Book summary “What is real?”: summary

Introduction

You live in a world governed by laws that physics has successfully decoded for centuries, yet the subatomic realm refuses to make intuitive sense. When you study quantum mechanics, you encounter a bizarre mathematical framework that predicts experimental outcomes with astonishing precision while denying the existence of an underlying reality. You are told to shut up and calculate rather than ask what particles actually do when you are not looking.

Adam Becker investigates how this anti-realist consensus took over physics and why a courageous group of scientists risked their careers to challenge it. The book traces the historical suppression of alternative theories and explains the foundational experiments that proved the universe is stranger than the Copenhagen orthodoxy admits.

• What does quantum mechanics actually say about the nature of the physical world?
• Why did generations of physicists abandon the search for an objective reality?
• How did pioneers like David Bohm and John Bell revive the quest for truth?

1. Foundations of Quantum Physics

Quantum mechanics rules the subatomic domain with astonishing precision. Yet the theory leaves you with a profound conceptual puzzle. It tells you how to calculate probabilities for experimental outcomes while refusing to describe what actually happens in the physical world.

The Copenhagen interpretation asserts that quantum physics does not describe an objective reality existing independently of observers, but rather serves merely as a mathematical tool to calculate the probabilities of experimental outcomes.

Niels Bohr and his colleagues argued that subatomic particles do not possess definite properties like positions until they are measured. Measuring forces an electron to assume a specific state, meaning observers actively produce the results of measurements rather than uncovering a pre-existing reality.

Pascual Jordan noted that when measuring an electron's position, we compel it to assume a definite position where previously it was neither here nor there.

The Copenhagen Orthodoxy

Niels Bohr formulated the principle of complementarity to argue that quantum objects require mutually contradictory descriptions, such as particles or waves, depending on the experimental apparatus. Bohr asserted that because observation unavoidably disturbs atomic phenomena, you cannot ascribe independent reality to quantum objects without a measurement apparatus. Consequently, properties like position and momentum, or wave and particle descriptions, are complementary perspectives that are never simultaneously valid but both needed for a complete description of experience.

Electrons in a cathode-ray-tube TV propagate outward like waves governed by the Schrödinger equation, but strike the phosphorescent screen at a single location like particles.

Albert Einstein strongly rejected the Copenhagen interpretation because it lacked a coherent story about an objective physical world existing independently of measurements. Einstein argued that physics must determine what actually exists rather than acting as an epistemology-soaked system that avoids describing the real world between measurements. He believed that the theory's reliance on observer-dependent collapse was deeply unsatisfactory.

Einstein dismissed the Copenhagen interpretation as a tranquilizing philosophy or religion providing a soft pillow for true believers, famously asking if a mouse or a single-celled creature was required to observe the universe for it to take a definite state.

The Measurement Problem

The measurement problem arises because wave functions evolve deterministically according to the Schrödinger equation until a measurement occurs, at which point the wave function violently and randomly collapses. While the Schrödinger equation predicts that wave functions change smoothly and continuously over time, the act of measurement causes an instantaneous collapse to a single point. This creates an unresolved contradiction regarding why laws of nature should change during a measurement and what specifically constitutes a measurement.

A ball in a box has a smooth, undulating wave function until its location is measured, causing the wave function to immediately collapse to a single spot.

2. Historical Debates and Alternative Theories

Historical conferences and early dissenting frameworks challenged the Copenhagen orthodoxy by proposing pilot waves and deterministic trajectories.

The Solvay Conference

The Fifth Solvay Conference in 1927 in Brussels is the legendary birthplace of quantum physics debates, where Einstein challenged the emerging Copenhagen interpretation. Seventeen of the twenty-nine total attendees won or would win Nobel Prizes. Ernst Solvay organized the first conference on quantum theory in 1911, and died in 1922.

At the 1927 Solvay Conference in Brussels, Einstein walked up to the chalkboard to sketch a thought experiment with electrons passing through a hole onto a phosphorescent hemisphere, challenging the instantaneous wave function collapse. While Louis de Broglie presented a deterministic pilot wave theory, Born and Heisenberg declared quantum physics a closed theory, and Einstein presented his first major thought experiment critiquing the Copenhagen interpretation.

De Broglie used Schrödinger's equation to suggest that particles are guided along pilot waves in a deterministic way while still satisfying Heisenberg's uncertainty principle because their exact paths remain hidden. De Broglie had defended his PhD thesis three years prior to the conference.

De Broglie presented his pilot wave theory at the 1927 Solvay Conference, but faced harsh objections from Wolfgang Pauli and Hans Kramers regarding particle collisions and mirror recoil.

The EPR Paradox

The 1935 EPR paper argued that quantum mechanics is incomplete because it forces a choice between nonlocality and lack of pre-existing properties. The paper was authored by Einstein, Podolsky, and Rosen, and used an indirect thought experiment involving two interacting particles to show that either quantum theory fails to describe reality completely, or it involves spooky action at a distance.

Podolsky leaked the paper to the New York Times before publication, causing a media frenzy with the headline EINSTEIN ATTACKS QUANTUM THEORY, which infuriated Einstein. The EPR paper was published in 1935, and May 4, 1935 was the date the New York Times published the leak.

Erwin Schrödinger discovered that entanglement is pervasive in quantum mechanics, compounding the dilemma posed by the EPR paper. Schrödinger showed that nearly any interaction between subatomic particles causes them to share a single wave function, meaning that Einstein's forced choice between nonlocality and incompleteness applies throughout quantum theory.

Schrödinger wrote to Einstein praising the EPR paper for calling dogmatic quantum mechanics to account, and subsequently introduced the term entanglement in his own papers.

3. Bohm's Pilot-Wave Interpretation

David Bohm developed a new interpretation of quantum physics featuring particles with definite positions guided by pilot waves, providing a deterministic picture of the subatomic world that was mathematically equivalent to normal quantum physics. You can see that subatomic particles exist with definite positions and trajectories whether anyone is looking at them or not.

David Bohm developed a consistent pilot-wave interpretation featuring particles with definite positions, facing severe institutional resistance during the Cold War.

Pilot-Wave Mechanics

Bohm's theory posits that particles are guided by pilot waves that determine their motion in an orderly, predictable fashion, eliminating the need for the paradoxes of the Copenhagen interpretation. You find that a complete description of reality with hidden variables is mathematically possible.

When Max Dresden presented Bohm's new work at the Institute for Advanced Study in Princeton in 1952, he was met with fierce hostility and accusations from luminaries, ending with Robert Oppenheimer's famous suggestion to ignore Bohm since he could not be disproved. Despite being mathematically rigorous and consistent, his theory faced heavy resistance and ostracism during the Cold War due to his political associations and the hegemony of the Copenhagen interpretation.

Persecution and Exile

David Bohm was born on December 20, 1917, and later became a Brazilian citizen on December 20, 1954. His association with the Communist Party during his time at Berkeley led to the denial of his security clearance for the Manhattan Project.

David Bohm was called before the House Un-American Activities Commission in 1949 and later moved into exile. On May 25, 1949, David Bohm appeared before HUAC, and on December 4, 1950, a US marshal arrested Bohm for contempt of Congress after he invoked the First Amendment. This led to his immediate suspension and banning from the Princeton campus. He was cleared of criminal charges in 1951, but ultimately forced to leave for Brazil due to continued blacklisting and passport confiscation.

The physics establishment united against Bohm's pilot-wave interpretation, dismissing it as artificial metaphysics. Wolfgang Pauli dismissed the theory as an uncashable check, and Léon Rosenfeld actively worked to prevent the publication of Bohm's paper in Nature.

4. Bell's Theorem and Nonlocality

John Bell proved through a mathematical inequality that local hidden-variable theories are incompatible with quantum mechanics, demonstrating that the universe is fundamentally nonlocal.

Deriving Bell's Inequality

Bell designed a thought experiment involving Ronnie the Bear's casino in Bellville, California, using inspectors Fatima and Gillian checking roulette wheels. By introducing imperfect correlations into the EPR-Bohm setup—such as rotating the axis of one polarizer—Bell showed that local hidden-variable theories cannot reproduce quantum predictions. Under local instruction sets, local theories must satisfy specific statistical limits like a 33 percent minimum expected match rate, which quantum mechanics violates with a 25 percent match rate for differently numbered wheels.

Bell's theorem leaves only three unequivocal possibilities:

• nature is nonlocal;
• we live in branching multiple worlds;
• quantum physics gives incorrect predictions.

Explaining away Bell's theorem requires accepting nonlocality, adopting Everett's many-worlds interpretation where multiple outcomes occur, or entertaining extreme superdeterminism where experimental settings are pre-conspired with particle behavior. Bohm's pilot-wave theory accepts explicit nonlocality, whereas Everett's interpretation avoids nonlocality by permitting multiple branching worlds.

Experimental Tests

John Clauser adapted an existing atomic polarization experiment to perform the first laboratory test of Bell's theorem. While a graduate student at Columbia, Clauser discovered J.S. Bell's paper and realized it could be tested in the laboratory. After communicating with Bell and teaming up with Stuart Freedman at Berkeley using borrowed equipment from Gene Commins, Clauser collected data showing that Bell's inequality was violated, validating quantum mechanics.

Clauser and Freedman collected two hundred hours of data using equipment borrowed from Gene Commins and a repurposed old telephone relay, eventually publishing their results in 1972. Bell showed that if measurements on separated entangled particles are independent of local hidden variables, certain statistical correlations cannot exceed a specific bound. Experiments later verified that quantum mechanics violates this bound, proving the universe is fundamentally nonlocal.

Alain Aspect successfully performed a landmark experiment testing Bell's inequality by switching polarizer angles while the entangled photons were actively in flight. To rule out the possibility that photons could communicate with the polarizer settings beforehand, Aspect designed a sophisticated setup to rapidly change polarizer orientations faster than the time it took light to travel from the source. Aspect spent six years assembling and testing his equipment in Christian Imbert's lab at the Institut d'Optique, publishing his groundbreaking results in 1982 using forty nanoseconds or less time required to switch polarizers and ten meters typical distance from source to polarizers.

5. The Fall of Positivism and the Future of Reality

The collapse of logical positivism and the rise of scientific realism have cleared the path for burgeoning research in quantum foundations and alternative interpretations.

The Fall of Positivism

Logical positivism asserted that unobservable entities were meaningless metaphysics. Willard Van Orman Quine destroyed this verification theory of meaning by proving that individual statements cannot be verified in isolation from the rest of human knowledge. In his 1951 paper Two Dogmas of Empiricism, Quine argued that our statements face experience as a corporate body. Quine spent time studying in Europe with Moritz Schlick, Philipp Frank, and Rudolf Carnap before publishing his critique at Harvard, building upon his 1932 PhD thesis on mathematical logic.

Scientific realism emerged as the consensus philosophy replacing positivism. Philosophers like J.J.C. Smart and Hilary Putnam argued that instrumentalism fails because the extreme predictive precision of quantum chemistry and other fields can only be understood if unobservable entities like electron orbitals actually exist.

Smart illustrated the failure of instrumentalism by pointing to the thermite reaction, where mixing powdered rust and aluminum triggers a runaway chemical reaction reaching 2,500 degrees Celsius that continues even underwater.

Current Research and Pluralism

Even though different interpretations of quantum mechanics yield identical experimental predictions, choosing an interpretation profoundly shapes the mental models and new theories scientists develop.

Positivist arguments claiming that interpretations are meaningless because they are empirically indistinguishable are false. Tycho Brahe's geocentric-heliocentric hybrid model gave identical mathematical predictions to Copernicus's heliocentric model in the 16th century, but the underlying worldview led to completely different sets of ideas about the universe. Subscribing to a specific interpretation guides how physicists search for physics beyond the current theory.

Despite decades of institutional discouragement, the field of quantum foundations is burgeoning with diverse alternative approaches aiming to solve deep problems like quantum gravity. You can see researchers actively exploring numerous interpretations and modifications of quantum theory:

• retrocausal approaches;
• superdeterministic local hidden-variables theories;
• gravity-induced collapse models;
• quantum field theory extensions.

Roger Penrose proposed that wave function collapse is a real, non-random process caused by gravity, uniting general relativity and quantum mechanics in a novel way.

10 Key Ideas

1. The Copenhagen interpretation asserts that quantum physics does not describe an objective reality existing independently of observers, but rather serves merely as a mathematical tool to calculate the probabilities of experimental outcomes.
2. The measurement problem arises because wave functions evolve deterministically according to the Schrödinger equation until a measurement occurs, at which point the wave function violently and randomly collapses.
3. Historical conferences and early dissenting frameworks challenged the Copenhagen orthodoxy by proposing pilot waves and deterministic trajectories.
4. The 1935 EPR paper argued that quantum mechanics is incomplete because it forces a choice between nonlocality and lack of pre-existing properties.
5. David Bohm developed a consistent pilot-wave interpretation featuring particles with definite positions, facing severe institutional resistance during the Cold War.
6. John Bell proved through a mathematical inequality that local hidden-variable theories are incompatible with quantum mechanics, demonstrating that the universe is fundamentally nonlocal.
7. Alain Aspect successfully performed a landmark experiment testing Bell's inequality by switching polarizer angles while the entangled photons were actively in flight.
8. The collapse of logical positivism and the rise of scientific realism have cleared the path for burgeoning research in quantum foundations and alternative interpretations.
9. Even though different interpretations of quantum mechanics yield identical experimental predictions, choosing an interpretation profoundly shapes the mental models and new theories scientists develop.
10. Despite decades of institutional discouragement, the field of quantum foundations is burgeoning with diverse alternative approaches aiming to solve deep problems like quantum gravity.