13

November 1944

Planck’s lifelong motto was Man muss Optimist sein, or “one must be an optimist,” but in late 1944, he must have let himself consider the unthinkable. He and Nelly had stretched and then exhausted their contact lists in the Reich. Only two questions remained. Would a person in power suffer a whim of sentiment? Failing that, would a Nazi underling risk his own standing to speak for the son of Max Planck? Max would have looked on the odds of Erwin’s release like Ludwig Boltzmann had once looked at those rare cases where a molecule bounced left instead of right and, for a split second, entropy in the universe took a tiny dip instead of marching ever upward—an event practically but not statistically impossible. And even if Erwin against odds evaded his sentence, the odds of surviving the war looked worse than ever. Erwin had already escaped an Allied bombing; it was, “really like a miracle,” Max wrote in a letter. Explosions obliterated one building, while next door, Erwin’s prison wing stood untouched.¹

Max wrote to Nelly on the first day of November.

Dear Nell! Never in my life have you been so close to me and have I felt so tightly connected to you as in these days, when both of us turn all thoughts and worries towards the one, who, of all the world’s men, is closest to us, who gives us the greatest security and foothold in the world, and who is to be taken away from us. … Let’s continue hoping that fate smiles on him again. Hopefully he stays healthy while imprisoned and can sleep properly, even without alcohol, his usual sleep tonic.²

In the closing months of 1944, streams of Berlin refugees swelled Rogätz to 10 times its prewar population. Max was still regularly seen walking through them on his way to a daily shave. Moving among these throngs, he must have occasionally seen a kindly pug dog face—Mops! In this Erwin vigil, subconscious excitement would surrender to conscious dread. And if some of the refugees recognized the celebrated scientist? His stooped and painful gait mirrored the empire’s expiring dreams, hatched when the fearless Max Planck stood erect, and demanded answers from the universe.

But in 1944, even his trusted dialogue with nature—halting, sometimes confusing, but always following certain rules, and always progressing—lay unrecognizable, like so many smoldering cities.

The key moment illustrating physics passing Planck took place in 1926. At the age of 68, he at first welcomed a new take on quantum theory called “quantum mechanics,” dreamed up by a young group of physicists from Göttingen. He was proud that the emergency funding apparatus, the Notgemeinschaft, had kept these young guns blazing. But in that year, he publicly announced that, whatever came next from the new theory, physicists could rest assured that the bedrock foundation of physics stood firm. Namely, they could still rely on experiments that took a true measure of natural world, without affecting it. That is, the exact wording of their laboratory questions would not affect the answers nature uttered in reply.

Physics had long relied on the notion that particles were being observed discretely, like so many animals in an enormous nature preserve. With experiments functioning as silent telephoto lenses, we could count on observing a particle’s natural state. But would the new brand of quantum theory render particles more like animals in a zoo, with behavior inseparable from the confines of the experiment? Could humans really trust what they observed as fundamental reality or were they looking at byproducts of their intrusions? In 1926, Planck reassured everyone that physics rules out that worry, “from the very beginning.”³ He couldn’t have been more wrong, and his timing couldn’t have been worse.

Within months, physicists started a new dialogue with the microscopic world, a dialogue marked by a bizarre mix of unprecedented predictability, on the one hand, and a chilly intellectual distance, on the other. Not only did the new physics admit that experiments could influence their subjects, but it also drew a curtain across the bars of the particle cages. It was as if the young physicists said, “Nature, we will make you a deal. As long as you act predictably, we will quit asking why you do things,” and nature readily accepted. The treaty persists in large part to this day, despite the protests of Planck, Einstein, and others.

From the time of his first scientific work onward, Planck had been fixated on nature’s motion from starting points to endpoints. In his 1879 doctoral thesis, he reworded Clausius’s second law of thermodynamics to implicitly include time’s arrow: The process of heat conduction cannot be completely reversed by any means. Nature somehow prefers the end state of a process, Z, to the starting point of the process, A. The cold cup of coffee never spontaneously regains its warmth as it sits forgotten on the counter. To Planck, thermal physics provided a windsock by which humanity could see the warm breeze of time, always blowing in one direction. Planck never lost his interest in “irreversibility,” and he believed that attempting to fully understand it was among the most important tasks for physics. In fact, he had put his finger on a concept that still plagues us nearly 150 years later. How does one reconcile beautiful and successful physical theories (like Newton’s laws of motion or Maxwell’s equations for electromagnetic waves), which show no preference for a direction of time, and somehow then derive a more comprehensive theory in which time does have a preference, as per reality? Most theories in physics would work just fine if you put them in reverse. Just like every up has a down, and every left a right, every positive axis of time has a negative axis trailing behind it, mathematically enticing real estate with no bids from the real world.

Planck longed to find some cause—a hidden archer launching time’s arrow. In 1897, he labeled this “the fundamental task of theoretical physics,” and it drew him further into black-body radiation. He became convinced that, in this unique problem, he would prove an irreversible pattern using only established and nonstatistical approaches. In that year, he published a five-paper series on the black-body problem called, “On Irreversible Radiation Processes,” but he couldn’t prove his title’s seductive notion.⁴

Instead of divining why time moves in one direction, he inadvertently opened a new field, quantum theory, with its own questions that would consume the attention of physicists for decades to come. He asked nature why time moves in one direction, but nature replied that he would be better off worrying about matter and energy.

Planck expected only glacial progress on these new questions. Einstein aside, few others made advances in quantum theory for the first decade after Planck’s discovery. In 1912, he concluded the preamble to his second quantum theory with a prediction of more slow going. “Any one who, at present, devotes his efforts to the hypothesis of quanta, must, for the time being, be content with the knowledge that the fruits of the labor spent will probably be gathered by a future generation.”⁵

But before this book hit the shelves in 1913, a Danish graduate student sidestepped Planck’s morose prediction (Figure 13.1). By March of that year, Niels Bohr presented a wholly new vision of the atom. He used the notion of discrete energies to shackle electrons within an atom. Now, instead of having any value of energy, the electrons had only a set of allowed quantized options—one, two, or three chunks of energy, but never anything in between. The model immediately provided, for instance, a conceptual way to understand the colors of fireworks—why different elements emit specific colors when excited or ignited. To be sure, Bohr’s model was not perfect, and fine-tuning would follow, but physicists began to realize a new era for light, energy, and fundamental matter. The number of publications involving the quantum idea nearly doubled from 1912 to 1913.⁶ The teenaged theory wasn’t just putting up a few posters—it wanted to knock out some walls.

Figure 13.1 Niels Bohr circa 1911.

Courtesy the Niels Bohr Archive, Copenhagen, and courtesy AIP Emilio Segre Visual Archives.

Fresh from his atomic model success, Bohr in 1917 began to build what would come to represent an impregnable fortress for quantum theory. His Institute for Theoretical Physics opened in Copenhagen four years later. There he regularly hosted the best and brightest theoretical physicists from around Europe and America for stints of intensive work and collaboration. Over the next decade, visitors included Werner Heisenberg, Max Born, Paul Dirac, Wolfgang Pauli, and Erwin Schrödinger, among others—the eventual prophets of modern quantum theory.

Bohr won the 1922 Nobel Prize in Physics for his brilliant 1913 “Trilogy” of papers concerning a quantum atom. The same year also settled a long-simmering question for Planck and most other physicists. Einstein had first suggested his lichtquant, or light quanta, in the miracle year of 1905. Planck rejected this notion outright and even had sincere doubts as late as 1918. As he wrote to his friend Hendrik Lorentz that spring, “the question whether light rays themselves are quantized or whether the quantum effect occurs only in matter is the first and most serious dilemma confronting the entire quantum theory.”⁷ In fact, very few physicists had accepted the reality of such light packets, but measurements from the American laboratory of Arthur Compton confirmed their existence by the end of 1922.⁸ (Even then, neither Einstein nor Compton had ever used the word “photon,” which appeared for the first time in a 1926 paper by the chemist Gilbert Lewis.)⁹

So now physics faced an uncomfortable fact that Einstein had suggested for some time: Light was composed of particles, but the particles could exhibit the character of waves. “Wavicles” are not much easier to grasp for the human mind now than they were in the early twentieth century, but it may be best understood as a vocabulary problem. The reality of the photon, unlike any object or quality of our everyday lives, is analogous to describing an artichoke to a person who’s never seen one. For size, weight, and color, you might describe it as “kind of like a pear,” but in terms of skin and texture you might say it’s “kind of like a pineapple.” The photon is most certainly an artichoke, but we humans are stuck trying to understand it as either pear or pineapple, so we force inappropriate concepts and constraints on nature. For the nano world, the images of billiard balls or ripples on a pond are not quite right, but these are the clumsy tools we carry from the macro world.

Work in the field of quantum theory shifted into higher gear in the 1920s. A French doctoral student Louis de Broglie wondered, since light waves could have the properties of particles, couldn’t particles also then act like waves? In essence, he asked the world of physics: What if all these tiny, invisible things are artichokes? Not knowing what to think of such a radical idea, his thesis advisor sent a copy to Einstein, and Einstein blessed it immediately.¹⁰

Meanwhile, Werner Heisenberg was leading a group to introduce a new quantum theory to the world. In 1925, he fled his Göttingen hay fever and retreated to Helgoland, a barren rocky island in the North Sea.¹¹ Just as 23-year-old Isaac Newton had fled a London plague and made critical advances amid bucolic scenery, 23-year-old Heisenberg penned the start of quantum “mechanics” in stark surroundings. Newton’s classical mechanics had described macroscopic objects like billiard balls, whereas quantum mechanics would do the same for something like de Broglie’s wavy electrons. Heisenberg’s version is still taught as “matrix mechanics,” since an electron’s properties, like energy, were for the first time partitioned over the face of a tic-tac-toe–like matrix of options. At least mathematically, an electron could now retain many options simultaneously. Some of the electron’s reality was over here, and another portion was over there.

At the same time, an alternate version of quantum mechanics arose from the mind of the young Austrian physicist Erwin Schrödinger. He published his “wave mechanics,” in 1926, a year before assuming Planck’s position at the University of Berlin. Planck followed Schrödinger’s work with great interest, writing letters about next steps to both Schrödinger and Hendrik Lorentz.¹² Planck hoped that Schrödinger’s theory offered a deterministic future to quantum mechanics—meaning A would still lead to Z somehow, instead of having just some probability of Z, mixed with a little probability of Y, and so on. Planck desperately wanted a quantum mechanics that could describe the path of an electron with the same confidence that a center fielder traces a flying baseball. Instead, some of the new quantum ideas threw many baseballs at once. All realities were valid and not even the baseball players could tell them apart until the moment the ball appeared in just one glove, with all other gloves, raised aloft, coming up empty.

Although Planck may have seen the writing on the wall, or in the matrices, he doubled down on his lifelong fundamental beliefs about humans, science, and the natural world. In the two aforementioned 1926 lectures, he assured audiences that, no matter where quantum theory went next, science would not have to worry about an experiment influencing its own subject, meaning that scientists would still be free to observe nature as if they were sitting in a perfectly camouflaged and scent-proof hunting blind—nature would behave as if nobody was watching. In the same year, he wrote a series of six letters to the philosopher Theodor Haering, explaining the importance of his stance. Planck claimed that picturing a universe where data are collected without altering the system under study, “is the basic presupposition of any sort of scientific knowledge.”¹³ Indeed, Planck must have at times viewed Heisenberg and his gang as barbarians who threatened to ransack science back to darker times.

In late 1926, Max Planck was 68 years old, while the average age of the new quantum innovators was just under 28. (I average a defensible but definitely debatable list of Bohr, Heisenberg, Schrödinger, Max Born, Paul Dirac, Pascal Jordan, and Wolfgang Pauli.) Youths were running with Planck’s ideas and taking them well beyond his comfort.

Einstein, Planck’s ally, watched the developments with great skepticism. The still creative and convention-free genius would never be an “establishment” physicist clicking his tongue at new ideas. But he also had to recognize the natural rise of a new physics generation. Shortly before turning 40, Einstein lamented that one naturally becomes “more blockheaded” with age and experience. Although many of us feel the slowing or perhaps stiffening of our minds, few have expressed it with Einstein’s playful eloquence. “The intellect gets crippled,” he wrote in 1918, with the ink barely dry on his general theory of relativity. “But glittering renown is still draped around the calcified shell.”¹⁴

Einstein initially sought to dismiss Heisenberg’s matrix mechanics, writing to Ehrenfest that Heisenberg had “laid a big quantum egg.” Even though the Göttingen crowd may believe their strange work, he wrote, “I don’t.”¹⁵ He locked his jaw and sharpened his pencil for battle.

By the end of 1926, three parallel versions of quantum mechanics had emerged. To Schrödinger’s wave mechanics and Göttingen’s matrix formulation, an English graduate student named Paul Dirac added a third and arguably most elegant version in his PhD thesis. The next tectonic shift in quantum theory arrived in 1927, when Heisenberg published his famous uncertainty principle, giving an electron’s position and speed a new sort of relationship. Given a wave–particle duality and thereby a more hazy or furry vision for an electron, one had to accept uncertainty in both its position and its speed—meaning the electron would inhabit a range of position and speed values instead of one exact value for each. Instead of being able to put one’s finger on the electron’s location—it’s right here—one would have to use both hands held apart—it’s somewhere in here. Moreover, the respective uncertainties of position and speed were now linked together. If a clever experiment narrowed the uncertainty in an electron’s position, the same experiment necessarily increased the uncertainty in its speed. When a scientist brings her hands together to locate the electron within, it squirts out like a watermelon seed with some new and unforeseen speed. The very probing of the electron, using light or anything else, would affect its state of being.

Shortly after reading Heisenberg’s paper, Planck wrote to Lorentz, and the two aging physicists could only shake their heads at what was happening. Max called the “ominous” uncertainty principle “an unacceptable limitation of the freedom of thought, and … a mutilation” of the sacred notion of pure and simple measurements.¹⁶ But in the end, Heisenberg helped the world better understand the true nature of Planck’s own constant.

When Planck had speculated in 1908 that spacetime was quantized, he was on the right track,¹⁷ and with our long hindsight, Heisenberg’s 1927 work shed the brightest light on Planck’s “quantum of action,” h. The uncertainty principle sets a limit to the possible precision of physical knowledge for a particle or a process. If we multiply the uncertainty of a particle’s position by the uncertainty in its momentum (or alternately the uncertainties of energy and elapsed time for a process), the result, as ordained by the universe, can never be smaller than Planck’s constant.¹⁸ In one sense then, h should be viewed as the fundamental grain size to the knowability of the physical universe. Planck himself eventually said that h “erects an objective barrier” between our experiments and ultimate precision, such that “progress will only give this barrier even sharper outlines than it had before.”¹⁹

Following the uncertainty paper, Heisenberg, Bohr, and Max Born fell into grueling round-the-clock discussions as to the nature of the emerging quantum reality. Allegedly, in their first meetings, Heisenberg was reduced to tears.²⁰ They eventually found enough common ground for a truce of sorts. That fall, the three men set out a new framework for what came to be called the “Copenhagen interpretation.” First, Bohr presented his principle of “complementarity,” meant to draw a bright line between the musings of the human mind on one side, and the complexities of the atomic world on the other. The pristine “natural” state of something like an electron, according to Bohr, included two equal sides like a coin (a wave side and a particle side), but the inner truth, balanced on the coin’s edge, could never be captured by the ungainly hands of human science, which typically bumped an electron enough to make it look mostly like a particle or mostly like a wave.

Heisenberg and Born, claiming they had completed quantum mechanics, unveiled their theory’s statistical underbelly, employing the Heisenberg uncertainty rule and declaring that the “waves” of Schrödinger’s theory were in fact just mathematical ripples of probability—the peak of a wave just showed the point of highest probability for a particle’s existence. With Schrödinger’s approach fully assimilated, they snuffed Planck’s hope for a more comfortable quantum future.

The Copenhagen interpretation has tempted many observers to run with it and claim scientists can no longer know or predict anything whatsoever, but nothing could be more incorrect. In terms of its experimental predictions, quantum mechanics and its descendants are still unsurpassed.

Quantum theory’s most compelling glory was its description of an element’s “atomic spectrum,” the specific set of light colors that it gives off under duress. The aforementioned case of a firework gives an example, in which different sizzling components give us different colors; a neon sign provides another. This is very different from the universal, atom-independent thermal radiation that Planck described. The distinct atomic spectra are essentially the fingerprints of underlying atomic architecture. The spectra shine forth only when an element is agitated by the outside world, whereas thermal radiation is ever-present, based only on temperature. Before quantum theory, the spectra were complete head scratchers. Quantum theory, starting with Bohr’s atom and moving through decades of improvements, learned to predict and understand these exact colors with ever-better and eventually ridiculous precision. At some point, even skeptics had to nod, sigh, and say, okay, this must be right. No theoretical model has been as precisely and eerily successful in describing nature’s behavior as quantum mechanics and its descendants. At the same time, perhaps no other model has admitted its own intellectual limitations as bluntly: The authors of Copenhagen drew a bright line between describing the measurements of evidence like the spectra, and actually claiming to know what exactly was happening under the atomic car hoods.

The Copenhagen interpretation threw up its hands at a complete blow-by-blow understanding of the subatomic world. It even cordoned off that terrain from curious human beings. Bohr said bluntly, “It is wrong to think that the task of physics is to find out how nature is.” Rather, he claimed the task of physics was only to find what humans could reliably say about nature.²¹ Well into the twenty-first century, many physicists still consider the Copenhagen interpretation our best and most honest assessment, whereas others are itching to challenge it. Physicist and Nobel laureate Steven Weinberg has arguably spent as much time thinking about quantum theory as any other living person. In his recent textbook, Lectures on Quantum Mechanics, Weinberg discusses how we can interpret the meaning of the wonderfully useful and successful machinery of the theory. He admits that, “it is hard to live with” a framework for computing predictions without a deeper understanding of these microscopic systems. “My own conclusion (not universally shared) is that today there is no interpretation of quantum mechanics that does not have serious flaws.”²² We may yet get to an underlying and more satisfying truth.

Planck’s friend and one-time assistant Max von Laue shook his head at the Copenhagen interpretation, with its focus on being finished and washing its hands. “Planck has mentioned conscientiousness and loyalty as the necessary character traits of a scientist,” he wrote. “I think that we should add patience.” And he would always see the Copenhagen interpretation as tinged with Europe’s pessimism after the Great War.²³ It was as if younger minds had decided that humans, judging by their senseless brutality, were not smart enough to win a true understanding of the atomic world. We were just not worthy.

Neither Planck nor Einstein would ever achieve comfort with quantum mechanics in this form, despite its eventual dominance. Physicist A. Douglas Stone recently relayed Einstein’s lifetime toil with quantum with a whiff of Dr. Frankenstein’s tragedy, in which Einstein eventually viewed a monster he had helped create with a mix of sympathy, regret, and horror.²⁴ Hoping to tweak this monster for the better, Einstein jousted with the new quantum mechanics for many years, proposing “gotcha” types of paradoxes to the new quantum apostles. But the paradoxes, after some intense work by Bohr or Born, were always resolved to the vindication of quantum mechanics, and Einstein would slink back to his mental workshop, determined to build a better trap.

In the most fascinating and still relevant example, Einstein and his co-authors proposed a ridiculous outcome from the Copenhagen interpretation. By considering two quantum particles “entangled” at the start of a thought experiment, and letting them fly off in opposite directions, Einstein showed that quantum mechanics would lead to faster-than-light communication. Since this was forbidden by the theory of relativity, something was amiss with either the universe or the new quantum mechanics. Entangled particles are like two travelers who share and pack a small suitcase, with just one white shirt and one blue shirt. Imagine the travelers become separated but, as quantum particles, they don’t really have to determine an outfit until a human spies one of them. When a human observes one particle, the measured particle must immediately call its partner and say, “Hey, I’m wearing white today. You’ve gotta wear blue.” Quantum mechanics requires a phone call so immediate that it must travel faster than the speed of light.

Einstein called this conundrum “spooky action at a distance,” and the EPR paradox (for authors Einstein, Podolsky, and Rosen), would either refute the Copenhagen interpretation or open an entirely new concept in physics. In time, experiments found that quantum action can indeed get spooky at a distance; entangled particles do somehow communicate immediately. Today, some researchers are even exploiting the effect to attempt commercial message encryption. The reality of this particular Einstein relic cracks another of our most comfortable bedrock human assumptions, locality. In shorthand: To make a dent in something, you must be standing near it or throw something or beam something at it. Interested readers can pursue the subsequent Bell’s theorem of 1964 for further blowing of mind and for further support of quantum mechanics, despite its cruel and steady march away from our intuition.²⁵ Suffice it to say, Einstein’s pithy formulation of the paradox undercuts his own claim that he was becoming more “blockheaded” with age.

“Quantum mechanics is certainly imposing,” Einstein once wrote to Max Born, setting up one of his most oft-quoted phrases. “But an inner voice tells me that it is not yet the real thing. The theory says a lot, but it does not really bring us any closer to the secrets of the Old One. I, at any rate, am convinced that He does not play dice.”²⁶

Planck and Einstein had both come to hold sacred reverence for the concept of causality. In Planck’s words, causality is, “the fact that natural phenomena invariably occur according to the rigid sequence of cause and effect. This is an indispensable postulate of all scientific research.”²⁷ But some believed (and believe, to varying degrees) that the probabilistic aspect of quantum mechanics deals a great blow to causality. Planck and Einstein trusted a universe where experiment A provided result B, reliably and definitely. But Heisenberg rejected such outdated notions. “When one wishes to calculate ‘the future’ from ‘the present,’ ” he said, “one can only get statistical results.”²⁸ Planck saw the younger generation as resigning themselves to ignorance,²⁹ and he must have heard in their statements echoes from Ernst Mach and the positivists.

Planck was 69 when he first confronted the Copenhagen interpretation. Two generations removed, he didn’t debate the young guns like Einstein did. Based on his writings, he came to accept the physics of quantum mechanics, showing one last burst of flexibility. But he fought to maintain his old comforts like causality via philosophical maneuvers. Copenhagen aggravated his old wounds from Mach, and he began a series of philosophical essays, intending to throw a life preserver to his beloved notion of causality.

Planck had what a physicist philosopher today would call a strong or strict version of causality, with little room for nuance: The universe operated via an irreversible march from known cause to certain effect. And at least to my reading, he sometimes conflated causality with the related but even more stringent idea of determinism. In twin essays on “Causation and Free Will” he confronted a very old dilemma: If we support an exact and predictable scientific world view, including the molecules in our brains, then how can we believe that our impulses are anything but pre-determined results? He wrote his essays, “to keep the lines of communication clear between serious science and the seriously thinking public.” And he wanted to counteract those who claimed causality was not relevant to quantum physics, an idea that had been “exploited by popularizers.”³⁰

First, he boldly summarized the “failed” efforts from millennia of philosophers to reconcile a clearly causal universe with the very “dignity of man.” Planck wondered, “how far science can help us out of the obscure wood wherein philosophy has lost its way.” In the end, he painted a fully causal universe, except for “one single point … the individual ego.” And science could never move beyond this point, because, “the observing subject would also be the object of research. And that is impossible; for no eye can see itself.” In these essays written circa 1930, Planck had his causal cake but enjoyed a cup of free will as well.³¹

Reactions to Planck’s earnest offerings were mostly negative, and among philosophers, even dismissive. Heisenberg wrote respectfully of Planck’s efforts, although he noted “little practical value.” Physicist Wolfgang Pauli, renowned for his blunt ways, labeled Planck’s thinking “sloppy.”³²

But Planck kept writing and speaking philosophically. Over the next 10 years, his positions softened somewhat. His essays “The Meaning and Exact Limits of Science” and “The Concept of Causality in Physics,” published posthumously, simultaneously move the goalposts of science and paint any refutation of causality as a misunderstanding. The goal of science was, he wrote, “the creation of a world picture, with real elements which no longer require an improvement, and therefore represent the ultimate reality.”³³ And yet, “the introduction of the elementary quantum of action destroyed this hope at one blow and for good.”³⁴ Even if Planck was no happier with the new physics than Einstein, he may have been more comfortable shifting his focus from the destination to the journey itself. “To some extent it is unsatisfactory but on the other hand it is proper and gratifying, for we will never come to the end, to finality. Scientific work will never stop, and it would be terrible if it did. … In science rest is stagnation, rest is death.”³⁵

Perhaps the best way to summarize Planck’s final stance on quantum mechanics would be this: He accepted the theory, equation by equation, but saw an explosion of falsehoods whenever someone applied it carelessly. He deplored a practice that still lingers: the vague application of quantum mechanics to biology and psychology. This seductive mixing began almost immediately after Heisenberg’s 1927 paper on uncertainty. Some conspicuously talented physicists like Pascual Jordan and Wolfgang Pauli were notable perpetrators, using quantum theory to support, respectively, the psychologies of Freud and Jung.³⁶

Planck especially cringed when others rushed to connect quantum ideas to philosophical conclusions. He bemoaned what he saw as unnecessary philosophical hand wringing over causality. For him, that fundamental philosophical pillar survived the quantum revolution with just a few bumps, scrapes, and clarifications. In this new scientific world, he thought those working at the philosophical extremes were doomed to frustration. Those obsessed with finding “a rule behind every irregularity” and likewise those who threw up their hands and saw “nature ruled exclusively by statistics,” were all being too intellectually rigid. “The law of causality is neither true nor false,” he wrote late in life. “It is rather … a signpost—and in my opinion, our most valuable signpost—to help us find our bearings in a bewildering maze of occurrences.”³⁷ In short, what would we do without it? Why would elderly Max put a needle to a phonograph record if he didn’t believe that action would cause a soothing Schumann arrangement to fill the room?

In late 1944, Planck’s chamber for such thoughts, his meditative library, was in ashes. But the bewildering world brought another random-looking occurrence, a surprising statistical blip. On November 9, the Otto Wolff Company received a letter from the offices of Heinrich Himmler, asking the company to inform Max Planck that his plea had not gone unnoticed and that Erwin’s sentence was, for now, suspended. The SS Reichsführer could justify converting the death sentence to life in prison.³⁸

Nelly received this incredible news first and caught Max briefly by phone. In a follow-up letter, he wondered if his cries of celebration could be heard all the way from Rogätz to Berlin. He wrote that life itself felt restarted. He immediately thanked Himmler for giving the family a reason to hope.³⁹ The Plancks received no further news in 1944, and Max’s New Year’s card to his friend and biographer Hans Hartmann underlines his state of mind and the state of Germany. He thanks the Hartmanns warmly for sending a letter, and he sympathizes with their woes: a younger son at risk in war; an older son facing grave injuries; and a damaged, barely livable home. He thanks them for asking after his granddaughter Grete Marie, saying she had fled Heidelberg, “with her four children helter skelter, overnight,” presumably in the midst of the ongoing air raids. Then he turns to Erwin. “The verdict hasn’t been carried out yet, and we still have some hope that the pardon will happen, given support from influential places. But I still feel since the verdict was so final that the sword of Damocles hovers over us constantly.”⁴⁰ He knew Himmler could be overruled, and Max Planck had already measured the Führer’s chaotic temperament in close quarters.