Erwin Schrödinger

Making Waves: John Gribbin on Why Erwin Schrödinger’s Famous Paradox is the Cat’s Meow

Best known for his cat thought experiment, Austrian physicist Erwin Schrödinger (1887–1961) contributed significantly to fields of wave mechanics and wave equation. In 1933, he co-won the Nobel Prize for Physics for the introduction of Schrödinger’s wave, which is still widely used in modern quantum theory.
British science writer, astrophysicist, and lecturer in astronomy at the University of Sussex, John Gribbin is the author of numerous books, including several about Schrödinger.

Simply Charly: You’ve written prolifically on a wide variety of science topics. In particular, you’ve written several books in which Erwin Schrödinger figures prominently, including In Search of Schrödinger’s Cat: Quantum Physics and Reality, which has become a perennial best-seller and, most recently, Erwin Schrödinger and the Quantum Revolution. What is it particularly about Schrödinger that sparked your interest in him over, say, other prominent architects of quantum mechanics like Paul Dirac or Werner Heisenberg?

John Gribbin: First, I should point out that I started out as a physicist (Ph.D., Cambridge). I even met Dirac, although literally only to say hello. So although I later diversified and now regard myself as mainly a writer with a background in science, not as a scientist who writes, this is a very longstanding interest! I became interested in Schrödinger through the cat puzzle and his distaste for the theory he had helped to found, which made him more interesting (to me) than, say, Heisenberg, who never had any doubts. Then there was his interesting personal life and his advanced age (for a mathematical physicist) when he made his breakthrough, so he was not your average scientist! He also looked a little like my father.

SC: Despite being one of the principal architects of quantum mechanics, a paucity of good biographies on Schrödinger exists today. Indeed, only one—Walter J. Moore’s Schrödinger: Life and Thought—comes to mind, aside from your own recent contribution. Why do you think there hasn’t been more written about Schrödinger’s life and work?

JG: I think there was at one time a certain reticence about delving into Schrödinger’s personal life, and it is impossible to understand the man without discussing his relationships with women. He was also for a long time regarded as having gone astray after developing his famous equation, which became the standard tool of quantum physicists, while at the same time they lapped up the so-called Copenhagen Interpretation, which he abhorred and which the cat puzzle was intended to ridicule. It was only after about 1982 when experiments showed that the Copenhagen Interpretation is indeed ludicrous, that his ideas became rehabilitated. Then Moore came along and produced a great fat book, which seemed to cover all the bases, and left little room for another biography. I had planned to write one about the same time, but decided to hold off for a decent interval before going ahead. Happily, this means that my book contains a discussion of recent developments in quantum theory, which confirm Schrödinger’s prescience.

Erwin Schrödinger

SC: Schrödinger had an abiding interest in philosophy—more so than any of his contemporaries. In particular, he had a fascination with the Eastern philosophy of Vedanta. How did this impact his scientific outlook?

JG: It was very important. In simple terms, he subscribed to the idea that what we perceive as reality is one aspect of a more complicated reality. This is related to the idea that all possible outcomes of quantum processes are real (the cat really is both alive and dead), and so is the idea of multiple universes—the Multiverse.

SC: Science writer Dick Teresi described theoretical physicists as the shooting stars of science:

“They do their best work in their 20’s, then seemingly burn out. Theorists commonly retire, intellectually speaking, by their 30’s to become ”elder statesmen” of physics. Four of the giants of quantum mechanics—Paul Dirac, Werner Heisenberg, Wolfgang Pauli and Niels Bohr—all crafted their greatest theories as very young men.”

However, at age 38, Schrödinger produced six exquisite papers, all written and published in a six-month period of theoretical research that is without parallel in the history of science. What do you feel triggered this six-month burst of creativity at such a late stage in his career?

JG: Moore suggests that Schrödinger’s burst of creativity was triggered by a passionate love affair with a woman whose identity is not known to us (although the evidence for the affair is compelling). It is certainly true that Schrödinger was, throughout his life, more creative intellectually when he was more active sexually; this, by the way, was also true of Einstein. However, I am not sure which is cause and which is effect. It seems that the euphoria of making a great discovery would stimulate the senses in more ways than one! But in this case, I think it is more significant because Schrödinger had been brought up in the tradition of 19th-century physics, and his great achievement with the wave equation seemed to him at first to be bringing order back into science and replacing weird ideas like quantum jumping and uncertainty with the familiarity of waves, harking back to Robert Maxwell and earlier. This cozy but superficial familiarity made physicists latch on to his equation as the preferred way of doing calculations, but it was part of Schrödinger’s genius to realize that he had not after all got rid of what he called “this damned quantum jumping,” leading him to say of the quantum theory he had helped to found: “I don’t like it, and I wish I’d never had anything to do with it.”

SC: In 1933, the Nobel Prize in Physics was jointly awarded to Erwin Schrödinger and Paul Dirac “for the discovery of new productive forms of atomic theory.” In Schrödinger’s case, he formulated in 1926 a wave equation that accurately gave the energy levels of atoms known as the Schrödinger equation. Can you elaborate a little more on his contribution?

JG: The Schrödinger equation jumped off from the suggestion by Louis de Broglie that electrons, previously regarded as tiny particles, could also be treated as waves. At that time (the 1920s), there was a puzzle about atomic structure, because the accepted model of an atom, devised largely by Niels Bohr, treated electrons as particles in orbit around a central nucleus, analogous to the way planets orbit the Sun. Each orbit corresponded to a certain energy, or “energy level.” But experiments had shown that, unlike planets orbiting the Sun, the electrons could not be in any orbit, but could only sit at certain energy levels, and not anywhere in between. They could, however, jump from one level to another, without passing through the space in between in the process. This is as if Mars, suddenly jumped into the orbit of Jupiter, without actually crossing the space in between. The wave idea explained the energy levels in terms of standing waves, like the notes on a plucked guitar string. The only “allowed” levels were now seen as ones corresponding to a whole number of wavelengths so that the wave could neatly fit around the atom, like the worm Ouroboros biting its tail. “Jumping” now involved changing the wavelength of the “electron” to another energy level, like playing harmonics of a fundamental note on a guitar. Schrödinger’s equation put all this on a formal basis and gave physicists the tool they needed to calculate things like the radiation of light from an atom (its spectrum), resulting from all the shifts from one harmonic to another.

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SC: Schrödinger found it difficult to find a stable academic post after leaving Germany in 1933—a seven-year odyssey finally concluding with a comfortable position at the newly formed Dublin Institute for Advanced Studies. Why did such a supremely eminent scientist like Schrödinger have a tough time finding an academic post?

JG: Fundamentally, because of his complicated sex life. He turned up in Oxford as a refugee from the Nazis with both a wife and an openly acknowledged and pregnant mistress, at a time when Oxford colleges thought it rather odd for a man to have even one wife. His temporary stay was not extended. Princeton was closed to him for the same reason.  It is a curiosity that avowedly Catholic Ireland was much more tolerant of such behavior!

SC: The Dublin Institute for Advanced Studies, where Schrödinger finally accepted a position, was a far cry from the more notable bastions of physics in Germany—like the University of Göttingen or the University of Berlin. How was he drawn to such an unusual place? And was he happy there?

JG: In 1938, as a direct result of Schrödinger’s dismissal from his Viennese post by the Nazis, wheels started turning to his future advantage. Eamon de Valera, who was Prime Minister of Ireland, had a passion for mathematics, and his pet project was to establish an Institute for Advanced Study in Dublin. When he heard of Schrödinger’s situation and realized that he might soon have to leave Austria, de Valera decided to try to make contact with him through intermediaries, offering him a post there.

Erwin’s wife, Anny’s, account of the Schrödingers’ flight from the Nazis is preserved in the Dublin archive; after I had written In Search of Schrödinger’s Cat, I heard another account from William McCrea, then (in the mid-1980s) a Professor at the University of Sussex, but from 1936 to 1944 Professor of Mathematics at Queen’s University of Belfast, and a frequent visitor to Dublin. When the Schrödingers arrived in Rome completely penniless, having abandoned everything at home so that the Nazis would not suspect they were leaving permanently, Erwin had to ask the taxi driver to tip the porter who carried their bags from the train, then got the commissionaire at the hotel to pay the taxi off, having convinced him that Erwin really was a famous scientist and a friend of the well-known Italian physicist Enrico Fermi. He then announced at the reception desk that Professor Fermi would pay their bill. The story, said McCrea, rings true, and “is entirely in character” for Schrödinger. Fermi, summoned by telephone, came to the hotel and gave them some money, but warned that they were scarcely out of danger in Fascist Italy (before the end of the year he would be forced to flee himself) and that letters were likely to be censored.

There was, however, a loophole. From the premises of the Papal Academy, inside the Vatican, Schrödinger was able to write to Professor Frederick Lindemann, in Oxford, to a friend in Zürich, and to de Valera, informing them that he was in Rome. Letters from the Vatican, recognized by Mussolini as a sovereign state, escaped the attention of the Italian authorities. It was easy to contact de Valera since he was President of the League of Nations at the time and in Geneva on League business. A couple of days later, while at the Academy, Schrödinger received a telephone call from the Irish Embassy, advising him to get out of Italy as soon as possible. Erwin spoke to de Valera himself that afternoon; the political situation was becoming more complex with Germany’s takeover of the Czech Sudetenland, and de Valera urged Schrödinger to get to England or Ireland before the likely outbreak of war.

The Irish Embassy provided the Schrödingers with first-class train tickets to Geneva, but as it was illegal to take currency out of Italy at that time, they left with only a pound. This led to an unanticipated complication: at the border, the train was stopped, and the Schrödingers were separated from one another and interrogated while their luggage was searched; Anny described it as “the fright of my life.” But the problem was not, this time, political.  The Schrödingers’ passports had visas for cross-Europe travel, and the customs authorities could not believe that people with first-class tickets and Europe-wide visas were traveling with only a single pound in their pockets. Their not illogical conclusion was that the couple must have been smuggling valuables in their luggage. When nothing was found, they were allowed to leave, on the same train, which had been held during the search.

De Valera met the Schrödingers in Geneva, where they stayed for just three days before moving on to England through France. Schrödinger worked hard to make the Institute in Dublin a success, not least out of gratitude to the Irish for rescuing him from a desperate situation, and later described the 17 years he spent in Dublin as the happiest of his life.

SC: Like Heisenberg’s Uncertainty Principle, Schrödinger’s name will be forever linked to the thought experiment he devised which has been eponymously dubbed Schrödinger’s cat. What did Schrödinger intend to illustrate with this thought experiment?

JG: The idea of the thought experiment (no cat has ever been subjected to such indignities) is that according to the standard version of quantum theory in the 1930s, the Copenhagen Interpretation (so-called because Bohr lived in Copenhagen), it is possible to set up a system where there is a precise 50:50 chance of what Schrödinger called a “diabolical device” being triggered and killing a cat in a closed room. The Copenhagen Interpretation asserts that until somebody looks to see what has happened, the cat is in a “superposition of states”, either both dead AND alive, or neither dead NOR alive, depending on how you choose to think about it, and then when you do look there is a “collapse of the wave function” onto one possibility. This is clearly nonsense. Although the Copenhagen Interpretation still has its adherents, it is slowly fading away in the light of the experiments referred to above. My own preferred alternative is the Many Worlds Interpretation (MWI, or Multiverse), which Schrödinger himself discussed in the early 1950s. In this picture, there are two universes, one with a dead cat and one with a live cat. Simple.

SC: In 1944, Schrödinger published What is Life?, a book that stimulated a number of young physicists to enter the rapidly burgeoning field of molecular biology and greatly influenced the co-discoverers of the structure of DNA—James Watson and Francis Crick. What did Schrödinger lay out in this book? And is it still relevant today?

JG: It has been accurately said that what was good in the book was not original, and what was original in the book was not particularly good. Schrödinger’s achievement was to promote and make physicists aware of pre-existing ideas about what we now call the genetic code, including the idea that there was a genetic code—that biological information could be stored in long-chain molecules like messages written in the letters of the alphabet. Physicists such as Crick read the book at a time when they were war-weary and profoundly disturbed by the direction being taken by physics following the development of the nuclear bomb, and were looking for peaceful uses for their talents. The book is still readable and interesting historically, although best understood through more modern commentaries (including, dare I say, my own In Search of the Double Helix).

SC: Although Schrödinger was one of quantum mechanics’ pioneers, he remained staunchly conservative in his attachment to classical physics. Why do you think such brilliant scientists like Schrödinger and, to a greater degree, Einstein became so disenchanted with the theory they had done so much to create?

JG: The case of Einstein is different. His disenchantment lay purely with the application of his ideas to the development of nuclear weapons. Schrödinger was a special case because of his age and background. He thought he was restoring common sense (19th-century common sense) to physics, and became disillusioned when that turned out not to be the case. Ironically, what looked, at the time, as his more wacky ideas related to MWI, turned out to lie at the heart of modern developments such as quantum computing.

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