The Holy Land of Physics: Quantum Mechanics Turns One Hundred
- Philip Ball
- 3 days ago
- 17 min read
Updated: 11 hours ago
Philip Ball on post-war seaside towns and the past and future of quantum mechanics
Maybe it’s because I’m an islander that Helgoland, a tiny outcrop of red sedimentary rock in the North Sea, felt familiar. Barely a kilometre and a half by half a kilometre and with a permanent population not much over a thousand, the island can be circumambulated in under an hour. There are few accessible beaches, saving those on the even smaller sister island of Düne across a 500 meter stretch of water—and in any event, a dip in the North Sea is not the most appealing prospect even in the summer.
And yet people do come here for holidays, just as they came to the coastal towns and villages on the Isle of Wight where I grew up. The slightly tatty apartments, the antiquated crazy golf course, the pervasive smell of chips and ice cream, the families seeming not quite sure how they will pass their days in the drizzle before riding the ferry home: all this is Proustian to me. So too is the visible legacy of wartime strategic importance, not least the empty bomb casings on display and the remnants of military fortifications facing the waves. The Heligoland Bight (an alternative spelling of the island’s name, which is thought to mean “Holy Land”) was the setting for one of the first major air battles of World War II, and the island was bombed early in the war because of its function as a port for German warships. There was extensive bombing again in 1945, when the island was evacuated, and in 1947 the British Royal Navy detonated one of the largest non-nuclear explosions in history to destroy the remaining military installations, leaving a crater on the southern tip that is still visible today.
It was not a hankering to recapture the sensations of my youth that brought me to Helgoland in June. If I want that, I can still get it from the Isle of Wight, as time-warped as ever (and frankly, with far better beaches, walking, and climate). I was there for a stranger reason.
In 1925, the German physicist Werner Heisenberg came to the island seeking relief from his terrible hay fever. (This story has led many people to believe the island has no trees, or perhaps any vegetation of any sort. Neither is the case.) Heisenberg had spent the previous winter and spring in Copenhagen at the Institute for Theoretical Physics created by Danish physicist Niels Bohr. In Copenhagen, Heisenberg, Bohr, and their colleagues had searched in vain for a theory of what was already being called quantum mechanics: a framework to explain the behaviour of atoms and subatomic particles that could substitute for the “classical” mechanics of Isaac Newton that described the laws of motion and forces at larger scales. On Helgoland, unencumbered by the impediments of daily life, Heisenberg is said to have had the conceptual breakthrough that led later that year to the formulation of the first genuine and effective theory of quantum physics.
To mark the centenary of this event, scientists at the Quantum Institute at Yale University in New Haven, Connecticut, and the Max Planck Institute for the Science of Light in Erlangen, Germany, convened a gathering of quantum physicists that is unlikely to be repeated or rivaled. There were four Nobel laureates present—Anton Zeilinger, Alain Aspect, David Wineland and Serge Haroche, all of whom have contributed to the understanding and applications of quantum mechanics—and there would have been five if one had not had to cancel. (Several of the quantum veterans present are now of an age where a journey like this, and the relative starkness of conditions on the island, pose challenges.) Other attendees had a comparably legendary status. Charles Bennett and Gilles Brassard are pioneers of the efforts to understand quantum mechanics today using the concepts of information theory, which is concerned with how information may be stored, manipulated and transmitted. Wojciech Zurek was a student of John Wheeler (who himself worked with Bohr on the physics behind nuclear fission) and has helped to clarify the mystery of what “making a measurement” means in the quantum world. Carlo Rovelli is best known for his exquisite books, beginning 10 years ago with his Seven Brief Lessons on Physics, and is currently seeking a theory that unites quantum mechanics with gravity (of which, more later).
And so it went on: the list of attendees was, for the cognoscenti, jaw-dropping. Someone quipped that if the ferry from Hamburg (a speedy catamaran) had sunk, so would have quantum physics for a generation. There was never any danger of that, although the choppy outward crossing might have left a few passengers nervous, and certainly left them feeling the worse for wear. A childhood of ferry crossings left me in good stead.
As the attendee list suggests, this is very much a field in which one has to speak of Grand Old Men. Even today, much of the research in quantum physics is male-dominated. A survey conducted by Nature on current attitudes towards “foundational” questions of meaning and interpretation drew a response that was almost 90% male, which is probably a fair reflection of the gender balance in the discipline. That being so, the organizers did a pretty good job of ensuring a degree of female representation, even if nothing like parity. The field has something of a history of overlooking contributions from women. Lise Meitner, the Austrian physicist who first understood that the experiments on uranium by Otto Hahn and Fritz Strassmann in 1938 had revealed nuclear fission, was omitted from the 1945 Nobel given to Hahn, and was forbidden even to join Hahn in the laboratory when she worked with him in Berlin before fleeing Germany, as a Jew, after the Anschluss. Historian and philosopher Elise Crull of the City College of New York spoke at the opening banquet in Hamburg about the work of mathematician Grete Hermann, who, among other contributions, pointed out a flaw in an important “proof” in quantum mechanics posited by the Hungarian mathematician John von Neumann, decades before that same shortcoming was highlighted in the 1960s. Hermann’s contribution is now becoming more recognized, but the work of many women early in the field remains largely unknown; Crull is a contributor to a new book Women in the History of Quantum Physics (Cambridge University Press, 2025) which highlights a number of them: Johanna van Leeuwen, Hertha Sponer, Lucy Mensing, Laura Chalk.
Gender balance is only one of the ways in which this field is showing some healthy changes. The caricature, which I have been probably as guilty as anyone else of perpetuating, is that after a hundred years physicists are still arguing about what quantum mechanics means. That’s not untrue, but the debate now is not merely a rehash of that between Bohr, Einstein, Heisenberg and Erwin Schrödinger in the 1930s. The emphasis of the debate is shifting in such a way that what once seemed like sterile deadlock has ceded to discussion of astonishing and deep connections between diverse areas of physics, as well as pointing towards new applications like quantum computers or quantum cryptography. The Grand Old Men were there to lend their prestige, but it was among the younger generation that one could discern the true vibrancy of the field.
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The weather when we arrived was another throwback to the island summers of my youth: grey, damp, and windy, forcing you to wonder if you’d brought enough clothes or how you were going to occupy yourself for a whole week. Bearded Nobel Laureates dragged suitcases along the slick wharf. We all queued patiently outside the tiny office in town where keys to guesthouses were handed out: check-in time, 3 p.m.
There is no record, to my knowledge, of what the weather was like when Heisenberg arrived in early June 1925, but if you were seeking a place to focus the mind in isolation, Helgoland is surely it. In the story he told decades later, he worked away in his lodgings on the problem of quantum mechanics, making mistake after mistake but gradually making progress towards his goal. At around 3 a.m. he was still at it, and it was then that his Eureka moment arrived:
At first I was deeply alarmed. I had the feeling that, through the surface of atomic phenomena, I was looking at a strangely beautiful interior… I was so excited that I could not think of sleep and so, as a new day dawned, I made for the southern tip of the island, where I had been longing to climb a rock jutting out into the sea. I did so now without too much trouble, and waited for the sun to rise.
Like many historians, I have doubts about this narrative. For one thing, Heisenberg was a notoriously unreliable narrator. (I argued in a friendly but impassioned way with Carlo Rovelli about this; Carlo, I felt, presented a misleading picture of what Heisenberg did and didn’t do in his wartime research on uranium fission for the Nazis. I’m convinced the historical evidence shows that the story Heisenberg later told was self-serving and exculpatory.) And whereas, like many who hear this tale, I had visions of Heisenberg striding for hours across the cliffs, I now know that barely any point on the island is more than a quarter of an hour from any other—the rock that now commemorates Heisenberg’s breakthrough is a mere ten minutes from the town. There is no “jutting rock” to scale on the southern tip, and the stack on the northern tip that meets this description is unscalable.
But more to the point, what Heisenberg did was not really as novel as is generally implied. By the 1920s there was a quantum theory of sorts, but not a quantum mechanics. That’s to say, there was a rather ad hoc collection of new ideas about how the world worked at the atomic scale, but no mathematical framework for making systematic calculations and predictions about it. Bohr had taken the quantum hypothesis first put forward by Max Planck in 1900 and then developed by Einstein in 1905, and shown that it could explain the precise frequencies of light absorbed and emitted by the hydrogen atom, the simplest of all atoms. According to this hypothesis, energy does not vary gradually at the finest of scales, but is divided into discrete chunks called quanta. Einstein proposed that the energy carried by light was like this: that light itself came in packets, later called photons.
Bohr applied this idea to the way electrons were believed to orbit the central nuclei of atoms, such that they could only reside in orbits with particular energies and could not have energies in between.
There is none of the famously counterintuitive nature of quantum mechanics in this early quantum theory. Sure, it was a surprise that energy and light were “quantized”—but why not? And soon enough Bohr’s quantum theory of the atom hit problems, not the least of which was that it didn’t seem to work for any atom more complicated than hydrogen, in which there are only two subatomic particles to contend with. Bohr and colleagues had tried all kind of tricks to extend the theory, even being prepared at one stage to jettison the sacred principle of the conservation of energy. Nothing seemed to work.
The usual story has it that on Helgoland Heisenberg took the crucial step of abandoning any notion of electrons traversing the atom in orbits, or indeed any physical picture of “what the electrons were doing” at all, and instead decided to build his mechanics using only quantities we could observe experimentally: the frequencies and intensities of the light emitted and absorbed by atoms. Heisenberg arranged these quantities in tables that could then be manipulated to make predictions about properties such as the positions and momenta of electrons.
But as I explained in my historical talk at the Hamburg launch banquet, this abandonment of a physical picture of electrons and orbits was already the standard view of Heisenberg’s professor at Göttingen, Max Born, and others. The advance Heisenberg made in June was to find a way of formalizing the problem mathematically. Even then, he didn’t really know what he had done. While undeniably brilliant, at 23 he had gaps in his knowledge of physics and maths you could drive the Helgoland ferry through, and so he did not recognize these tables as what mathematicians call matrices. That connection was made by Born, who coined the term matrix mechanics for Heisenberg’s method. It was Born too, along with his other young assistant Pascual Jordan, who took Heisenberg’s ideas and brought them together into a proper quantum mechanics in a paper they published in September. The theory was fully presented in a subsequent paper by all three men, completed in November.
This “Three-Man Paper” secured the departure from classical physics, but the real strangeness was still to come. In 1927 Heisenberg showed that the peculiarity of matrix algebra—that the order in which you did the multiplication mattered, in contrast to the fact the 2x3 is no different from 3x2—implied that there was an irreducible “uncertainty” in nature. (It’s debatable if that is the best word; some prefer to talk about undecidability.) If we wish to know some things about a quantum object, that might preclude all possibility of knowing certain other things. Most notably, the more precisely we measure the speed of, say, an electron (strictly, the momentum: speed x mass), the less precisely we can know its position at the same instant in time. This is Heisenberg’s Uncertainty Principle.
Heisenberg tried—despite his earlier injunction to give up hope of explaining quantum phenomena using a physical picture of objects moving in time and space – to offer a rationale in terms of the way any measurement on a quantum particle must inevitably disturb it. If we want to see an electron, we must bounce a photon off it, which changes its motion. But this is a classical argument and gives the wrong idea. It seems that the constraint on what is knowable about the world is fundamental. And by knowable, I don’t mean “how much we can know,” but rather, “what can meaningfully be considered real at all.”
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This limit was made all the more apparent when, in 1935, Einstein made explicit a feature of the theory that had already been implicit (and in fact which Grete Hermann had already alluded to earlier that year). It is a phenomenon that Schrödinger called entanglement, and which he—with good reason—claimed was thedistinguishing characteristic between quantum and classical physics.
Entanglement arises between two particles when they interact: when they “feel” one another in some way. Two gas molecules that collide become entangled thereafter, for example. According to quantum mechanics, the two objects are then no longer independent of one another: in some sense quantum mechanics considers them to be a single entity. In technical terms, it means that their properties are correlated, or that they share “mutual information.” This in itself is not so odd: a pair of gloves are correlated, such that if one is left-handed, the other must be right-handed. But entangled quantum objects are more strongly correlated than classical ones can ever be.
Here’s what that means. The odd thing about quantum mechanics is that it seems in general not to assign definite properties to objects until we measure them; before that point, all we can say is what the possible outcomes of such a measurement might be, and what the relative probabilities of those outcomes are. It’s not that we just haven’t looked yet; it’s that there is no obvious meaning to the question of what the properties arebefore we look.
This is what Einstein could not accept, famously asking on one occasion how it could be reasonable to doubt that the moon was there before we look at it. This seems like a merely philosophical question, since we can’t know unless we look. But entanglement brings that issue to a head. If we make a measurement on one of a pair of entangled particles and thereby “force nature to make a choice” from the possible outcomes of the measurement, the correlations imply that we also force the choice for the entangled partner. It is as if the information about the outcome of our measurement is transmitted instantaneously to the other particle. But this, as Einstein rightly pointed out, contradicts his theory of special relativity, which states that no signal can travel faster than light.
The ”paradox” hung in the air for another three decades, until the Irish physicist John Bell proposed an experiment that could distinguish between Einstein’s preferred solution – that the particles somehow had the values of those properties determined all along – and what quantum mechanics seemed to say, which is that indeed the states of both particles were determined by a measurement on one of them. Bell’s experiment was first performed by John Clauser – the missing Nobel laureate on Helgoland – and his student Stuart Freedman at the University of California at Berkeley in 1972, using lasers. The result supported the picture given by quantum mechanics (contrary to Clauser’s expectation), and seemingly refuted Einstein. Bell’s test was performed more rigorously by Aspect in the 1980s and by Zeilinger in the 1990s, and every version of it has been consistent with quantum mechanics.
What does this mean? What it doesn’t mean is that indeed a signal passes faster than light between the two entangled particles. Rather it suggests that quantum physics is nonlocal: to put it one way, we cannot consider that the properties of a quantum object necessarily reside locally on that object. Or better, perhaps: that we must give up the idea that what appear to be two entangled particles really are separate entities, and accept that they have become a single one. “There is no story in space-time that tells us how it happens,” physicist Nicolas Gisin of the University of Geneva said at the meeting.
This is dizzying —because atoms and particles are interacting all the time in our world, seeming to weave them all into an entangled web. But here’s another way to think about it: entanglement seems to collapse our conventional notion of space. The entangled pair can be as far apart as you like—in principle, on opposite sides of the galaxy—and still be “in touch.” Experiments like Zeilinger’s have demonstrated this effect over many kilometres of optical fibre for entangled photons, and another speaker at the meeting, Jian-Wei Pan of the University of Science and Technology of China in Hefei, has demonstrated entanglement between particles connected via a telecommunications satellite. This has led some researchers to propose that space itself is not a primary feature of nature—the stage on which particles dance—but is something that emerges from an underlying web of entanglements.
That extraordinary idea was discussed by Juan Maldacena, a physicist at the Institute for Advanced Study in Princeton who has acquired a justified reputation for formidable smartness, in a talk that was much more comprehensible than I’d expected. The exciting thing here is that it seems to hint at a link between quantum mechanics and the theory we use to describe spacetime: general relativity.
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If you ask physicists what is the biggest outstanding question in their subject, many will say that it is the problem of how to unite those two theories. At the meeting, quantum theorist John Preskill of the California Institute of Technology (trust me, each of these names has rock-star status in the field) said precisely that. One often hears that the best hope for that reconciliation lies in the abstruse mathematics of string theory, which so far has not been able to furnish a single prediction anywhere close to being testable with current methods. But there was no string theory on Helgoland. The talk was instead of the deep connections between quantum mechanics and gravitation that seem to be revealed by couching them in terms of what can and can’t be done with information.
For example, the prohibition in special relativity about faster-than-light communication is expressed as what quantum information theorists call a “no go theorem”: basically, an axiom that whatever else your ideas permit, they mustn’t permit this. “This” being, in that context, “signaling”: sending a message faster than light. One puzzle about quantum mechanics is that it seems possible in theory to imagine correlations between objects that are even stronger than those seen in quantum physics, still without violating the “no signaling” rule. The question is then: why don’t we see those limits in nature instead? No one knows, but if we understood why, that might offer some deeper insight into quantum theory itself.
The truth is that we don’t even know for sure if it’s right to be searching for a theory of quantum gravity – which is to say, a theory in which the gravitational force appears as a “quantum” force, transmitted via the exchange of quantum particles just as are the other three forces of nature. (Electromagnetic forces between objects, for example, can be described as an exchange of the quantum particles of light: photons.) Gravity is assumed to have an associated particle called the graviton, but no one has ever detected one. As Flaminia Giacomini of ETH Zürich admitted, “We have no experimental evidence that we should quantize gravity.”
Several speakers suggested ways in which we might obtain it. Igor Pikovski of the Stevens Institute of Technology in New Jersey proposed that a variation of a method once proposed to search for gravitational waves—ripples in spacetime caused by extremely violent astrophysical processes such as the merging of two black holes—might be adapted to spot single gravitons. Another approach, first proposed by Richard Feynman in 1957, is to try to entangle two objects via their gravitational attraction: something that is possible only if gravity is indeed a quantum force. That’s extremely tricky to do experimentally, because it demands objects big enough to have a significant gravitational field but small enough to still show quantum behaviour, which gets washed away by the time objects reach the scale of footballs, say. Lumps of silica the size of the smallest bacteria – perhaps a few tens or hundreds of nanometres across—might fit the bill, but only if they are very cold. Experimental techniques aren’t there yet, but are getting close.
Such methods might also soon be capable of putting a living object like a bacterium or a virus (whether the latter are truly alive is still debated) in a quantum superposition: for example, so that it might be observed in either of two possible locations. Such an achievement would doubtless be trumpeted in the media as a sort of realization of Schrödinger’s famous thought experiment with a cat, even if would not involve the bacterium being “simultaneously live or dead”, as in Schrödinger’s original proposal. (It is in fact not obvious how one could ever create a superposition of “live” and “dead” for an organism, one reason why the thought experiment tends to get more attention than it deserves.) If that were done, “all hell would break loose in the public sphere", commented Zeilinger, who confessed that such a superposition of a living thing was his “longstanding dream.”
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My guess is that the village hall on Helgoland has never before heard such esoteric discussions, nor will again any time soon. Situated next to the crazy golf course, it looked more accustomed to Christmas raffles and “Sounds of the Sixties” nights. All this lent a surreal feeling to the week. On one’s wanderings in search of snacks to hoard for sustenance after the late-night discussions finally ended, you were liable to overhear discussions about Hamiltonians, quantum error correction, or zero-point motion from the tables outside every small restaurant. The common practice for senior speakers to fly in, give a talk, and fly out again, simply wasn’t possible here, and as the days passed and the weather gradually improved, everyone seemed to relax into the liminal feeling so characteristic of islands.
Did this strange week of isolation and insularity resolve any of the old debates in quantum theory? Not really. Theoretical physicist Gemma de les Coves of the Universitat Pompeu Fabra in Barcelona began her talk with a bold confession: “I do not understand quantum mechanics,” her opening slide announced. (Others agreed; “Maybe they say they do, but I don’t believe it,” said Renato Renner of ETH Zürich.) And while some of the Old Timers seem at least to have found their own way of rationalizing it (Zeilinger: “The quantum state only describes our knowledge, not the world out there”), they didn’t agree among themselves even on very basic issues. When Gisin stated that he believes “measurements give outcomes”, it seemed like an almost crudely obvious thing to say—except that, in the Many Worlds interpretation of quantum theory devised in 1957, which now has some prominent advocates, this cannot really be said to be so. In that view, measurements give every outcome conceivable, each in a different universe, so that one can’t truly say “this (and not that) happened”.“How much common sense can one hope to retain from a theory?", de les Coves astutely asked. She admitted to being happy to give up a lot, but we can't give it up completely without relinquished all sense of knowing anything.
Yet despite Renner’s suggestion that as far as interpretations are concerned, “it’s still the same questions, still the same arguments,” we boarded the ferry home. The weather having improved considerably, ice creams were needed, at last, and we were feeling more optimistic. Not because we are significantly closer to the “correct interpretation” of quantum mechanics—it’s still very hard to figure out how experiments could distinguish many of them (and there are dozens, at least)—but because the connections now evident between interpretations and applications, between ideas about gravity and spacetime and quantum communication and computation, between the concepts of measurement and the incredible precision with which it can be done, have revealed an unguessed richness about the theory and the ways it can be used. If the old questions remain unanswered, that may be in part because we now have better ones. And that’s surely some kind of progress.
Philip Ball is a scientist, writer, and a former editor at the journal Nature. He has won numerous awards and has published more than twenty-five books, most recently How Life Works: A User’s Guide to the New Biology; The Book of Minds: How to Understand Ourselves and Other Beings, From Animals to Aliens; and The Modern Myths: Adventures in the Machinery of the Popular Imagination. He writes on science for many magazines and journals internationally and is the Marginalia Review of Books' Editor for Science. Follow him @philipcball.bsky.social
