaches an annual science journalism workshop at the National Centre for Biological Sciences in Bangalore, India. He is a freelance feature editor for the Proceedings of the National Academy of Science’s “Front Matter” and has written for National Geographic News, Discover, and Matter. He has been a columnist for PBS NOVA’s The Nature of Reality blog. He won the UK Institute of Physics’ Physics Journalism award and the British Association of Science Writers’ award for Best Investigative Journalism. His first book, The Edge of Physics, was voted book of the year in 2010 by Physics World. He lives in Bangalore, India, and Berkeley, California.

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Displaying 1 - 30 of 107 reviews
Profile Image for Max.
341 reviews299 followers
August 26, 2020
If you are fascinated by the classic double-slit experiment this is your book. Written for the nonscientist in straightforward language, it is for readers with a strong interest in quantum mechanics. Of popular physics books I have read, this one is unique in its focus on the double-slit experiment. Most of the experiments use an interferometer to duplicate and go beyond the original experiments. One experiment traverses over 100 kilometers going between two islands. Ananthaswamy uses each experiment to illustrate different aspects of the puzzling results. He discusses these with many of the scientists who performed them getting their insights. He even interviews Roger Penrose in Oxford about his pet interpretation of quantum mechanics. The book includes snippets of history of the development of quantum mechanics covering the many different takes on what it means. At the end he runs through the main interpretations but doesn’t take a position on any of them. Ananthaswamy is a science journalist who is well versed in his subject, not a physicist backing one particular view. This nonpartisan approach was refreshing in this type of book.

The heart of the book is about the experiment itself showing us the many ways it has been done over the years. We get useful diagrams of the different interferometer setups with clear explanations of what happens. The first experiments are the simplest. They show the particle-wave duality, the interference pattern created by a single particle and that any sort of measurement or detection causes the particle to take a definite path. While this may seem like standard fare, there are many nuances in the different setups that demonstrate a variety of indirect measurements changing the particle’s behavior. Ananthaswamy goes on to describe experiments using entangled pairs of particles where one contains information about the other and shows that erasing the information reverses the act of measurement reversing the other particle’s quantum state. Particularly stunning is that this reversal works even if that information is erased after the other particle has finished the experiment seemingly going back in time to change the results. As an encore he demonstrates how to detect an object without anything encountering it. The whole series of experiments left my head spinning.

At the end he devotes a few chapters to the interpretations. He pretty much assumes the reader is familiar with the Copenhagen interpretation and goes through the de-Broglie-Bohm theory, GRW theory, QBism, Many Worlds theory and briefly mentions a few others including Penrose’s. He doesn’t favor any one, but believes all should be kept in mind because even if all are wrong, one might lead to a new idea, a new theory that may provide a better explanation. He provides some examples of new lines of thought. But it wasn’t Ananthaswamy review of the theories that made this book a winner. It was his descriptions of the many experiments that made clear that acquiring or erasing any form of information that defined a particles’ path changed its quantum state, even apparently going back in time to do so. Ananthaswamy leaves us with no doubt about how profoundly mysterious the quantum world is.
Profile Image for David Katzman.
Author 3 books446 followers
January 10, 2019
Ananthaswamy introduces extremely complicated ideas in a friendly conversational manner. Admittedly, even for those familiar with quantum physics, it’s easy to get lost in some of the experiments and theories. But despite the times when I was lost, it was a fantastic overview of many of the current diverse theories that attempt to explain the mysteries of quantum physics.

The Double-Slit Experiment is one of the earliest physical experiments that demonstrates many of the mind-bending attributes of particles, particularly photons and electrons. I was surprised to learn that they’ve even demonstrated these quantum effects with particles larger than atoms. I also had no idea there were so many variations of the Double-Slit Experiment, and that it is still being used to test new quantum theories and variations of existing theories. Significant portions of the book explain this experiment in detail and numerous variations of it. This is where I occasionally got lost. Some of the experimental variations are quite complex. But it’s a worthy effort to follow because—as abstract as quantum theory is—the results of it are borne out in experiments. It’s not just abstract philosophy, it produces results.

By means of the Double-Slit Experiment, Ananthaswamy takes us through two sides of Quantum Mechanics. The strangeness and mysteries of particle behavior and the various theories that attempt to explain that weirdness. And between these two sides sits the Schrödinger Equation that mathematically calculates the behavior of particles going through the experiment (as well as, of course, in other contexts, such as particle behavior around atoms).

The weirdness of quantum mechanics includes, but is not limited to, wave/particle duality, entanglement, spooky action at a distance (also called non-locality), and the very nature of probability versus randomness.

As he takes us through various versions of the experiment, and stories about the various physicists who have contributed to quantum mechanics, he also takes us through a survey of the majority of currently en vogue quantum theories that try to explain the weirdness. From the conservative Copenhagan interpretation to Multiple Worlds Theory to some that I had never even heard of! He explains the strengths and weaknesses of each and what they do and don’t explain successfully. It was fascinating to learn about what some of the holes are, and it certainly appears that there is no entirely satisfactory theory in place yet. For example, the Copenhagan theory fails to explain where the transition occurs from quantum to non-quantum observation. It also forces you to accept non-locality, meaning particles that are billions of miles apart could instantaneously affect each other, far faster than the speed of light. The Multiple Worlds theory solves both of those particular issues (and many others), but it fails to explain what “probability” means in the context of the Schrodinger’s equation. It fails to justify the randomness that we “observe” from repeated experimentation. There are some ideas how to deal with this, but none seem entirely satisfactory yet.

Ananthaswamy does a nice job explaining most of the theories, again, admittedly, with some difficult-to-understand parts. But given the complexity of the issues, I think it was commendable. Overall, a fascinating and wonderful book.
Profile Image for Brian Clegg.
Author 185 books2,513 followers
August 20, 2018
It's sometimes hard to imagine that there's anything new to say about the basics of quantum physics, yet Anil Ananthaswamy manages this in a twofold manner (appropriately, given the title). Through Two Doors at Once does so by using the double slit experiment as a constant reference point throughout the book, and by bringing in a number of the more modern variants on the experiment which rarely feature in popular accounts of quantum theory.

Strictly, the book should probably be called 'Through Two Doors at Once and Spooky Action at a Distance plus Things That Have a Similar Effect', as it uses both the double slit experiment and the EPR entanglement thought experiment, plus modern experiments which don't, for example, involve slits but rather beam splitters that are their logical equivalent - but I have to admit, that would be a clumsy title.

Ananthaswamy gives us a good overview of the development of quantum physics - sometimes quite summary - but by making repeated use of the double slit, going all the way back to Thomas Young, but also looking at the quantum specifics, he both helps the reader get a better feel for just why quantum physics can seem strange and also what the different interpretations, from Copenhagen to Many Worlds, tell us about what we can and can't know of what's happening inside the experiment.

The part of the book covering interpretations is perhaps slightly less effective than the rest, because, in the end, unless you are an enthusiast for a particular interpretation, the diversity of ideas tends to obscure, rather than help get a better understanding. (We still have to come back to Feynman's crushing '[Y]ou think I’m going to explain it to you so you can understand it? No, you’re not going to be able to understand it... You see, my physics students don’t understand it either. This is because I don’t understand it. Nobody does.')

The best part of this section is the explanation of the Bohm/deBroglie interpretation where there is both a wave and a particle, though there is one minor problem here, as we are told that making a strong measurement in the Bohm model leaves particles where you don't expect them to be - but are not told why the strong measurement of the particle causing a scintillation on a screen does leave them where we expect them to be.

For me, the only real improvement would have been to put a bit more character into the historical context: it's rather dry and summary. So, for example, we are not told about Einstein's dismissive 'Ist mir Wurst' remark about EPR's confusing use of two measurements. Another example: John Wheeler is described as coining the term 'black hole', rather than giving us the more interesting actual story. There's enough to get the point, but it could have been made more engaging.

Overall, though, Anathanswamy cleverly comes at quantum physics from a different direction, and as a result, adds to the picture we get from most popular titles. We really get into why the double slit plus entanglement are often described as the central mysteries of quantum theory, and though they can still send the brain spinning, there's the best description of many of the more recent experiments I've seen - useful as they can seem a little pointless without this kind of in-depth picture. An excellent addition to the 'Quantum physics for the rest of us' shelf.
Profile Image for Cindy.
145 reviews56 followers
September 15, 2020
This book was the Bohm dot com! Have you heard of Bohm, of Bohmian mechanics? Real cool guy with a real cool theory about how the world works. The theory is not very popular (blame the politicians), but I want it to be popular! I'm too dumb to explain it, but you guys should read this, and it'll give you a little introduction. It's a theory to list along with Copenhagen and Many Worlds. Also, don't be mad if you hate this book for the first 70ish pages. It gets much, much better after that. Do you know how to detect a bomb without detecting a bomb? Read to find out.
The diagrams are clear and lovely. Apparently, they don't use double slit experiments too much anymore. They use interferometers, and the author goes into super extreme detail on them. It's funny, there are actually some new developments about the Diosi-Penrose theory, and I was reading that exact part of the book right when the article popped up on my phone. Naughty, naughty Google. But also, what a coincidence.
Recommend. 5 stars for the thought-provoking info.
Profile Image for Greg.
1,771 reviews18 followers
May 29, 2019
You face 2 doorways: 1) This book sits on a table beside a nice comfy chair or 2) A sheet of paper on said table has just four blurry words that are the final sentence of the book, but you have to walk through the doorway to read the words clearly. What do you do? It doesn't matter at all it seems to the author. After all, his last sentence is "The case remains unsolved." But it's the trip that counts. Walk forward, and you've walked through 2 doors at once! Fun and challenging...with a cat, natch.
Profile Image for Ken Rideout.
371 reviews12 followers
September 8, 2018
The first half of this book is simply a thorough review of Young's Two Slit experiment as it applies to quantum mechanics. If you've ever taken a first year physics course you will find nothing new here.

The second half is much more interesting as the author takes you a tour of the more recent, mind boggling tests of entanglement in quantum physics which seem to definitively prove Einstein wrong and imply the Universe is non local in someway. However, even though the results are fascinating, I found the detailed explainers of the experiments and the back stories of the scientists exhaustively detailed. Finally, the author spends some time on the various non Copenhagen interpretations of Quantum Mechanics. This is arguably the most important part of the book and, yet, somehow also got bogged down in the backstory of the personalities involved and suffered from a muddled exposition (at least it was hard for me to extract the key differences).

The idea of keeping the double slit experiment central to the explorations of all the big ideas in quantum physics is a cool one, but ultimately the book felt to me like it wandered all over the place (like ideas about the meaning of the wave function itself). I did finish with a much better understanding of the philosophical problems in the Copenhagen Interpretation (namely, the arbitrary nature of the wave function collapse) and a better understanding of Bell's Theorem, but I don't think I will be recommending this particular book to anyone looking to explore the philosophy of quantum mechanics.
Profile Image for Peter Tillman.
3,551 reviews310 followers
Want to read
August 4, 2018
"Richard Feynman said that the double-slit experiment seemed to have been ‘designed to contain all of the mystery of quantum mechanics.’

Are you facing a difficult decision? Quantum physics may have the answer, says science journalist Anil Ananthaswamy. Type your dilemma into the Universe Splitter app, tap a button, and a signal will be sent to a laboratory in Switzerland. A message will come back telling you which choice to make, along with the assurance that there is now another universe where your double can opt for the alternative.

Mr. Ananthaswamy was shown this high-tech coin toss by Sean Carroll, a professor of theoretical physics at Caltech and an advocate of the “many worlds” interpretation of quantum mechanics. Mr. Ananthaswamy’s “Through Two Doors at Once” is a challenging and rewarding survey of how scientists like Mr. Carroll are grappling with nature’s deepest, strangest secrets. ..."
Profile Image for Jeremy.
407 reviews7 followers
December 26, 2018
Gets at the foundations of quantum physics by detailing the history of a specific type of experiment. Well worth it even for those of us who don’t think we want to read another popular book on quantum physics. Very rewarding read.
Profile Image for Santiago Ortiz.
94 reviews169 followers
October 31, 2018
Experiments and their -sometimes astonishing or even confusing– results are the true doors to physics, and they should precede interpretations and model equations.

"I will not describe it in terms of an analogy with something familiar. I’ll simply describe it."
Profile Image for Ramnath.
11 reviews2 followers
February 14, 2023
This was a captivating story from start to finish.

Unlike a number of other books that discuss the theoretical (often veering into philosophical) aspects of modern physics, this one is pleasantly grounded in experimental observation (the "double-slit experiment" to be precise).

Towards the end, I wish I could have read more about experimental tests for the Bohmian view of the universe and the Many Worlds hypothesis, but I guess the lack of ideas here is more a reflection of the current state of fundamental physics, and not a shortcoming of the book itself.
Profile Image for Todd.
160 reviews7 followers
August 19, 2018
Do you know about the double slit experiment? Well, you only think you do. The double slit experiment wasn't just done 100 years ago and then physicists moved on. Variations on this well-known experiment are happening today with significant implications for our modern understanding (or lack thereof) of quantum mechanics.

When I read a book like this, I'm reminded how lacking science education can be at times. If the double slit experiment is covered in physics or chemistry classes, it's usually mentioned in reference to the "wave-particle" duality of light. The best science teachers might even shoot a laser pointer through some diffraction gratings to demonstrate interference. Now, that's all fine as far as it goes, but there is so much more to this experiment and a modern understanding of quantum mechanics to just stop there. These experiments are the perfect opportunity to teach students that fundamental questions remain and that, with the right mindset and training, they could join the conversation and that it does not take genius to partake. Genius has been a part of this story to be sure, but at this point the full set of issues are readily described and understandable by the willing. If better answers to quantum mechanics are out there, they will be found by dedication, collaboration, and inspiration. The days of the loan genius solving complex problems is past.

Ananthaswamy's writing is clear and concise throughout. Like most popular science books, numbered footnotes are exchanged for text-based references. Yes, not seeing those damn little numbers may improve readability, but I'm the type of reader who likes to know exactly when something is being reference. Diagrams are employed occasionally and so the science can be, at times, rigorous for a popular science treatment. There were times when more detail, even if just as an appendix, was called for but this is a minor point of personal preference on may part. Strongly recommended for the quantum curious.
Profile Image for Nilesh Jasani.
970 reviews126 followers
June 2, 2019
This short book treads territories where no other popular books on quantum physics have gone for this reader. And it glides through the difficult terrains in the most understandable ways possible.

Quantum physics beyond the uncertainty realms and Copenhagen interpretation is complicated to make sense of. Both thought experiments and their real-life versions, many including the double slits, are hard to follow in implications and on their importance. Mr. Ananthaswamy wastes no time on concepts usually discussed in popular science books. The short book starts almost where the famous Bohr/Einstein linked sagas end.

Bohm’s pilot wave, Elliott’s many worlds, and two other possible interpretations are explained in a way that makes it easy for readers to understand where they differ against each other and from the Copenhagen interpretation. More remarkably, the author wonderfully explains various complicated experiments involving Mach-Zehnder Interferometer, including Wheeler’s Delayed Choice and various Quantum Eraser varieties. While many of these topics are covered in other popular books, Mr. Ananthaswamy is able to shine the light on them in a new way in almost every single section of the book.

A must-read for anyone interested in quantum spookiness for decades to come.
Profile Image for R Nair.
122 reviews52 followers
August 5, 2019
The initial apprehension that this may be the retelling of the same double-slit experiment that is contained as an introduction to quantum mechanics in most text books is needless. Instead the author uses the double slit as a thread winding its way through the different approaches to quantum mechanics and the complex modern experiments that essentially boil down to it. Also one of the best non-technical explanations of quantum Bayesianism (QBism) in recent times. All in all a great book to look at quantum mechanics through the double slit experiment (pun intended).
Profile Image for Jonna Higgins-Freese.
705 reviews44 followers
October 5, 2018
I loved this, because I am currently obsessed with quantum physics and physics in general. This was a great explanation of the history of these experiments and the different ways in which they can be interpreted -- and the further experiments that flow from those interpretations.
Profile Image for Sean.
19 reviews4 followers
January 18, 2019
In another world, I gave this book 5 stars.
Profile Image for Mason.
Author 1 book29 followers
October 12, 2022
I loved this book! It had been on my to-read list for a long time (I’m acquainted with the author, and had beers with him when he was working on the book). I wish I’d read it a lot sooner.

One aspect I really liked is that it clearly explained the details of experiments that probe the strange behavior of quantum systems. I’ve read a number of popular books on the topic, but none that I’ve read cover these experiments in enough detail to really grasp what they’re measuring and why they’re confounding. Also, it includes not only now-classic experiments—such as those that earned Aspect, Zeilinger, and Clauser the 2022 Nobel Prize in Physics—but also newer, cutting-edge experiments to try to get a better handle on what’s actually going on.

For background, I got my bachelor’s degree in physics almost 25 years ago, and am curious to catch up on developments, to see if anyone has made progress on understanding the problem of what is measurement in quantum mechanics, in which the wave function “collapses,” according to the traditional interpretation. In college, I found the explanations of this unsatisfying, but I didn’t know enough at the time to understand why. Now I know that nobody really understands quantum mechanics, as Feynman put it.

My understanding of the details of quantum mechanics are rusty enough that I don’t want to try to wade through the original literature. So I really appreciated that this book took the reader through the details. It was enough to satisfy me, and to give an idea of how these experiments actually work.

I also really liked how the book considered many different interpretations—the traditional Copenhagen interpretation, the many-worlds interpretation, Bohmian versions with hidden variables, and more—in an even-handed way. Then the book wraps things up in an elegant way by discussing how these various interpretations might have more in common than it seems, and that they might turn out to be different ways of looking at the same underlying reality.

As I was reading it, I was thinking of what Feynman said in The Character of Physical Law: “… every theoretical physicist that’s any good knows six or seven different theoretical representations for exactly the same physics and knows they are all equivalent…” Soon after I thought of this, Through Two Doors gave that same Feynman quote! So I felt the book was doing a nice job of leading the reader along to see the advantages of considering many different interpretations. As Feynman argued, holding multiple representations in mind can be useful since they can spark different ideas. For trying to explain some unexplained anomalies, it could be that a tweak to one representation would suddenly explain those anomalies, whereas there wouldn’t be an obvious explanation (or new representation) if looking at the phenomenon in another way. I like this pluralistic approach.

It seems like many of the physicists that the author interviewed have become adherents of one view or another (even if they had switched views, previously having been an adherent of a different view). It smacks of tribalism and religiosity.

I get that some interpretations can seem intuitive to one person but not another. I personally think the many-worlds interpretation—as I understand it, at least—is untestable and fuzzy. It avoids the measurement problem by introducing the concept of many worlds which isn’t parsimonious at all, and which we have no evidence for. Others (such as Sean Carroll) think many worlds are perfectly fine.

I think it’s fine for one idea to not be as intuitive or seem as plausible for a particular physicist, as long as they can still consider seriously a number of other representations. It’s when people get wedded to one view and then try to defend it that I think it’s more problematic. If there’s evidence for or against a certain view, that’s fine, but the problem in quantum mechanics is that all of these theories are compatible with the evidence so far, as far as anyone can tell.

The book ends on a hopeful note, with physicists still devising new versions of the double slit experiment that they hope will reveal new aspects of the behavior that might shed light on what’s going on. I’m rooting for the realists who are trying to go beyond the Copenhagen interpretation, to see if there’s more we can say about what’s happening, about what the quantum world is like, and why it shows behavior that’s weird by our standards in the macroscopic world.
74 reviews1 follower
January 13, 2019
Anil tries gamely to give the reader an overview of the many schools of thought currently extant in QM through the lens (hah) of the double slit experiment. I remember the experiment fondly because a variant (with the slits immersed in some kind of liquid) was a question in one of my entrance exams and which I knew how to solve. I followed along for the first few chapters but the complexity of it all and the need to follow stuff like entanglement without recourse to mathematics quickly overwhelmed me. I felt like most of these concepts are best explained interactively in a classroom setting and even then it is debatable how much of it can be grasped without the necessary math. Overall, I learned quite a few high level things about QM that I didn't know previously but my one take away is that I understand QM even less than I thought I did and I used to think I didn't understand it at all :)
Profile Image for Roberto Rigolin F Lopes.
363 reviews95 followers
September 19, 2020
Newton (1643-1727) suggested that light is made of particles but Huygens (1629-1695) formulated a wave theory of light. Particles or waves? Both! (you can shout today). But in 1793 Thomas Young came up with a simple experiment showing that light behaves as a wave. In a dark room, he made a small hole in the window-shutter, using a needle, to let a ray of sunbeam enter the room. Then, he brought into the beam a small piece of paper (85 mm), and studied its shadow to see the diffraction pattern of ONE wave. One century later, in 1909, Taylor did a similar experiment with two small holes (double-slit) creating a diffraction pattern from TWO waves. From the 1960s, physicists started using electron beams and biprisms instead of slits in much more complex setups including two islands and satellites. This book compiles the history of how this striking experiment has been evolving in the last three centuries to unveil the wave-particle nature of light and to study the nondeterminism of Nature anticipated by Quantum Mechanics.
144 reviews2 followers
January 11, 2019
Simply mindblowing. Quantum mechanics has always been a topic of interest but a very daunting exercise to read because of the sheer volume of work out there.
The book does a great job of explaining the evolution, philosophy, and differing opinions through the lens (or slits) of a single (double?) experiment.
Thoroughly enjoyed it. Can't say that I understand the subject fully, specially the last two chapters on Everett's Many Worlds Interpretation and Penrose/GRW's quantum gravitation. Definitely merits a re-read.
Profile Image for Carmen Tracey.
76 reviews35 followers
March 5, 2019
This book was very clearly written and concise, and it was definitely an enjoyable read. I listened to the audiobook version, and the narrator did a great job. I don't think I'm ever going to understand quantum physics, though. :D
Profile Image for Mack .
1,496 reviews50 followers
December 6, 2020
I really like a concise, broad-ranging book that gives not only a good overview of an area of interest but does it with concise writing. Ananthaswamy has created just such a work and covers many, many aspects of the two-slit experiment. An enjoyable, educational read.
Profile Image for Max.
8 reviews1 follower
July 24, 2021
An amazing read. It is not all that well-written and a few concepts—including, annoyingly, the uncertainty principle—are not explained in enough detail, but it is unputdownable on strength of subject matter alone
236 reviews
January 27, 2021
Very clear discussion on two-slit, with follow-on discussion of theories.
Yes, it took me four months to finish reading - not a page-turner.
I enjoyed Anil's writing.
908 reviews36 followers
June 14, 2021
Best explanations I have seen of a lot of quantum phenomena, embodied in the double-slit experiment and the interferometer-based experiments that have supplanted it. I highlighted a lot of passages that were interesting to me. Even so, I found this pretty heavy going and at a certain point I knew my brain was no longer ready for the information. I continued reading anyway, because I believe in bombarding my brain with information so that it'll be more ready next time.

Eventually the experiment descriptions gave way to discussion of alternatives to the Copenhagen interpretation such as the DeBroglie-Bohm interpretation, many worlds (always a trip), and QBism. While these were easier to grasp, I found them a bit less interesting as there seems to be very little we can do experimentally to tell between them.
Profile Image for Sudharshan Viswanathan.
78 reviews3 followers
September 10, 2018
My attempts at trying to understand QM is restricted to a first course in Undergraduate level. It was sufficiently discouraging for me to never bother attempting to try to understand it again.

Curiously enough, my physics professor has been a big fan of the Young's Double Slit experiment. An introduction to YDSE at high school gave a very wave like treatment to explain the interference patterns.

Now, arriving at this book - I would think that this was the best way of probably introducing someone to QM. It starts with the YDSE and beautifully builds up the evolution of thought through the ages. As I was reading I was left wishing that in my first course to QM, if this introduction had been used to motivate the mysterious and magical Schrodinger wave equation, the Heisenberg's uncertainty principle etc with the experiments that lead to the formalism, I am convinced that it would have been a persuasive introduction.

Any introduction course without mentioning the simultaneous discovery of the atom model, the formalization of Maxwell's laws, the blackbody radiation problem leading eventually to the Bohr model isnt motivating the subject well IMO.

Subsequent topics such as the action at distance and non-locality, Bohmian mechanics were discussed with so much intuition (even when the underlying physics lacks it) that it never felt like a hard book to read though at moments it might take some time and thought for the implications to sink in.

Thoroughly enjoyed the book and would recommend it anyone who wants a nice primer on Quantum world without having to look through the math.
Profile Image for Chris Esposo.
671 reviews31 followers
June 22, 2020
Half of this book is an excellent, well-structured, introduction to the wave-particle duality phenomena, and entanglement, with emphasis of setting up, and analyzing, Mach-Zehnder interferometers to understand these unintuitive and abstract phenomena. The other half, though not as instructional, nor grounded as the first, is still a great introduction to alternative (non-Copenhagen) theories of quantum mechanics, and most importantly, introduces a category system for one to compare/contrast the various alternative interpretations, which helps understand the debates between the various practitioners and the differing consequences (if any) resultant from their deviations.

The book is unique relative to the other layman books I’ve read on QM exactly because it takes care to explain/illustrates interferometers, which is critical for a layman to understand the mechanics of the experiments described in the text. Interferometers are lens systems where light, or individual photons, are emitted from an origin/source point to a lens-node network, where each lens/node either transmits, or reflects, the light/photon onto another node in the apparatus. In this way, the device sort of reminds one of a Rube Goldberg device, or perhaps a causality diagram, or more generally, a di-graph. The light/photons ultimately end up in one or several terminal locations, based on the positioning of the lens sequence, and depending on where they end up, one can make inference on what may have happened to the light/photon whilst it traversed the system.

Of course, as the title suggests, understanding this system is in service to understanding the central topic, the double-slit experiment, and QM more generally. The author works through this material chronologically, first introducing the original double-slit experiment, which tipped off the physicists of the day to the strange nature of light, exhibiting both wave-like and point-like behavior. The book uses this conundrum to motivate the development of QM, and specifically the founding of the Cophennehange Interpretation of QM, pioneered by Bohr & Heisneberg. Here, we are first introduced to the notion of “realism” and “anti-realism”, the first of many splits in the foundational debates physicists have on the nature of reality.

Essentially, one camp, the realist-camp, believed that physical phenomena had to abide by certain “rules” or intuitions that were developed from the observation of classical mechanics, namely, that phenomena exhibited a property that would later be labeled “locality”, which is that interactions between two objects are transmitted in real-space either by direct contact, or force, as classically understood, via a field. Einstein and his acolytes were the main proponents of this camp. The other camp, believed that locality was a unneeded, and that the notion of “realism” was just a bias developed by the limited sense-ability of humans, and although we could not really understand the nature of the mechanics described in QM from a “real-space” perspective, the theory of QM was able to describe and characterize these systems with preternatural accuracy, thus demonstrating the merit of their ideas.

Specifically, the point of contention between these two camps was “completeness” of QM, Einstein agreed that the theory was successful, but he believed it was not the “final” answer to the question of quantum phenomena, specifically, he was in search of a deterministic theory. Whereas, proponents of QM as the “best” (and possibly the final) point of question believed that no deterministic theory could exist, and in fact, the nature of reality was intrinsically probabilistic. The issue arose from the apparent arbitrariness of what became known as Born’s rule, which is that the norm of the wave function resulted in the probability of the particle being found in a given point in real-space. Einstein and others believed that there must have been other variables not observed or accounted for by the current QM and that the current probabilistic nature of the theory was a tentative solution to the description of the system.

This debate, would eventually be resolved in the 60s and 70, by the likes of Bell, Aspect, and Zeillinger, who equipped with a more refined version of the double slit experiment, whereby the system could isolate and pass through just one photon of light into the interferometer at a time, started to really observe the oddity of the quantum phenomena, namely the self-interfering nature of photons, which put to rest the notion that particles were either/or wave-like or point-like, as a physicist of the era stated, such a notion is a ‘category-error’ particles, are neither, in fact they are both, or perhaps better stated, they are something else entirely.

It was around this time that Bell postulated his famous inequality, and the book goes into adequate detail describing it, and the experiments constructed to prove (or disprove) it. These experiments ended up being just larger versions of the first interferometers, where each photon beam would be sent literally kilometers to another lens, instead of feet. This increase in scale was designed to remove the possibility of hidden variables affecting one point in the node relative to another.

Although this material is covered in other books, most notably, Zeillinger’s own layman book “Dance of the Photons”, this one is unique in that it really makes interferometers central to the analysis. In fact, I now understand the Eleiter-Vaidman bomb experiment much clearer because of the exposition in this text.

The second half of the book is much less stellar, as it covers a lot of the same “debates on the foundations” material other recently published layman books on QM and quantum information have gone over. However, it does this in a much more organized manner, introducing a simple typology scheme, whereby each theory is either epistemic (making no claim on the nature of reality) or ontological (claiming the theory describes reality), as well as whether it is realist or not. In this manner, all popular interpretations from Copenhagen, Bohm-pilot wave, to many-worlds and QBism, can be put into one of the 4 combinations.

Overall, I have to highly recommend this book, especially for those that may have some technical skill in other areas, and want to understand QM better. I imagine for actual physicists this material is of no real value, but for those who are looking to broaden their horizon from an interdisciplinary perspective, I believe this book, and Zeillinger’s book are a good place to start (outside of a traditional textbook).
Profile Image for Jerry Pogan.
995 reviews26 followers
November 16, 2018
Probably one of the smartest things I've ever done in my, almost, 72 years was to not choose Quantum Physics as a career path (Not that I actually ever considered it). The reason is because of all the things I'm woefully ill equipped to understand this is at the top of the list. I simply can't wrap my head around it. I've read dozens of books on the subject and this book, probably, comes closest to being somewhat comprehensible to me. Sarah (one of the Goodread reviewers I follow) said it best when she said she understands in "flashes" because that best describes what my experience was when reading this book, I understood what Ananthaswamy was saying in occasional "flashes".
The book is absolutely fascinating and talks about many of the pioneers of the subject as well as current physicists and goes into great detail on thought exercises and experiments involving the double slit. He describes what the experiments were attempting to explain and some of the conclusions they came to. I just wish I could actually understand it but I often found that I not only didn't really understand the answer, sometimes, I didn't even understand the question. Regardless of my ability to understand, though, I'm fascinated with the subject.
Profile Image for Jimbo.
281 reviews11 followers
September 7, 2021
Updated review after relistening to the audiobook.

Ananthaswamy is one of my favorite non fiction writers. Although most pop science books follow the same structure of mixing science, journalism, and history, Anathaswamy captures an appropriate amount of drama without it feeling phony or exaggerated.

I enjoyed the history and progression of ideas. Each interpretation of the double slit experiment sounded perfect, but I knew something better was coming because the book wasn't finished.

Overall the book is not very practical. This book won't make you a better person or help you out in life, but if you're curious about quantum physics or wildly interesting ideas/experiments. This book is perfect. The philosophy of the theories is most engaging.

I look forward to whatever Anathaswamy publishes next.
Profile Image for Doug.
294 reviews14 followers
November 6, 2018
I think it’s about time to quit hoping that someone will be able to dumb-down quantum physics to the point that I understand everything I know. This was a decent effort (I think Carlo Rovelli does it as well as anyone). I believe that the best thing about this book was the insightful anecdotes about some of the giants of the field. Some of them even made me feel better about my lack of understanding-apparently even Einstein got a few things wrong. A worthwhile read for those with an interest in the subject.
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