Monday, August 13, 2018

Book Review: “Through Two Doors at Once” by Anil Ananthaswamy

Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality Hardcover 
By Anil Ananthaswamy
Dutton (August 7, 2018)

The first time I saw the double-slit experiment, I thought it was a trick, an elaborate construction with mirrors, cooked up by malicious physics teachers. But no, it was not, as I was soon to learn. A laser beam pointed at a plate with two parallel slits will make 5 or 7 or any odd number of dots aligned on the screen, their intensity fading the farther away they are from the middle. Light is a wave, this experiment shows, it can interfere with itself.

But light is also a particle, and indeed the double-slit experiment can, and has been, done with single photons. Perplexingly, these photons will create the same interference pattern; it will gradually build up from single dots. Strange as it sounds, the particles seem to interfere with themselves. The most common way to explain the pattern is that a single particle can go through two slits at once, a finding so unintuitive that physicists still debate just what the results tell us about reality.

The double-slit experiment is without doubt one of the most fascinating physics experiments ever. In his new book “Through Two Doors at Once,” Anil Anathaswamy lays out both the history and the legacy of the experiment.

I previously read Anil’s 2013 book “The Edge of Physics” which got him a top rank on my list of favorite science writers. I like Anil’s writing because he doesn’t waste your time. He says what he has to say, doesn’t make excuses when it gets technical, and doesn’t wrap the science into layers of flowery cushions. He also has a good taste in deciding what the reader should know.

A book about an experiment and its variants might sound like a washing list of technical detail with increasing sophistication, but Anil has picked only the best of the best. Besides the first double-slit experiment, and the first experiment with single particles, there’s also the delayed choice, the quantum eraser, weak measurement, and interference of large molecules (“Schrödinger’s cat”). The reader of course also learns how to detect a live bomb without detonating it, what Anton Zeilinger did on the Canary Islands, and what Yves Couder’s oil droplets may or may not have to do with any of that.

Along with the experiments, Anil explains the major interpretations of quantum mechanics, Copenhagen, Pilot-Wave, Many Worlds, and QBism, and what various people have to say about this. He also mentions spontaneous collapse models, and Penrose’s gravitationally induced collapse in particular.

The book contains a few equations and Anil expects the reader to cope with sometimes rather convoluted setups of mirrors and beam splitters and detectors, but the heavier passages are balanced with stories about the people who made the experiments or who worked on the theories. The result is a very readable account of the past and current status of quantum mechanics. It’s a book with substance and I can recommend it to anyone who has an interest in the foundation of quantum mechanics.

[Disclaimer: free review copy]


  1. What I don't understand is, why physicist get all worked up about

    "a finding so unintuitive that physicists still debate just what the results tell us about reality."

    but nobody says, "hey its just a wave going through the two slits, but we cannot yet explain how a wave front is able to deposit a blip of energy at a localized position.

    What are the arguments why this is so much more impossible than a "particle taking two paths at once" that nobody explains it like that?

    1. You're just considering the base experiment. It's the myriad permutations of the experiment that get truly bizarre. From this week's Nature:

      "Odder, the pattern vanishes if we use a detector to measure which slit the particle goes through: it’s truly particle-like, with no more waviness. Oddest of all, that remains true if we delay the measurement until after the particle has traversed the slits (but before it hits the screen). And if we make the measurement but then delete the result without looking at it, interference returns."

  2. Definitely my next purchase. I met the double slit in high school physics and have battled it ever since. A detailed account its variations can not help but be fascinating,
    Thank you for publishing your review.

  3. Does the book explain how we could be sure in the early single-photon experiments that the photons were indeed going one by one?
    I would love to get a detailed answer to this question!

  4. …1) Large molecules diffract, (10,123 amu[1]). An entity('s wavefunction) traverses all slits simultaneously. Tracing pathways collapses the pattern ̶ no dissipation.

    …2) Hund's paradox[2] says chirality's persistence demands continuous wavefunction collapse. Absent dissipation, chiral entities sum to achiral wavefunctions, then racemic observables.

    …3) Beam through a grating cold (~2 kelvin)[3]), low molecular weight, rigid, internally multiply-connected, resolved extreme chiral divergent molecules (stereograms[4]) with no classical path to racemization ̶ rigid extreme shoes without vibrational transitions (~140 kelvin cutoff).

    …4a) The expected pattern emerges when 100% left or right shoes enter. A racemized mixture must exit. Classical inversion requires at least 350 kcal/mole. Grating near-field time interval is hugely insufficient given kinetics. Chemistry is falsified.

    …4b) No pattern emerges. The beam remains homochiral. Quantum mechanics is falsified.

    …4c) Somewhere in-between. Everybody publishes.

    Aristotelian QM is insufficient. LOOK. Nanogram quantitative enantiomeric excess is easy[5].

    [1] arXiv:1703.02129
    [2]DOI:10.1103/PhysRevLett.103.023202, DOI:10.1088/1361-6455/aa5115, DOI:10.1039/C2CP40920H
    [3] DOI:10.1002/anie.201704221

  5. Thank you, I'll order the book. Silly question; Do these Bell inequality/ photon entanglement experiments mean the end of hidden variable theories? Or is it only what is called "realistic local hidden variable theories"?

  6. Perhaps someone could just do Uncle Al's experiment and get him off our backs. Something to do with chiral molecules.

  7. Just ordered the book.
    Can hardly be worse than the neverending dispute that recently took off on this blog and is continued next door, between philosophers who don't fit through one door together (that's a dutch expression, badly translated).

  8. Feynman said something to the effect that understanding the double slit experiment is an understanding of most of quantum mechanics. The basis states from each slit separated by a distance D e^{ikx} and e^{ik(x+D)} give an interference term that is oscillatory on the screen. If you place a spin state at a slit, then you exchange the interference phase with an entanglement with the spin.

    Quantum interpretations are attempts to make quantum mechanics conform to our standard ideas or intuition about the nature of things. Physics has a long history of drifting away from our intuition. In our ordinary lives we live as Aristotelians. Newton's laws with friction, air drag and local gravity is similar to what Aristotle thought of physics. In order to move things need a force or impetus. Things tend to seek the ground and so forth. Teaching elementary physics is not easy because you have to break up the intuitions young people have about things. Of course relativity changes our intuition about time and space and quantum mechanics goes further by demolishing our ideas about reality. Quantum interpretations are really just attempts to make quantum mechanics hook into our intuitions about reality. They all suffer from problems. They are then at best heuristic crutches that can be used in explaining and understanding different problems. I think quantum gravitation will take us further away from our ordinary intuition about reality.

    Decoherence and gravity is an interesting conjecture. Penrose suggested a sort of metric change with a measurement. Spacetime as comprised of massive entanglements of states, states that have some duality with CFT or QFT states we ordinarily observe (AdS ~CFT duality etc), it is not unreasonable to think that decoherence is a case of superposition or entanglement phase being absorbed into spacetime. Certainly in the case of an EPR pair and a black hole this is the case. The decoherence of a quantum state entangled with a state approaching a black hole is a case of an entanglement phase being transferred to the black hole. It is a sort of quantum teleportation, but there can't be a classical signal from the interior to the exterior to inform the exterior observer on the state of the BH interior. Penrose calls this an R-process that is a real collapse, while we might also interpret this as a coarse grained understanding of how entanglement phase shifts from an EPR pair to a black hole.

    Roger Penrose suggested in his Road to Reality a type of experiment to show how a wave function collapse resulted in a metric change. It is a sort of Fabry-Perot interferometry across astronomical distances with spacecraft. It strikes me as a worthwhile project to undertake. Penrose thinks it is his R-process, but really in general it would just illustrate how QM and GR are related.

    Related to Penrose's idea is that of Pullin in the Montevideo Interpretation. The Montevideo Interpretation that suggests a role for gravitation in apparent wave function collapse.

  9. I went quickly through the book. Nothing new, when the whole environment is considered as the common wave environment, no contradictions occur.

    Is it so hard to imagine there are no particles at all? We are all samples of the common wave environment too, naturally. ;)

  10. Harald, we can explain how a wave front is able to deposit a blip of energy at a localized position. Check out the optical Fourier transform. However people do like to peddle mystery and write whole books about it.

    John Duffield

  11. Thought you'd want to know that your 'buy my book' widget is dead? Just a blank square.. presumably that's not a good thing! (i've already bought your book previously :) )


  12. I will probably buy this book soon, because I love plowing the same ground over and over again. BTW the science historian Robert Crease has a delightful chapter on the two-slit experiment in his book "The Prism and the Pendulum." He writes very well.

  13. Thanks for the review. The booked has been purchased.

  14. Another one for my list. By the way, I ordered your book, but for some reason the box wasn't ticked to ring me when it arrived, so when I didn't collect it, it got sent back. I have since re-ordered it.

    People here might be interested in my reviews of (mostly) popular-science books (mostly) about cosmology and astrophysics.

  15. Herald,

    "...but we cannot yet explain how a wave front is able to deposit a blip of energy at a localized position."

    The photon as a minimum wavepacket (quantum) for a given energy content explains it quite well.

  16. Even crazier is the quantum eraser experiment which seems to violate causality (but does not I think):

    Here's a fun challegne: I wonder if you could explain it without looking up at references?

  17. Just finished the book. It was worth reading and I learned quite a bit from it, but it leaves many questions unanswered--mostly because there is no clear or unanimous answer to those questions. However the author makes some statements about which models are deterministic and which are local which I have seen disputed here in comments. So I wonder if he might have been less definite if he had spoken to some more people and gotten some more points of view. Well, in hindsight I suppose there are always more questions to ask.

    Still I wish there had been more explanation of the Many Worlds model. He explained that its main drawback is that it provides no mechanism for the Born Rule (at least, none that is widely accepted), but left me wondering what the rules are for the entanglement and interference of "worlds". In particular, is it true that there must be one world in which the beam-splitter always sends the photon or particle in the same direction, or is there some sort of interference or rule which prevents this? (If worlds just branch automatically, one world for left and one for right at the splitter, then at the next particle the left-world would branch into left-left and left-right, and so on, so there would be two worlds, left-left-...-left and right-right-...-right in which quantum mechanics could never be detected, which seems like a paradox or at least rather singular.)(And many worlds in which QM almost never is detectable.) (Granted, there would be a minuscule probability for someone to find oneself in such a world.)

    Overall, I rated the book as a B+, because I learned something but would like to have learned a bit more. This might be unfair as perhaps no one could have done a much better job on the material, but I can only judge based on my own sense of satisfaction.

  18. This is one book I'll definitely be adding to my physics book collection.

  19. I'm going to have to buy this book, it sounds interesting.

    sean s.

  20. I do believe that something is being missed in all discussions of the two slit experiment namely that the interference pattern only occurs in the Far Field or Fraunhofer region. If the screen were close up in the near field region all one would see is two bands close to the slits.

    If that is the case whatever is happening to cause the interference pattern cannot be attributed to a particle passing through two doors at once. As the book and many others clain

  21. I’m 30% thru - just finished chapter 3. Is it possibe that the photon is accompanied by a carrier wave? The carrier wave has enough energy to split at the splitters, but the photon is the minimum energy so can not split. The interference seen in the experiments would be from the carrier wave. Is this what is called a hidden variable theory?

  22. @Eusa

    You wrote,

    “Is it so hard to imagine there are no particles at all? We are all samples of the common wave environment too, naturally. ;) ”

    Yes, because I think it to be incorrect. Energy is only carried by particles.


  23. My uneasiness with the Many Worlds section of the book has collapsed or decohered as follows. The issue with probabilities is presented as: suppose there is a beam-splitter which has a 75% probability of reflection and 25% chance of transmission (rather than the usual 50-50). How can the MWI explain this? This seems to assume that there would be one branch for reflection and one branch for transmission.

    This seems easy to explain. Since I like definite mechanisms, I will assume a glass pane is half-silvered with a very fine distribution such that a photon in a beam-splitter experiment has a 50% chance of hitting the silvered (mirror) part, and if it hits a clear part, has the normal 50% chance of being reflected or transmitted. A la Feynman ("QED"), the photon acts as though it takes all possible paths (and some of them interfere with each other) in the experiment, so there are not just two, but many MW branches. Of those, 75% will contain a reflection and 25% a transmission, so the experimenter has a 75% chance of finding herself in a branch with a reflection. What's the problem?

    Reply to John Mark Morris: carrier waves will be covered later in the book (albeit not in great depth).

  24. @Michael Kovari:

    Particles traversing a grating diffract and interfere, arXiv:1703.02129, “Through Two Doors at Once,” quantum mechanics’ beating heart. Falsify that in one day in existing equipment repeating published experiments. Kill a postulate.

    Remove the grating and GR is falsified the second day. Kill a postulate. No prior observation is contradicted in any venue at any scale; Galileo and Popper versus Aristotle and Bayes.

    A 12 ga shotgun firing one ounce of birdshot is ignored by a leather jacket 10 meters distant. Substitute a one ounce Foster slug and it will explode your insides 200 meters down range. If physical theory's most esoteric echelons fear millitorr analytical chemistry, they should. Birdshot derived papers versus slug empirical experiments – look.

    Science is blood sport not social justice.

  25. "Perhaps someone could just do Uncle Al's experiment and get him off our backs. "

    I've often thought this as well. If he had put all the time and effort he has put into comments (not just here), couldn't he have done the experiment by now?

  26. Gosh, I was going to buy the book two weeks ago, then got distracted. But with a call to a Barnes and Noble in Massachusetts, discovered they carried the book. Can't wait to start reading it!

  27. Anil Ananthaswamy's book "Through Two Doors at Once..." is absolutely marvelous! It's everything that Sabine said it would be in her review.

    And, just a short time ago, I printed out the paper "Strong lensing with superfluid dark matter", that Sabine, and her grad student, Tobias Mistele, co-wrote. It's quite technical, and I do hope that when she returns from her vacation that she might find the time to give a review of that theory/model.

  28. After a bit of a hiatus finally got back to Anil's book. The delightfully detailed description of the Tenerife delayed choice experiment truly brings into focus the mind boggling aspects of Quantum Mechanics, which are so inexplicable from a classical perspective.

    The mention of steel shipping containers, to house the experimental apparatus, brought to mind the steel shipping containers we brought on our oceanographic research cruises, also to house our experimental equipment. But our experiments, studying wave structure and ocean chemistry, had nowhere near the exquisite sensitivity and precision of the ground-breaking Tenerife experiment.

  29. One thing that has always intrigued me is whether, for non-local interactions as described in Anil's book, it is necessary for the transmit point and detection point to be physically connected by a pathway, be it a fiber optic cable, or direct line of sight as in the Tenerife experiment?

    To clarify what I am asking imagine an alternative experiment where a pair of photons become entangled, and are allowed to circulate in separate fiber optic loops. Move the loops to separate sealed rooms, so there is no possibility of direct line of sight between them. Under this circumstance would measurement of the spin of one of the photons immediately result in the opposite spin being detected in the other photon?

  30. Some of the first diagrams of chapter 1 of the book seem incorrect. These are the sand particles distribution through any of the vertical slits. The diagrams indicate a normal distribution of sand particle strikes along the screen, centered on the center of the slit(s). It seems sand particles should have a square wave. That is, the distribution of sand particles going through the slit should be equal across the slit, then none to left or right of the slit. In other words, there is no particle - wave duality phenomena with a sand particle. Why do the diagrams show a normal distribution instead of square distribution?

    The book's text associated with these diagrams is, "Lets take it that the source is firing grains of sand at high enough speeds that they have straight trajectories. When we do this, the grains of sand that get through the slits mostly hit the region of the screen directly behind the open slit, with the numbers tapering off on either side. " The 'distribution of particles (y) along screen (x)' does not show this. The trace shows a normal distribution, which seems incorrect.

    Also the 5th diagram of chapter 1 shows the slits with the slit surface horizontal (normal to gravity), and sand being poured out of pail and falling into piles below the slits. The resulting piles have peaks. In which case the particle distribution might be considered as a normal distribution. But, in this case, the distribution of sand dues to piling into hills in a gravity field is not relevant to the particle through a slit experiment, right?


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