The One E-Book

Heinrich Päs

• Herausgeber: Icon Books
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2024

Seitenzahl: 542

Beschreibung

In The One, particle physicist Heinrich Päs presents a bold idea: fundamentally, everything in the universe is an aspect of one unified whole.This idea, called monism, has a rich 3,000-year history: Plato believed that 'all is one', but monism was later rejected as irrational and suppressed as a heresy by the medieval Church. Nevertheless, monism persisted, inspiring Enlightenment science and Romantic poetry.Päs shows how monism could inspire physics today, how it could slice through the intellectual stagnation that has bogged down progress in modern physics and help science achieve the 'grand theory of everything' that it has been chasing for decades.Blending physics, philosophy, and the history of ideas, The One is an epic, mind-expanding journey through millennia of human thought and into the nature of reality itself.

Leseprobe

‘The history is thoroughly researched, the physics is cutting edge and Päs’s larger point resonates: much, or maybe all, of what we take for reality is an artifact of our limited perspectives.’

AMANDA GEFTER, Scientific American

‘“It has always been the dream of philosophers to have all matter built up from one fundamental type of particle,” said Paul Dirac in 1930. With expert guidance from Heinrich Päs, in The One we glimpse the scale and grandeur of the dream in one of its modern forms: everything is quantum information.’

JIM BAGGOTT, AUTHOR OF ATOMIC AND QUANTUM REALITY

‘Usually we say the universe is made of particles, but Päs shows how quantum physics inverts that. The whole comes first, not the parts – the parts come from fragmenting the whole. I’ll never see reality the same way again!’

GEORGE MUSSER, AUTHOR OF SPOOKY ACTION AT A DISTANCE

‘Are we one with the universe? It is a question as old as mankind … But Päs is ready for the challenge and delivers an original and fresh account of both the history and the science of monism. An enticing read for those who seek to understand their place in nature – and who does not?’

SABINE HOSSENFELDER, PHYSICIST AND AUTHOR OF EXISTENTIAL PHYSICS

‘Päs delivers an entertaining and enlightening tour of physics, religion, and philosophy, yielding a monistic vision of fundamental reality as a vast unified whole: The One. In place of the pluralist image of a world composed from many little particles, Päs offers an image of one entangled cosmos from which all else emerges. The result is an important new program for physics based on quantum cosmology, from which space, time, particles, and all the rest are to be derived from the universal wave function via decoherence – a new hope for our understanding of fundamental reality.’

JONATHAN SCHAFFER, RUTGERS UNIVERSITY

ALSO BY HEINRICH PÄS

The Perfect Wave

Published in the UK in 2023 by

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ISBN: 978-183773-029-2

e-BOOK ISBN: 978-183773-031-5

Text copyright © 2023 Heinrich Päs

Illustrations by Frigga Päs

Wheeler’s U was first published in Zurek 1990, p. ix.

Reprinted with permission of the estate of John Archibald Wheeler.

The author has asserted his moral rights.

No part of this book may be reproduced in any form, or by any

means, without prior permission in writing from the publisher.

Interior design by Jeff Williams

Printed and bound in the UK by TJI.

For Sara You are the One for me

CONTENTS

Introduction: Stargazing

1. The Hidden One

2. How All Is One

3. How One Is All

4. The Struggle for One

5. From One to Science and Beauty

6. One to the Rescue

7. One Beyond Space and Time

8. The Conscious One

Conclusion: The Unknown One

Acknowledgments

Further Reading

Glossary

Notes

Bibliography

From all things One and from One all things.

—HERACLITUS

There are many indications that, following the recursive pattern of scientific revolutions, we are now witnessing the beginning of the phase of crisis . . . This is the most complex and intense moment of scientific research, when revolutionary and unprejudiced ideas are needed for a real paradigm change.

—GIAN GIUDICE, HEAD OF THE THEORETICAL PHYSICS DEPARTMENT, CERN

To lend wings to physics once again.

—FRIEDRICH WILHELM JOSEPH SCHELLING

INTRODUCTION

Stargazing

ONE VERY EARLY MORNING IN MID-OCTOBER 2009, I was waiting alone in a deserted and pitch-dark alley in San Pedro, in the middle of the Chilean Atacama Desert, one of the driest spots on earth. Above me, countless stars were sparkling, so mesmerizing that I struggled to keep an eye out for the tour guide’s truck, coming to pick me up for a trip up to the Altiplano to watch the flamingoes stalking a secluded salt flat in the first light of the rising sun. Never before or after have I seen a more magnificent sky, though there have been other, similarly magical moments: counting shooting stars from the deck of a sailboat while crossing the Baltic Sea, practicing full-moon surfing off Waikiki Beach in Hawaii, or stepping out of a ski cabin at night, halfway up a mountain in the Austrian Alps, only to be stopped in my tracks by the bright band of the Milky Way’s galactic disk. In such moments, I have felt entirely small and insignificant and yet, at the same time, strangely at home in the universe.

But what does it mean to feel at home in the universe? What do we actually mean when we talk about the “universe”? Etymologically, the word comes from the Latin universum, meaning something like “all things combined together into one.” Yet, when we speak of the universe, we usually refer to outer space, our cosmic environment, stars, planets, galaxies, a vast realm filled with countless objects. Apparently, what we refer to as the “universe” and what the term actually means have little in common, if anything at all.

Almost all the celestial objects you can identify in the night sky belong to our own galaxy, the Milky Way, which in total hosts more than one hundred billion stars. And the Milky Way itself is only one among about a trillion galaxies. As impressive as these numbers are, these visible objects are only the tiniest part of the entire universe. For every star you can spot out there, there exists about ten times more mass in nonluminous matter, such as gas clouds billowing around in interstellar space. Even more so, for all ordinary matter there exists five times as much mass in “dark matter,” expected to be made of exotic, unknown particles floating across the universe. And finally, there exists three times more “dark energy,” the puzzling fuel that drives the fabric of space-time to expand faster and faster.

So much for our “universe.”

But according to modern cosmology, maybe even our universe is not everything—there may be more than just a single universe. Cosmologists now describe an epoch of accelerating expansion in the very early times, called “cosmic inflation.” The inflationary period terminates in a hot plasma, which we can identify with the Big Bang. But nobody knows what happened before inflation, or for how long it lasts. Maybe inflation goes on forever, and is maybe still going on outside our own universe, in other regions of a “multiverse”. In that case it may continue to produce innumerable other “baby universes,” popping up in an eternally inflating space. This, actually, is quite possible.

But that is not enough to account for “everything” either. Not even close! Beyond parallel universes, dark energy, dark matter, and trillions of galaxies with a hundred billion stars each, there may yet exist a realm of infinite possibilities, where everything that, in principle, could exist actually does. There you would find innumerable copies of yourself, my cat, your dog, the flamingoes of the Altiplano, everyone, of all stars and galaxies and everything mentioned above. These parallel realities are the different branches of Hugh Everett’s infamous “many worlds” interpretation of quantum mechanics. In fact, they constitute another—arguably more fundamental—layer of multiverse. Increasingly more physicists are willing to accept now that they are an inherent prediction of quantum mechanics—that a functional notion of quantum mechanics is increasingly difficult to sustain without “many worlds.”

And even this is not the end of the story. In addition to these parallel worlds, the quantum world contains infinite arbitrary “super­positions” of these realities. These are realities in which cats are half dead and half alive and where you are not either sitting in a chair and reading a book in the United States or driving a rental car through Europe but where both activities and places are mixed up in a way implying one cannot decide which one is true. The quantum realm encompasses everything that could be and all possible blends of these would-be realities.

Yet, standing there, under the stars, I still felt that feeling that many humans have shared: that I was somehow one with the vastness beyond myself. Is there a more daring, courageous, and flat-out overwhelming thought than to conceptualize “the whole material world,” everything from “celestial bodies” to “life upon the earth” and from the “nebula stars to the mosses on the granite rocks,” as the great German naturalist and discoverer Alexander von Humboldt portrayed the universe, as “One”?1

It seems bizarre to believe all of this could be connected. It sounds like a fairy tale fabricated by mystics or madmen. Yet the conviction that the universe is all “one” and the experience that it is comprised of many things have been an enduring conflict for humanity since its earliest days. “From all things One and from One all things”: twenty-five hundred years ago, the Greek philosopher Heraclitus had expressed the thought of an all-encompassing universe in its most radical way.2 This notion that there is but one object in the universe, the universe itself, is known to philosophers as “monism,” from the ancient Greek monos, meaning “unique.” It has inspired Plato’s dialogues, Botti­celli’s painting The Birth of Venus, Mozart’s opera The Magic Flute, and a major part of Romantic poetry from Goethe to Coleridge and Wordsworth. It has traveled with James Cook’s ships around the world and driven several of the founding fathers of the United States of America, even making its way into the US Declaration of Independence as “Nature’s God.” The One has had such an influence on the world of ideas, on the arts and humanities, that its importance as a scientific concept is often overlooked. Taken at face value though, the hypothesis that “all is One” isn’t a statement about God, spirits, or subjective mental states; it is a statement about nature, about the particles, planets, and stars out there.

As a theoretical physicist, for the past twenty-five years I have worked to figure out how tiny particles compose the world. Particles have thrilled me since the very first time I heard about them. Yet, fascinating as they are, what truly captivated me about these particles is how they can serve as a tool to uncover the foundations of reality. “What is everything made of? ” was a question that started to occupy me when I still was in high school. This fascination was what got me into physics, earning me a PhD and finally a professorship. Particles kept me going when I was struggling with math, incomprehensible language, and feelings of inferiority. And particles were the driving force of my work when, over the following decades, I published more than eighty papers in refereed journals, when I wrote a Scientific American cover feature that got reprinted next to a piece by Stephen Haw­king, and when my research made it three times onto the cover of New Scientist magazine. Of course, I’m not alone in this endeavor. I’m just a modest contributor in a global enterprise. There are some ten thousand researchers all over the world, including some of the most brilliant minds on the planet, working restlessly to find out how particles ultimately constitute what we see around us.

Now I believe we are on the wrong track.

Don’t get me wrong. Science’s most important task is to predict and explain the outcome of experiments, observations, and events. And particle physics does that with an unrivaled accuracy. Starting with a set of equations that fits onto a coffee mug, particle physicists predict the results of their experiments with a precision that would correspond to knowing the distance between London and Berlin up to less than a millimeter. But while particle physics is still more precise than any other discipline in science, it doesn’t tell the full story. Because if we pay attention to the full story, we will see that particles do not compose the world; it is the other way around.

Ever since the discovery of the atom, physicists adhered to the philosophy of reductionism. According to this idea, nature could be grasped in a unified understanding by decomposing everything around us into pieces made up from the same tiny constituents. According to this common narrative, everyday objects such as chairs, tables, and books are made of atoms, atoms are composed of atomic nuclei and electrons, atomic nuclei contain protons and neutrons, and protons and neutrons consist of quarks. Elementary particles such as quarks or electrons are understood as the fundamental building blocks of the universe. Over the past fifty years, to work out and concretize this view, hundreds of thousands of pages have been filled with sophisticated equations full of strange symbols. To test these ideas, gigantic particle smashers have been built, tubes many miles long and worth billions of dollars, to accelerate subatomic matter close to the speed of light, let it crash together with violent impact, and search for even smaller or as-yet undiscovered pieces. With the help of NASA and the European Space Agency, engineering marvels have been launched into space to eavesdrop on the earliest incidents in the universe to understand how the world looked when it was but a soup of hot particles.

This philosophy has been tremendously successful, but there is a blind spot. Atoms, protons and neutrons, electrons and quarks are described by quantum mechanics. And according to quantum mechanics, it is, in general, impossible to decompose an object without losing some essential information. Particle physicists strive for a fundamental description of the universe, one that discards no information. But if we take quantum mechanics seriously, this implies that, on the most fundamental level, nature cannot be composed of constituents. The most fundamental description of the universe has to start with the universe itself.

Like any other professional physicist, I work with quantum mechanics on a daily basis. We use quantum mechanics to calculate and predict the results of the experiments, observations, and problems that interest us, be it particle collisions in giant accelerators, scattering processes in the primordial plasma of the early universe, or the behavior of electric or magnetic fields in a solid-state lab experiment. But while we almost always adopt quantum mechanics to describe specific observations and experiments, we usually don’t apply it to the entire universe.

This has a mind-boggling consequence. As I will argue in this book, once quantum mechanics is applied to the entire cosmos, it uncovers a three-thousand-year-old idea: that underlying everything we experience there is only one single, all-encompassing thing—that everything else we see around us is some kind of illusion.

Admittedly, the claim that “all is One” doesn’t sound like an ingenious scientific concept. On a first glance, it sounds absurd. Just look out the window. Most of the time there will be more than one car in the street. It takes two persons (at least!) for a love affair, “two or three” believers are required to hold a Mass, and twenty-two players are needed for a proper soccer game. Ages ago, astronomers convinced us that Earth is not the only planet in the universe, and today modern cosmology knows virtually innumerable stars.

But quantum mechanics changes everything. In quantum systems, objects get so completely and entirely merged that it is impossible to say anything at all about the properties of their constituents anymore. This phenomenon is known as “entanglement,” and while it was pointed out by Albert Einstein and collaborators some eighty years ago, it is only now getting fully appreciated. Apply entanglement to the entire universe and you end up with Heraclitus’s dogma “From all things One.”

“Hold on,” you may object. “Quantum mechanics applies only to tiny things: atoms, elementary particles, maybe molecules. Applying it to the universe doesn’t make sense.” You will be surprised to learn that there are increasingly many good hints that this conviction is wrong. Between 1996 and 2016 alone, six Nobel Prizes were awarded for so-called macroscopic quantum phenomena. Quantum mechanics seems to apply universally, a finding whose consequences are just starting to be explored.

You may throw up your hands and protest that such a discussion is pointless. Physics seems to work just fine without any such metaphysical pondering. Fact is, it doesn’t. At present, physics is facing a crisis that forces us to reconsider what we understand as “fundamental” in the first place. Right now, the most brilliant particle physicists and cosmologists are alienated by experimental findings of extremely unlikely coincidences that so far defy any explanation. At the same time, the quest for a theory of everything is bereaving physics of its foundational concepts, such as matter, space, and time. If these are gone, what remains?

Quantum cosmology implies that the fundamental layer of reality is made neither of particles nor of tiny, vibrating, one-dimensional objects known as “strings,” but the universe itself—understood not as the sum of things making it up but rather as an all-encompassing unity. As I will argue, this notion that “all is One” has the potential to save the soul of science: the conviction that there is a unique, comprehensible, and fundamental reality. Once this argument holds sway, it will turn our quest for a theory of everything upside down—to build up on quantum cosmology rather than on particle physics or string theory (currently the most popular candidate for a quantum theory of gravitation). Such a concept further implies the need to understand how it is possible that we experience the world as many things if everything is “One,” after all. This is ensured by a process known as “decoherence,” which is essential to virtually any branch of modern physics. Decoherence is the agent protecting our daily-life experience from too much quantum weirdness. And it realizes the rest of Heraclitus’s tenet: “from One all things.”

As a consequence, we will have to work out how such a notion changes our perspective on philosophy’s deepest questions—“What is matter? ” “What is space? ” “What is time? ” “How did the universe come into being? ”—and even on what religious people call “God” (since for centuries, the concept of an all-encompassing unity was identified with God). We will also have to confront why monism is not more popular, if it follows so straightforwardly from quantum mechanics. Why does it sound so bizarre to us? Where does our intuitive, deprecative reflex come from? To really understand this bias, we have to venture into the history of monism.

The One is the story of both a serious crisis in physics and the half-forgotten concept that has the potential to resolve it. It explores the idea that “all is One,” that matter, space, time, and mind are all just artifacts of our coarse-grained perspective onto the universe. Along the way it narrates how the concept evolved and shaped the course of history, from ancient times to modern physics. Not only did monism inspire the art of Botticelli, Mozart, and Goethe, but it also informed the science of Newton, Faraday, and Einstein. Even now, monism is becoming a tacit assumption underlying our most advanced theories about space and time. This is a story full of love and devotion, fear and violence—and cutting-edge science. In no small way, this is the story of how humanity became what it is.

1

THE HIDDEN ONE

QUANTUM MECHANICS IS THE SCIENCE BEHIND nuclear explosions, smart phones, and particle collisions. But it is more than that. It sketches a hidden reality beyond what we experience in our daily routines and holds within it the power to transform our notion of what is real—provided that it is taken seriously as a theory about nature. And therein lies the debate that begins our journey: How can we know that there exists something hidden that we can’t experience directly? Doubts about this question launched the debate that ultimately returned the notion that “all is One” to the science most concerned with the separate identities and behaviors of the universe’s most finicky bits and pieces.

Wheeler’s U

“This guy sounds crazy. What people of your generation don’t know is that he has always sounded crazy,” Richard Feynman told Kip Thorne when the two had lunch together at an Armenian restaurant near the California Institute of Technology in 1971. Pointing at John Wheeler, their common PhD advisor sitting next to them, he went on, “But when I was his student, I discovered that, if you take one of his crazy ideas and you unwrap the layers of craziness one after another like lifting the layers off an onion, at the heart of the idea you will often find a powerful kernel of truth.”1

John Archibald Wheeler belongs among the most influential physicists of the twentieth century. In his manners, his lifestyle, his political views, and his appearance, Wheeler was a rather conservative person. But in his ideas about physics, he displayed his wild side. This trait included a lifelong fascination with explosions, which almost cost him a finger when, as a young boy, he played with dynamite caps in his parents’ vegetable garden. Working with Niels Bohr, the “grand old man” of modern physics, Wheeler later demonstrated that the atomic nuclei Uranium-235 and Plutonium-239 were possible candidates for nuclear fission and worked out how it might be accomplished.2 That paper got published in 1939 on the day Adolf Hitler invaded Poland, and six years later those were the isotopes that fueled the explosion of the nuclear bombs deployed on Hiroshima and Nagasaki to terminate World War II.

When the Soviet Union tested its own first nuclear bomb in 1949, Wheeler and his students joined Edward Teller and Stan Ulam to become instrumental in the development and realization of the hydrogen bomb, taking advantage of nuclear fusion to fuel even more tremendous detonations. Wheeler’s beloved brother Joe, fighting with the Allied forces against the German army in Italy’s Po Valley, had been killed in action in October 1944 only weeks after Wheeler, who at that time was working on the development of the nuclear bomb, had received his postcard carrying only two words: “Hurry Up!” Ever after, Wheeler felt he had a “duty to apply [his] skills to the service of his country,” as he explained in his memoirs.3 Yet as much as he desired to “keep America strong,” his heart was consumed by a deeper spirit of inquiry.4 “From my earliest student days, I was most intrigued by questions about fundamentals. What are the basic laws that govern the physical world? How is the world, at the deepest level, put together? . . . What are the unifying themes? In short, what makes this world we live in tick? ”5 Wheeler loved to ask the deep questions: “How come the quantum? ” “How come the universe? ” “How come existence? ”6 “How come time? ”7

His roughly fifty PhD students included superstars of physics, among them Richard Feynman, Kip Thorne, and Hugh Everett. Wheeler’s discussions with Feynman paved the way for the quantum version of electrodynamics—providing a role model for any subfield of modern particle physics and earning Feynman the 1965 Nobel Prize. Together with Thorne and other students, Wheeler made Albert Einstein’s theory of general relativity a respectable scientific topic again, culminating in the recent discovery of gravitational waves for which Thorne received the Nobel Prize in 2017. Wheeler also became the “grandfather” of the burgeoning field of quantum information8—the theory behind Google’s, IBM’s, Microsoft’s, Intel’s, and NASA’s recent efforts to revolutionize computing—with his continuing interest in the foundations of quantum mechanics: the weird physics governing the microcosmos, for which Everett proposed his equally congenial and controversial interpretation suggesting the existence of many parallel realities, or “worlds.” Finally, to top it all off, Wheeler is enshrined in the name of the Wheeler-DeWitt equation, the quantum equation for the wave function of the universe and starting point for much of Stephen Hawking’s work on cosmology.

In addition to his accomplishments, Wheeler was famous for coming up with catchy phrases and names for new concepts. He popularized the name “black hole” for the timeless corpses of burned-out stars and coined the name “wormhole” for hypothetical, handle-like shortcuts between faraway regions in the universe. He deployed the term “Planck scale” for the realms at tiny distances and extremely high energies where space and time themselves exhibit quantum properties and the name “quantum foam” for the bubbly consistency space and time supposedly have in these realms. And just as much as Wheeler loved catchy phrases, he loved to illustrate complicated concepts with simple sketches and diagrams.9 Suitably, Wheeler’s most enigmatic heritage is a little sketch depicting the history of the universe.

John Archibald Wheeler and his “U.”

“There’s the letter U. The U starts with this thin stem at the beginning when the universe is small. This stem gets fatter as we go up to the other side of the letter and at a certain point it’s terminated by a big circle. And there is an eye sitting in there looking back to the first days of the universe,” Wheeler said, describing his drawing that illustrates the evolution of the universe up to the emergence of conscious observers.10 Indeed, as Wheeler emphasized, “We ourselves can get and do get radiation today from the early days of the universe,” before he launched a bold speculation: “Insofar as the active observation has anything to do with what we ascribe reality to . . . then we can say this observer who was brought into existence by the universe has by his act of observation a part in bringing that universe itself into being.”11

Was one of the most eminent physicists of the twentieth century seriously claiming that it is we who are responsible for the existence of the universe? That we ourselves, just by observing the world, create space and time and matter? And that this influence travels back in time to the beginning of everything and brings the universe into existence?

How can we follow Feynman’s advice, unwrap the layers of craziness, and make sense out of “Wheeler’s U”—assuming we discard the unsettling possibility that each time we look out the window, we unknowingly travel back to the beginning of time to initiate the Big Bang? After all, without constant acts of time travel, we obviously cannot really change the course of the early universe, to say nothing of actually creating it. The only possibility in which we “by our act of observation” can have “a part in bringing that universe itself into being” is by employing a radical reinterpretation: to adopt that what we experience as universe and as its history is only a specific perspective onto a more fundamental, hidden reality.

To make this point clear, consider a cylindrical object, such as a can of Coke. Depending on whether we view it from above or from the side, the same can may be seen as a circle or as a rectangle. In this specific sense we can be held responsible for “creating” either the circle or the rectangle without really doing anything to the can itself, just by adopting a particular perspective.

A can of Coke may look like a circle or a rectangle, depending on the perspective.

We can obtain an even better understanding by comparing cosmic history with an old Hollywood movie. When we watch a film like Bringing Up Baby, the 1938 American screwball comedy starring Katharine Hepburn and Cary Grant, we experience a hilarious plot about a paleontologist trying to assemble the skeleton of a huge dinosaur, a project disrupted when he meets the crazy but beautiful Susan, who owns a tame leopard, and Susan’s aunt’s dog steals the last missing bone and buries it somewhere. But the story we experience in the theater is not really stored on the roll of film. Instead, a traditional movie projector displays the information on the film, one picture after another, flashed so quickly that the viewer has the impression of an unfolding story line. Again, the story is not really on tape; it is created by the viewer’s perspective onto the projected film. The story is created by us watching it, while the original source of information remains unswayed, mounted on the projector. In the same way, cosmic history may be understood as what we experience, created by our perspective onto a fundamental “quantum reality.”12

What is reality—the film roll mounted on the projector or the leopard story unfolding on-screen?

The Hollywood movie plot interpretation of cosmic history: Is the evolution of the universe just a projection of a more fundamental quantum reality?

(Credit: Shown on the screen is the Hubble Ultra Deep Field [HUDF]: original image by NASA, the European Space Agency, and S. Beckwith [Space Telescope Science Institute] and the HUDF Team, cropped and warped by the author.)

This “Hollywood movie plot interpretation of cosmic history” offers an astonishingly accurate picture of how quantum mechanics works, highlighting the most important question quantum mechanics forces us to ask: What is reality? Is it the light bulb and the collection of pictures stored on the film roll inside the projector, or is it the story experienced on-screen?

Even today, there are two camps of physicists and philosophers arguing fervently about exactly this question. The orthodox “Copenhagen” interpretation of quantum mechanics, advocated by Niels Bohr, Werner Heisenberg, and the overwhelming majority of physicists, insists that the movie plot constitutes reality. For many decades only a small number of outcasts, including (at least for some time) Erwin Schrödinger, Wheeler’s student Hugh Everett, and the German physicist H. Dieter Zeh, populated the “projector camp.” This renegade view, however, is getting increasingly popular. It is part of a controversy that originated in the 1920s, as physicists fought out the question of how strange reality really is.

Mountaineering Through Thick Fog

Quantum physics grew out of an old argument about whether light and matter consist of particles or waves. In the first quarter of the twentieth century, a number of groundbreaking experiments had shown that both are the case. On one hand, the properties of electrons produced by light shining on material surfaces and the spectra of electromagnetic radiation emitted by heated objects could only be explained if light, so far considered to be a wave, consisted of indivisible energy portions, or “quanta.” On the other hand, it had been demonstrated that the electron—so far known as a particle—also possessed the properties of a wave. In contrast to a particle, a wave has no accurately defined position; it is extended, or “nonlocal.” If, for example, an electron is described by a wave, it exists at various different places at the same time until it is measured. In that instant the electron seems to collapse into a defined position that in general cannot be predicted accurately beforehand. Even worse, this puzzling behavior was only one aspect of a persistent problem: there was no consensus on what was more fundamental, particles or waves. Were particles just a secondary property of waves, or were waves just how particles behaved under certain circumstances? Or were both only imperfect projections of a deeper reality?

Who were the protagonists of these discussions, and why do the questions they raised occupy physicists even today? Above all, it was the ingenuity and work of four men, as well as their weaknesses, interactions, and personal relationships in the decade between 1925 and 1935, that gave rise to our understanding of quantum mechanics.

Albert Einstein, born in 1879 and aged forty-six in 1925, was the most famous among the quartet.13 With his maverick working style, Einstein was the archetype of the solitary genius. Working isolated from his colleagues as a clerk in a Swiss patent office, he published several groundbreaking works that included not only his theory of special relativity but also his hypothesis that light might consist of “quanta”—at least as a heuristic guess. While this idea earned Einstein the Nobel Prize in 1921, he never felt quite comfortable with the dual nature of light. For a time, Einstein almost completely abandoned his work on quantum physics to concentrate on generalizing his theory of relativity to include gravitation. In 1914, Einstein hadn’t been able to resist leaving his beloved Switzerland and returning to his native Germany, where he had been offered a prestigious triple position, becoming the director of the newly founded Kaiser Wilhelm Institute for Physics, as well as a research professor at the University of Berlin and a member of the Prussian Academy of Science. Still, most of the time Einstein preferred to work alone.

Quite the contrary was Niels Bohr’s approach to science. Born in Copenhagen in 1885, the thirty-nine-year-old Dane was a team player, having been a top-level goalkeeper in the squad of his brother Harald, who had won the silver medal with the Danish national team in the first Olympic soccer tournament in 1908. Niels Bohr loved to interact with a large group of young scientists being employed at or visiting his Institut for Teoretisk Fysik in Copenhagen, a place that soon developed into a Mecca for aspiring quantum physicists. Gathering the most brilliant young minds from all over the world around him, he became a mentor and fatherly figure not only to John Wheeler, who later adopted Bohr’s style, but to a growing number of top-notch scientists who would move on to occupy professorships and research positions around the globe. While Einstein was concentrating on his theory of general relativity, Bohr had developed an imperfect yet useful model of the atom that resembled a miniature planetary system, with one notable exception: in Bohr’s atom there existed only a restricted number of allowed orbits, which were later explained as standing waves by the French physicist Louis de Broglie. Assuming that electrons were hopping from one allowed orbit to another, without ever residing in the space in between, a process labeled by the physicists as “quantum jumping,” Bohr’s model allowed for the first time calculation of the characteristic frequencies at which hydrogen would absorb and emit light. It also secured Bohr the Nobel Prize in 1922.

Erwin Schrödinger, born in Vienna in 1887 and now thirty-seven years old, was a late bloomer and unconventional bon vivant with broad interests ranging from wine, theater, poetry, and art to Greek and Asian philosophy. Schrödinger’s career had gotten interrupted when he was drafted to serve as an artillery officer in the Austrian army in World War I. Bored by the monotony of his daily routines and frustrated by the incapacity of his commanders, he immersed himself in physics books to keep sane. With his wife, Anny, whom he had married in 1920, he lived in an open relationship; both had affairs on the side. Schrödinger even kept a diary of his sexual encounters. Still, to Anny, he was a “racehorse” she wouldn’t trade for a “canary.”14 After the war, Schrödinger had a string of brief employments in Jena, Stuttgart, and Breslau, until in 1921 he finally secured the professorship in Zürich previously held by Einstein. But then he fell ill: diagnosed with suspected tuberculosis, Schrödinger had to take a rest cure and stayed for nine months in the Swiss Alpine resort of Arosa. Thus, in 1925 the Austrian suffered from feelings of inferiority, unsure whether he would be able to leave a lasting mark in physics.

Werner Heisenberg, arguably the most ingenious among Bohr’s protégés, was by far the youngest of the four men. Born in 1901, by 1925 the twenty-three-year-old Heisenberg had already earned himself a reputation as a physics prodigy. Since early childhood he had trained in solving mathematical puzzles and games, and he remained extremely ambitious. In his free time he loved to camp out and hike in the Bavarian mountains with friends from his Pathfinder group, the German variety of the Boy Scouts. Thus, when Heisenberg, now working as a postdoc at the University of Göttingen, struggled with the problem of the intractable electron orbits around the atomic nucleus in the spring of 1925, he compared this situation to an ascent in the Alps that he and his friends had undertaken the previous fall, during which they got lost in thick fog: “After some time we entered a totally confusing maze of rocks and pines . . . where we by no stretch of the imagination could find our path.”15

A few months later, in May 1925, Heisenberg, plagued by hay fever, took sick leave to travel to the small island of Helgoland—a red rock devoid of bushes and meadows some forty nautical miles off the German coast in the North Sea. On Helgoland—inspired by the philosophy of positivism—he tried something new to tease out what happens inside the atom.

The credo of positivism holds that scientific theories should be based exclusively on what is observable in experiments. Scientists were urged to stick to what they saw in front of them, what they could measure and manipulate, rather than to theorize about an unobservable reality underlying the obvious phenomena. In other words, they were to concentrate on the on-screen reality and refrain from musing about the projector and film roll creating it. Accordingly, Heisenberg discarded the unruly electron orbits altogether. Viewed this way, the problem faced appeared vaguely reminiscent of the mathematical puzzles his father had assigned to him when he was a young boy. In his childhood, Heisenberg had excelled at these games, easily outperforming his elder brother. And indeed, Heisenberg now managed to find a solution where no one else had been able to before. In an ingenious act, Heisenberg developed an abstract formalism that allowed him, in a hard night’s work, to calculate the energy levels of a simplified version of the atom, an oscillating spring. It was a “veritable calculation by magic,” Einstein judged later.16 Only a few months later, in early 1926, Heisenberg’s friend Wolfgang Pauli adopted Heisenberg’s formalism to calculate the energy levels of the hydrogen atom. Heisenberg and Pauli were enthusiastic: “Through the surface of atomic phenomena, I was looking at a strangely beautiful interior . . . nature had so generously spread out before me,” Heisenberg wrote, describing his exaltation;17 Pauli rejoiced that he had found “a new hope, a new enjoyment of life.”18

For matrices and rotations, the order matters.

Wunderkind Against Racehorse, Particles Versus Waves

Without a doubt, Heisenberg’s glorious insight marked one of the greatest breakthrough moments in physics. But it also established the view that what happened inside the atom would elude any intuitive understanding. This became evident when Erwin Schrödinger in December 1925 found an equation that described the electron as a wave.

By way of backdrop, Schrödinger’s marriage was in trouble. His wife, Anny, had had an affair with his best friend, the mathematician Hermann Weyl, while Weyl’s wife had fallen in love with the physicist Paul Scherrer. This was too much even for the “racehorse” Schrödinger, who decided to hook up with an old girlfriend and leave Zürich to spend Christmas in Arosa. Only a couple of weeks before his trip, Schrödinger had become aware of de Broglie’s hypothesis that electrons could be understood as waves. What was missing in this picture was an equation that described the energies and the time evolution of such a wave. During his two weeks in Arosa, Schrödinger must have experienced “a late, erotic outburst,” Weyl imagined.20 In fact, when Schrödinger returned to Zürich in early January, he carried with him the first sketch of an equation for quantum waves, convinced that “if I can only . . . solve it, it will be very beautiful.”21 With help from Weyl, by the end of January Schrödinger had not only solved his equation but also determined the hydrogen spectrum and submitted his results for publication.

Now there were two competing theories on the market. One described nature in terms of particles moving from one place to another via quantum jumps governed by probability rules, while the other described it through “deterministic,” continuous waves. Once the state of Schrödinger’s wave was known at one instant in time, its future evolution could easily be determined. In contrast to Heisenberg’s matrix mechanics, Schrödinger’s wave mechanics was elegant and intuitive, and mastering it didn’t require mathematical tools the physicists weren’t familiar with. Particles, Schrödinger concluded, would soon turn out to be nothing but a bunch of overlapping waves producing a lump of energy, similar to the occasional freak wave in the ocean.

Heisenberg was not convinced. “The more I think about the physical portion of the Schrödinger theory, the more repulsive I find it,” he wrote to Pauli. “What Schrödinger writes about the visualizability of his theory is probably not quite right, in other words it’s crap.”22 Even when Schrödinger had demonstrated that his approach reproduced the same results as Heisenberg’s, the dispute raged on. Indeed, it turned out that Schrödinger’s waves had problems. When interpreted as oscillating fields in normal space, they would dissolve too quickly to account for the particle-like behavior observed in experiments. Max Born showed that the wave’s amplitude could be interpreted so as to provide the probability of finding a particle at the corresponding location,23 and afterward he considered Schrödinger’s quantum wave not as a real object but merely as a tool, “something purely mathematical,” as Born described it.24 By stipulating that the laws of quantum physics would yield only probabilities instead of concrete cause-and-effect relationships, Born sacrificed the principles of “causality” and “determinism” at the core of the old, “classical” physics since Isaac Newton: that nothing in the world of physics would happen without a cause and that knowing the exact state of a physical system at one time would make determining its future behavior possible. Bohr and Heisenberg agreed with Born. They appreciated that Schrödinger’s formalism simplified many calculations but similarly dismissed the possibility that Schrödinger’s waves had anything to do with the reality inside the atom. “Although Bohr was normally most considerate and friendly in his dealings with people, he now struck me as an almost remorseless fanatic, one who was not prepared to make the least concession or grant,” Heisenberg remembered later.25

At this point, Albert Einstein felt increasingly uneasy. In the following spring of 1926, Heisenberg traveled to Berlin to give a colloquium. After his talk, Einstein invited the young man into his apartment. As soon as they arrived there, Einstein started to challenge Heisenberg’s approach. Heisenberg would split the world into two separate realms: our daily-life, classical world (where objects have defined locations and properties and where causal physical laws determine their future) and a quantum realm that couldn’t be described in everyday language. Even worse, Einstein criticized, since Heisenberg’s formalism completely abandoned the notion of electron orbits inside the atom, it failed to elucidate the real nature of the quantum realm; it only summarized the observer’s knowledge about the outcome of measurements. “You are moving on very thin ice,” he warned Heisenberg.26 Quantum mechanics had to be incomplete, Einstein felt. There had to be a hidden reality underlying the phenomena. Heisenberg left the meeting disappointed that he hadn’t been able to convince the man he admired so much. Nevertheless, some of Einstein’s arguments struck a nerve.

Right after meeting with Einstein, Heisenberg faced a tough choice. Having planned to accept another postdoc position with Niels Bohr in Copenhagen, the brilliant young man had also been offered a professorship in Leipzig. Less than three years earlier Heisenberg had almost failed his PhD exam, when he couldn’t answer simple questions about the resolution of a microscope or a telescope or the functioning of a battery. Wilhelm Wien, the Nobel laureate of 1911 and head of experimental physics, had been frustrated about the young theorist’s poor performance in his experimental lab course even before and was only grudgingly convinced by Heisenberg’s advisor, Arnold Sommerfeld, to let the candidate pass, with a less-than-mediocre grade. Horrified, Heisenberg had literally fled Munich, taking the overnight train to Göttingen only to appear in front of Max Born the next morning, an embarrassed expression on his face and unsure whether he was still welcome to occupy his upcoming postdoc position. Now he was about to turn down the offer of a professorship in Leipzig, a tremendous honor for a scientist of his young age, in a time when hunger, poverty, and housing shortages were still common in postwar Germany. While his father, himself a professor of Byzantine studies, had urged him to accept the Leipzig position, Einstein and other senior physicists advised him to work with Bohr. Heisenberg decided to play for high stakes and went to Copenhagen. “I will always receive another call; otherwise I don’t deserve it,” he assured his parents.27

The stage was set for the development of the Copenhagen interpretation of quantum mechanics, a blessing and a curse for almost a century of research on the foundations of physics.

It Ain’t What the Moon Did

By February 1927, Heisenberg’s optimism was waning. Right after arriving at the Danish capital six months earlier, he and Bohr started their struggle to make sense out of quantum mechanics. While Heisenberg was perfectly happy with a mathematical formalism spitting out probabilities, Bohr insisted that physics should be framed in everyday language. As he detailed later, “By the word ‘experiment’ we refer to a situation where we can tell others what we have done . . . [T]herefore . . . the results of the observations must be expressed in unambiguous language”; he concluded, “All evidence must be expressed in classical terms.”28 Heisenberg pushed back: “When we get beyond this range of classical theory, we must realize that our words don’t fit.”29

The two men were at odds about another point too. While Heisenberg stuck exclusively to the idea of particles, Bohr wanted to incorporate Schrödinger’s waves as well. Heisenberg had tried to discuss the matter with Schrödinger the previous summer, only to be chided again by Wilhelm Wien: “You must understand that we are now finished with all that nonsense about quantum jumps,” the older man had told a dismayed Heisenberg before Schrödinger could even start to answer.30 Now Bohr decided to invite Schrödinger to Copenhagen to discuss their inconsistent interpretations face-to-face. Schrödinger visited in September, fell ill, and was nursed by Bohr’s wife, while Bohr sat on the edge of his bed, urging him to relent and accept that his theory was wrong. It didn’t help; Schrödinger left without an agreement reached. In the next months, Heisenberg and Bohr continued their discussions day after day, often late into the night in an increasingly tense atmosphere. When, after many hours, both men found themselves almost in despair, Heisenberg would try to free his mind, strolling in the neighboring Faelled Park and asking himself again and again, “Can nature possibly be as absurd as it seem[s]? ”31

Finally, Bohr decided he needed a break and set off on a four-week skiing holiday in Norway. Left behind, Heisenberg went on to ponder the problem of electron paths, only to hit “insurmountable obstacles” once again. “I began to wonder whether we might not have been asking the wrong sort of question all along,” he remembered later.32 Suddenly, Heisenberg recalled one of the arguments Einstein had raised against his first approach on quantum mechanics: “It is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe.”33 Einstein’s argument is known to philosophers as the “Duhem-Quine thesis”: in order to extract an experimental result from an observation, it is necessary to understand what is happening during the measurement and how exactly the measurement apparatus and our perception function. “You must appreciate that observation is a very complicated process . . . Only theory, that is, knowledge of natural laws, enables us to deduce the underlying phenomena from our sense impressions,” Heisenberg remembered Einstein arguing.34 If theory determines what we can observe, Heisenberg thought, shouldn’t it also determine what we can’t observe?

Long after midnight, Heisenberg set off for another walk in the dark Faelled Park, and there he had the idea that would evolve into his famous uncertainty principle. Knowing the path of a particle would imply that one would know both the particle’s location and its direction, its velocity at different instants of time. But when an experimentalist observes an electron in a cloud chamber, she doesn’t observe the path itself but rather a sequence of localized interactions. As the water drops indicating the particle’s position are much larger than the electron itself, this doesn’t necessarily mean that both position and momentum (i.e., velocity times mass) are accurately known.

Checking this idea with his matrix formalism, Heisenberg discovered that it, in fact, wouldn’t allow a simultaneous accurate determination of both position and momentum. In Heisenberg’s version of quantum mechanics, matrices represented the measurement of observable quantities, such as position or momentum. The product of two matrices depends, however, on the order in which they are multiplied. This strange multiplication rule implied that it made a difference which quantity was measured first: determining a particle’s position and after that its momentum would result in a different outcome than measuring in the reverse order. Now Heisenberg was able to represent what Wolfgang Pauli had described to him in a letter he had received in October: “One can see the world with the p-eye [i.e., momentum] and one can see the world with the q-eye [i.e., position], but if one opens both eyes together, then one goes astray.”35 As a consequence, the exact position and the precise momentum or velocity of a particle can’t be measured at the same time—there always remains an uncertainty. Either the position remains unknown, or the momentum remains unknown, or both quantities are known only with limited precision.

Heisenberg felt vindicated: if a particle’s position and velocity couldn’t be pinpointed at the same time, it didn’t make sense to talk about electron paths inside the atom. Either one doesn’t know where the electron is, or one is ignorant about the direction in which the electron moves. With this insight, Heisenberg thought he had identified the origin of the breakdown of causality. “What is wrong in the . . . law of causality, ‘when we know the present . . . , we can predict the future’ is not the conclusion but the assumption,” he wrote. “Even in principle we cannot know the present in all detail . . . [I]t follows that quantum mechanics establishes the final failure of causality.”36

When Bohr returned from the ski slopes to Copenhagen, he brought only consternation. He promptly found a mistake in Heisenberg’s argument and told him to rewrite the paper. At this point Heisenberg literally broke down in tears.37 What Bohr had realized was that Heisenberg’s uncertainty, rather than invalidating the quantum wave picture, was in fact a typical behavior for waves. Long, plain wave trains have a well-defined momentum, but when these waves encounter a barrier with a narrow hole in it, they produce a circular wave behind it, spreading out in all possible directions. Limiting the wave to a narrow distance to determine its location thus results in a diverging momentum. Bohr thus identified the particle-wave duality as the core element of quantum mechanics and came up with an interpretation of his own: “complementarity.”

Illustration of Heisenberg’s uncertainty principle: plain wave trains with a defined momentum spread out into all possible directions after hitting a barrier with a narrow hole in it.

Complementarity soon became the centerpiece of the Copenhagen grasp of quantum mechanics. But what exactly is complementarity? On a visit to Moscow, Bohr had scribbled the gist of his idea on the blackboard of his host’s office: “Opposites [such as particles or waves] do not contradict but rather complement each other.”38 According to Bohr both views, understanding matter as particles or as waves, had their justification, and each would reveal crucial information even though they seemed to contradict each other. As Bohr explained, “Evidence obtained under different experimental conditions cannot be comprehended within a single picture . . . [O]nly the totality of the phenomena exhausts the possible information about the objects,” and “the study of the complementary phenomena demands mutually exclusive experimental arrangements.”39

Heisenberg stubbornly refused to alter his paper, however. He had come to believe that his professional future now depended on a refutation of Schrödinger’s wave mechanics, and he was convinced that he needed a fast publication to receive another job offer: “I have come to be in a fight for the matrices and against the waves,” a “quarrel with Bohr,” Heisenberg wrote to Pauli.40 At this point the controversy between Heisenberg and Bohr escalated into a personal conflict. Finally, Heisenberg gave in and supplemented his uncertainty paper with a postscript, admitting that “recent investigations of Bohr have led . . . to an essential deepening and sharpening of the analysis . . . attempted in this work.”41

This painful compromise between Heisenberg and Bohr became the centerpiece of what would be known later as the “Copenhagen interpretation,” which would dominate both the thought of scientists and the textbook expositions future generations of physicists would learn from for at least the next fifty years. It would provide physicists with a working framework to approach the quantum mechanical problems they faced in atomic, nuclear, and solid-state physics, but it came with a price tag. In the Copenhagen philosophy, the act of measurement played a crucial role. According to Heisenberg, “Everything observed is a selection from a plenitude of possibilities,” and only what was finally observed was considered “real.”42 This notion of a reality produced by the act of observation later fueled Wheeler’s speculation about our creating the universe.

Coming back to the Hollywood movie plot interpretation of reality, the phenomenon of complementarity may be illustrated by a single film roll featuring different overlapping movies. Depending on the color of the light source or the angle of the projection, instead of Bringing Up Baby, the 1985 science fiction blockbuster Back to the Future might show up on the screen, to the bewilderment of a moviegoer ignorant of what kind of film roll is being loaded into the projector.

But as interesting as the concept is, the actual workings of complementarity remained somewhat vague. Bohr himself entertained at least two different versions of complementarity at different times. One described the relation of different projections or on-screen realities, such as particles versus plain wave trains with well-defined momentum. Another characterized the relation of on-screen realities and the underlying projector reality, the latter being described by Schrö­dinger’s wave equation.43

At this point the physicists should have asked themselves what constituted these complementary observations. What kind of foundational reality underlay such conflicting experiences? In fact, whenever in the history of science physicists had discovered a more fundamental theory with a broader range of applications, they worked out how the old successful but limitedly valid theory could be understood as a limiting case of the new theory with its novel concept of reality. A famous example is Newtonian physics, which can be obtained as a low-energy limit of Einstein’s special theory of relativity. But this was not done for quantum physics. In contrast to classical physics, quantum physics was capable of describing atomic and subatomic phenomena. Nevertheless, the Copenhagen physicists didn’t understand classical physics as a limiting case of a more fundamental quantum or projector reality; rather they saw quantum mechanics as an instrument to obtain knowledge about classical objects experienced on-screen. The protagonists of the new quantum paradigm didn’t take the plunge and explore the new reality hidden behind the quantum measurements. They left the quantum revolution unfinished. Instead, the Copenhagen interpretation evolved from compromise into dogma.

Naturally, Einstein wasn’t happy. “The Bohr-Heisenberg tranquilizing philosophy—or religion?—is so delicately contrived that for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused,” Einstein judged.44 He maintained, “Does the moon exist only when you look at it? . . . I still believe in the possibility of a model of reality . . . that represents things themselves and not merely the probability of their occurrence.”45 Even more critical was the later assessment of H. Dieter Zeh: “This was an ingenious pragmatic strategy to avoid many problems, but, from there on, the search for a unique description of Nature was not allowed any more in microscopic physics . . . Only few dared to object that ‘this emperor is naked.’ ” 46

A World Split Apart

The next time Heisenberg and Einstein met was one and a half years later, at the Solvay Conference in Brussels. It was probably the most famous science meeting in history, with a participants’ list that even today reads like a “who’s who” of physics: Albert Einstein, Niels Bohr, Marie Curie, Max Born, William Bragg, Léon Brillouin, Arthur Compton, Louis de Broglie, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, Max Planck, Erwin Schrödinger, and others. They gathered in October 1927 to discuss “electrons and photons” (photons being the specific quanta of the electromagnetic field) and the “new quantum mechanics.” In Brussels, Bohr’s and Einstein’s views of the microcosm clashed, giving rise to the controversy that is ongoing even today. In subsequent years Bohr had to refute Einstein many times, one argument after another, typically expressed in the form of hypothetical thought, or Gedanken, experiments. But while Bohr succeeded case after case, he developed an interpretation of quantum mechanics that would become more and more absurd.

According to Bohr’s and Heisenberg’s Copenhagen interpretation, quantum mechanics was no longer a theory about nature. It was a theory about the experimentalist’s knowledge about nature: a humanities concept rather than science. “One might be led to the presumption that behind the perceived statistical world there still hides a real world in which causality holds. . . . [S]uch speculations seem . . . fruitless and senseless. Physics ought to describe only the correlation of observations,” Heisenberg argued.47 Similarly, Bohr saw an “impossibility . . . of drawing any sharp separation” between the quantum object itself and its observation, “between . . . atomic objects and their interaction with the measuring instruments.”48 According to the Copenhagen physicists, atomic objects obtained their reality from the act of measurement. For Bohr, reality was like a movie shown without a film or projector creating it: “There is no quantum world,” Bohr reportedly affirmed, suggesting an imaginary border between the realms of microscopic, “unreal” quantum physics and “real,” macroscopic and classical objects—a boundary that has received serious blows by experiment since.49 By installing this duality, Bohr enshrined what Einstein already had accused Heisenberg of doing when they debated in Berlin: Bohr had split the world apart.

For the fellow physicists in Brussels, however, Einstein’s stubborn criticism arguing for an objective reality beyond what can be observed appeared increasingly as the obstinacy of an aging reactionary rather than as indication of a blind spot in the understanding of the foundations of physics. Einstein’s friend Paul Ehrenfest captured the general impression of most physicists when he scolded, “Einstein, I’m ashamed of you, you are arguing against the new quantum theory just as your opponents argue about relativity theory.”50