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

by Ananthaswamy, Anil

  “From the viewpoint of the theory” : Ibid., 254.

  a conference in October 1962 in Cincinnati, Ohio : A transcript of the conference is available in Everett, The Everett Interpretation , 270.

  “It seems to me that if this is the case” : Everett, The Everett Interpretation , 273.

  “Somehow or other we have here” : Ibid.

  “Yes, it’s a consequence” : Ibid., 274.

  “You eliminate one of the two” : Ibid., 275.

  “Each individual branch” : Ibid, 276.

  “This universe is constantly splitting” : Bryce DeWitt, “Quantum Mechanics and Reality,” Physics Today 23, no. 9 (Sep 1970): 30.

  “I still recall vividly the shock” : Ibid.

  Universe Splitter : https://itunes.apple.com/us/app/universe-splitter/id3292 33299.

  “started to sound a little bit too practical” : David Wallace, “The Emergent Multiverse: The Plurality of Worlds—Quantum Mechanics,” February 21, 2015, https://youtu.be/2OoRdyn2M9A?t=183 .

  “I think one should invoke” : Everett, The Everett Interpretation , 278.

  “So the parallel universes are cheap” : Paul Davies and Julian Brown, eds., The Ghost in the Atom (Cambridge: Cambridge University Press, 1993), 84.

  “if the other universes” : Frank Wilczek, “Remarks on Energy in the Many Worlds,” Center for Theoretical Physics, MIT, Cambridge, Massachusetts, July 24, 2013, http://frankwilczek.com/2013/multiverseEnergy01.pdf .

  probability as something subjective : Charles Sebens and Sean Carroll, “Self-Locating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics,” British Journal for the Philosophy of Science 69, no. 1 (Mar 1, 2018): 25–74.

  “rather than relinquishing” : Christopher A. Fuchs, “On Participatory Realism,” June 28, 2016, https://arxiv.org/abs/1601.04360 .

  “built on billions upon billions” : John Wheeler paraphrased in Ibid.

  Quantum Bayesianism : Carlton Caves, Christopher Fuchs, and Rüdiger Schack, “Quantum Probabilities as Bayesian Probabilities,” Physical Review A 65, no. 2 (Jan 4, 2002): 022305.

  Einstein is thought to have : Matthew Leifer, “Is the Quantum State Real? An Extended Review of ψ-ontology Theorems,” Quanta 3, no. 1 (Nov 2014): 72.

  note that the same argument : Ibid.

  “from the third-person perspective” : David Wallace, The Emergent Multiverse: Quantum Theory according to the Everett Interpretation (Oxford: Oxford University Press, 2012), 310.

  “From the perspective of a given experimenter” : Ibid.

  “Quantum mechanics, in the QBist interpretation” : Christopher Fuchs, David Mermin, and Rüdiger Schack, “An Introduction to QBism with an Application to the Locality of Quantum Mechanics,” November 20, 2013, https://arxiv.org/pdf/1311.5253.pdf .

  “Science is about the interface” : David Mermin, “Why QBism Is Not the Copenhagen Interpretation and What John Bell Might Have Thought of It,” September 8, 2014, https://arxiv.org/pdf/1409.2454.pdf .

  “as a nutcase” : Christopher Fuchs, Coming of Age with Quantum Information: Notes on a Paulian Idea (Cambridge: Cambridge University Press, 2011), Kindle edition.

  “We need to talk” : Ibid.

  “Great talk!” : Ibid.

  “No, it wasn’t!” : Ibid.

  “You know what I would do” : Ibid.

  “The est foundation’s physics conferences” : David Kaiser, How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival (New York: Norton, 2011), 189.

  In one talk, he asked his audience to consider two theories : Richard Feynman Messenger Lectures on the Character of Physical Law, Lecture 7, “Seeking New Laws,” November 1964, http://www.cornell.edu/video/richard-feynman-messenger-lecture-7-seeking-new-laws .

  “In order to get new theories” : Ibid.

  “In other words, although they are identical” : Ibid.

  This inspired Wiseman : Jay Gambetta and Howard Wiseman, “Interpretation of Non-Markovian Stochastic Schrödinger Equations as a Hidden Variable Theory,” Physical Review A 68 (Dec 9, 2003): 062104.

  The results they obtained are eerily similar : Michael Hall, Dirk-André Deckert, and Howard Wiseman, “Quantum Phenomena Modeled by Interactions between Many Classical Worlds,” Physical Review X 4 (Oct 23, 2014): 041013.

  “If Born’s rule fails” : W. H. Zurek, quoted in Urbasi Sinha et al., “A Triple Slit Test for Quantum Mechanics,” Physics in Canada 66, no. 2 (Apr/Jun 2010): 83.

  sometimes the triple-slit : G. Rengaraj et al., “Measuring the Deviation from the Superposition Principle in Interference Experiments,” November 20, 2017, https://arxiv.org/abs/1610.09143 .

  Or is it equal to the amplitude : Rahul Sawant et al., “Nonclassical Paths in Quantum Interference Experiments,” Physical Review Letters 113, no. 12 (Sep 19, 2014): 120406.

  “Any . . . situation in quantum mechanics” : Feynman Messenger Lectures, Lecture 6, “Probability and Uncertainty: The Quantum Mechanical View of Nature,” http://www.cornell.edu/video/richard-feynman-messenger-lecture-6-probability-uncertainty-quantum-mechanical-view-nature .

  ACKNOWLEDGMENTS

  I remember being thrilled by Gary Zukav’s Dancing Wu Li Masters when I read it in the 1980s. The mysteries of quantum physics came alive. The book had, of course, a description of the double-slit experiment, besides a lot else. Then, as a journalist, I too started writing stories about quantum mechanics and encountered the iconic experiment at every turn. An idea took shape: a story about quantum physics told from the perspective of the double-slit experiment. But it remained on a back burner for years.

  Thanks to my editor, Stephen Morrow, for seeing the possibilities and making me revisit the idea and then seeing the book through to the end. Thanks also to Madeline Newquist and others at Dutton and to my agent, Peter Tallack, for their help in making this happen.

  A book like this needs an illustrator. Thanks to my friend Ajai Narendran for introducing me to Roshan Shakeel. Roshan turned my half-baked sketches into clean, sharp drawings that wonderfully complement the words. I’m grateful to Roshan for his unflagging efforts, and to Ajai for supporting and encouraging both me and Roshan.

  I’m grateful also to Rob Sunderland, Lis Rasmussen, Felicity Pors, and the rest of the staff at the Niels Bohr Archive in Copenhagen, Denmark, for all their help in accessing historical documents.

  I leaned heavily on physicists to explain quantum mechanics to me. They took the time, either in person or by phone or email, to enlighten me on the many conceptual issues that make quantum physics so confounding and enthralling; many read parts of the book and caught errors and suggested changes. I’m grateful to (in order of the chapters): Lucien Hardy, Alain Aspect, Philippe Grangier, David Albert, Tim Maudlin, Anton Zeilinger, Marlan Scully, Rupert Ursin, Xiao-Song Ma, Lev Vaidman, Sheldon Goldstein, John Bush, Tomas Bohr, Chris Dewdney, Basil Hiley, Aephraim Steinberg (for many discussions and meetings over the years), Roderich Tumulka, Roger Penrose, Markus Arndt, Dirk Bouwmeester, Sean Carroll, David Wallace, Howard Wiseman, Chris Fuchs, David Mermin, David Kaiser, Leonard Susskind, Urbasi Sinha, John Sipe, and Neal Abraham.

  I’m especially grateful to John Bush for vetting almost all the chapters. His enthusiasm was infectious. And a special thanks to Antoine Tilloy for reading and commenting on the whole book. Thanks also to my friends Sriram Srinivasan and Varun Bhatta for their input. Most of all, thanks to Adam Becker for his support throughout the writing of this book—including animated discussions over innumerable coffees and lunches in Berkeley—and for catching some errors in the final draft.

  Any errors that remain are, of course, my responsibility.

  Thanks to Banu and Ramesh for hosting me for months in Arlington and Amherst, as I went about meeting the coterie of quantum physicists on the US East Coast; to Caroline Sidi for hosting me in Paris; to Gita Suchak for making me feel at home in London; and to Rao Akella for help finding quotes for the epigraphs. And last, but by no means least, my thanks to m
y family back in India for their support, particularly my parents.

  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

  INDEX

  The page numbers in this index refer to the printed version of this book. The link provided will take you to the beginning of that print page. You may need to scroll forward from that location to find the corresponding reference on your e-reader.

  Albert, David, 79 –80, 182 –83, 195

  anti-particles, 141 –44

  Arndt, Markus, 195 –205

  Aspect, Alain

  and Bell’s theorem, 102 , 105 –6

  and Bohmian mechanics, 164

  and Dalibard, 196

  and nonlocality–relativity conflict, 94

  and QBism, 250 , 253

  and quantum eraser, 110 –11, 113 , 140

  and single-particle experiments, 60 –64, 68 , 74 –75, 84 –86, 88 –91

  Ball, Philip, 32

  Bayes, Thomas, 242 –43

  Bayesian probability, 248 . See also Quantum Bayesianism (QBism)

  beam-splitters

  and Bohmian mechanics, 170 –71, 177

  and decision-making app, 216

  and many worlds theories, 228 , 228 –33, 229 , 238 –39

  and Penrose’s thought experiment, 189 –90

  and QBism, 247 , 247

  and quantum eraser, 118 –20, 123 , 126 , 134 , 137 , 143 –44

  and single-particle experiments, 64 –65, 68 –75, 75 –80

  superposition mirror in, 207 –12

  and universal wavefunction, 219 –21, 220

  See also Mach-Zehnder interferometers

  Belinfante, Frederik, 167

  Bell, John, 61 –62, 94 , 101 –2. See also Bell’s theorem

  Bell’s theorem

  and Bohmian mechanics, 164 , 176

  and molecular interferometry, 196

  and nonlocality–relativity conflict, 94

  and QBism, 251 –52

  and quantum eraser, 110 , 128 , 144

  and single-particle experiments, 61 –62

  testing of, 102 –6

  Bohm, David (and Bohmian mechanics)

  background and early works, 147 –51

  debate on Bohmian mechanics, 181 –85

  de Broglie-Bohm theory, 156 –57, 203 , 246 , 250 , 252 , 260 , 261 –62

  and de Broglie’s double-wave solution, 161

  and dissent on Copenhagen interpretation, 217

  and EPR argument, 98

  and ESSW argument, 175 –77

  and Goldstein, 151 –52, 154

  hidden variable theory of, 101 –2

  influence of Bohmian mechanics, 162 –66

  and many worlds theories, 235 , 239 –40, 261 –62

  and molecular interferometry, 202 –3

  and particle trajectory research, 166 –67, 168 , 172 –75, 179 –81, 180

  and philosophical implications, 235

  and pilot-wave theory, 155 –58

  and QBism, 244 , 250 –53

  Bohr, Niels

  and Bell’s theorem, 61

  and Bohmian mechanics, 147 –48, 150 –52, 155

  and Einstein, 52 –58

  and Fifth Solvay conference, 40

  and many worlds theories, 233

  and models of atomic structure, 35 –38

  and molecular interferometry, 203

  and nonlocality–relativity conflict, 94

  and origin of double-slit experiment, 3

  and QBism, 244

  and quantum entanglement, 96

  and quantum eraser, 110 , 113 –14, 129

  and Schrödinger, 49 –52

  and single-particle experiments, 86 , 89 –90

  and universal wavefunction, 222 –23

  and Wheeler, 218

  Bohr, Tomas, 159

  Born, Max, 41 –42, 46 –48, 107 , 239 –42, 263 –64

  Born rule, 48 , 239 –42, 263 –64, 265

  Bouwmeester, Dirk, 143 –44, 206 , 210 –11, 213 –16

  Bozic, Danino, 94

  Broglie, Louis de

  and Bohmian mechanics, 155 –58, 161

  de Broglie-Bohm theory, 156 –57, 203 , 246 , 250 , 252 , 260 , 261 –62

  and molecular interferometry, 196 , 198 , 202 –4

  and problems with quantum mechanics, 189

  and quantum eraser, 146

  and single-particle experiments, 66

  and wave-particle duality, 24 , 38 , 45 –46

  Brougham, Henry, 15

  Bush, John, 157 , 159 –61

  Carroll, Sean, 227 –28, 230 –34, 237 –41

  Casimir, Hendrik, 58

  causality. See determinism/indeterminism

  Caves, Carlton, 245

  classical physics

  and causality, 48

  classical-quantum boundary, 190 , 192 , 195 , 203 , 205 , 214 , 250 , 252

  and indeterminism, 39

  and photoelectric effect, 26 –27

  and wave-particle nature of light debate, 23 –24

  Clauser, John, 104 , 106 , 110 , 164

  Cohen-Tannoudji, Claude, 61

  Coleman, Sidney, 255

  collapse of wavefunctions

  and Bohmian mechanics, 165 –69

  and future of quantum research, 257 –58

  gravitational collapse theory, 190 –93, 203 –6, 209

  and many worlds theories, 215 –16, 230 –31, 234 –35, 239 , 261 –62

  and nonlocality–relativity conflict, 95

  and QBism, 244 , 246 , 248 –50, 252 –53

  and quantum eraser, 114 –16, 130 –35, 139 –40, 145

  and single-particle experiments, 80 –81, 83

  spontaneous collapse, 190 –91, 193 –95

  and superposition mirror interferometer, 209 –12, 214 –15

  and universal wavefunction, 220 –21

  complementarity, 52 , 57 , 86 , 90 , 113 –14, 128

  complex numbers, 43 –44, 47 –48

  configuration space, 163 , 259 –61

  constructive interference, 14 , 20 –21, 75 , 77 , 141 , 208

  Copenhagen interpretation of quantum mechanics

  and Bohmian mechanics, 147 –51, 153 –57, 159 , 161 –62, 182 –83

  dissenting opinions on, 217 –18

  and EPR argument, 96 , 100

  and hidden variable theory, 146

  and many worlds theories, 229 , 231 , 233 –35, 235 , 239 , 261 –62

  and molecular interferometry, 203

  origin of, 54 –57

  and Penrose’s thought experiment, 190

  and QBism, 244 , 246 , 248 –50, 252 –53

  and quantum eraser, 110 , 129 –30, 133 , 145

  and single-particle experiments, 80 –81, 83 , 85 , 88

  and spontaneous collapse, 192 , 193

  and universal wavefunction, 219 –20, 222 –25

  Copernicus, Nicolaus, 53

  “corpuscular theory of light ,” 8 , 11 , 29

  Couder, Yves, 158 –59

  Couprie, Benjamin, 39

  Curie, Marie, 39 , 112

  Dalibard, Jean, 196

  Davies, Paul, 236

  de-Broglie-Bohm theory, 156 –57, 203 , 246 , 250 , 252 , 260 , 261 –62

  Deckert, Dirk-André, 259

  decoherence, 197 –98, 209 , 214 , 230 –31, 233 , 238 , 241 , 252

  delayed-choice experiments, 86 –91, 108 , 113 , 121 , 123

  Dennett, Daniel, 243 –45

  density matrices, 230 –31

  d’Espagnat, Bernard, 93 –94

  destructive interference, 14 , 20 , 75 –77, 208

  determinism/indeterminism

  and Bohmian mechanics, 151 , 153 –56, 165 , 169 , 176

  and EPR argument, 100 –101

  and first double-slit experiments, 21

  and many worlds theories, 239 , 261 –62

  and matrix mechanics, 44 –45

  and quantum eraser, 139 , 145

  and radioactivity, 39

  and single-pa
rticle experiments, 60

  and universal wavefunction, 218 –19, 221

  and wave mechanics, 48

  Deutsch, David, 234 , 236 , 241

  Dewdney, Chris, 166 , 173

  DeWitt, Bryce, 224 –27

  Diósi, Lajos, 193

  Dirac, Paul, 42 , 84 , 189 , 225

  double interferometer experiments, 140 –44

  double-wave solution, 157 , 161

  Drühl, Kai, 113 –16

  Düker, Heinrich, 65 –66

  Dürr, Detlef, 162

  Eddington, Arthur, 59

  Ehrehfest, Paul, 52 –53, 55

  Einstein, Albert

  and Aspect’s education, 60 –61

  and Bell’s theorem, 104 , 106 –7

  and Bohmian mechanics, 148 –52, 155

  and Bohr-Schrödinger discussions, 50

  debates with Bohr, 52 –58

  and dissenting opinions on Copenhagen interpretation, 217 –18

  and EPR argument, 100 –102

  and Fifth Solvay conference, 39 –40

  and many worlds theories, 238

  and models of atomic structure, 37 –38

  and nonlocality–relativity conflict, 94 –98

  and origin of double-slit experiment, 3

  and photoelectric effect, 16 –17, 27 , 29 –32

  and problems with quantum mechanics, 189

  and QBism, 246

  and quantum eraser, 110 –12, 113 , 139 –40

  and single-particle experiments, 70 , 80 , 82 , 85 –86, 89 –91

  and wave-particle duality of matter, 45

  electromagnetism

  and Bell’s theorem, 102 –3

  and Bohmian mechanics, 154

  and Bouwmeester’s work on Maxwell’s equations, 206

  electromagnetic fields, 26 , 26 , 32

  electromagnetic wave view of light, 24 –26, 33 , 63 , 103

  and Maxwell’s equations, 32 , 35

  and models of atomic structure, 37 –38

  and photoelectric effect, 26 –27, 29 –30

  and quanta of light, 32 –33

  and quantum gravity theories, 188

  and single-particle experiments, 63 , 77

  and wave-particle nature of light debate, 24 –26

  electrons

  discovery of, 28 –31

  and double interferometer experiments, 140 –44

  and Feynman on double-slit experiment, 6

  and first double-slit experiments, 18

  and matrix mechanics, 42 –45

  and models of atomic structure, 36 –37