Foreword

Physics has more than its share of mind-bending ideas: the slowing clocks and shrinking meter sticks of relativity; enormous coagulations of matter, like black holes, that can rupture space-time itself. Yet the strangest ideas of all are clustered in quantum theory, physicists’ remarkably successful description of matter and energy at atomic scales. Here we find descriptions of objects that seem to act as if they were in two places at once; of particles that can tunnel through walls; of Erwin Schrödinger’s twice-fated cat, trapped in a zombie-like state of being both alive and dead. For all that, Schrödinger himself declared one idea in particular, quantum entanglement, to be “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.”¹

Schrödinger had done so much to contribute to quantum theory; his “wave function,” obeying an equation he first published in 1926, remains central to physicists’ efforts to describe quantum systems quantitatively. Almost a decade later, in 1935, Schrödinger coined the term “entanglement,” though by then his enthusiasm for quantum theory had begun to wane. That same year his friend Albert Einstein teamed up with two younger colleagues, Boris Podolsky and Nathan Rosen, to issue his own, latest challenge to quantum theory. In their famous “EPR” paper (named for the authors’ initials), they described a system involving a pair of entangled particles emitted from a central source. Physicists could perform various measurements on one particle, and thereby learn something about the second particle, far off in the distance. Indeed, the EPR authors concluded, physicists should be able to glean more information about the far-away particle than could be accounted for within quantum theory.²

Each particle, it seemed self-evident to Einstein and his coauthors, should possess definite properties on its own, independent of what physicists happened to choose to measure. If physicists elected to measure the first particle’s position at a given moment, for example, they would learn about the position of the second particle, which had headed off at the same speed but in the opposite direction from the first particle. Or the physicists might choose to measure the momentum of the first particle, and thereby learn about the second particle’s momentum. But surely the second particle had definite values for these and any other properties the physicists might have chosen to investigate, regardless of what choices the physicists had made. After all, Einstein’s own theory of relativity made clear that no signal or influence could travel faster than the speed of light—so nothing the physicists might have chosen to do to the first particle should have been able to affect the second particle, which had traveled so far away. If relativity really set an absolute speed limit on how quickly A could influence B, then the second particle would need to carry all its own information with it, as it traveled through space; there would be no time to receive an update on what values for various properties it should have, based on the outcomes of measurements on the first particle. Therefore, the EPR authors concluded with a flourish, there existed “elements of reality”—real, definite properties of that second particle—about which quantum theory offered no description. Quantum mechanics, they argued, was incomplete.³

Within weeks a response came from Niels Bohr, the Danish physicist who had helped to craft quantum theory and who served as a kind of spokesperson for the emerging work. Bohr’s response to EPR was rapid, but abstruse; to this day, it remains difficult to parse Bohr’s argument. Central to his response, however, was a denial that objects in the subatomic realm really must carry complete sets of properties on their own. Rather, Bohr insisted, a particle might have no definite value of, say, momentum, until subjected to a particular measurement—as if a person had no definite weight until stepping on a bathroom scale. (Years later, Einstein famously asked a colleague if quantum physicists really believed that the moon was only there when someone chose to look.) Most important to physicists at the time, it seems, was that Bohr had responded at all. More recent scholarship has indicated that Einstein and Bohr were largely talking past each other—a series of miscommunications exacerbated by the rise of fascism in Europe, which had driven Einstein to emigrate to the United States, thereby ending the late-night, face-to-face discussions that the two had enjoyed during happier times. Each physicist died, decades later, having failed to convince the other when it came to quantum entanglement.⁴

The debate seemed to linger, with no clear resolution, for years. A young physicist from Northern Ireland, John S. Bell, shared many of the reservations about quantum theory that Einstein had expressed. Indeed, Bell had nursed a private concern about the topic since his student days, growing frustrated when students and teachers seemed to parrot Bohr’s responses. Bell was quickly counseled to keep such “philosophical” concerns to himself—by the 1950s, quantum mechanics had moved to the very center of physicists’ expanding efforts to understand everything from nuclear reactions to superconductivity to the properties of little devices like transistors. In every single case, the equations of quantum theory provided a remarkable match to experiments. So why, Bell’s teachers pressed, should they continue to fret over the abstract questions that had distracted old-timers like Einstein and Bohr?⁵

Bell dove into mainstream topics for his research in high-energy physics, even as his thoughts kept returning to nagging questions about quantum entanglement. Finally, during a sabbatical in the United States in 1964, Bell brought many of his ideas to fruition. He tweaked the famous EPR thought experiment, focusing on specific combinations of measurements that physicists could perform on each of the two entangled particles. In just a few lines of algebra, Bell demonstrated that Einstein’s pair of assumptions—that particles carry definite properties on their own, prior to and independent of measurement, and that no influence can travel faster than light—necessarily led to a contradiction with quantum theory. Bell identified a quantitative upper limit for how often the outcomes of certain combinations of measurements on the two particles could ever line up, if they behaved in accordance with Einstein’s assumptions. If, instead, the particles were governed by quantum theory, then the measurements on each particle should be more strongly correlated, surpassing the upper bound that Bell had derived. If quantum theory were true, in other words, then performing a measurement over here really would seem to affect the behavior of some other tiny bit of matter, observed arbitrarily far away.⁶

On paper, Bell showed, the contrast was as clear as day: an Einstein-like world limited to one side of the bound; a quantum-mechanical world clearly surpassing that limit. The central question that had absorbed Einstein and Bohr for decades could be posed in a laboratory, not just debated in a smoke-filled room. Bell published his paper, and then … nothing. Years went by before he heard so much as a peep of interest from the physics community.

In time, Bell’s elegant paper happened to catch the eye of a few unconventional physicists, who recognized the magnitude of Bell’s achievement. If they followed Bell’s reasoning, and really conducted experiments of the sort he had described, they might be able to learn something deep about how the world works. Pioneering physicists like Abner Shimony, John Clauser, Michael Horne, Stuart Freedman, Alain Aspect, and a handful of others began to realize that by testing Bell’s inequality in a laboratory, they could subject abstract, metaphysical mysteries to experimental investigation. Hence the term used in George Greenstein’s lovely book: experimental metaphysics.⁷

For nearly half a century, physicists have subjected Bell’s inequality to experimental test. Every single published result has been consistent with the predictions of quantum theory, showing correlations among measurements on pairs of entangled particles in excess of Bell’s bound. Yet from the start, Bell, Shimony, Clauser, Horne, Aspect, and others have recognized that each of these tests has been subject to one or more “loopholes”: little conceptual escape hatches by which an Einstein-like interpretation could still account for the experimental results.

Perhaps the particles, or other elements of the apparatus, had shared information (at or below the speed of light) during a particular series of measurements, thereby arranging for the measurements to line up the way they did. Or perhaps the detectors that had been used to measure the particles were inefficient, and failed to register any definite outcome some fraction of the time. Then it would be possible, at least in principle, for the strong correlations that showed up in those measurements that were successfully recorded to be but a rare statistical hiccup, some fluke that would have been washed out had all of the particles actually been measured. Or perhaps the striking correlations arose from some common cause, deep in the experiment’s past, which had somehow nudged the selection of measurements to be performed and tipped off the particles in advance. After all, as Schrödinger himself acknowledged back in 1935, one should hardly be surprised when a student aces an examination, if she had received a copy of the questions ahead of time.⁸

Experimental groups around the world have tackled these loopholes, usually one at a time, since the early 1980s.⁹ Only as recently as 2015 have various groups succeeded in measuring significant violations of Bell’s inequality while closing not one but two of the stubborn loopholes.¹⁰ More recently, my own colleagues and I have conducted experiments to address that strange, third loophole—the one regarding the seemingly random selection of measurements to perform on the entangled particles—by turning the universe itself into a pair of random-number generators. Yet again we have found—as have our colleagues around the world—the strong correlations that Bell had first identified.¹¹

Why go through all the trouble? Aren’t the dozens of previous experiments sufficient for us to declare the question settled? More than just stubbornness is at stake. In fact, quantum entanglement now lies at the center of a booming field dubbed “quantum information science,” which promises major new devices. Quantum encryption and quantum computers—each of which has progressed well into the beta-testing stage—will function as promised only if entanglement is real. If, for some reason, entanglement were merely an artifact, and the world really were governed by Einstein’s assumptions, then quantum encryption simply would not be secure, and quantum computers would fail to deliver the anticipated exponential speed-up compared to ordinary machines. These days, practical technologies built around quantum entanglement constitute a multibillion-dollar industry—contributing a new set of imperatives to keep testing Bell’s inequality, in addition to the deep, metaphysical questions.¹²

Despite all the theoretical progress over the past half century, and the recent experimental advances in addressing various loopholes, we are still left with vexing questions. How could the world really work that way? How could two little specks of matter act in concert, even after they had moved arbitrarily far apart? In recent years, physicists have gone to extremes trying to articulate, in everyday scenarios, what entanglement implies about the world. There have been elaborate fables about cops chasing quantum robbers; quantum soufflés that do or do not rise; tales of twins who order drinks in bars an ocean apart; and so on.¹³

Amid all these discussions, George Greenstein’s book is a special delight. With patience and clarity, Greenstein guides readers along this extraordinary conceptual journey. We are neither hushed nor rushed; never reprimanded, as John Bell himself had been as a student, to simply take this or that result on faith. Rather, Greenstein shares his own earnest struggles to come to grips with the ideas, to sit with them, to try to puzzle through what they might imply about the workings of nature at its most fundamental. And he does all this with simple illustrations and virtually no mathematics. His book is an invitation and a primer for those new to the topic, and a timely reminder to fellow specialists, who have long since grown comfortable with the mathematical formalism of quantum theory, that this central element of physicists’ toolkit retains a beguiling strangeness at its core.

David Kaiser

Germeshausen Professor of the History of Science and Professor of Physics

Massachusetts Institute of Technology

October 2018

Notes

1.  Erwin Schrödinger, “Discussion of probability relations between separated systems,” Mathematical Proceedings of the Cambridge Philosophical Society 31 (1935): 555–563, on 555.

2.  Schrödinger, 555; Albert Einstein, Boris Podolsky, and Nathan Rosen, “Can quantum-mechanical description of physical reality be considered complete?,” Physical Review 47 (1935): 777–780.

3.  Einstein, Podolsky, and Rosen, “Can quantum-mechanical description of physical reality be considered complete?,” 777.

4.  Niels Bohr, “Can quantum-mechanical description of physical reality be considered complete?,” Physical Review 48 (1935): 696–702. Abraham Pais recounts the story of Einstein asking him about the moon in Pais, “Subtle Is the Lord …”: The Science and the Life of Albert Einstein (New York: Oxford University Press, 1982), pp. 5–6. On the Einstein–Bohr debate, see especially Don Howard, “Einstein on locality and separability,” Studies in History and Philosophy of Science 16 (1985): 171–201; Don Howard, “‘Nicht sein kann was nicht sein darf,’ or the prehistory of EPR, 1909–1935: Einstein’s early worries about the quantum mechanics of composite systems,” in Sixty-Two Years of Uncertainty: Historical, Philosophical, and Physical Inquiries into the Foundations of Quantum Mechanics, ed. Arthur I. Miller (New York: Plenum, 1990), pp. 61–112; Arthur Fine, The Shaky Game: Einstein, Realism, and the Quantum Theory (Chicago: University of Chicago Press, 1986); and Mara Beller, Quantum Dialogue: The Making of a Revolution (Chicago: University of Chicago Press, 1999).

5.  See esp. Jeremy Bernstein, “John Stewart Bell: Quantum engineer,” in Bernstein, Quantum Profiles (Princeton: Princeton University Press, 1991), 3–91; Louisa Gilder, The Age of Entanglement: When Quantum Physics was Reborn (New York: Knopf, 2008); David Kaiser, How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival (New York: W. W. Norton, 2011); Olival Freire, Jr., The Quantum Dissidents: Rebuilding the Foundations of Quantum Mechanics, 1950–1990 (New York: Springer, 2014); and Andrew Whitaker, John Stewart Bell and Twentieth-Century Physics (New York: Oxford University Press, 2016).

6.  John S. Bell, “On the Einstein Podolsky Rosen paradox,” Physics 1 (1964): 195–200.

7.  See esp. John Clauser, Michael Horne, Abner Shimony, and Richard Holt, “Proposed experiment to test local hidden-variable theories,” Physical Review Letters 23 (1969): 880–884; Stuart Freedman and John Clauser, “Experimental test of local hidden-variable theories,” Physical Review Letters 28 (1972): 938–941. On the early experimental efforts to test Bell’s inequality, see also Gilder, Age of Entanglement, chapters 30–31; Kaiser, How the Hippies Saved Physics, chapters 3 and 8; and Freire, Quantum Dissidents, chapters 7–8.

8.  John Trimmer, “The present situation in quantum mechanics: A translation of Schrödinger’s ‘Cat Paradox’ paper,” Proceedings of the American Philosophical Society 124 (1980): 323–338. Schrödinger’s paper was originally published in three installments as “Die gegenwärtige Situation in der Quantenmechanik,” Naturwissenschaften 23 (1935): 807–812, 823–828, and 844–849. For recent discussion of loopholes in experimental tests of Bell’s inequality, see J.-A. Larsson, “Loopholes in Bell inequality tests of local realism,” Journal of Physics A 47 (2014): 424003; and N. Brunner, D. Cavalcanti, S. Pironio, V. Scarani, and S. Wehner, “Bell nonlocality,” Reviews of Modern Physics 86 (2014): 419–478.

9.  The first loophole to be addressed experimentally concerns “locality,” that is, whether information from one side of the apparatus could have affected the other side during the conduct of a given experiment. See Alain Aspect, Jean Dalibard, and Gérard Roger, “Experimental test of Bell’s inequalities using time-varying analyzers,” Physical Review Letters 49 (1982): 1804–1807; Gregor Weihs, Thomas Jennewein, Christoph Simon, Harald Weinfurter, and Anton Zeilinger, “Violation of Bell’s inequality under strict Einstein locality conditions,” Physical Review Letters 81 (1998): 5039–5043.

10.  B. Hensen et al., “Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers,” Nature 526 (2015): 682–686; M. Giustina et al., “Significant-loophole-free test of Bell’s theorem with entangled photons,” Physical Review Letters 115 (2015): 250401; L. K. Shalm et al., “Strong loophole-free test of local realism,” Physical Review Letters 115 (2015): 250402; W. Rosenfeld et al., “Event-ready Bell test using entangled atoms simultaneously closing detection and locality loopholes,” Physical Review Letters 119 (2017): 010402; and M.-H. Li et al., “Test of local realism into the past without detection and locality loopholes,” Physical Review Letters 121 (2018): 080404.

11.  Jason Gallicchio, Andrew Friedman, and David Kaiser, “Testing Bell’s inequality with cosmic photons: Closing the setting-independence loophole,” Physical Review Letters 112 (2014): 110405; Johannes Handsteiner et al., “Cosmic Bell test: Measurement settings from Milky Way stars,” Physical Review Letters 118 (2017): 060401; Dominik Rauch et al., “Cosmic Bell test using random measurement settings from high-redshift quasars,” Physical Review Letters 121 (2018): 080403. See also C. Abellán et al. (The Big Bell Test collaboration), “Challenging local realism with human choices,” Nature 557 (2018): 212–216.

12.  Jason Palmer, “Subatomic opportunities: Quantum leaps,” Economist (March 9, 2017).

13.  Kurt Jacobs and Howard Wiseman, “An entangled web of crime: Bell’s Theorem as a short story,” American Journal of Physics 73 (2005): 932–937; Paul Kwiat and Lucien Hardy, “The mystery of the quantum cakes,” American Journal of Physics 68 (2000): 33–36; Seth Lloyd, Programming the Universe: A Quantum Computer Scientist takes on the Cosmos (New York: Knopf, 2006), 120–121; see also Kaiser, How the Hippies Saved Physics, pp. 37–38.