The United Nations has designated 2025 the International Year of Quantum Science and Technology. Many physics magazines and journals have taken the opportunity to publish more articles on quantum physics than they usually do, and that has meant quantum physics research has often been on my mind. Nirmalya Kajuri, an occasional collaborator, an assistant professor at IIT Mandi, and an excellent science communicator, recently asked other physics teachers on X.com how much time they spend teaching the interpretations of quantum physics. His question and the articles I’ve been reading inspired me to write the following post. I hope it’s useful in particular to people like me, who are interested in physics but didn’t formally train to study it.

Quantum physics is often described as the most successful theory in science. It explains how atoms bond, how light interacts with matter, how semiconductors and lasers work, and even how the sun produces energy. With its equations, scientists can predict experimental results with astonishing precision — up to 10 decimal places in the case of the electron’s magnetic moment.

In spite of this extraordinary success, quantum physics is unusual compared to other scientific theories because it doesn’t tell us a single, clear story about what reality is like. The mathematics yields predictions that have never been contradicted within their tested domain, yet it leaves open the question of what the world is actually doing behind those numbers. This is what physicists mean when they speak of the ‘interpretations’ of quantum mechanics.

In classical physics, the situation is more straightforward. Newton’s laws describe how forces act on bodies, leading them to move along definite paths. Maxwell’s theory of electromagnetism describes electric and magnetic fields filling space and interacting with charges. Einstein’s relativity shows space and time are flexible and curve under the influence of matter and energy. These theories predict outcomes and provide a coherent picture of the world: objects have locations, fields have values, and spacetime has shape. In quantum mechanics, the mathematics works perfectly — but the corresponding picture of reality is still unclear.

The central concept in quantum theory is the wavefunction. This is a mathematical object that contains all the information about a system, such as an electron moving through space. The wavefunction evolves smoothly in time according to the Schrödinger equation. If you know the wavefunction at one moment, you can calculate it at any later moment using the equation. But when a measurement is made, the rules of the theory change. Instead of continuing smoothly, the wavefunction is used to calculate probabilities for different possible outcomes, and then one of those outcomes occurs.

For instance, if an electron has a 50% chance of being detected on the left and a 50% chance of being detected on the right, the experiment will yield either left or right, never both at once. The mathematics says that before the measurement, the electron exists in a superposition of left and right, but after the measurement only one is found. This peculiar structure, where the wavefunction evolves deterministically between measurements but then seems to collapse into a definite outcome when observed, has no counterpart in classical physics.

The puzzles arise because it’s not clear what the wavefunction really represents. Is it a real physical wave that somehow ‘collapses’? Is it merely a tool for calculating probabilities, with no independent existence? Is it information in the mind of an observer rather than a feature of the external world? The mathematics doesn’t say.

The measurement problem asks why the wavefunction collapses at all and what exactly counts as a measurement. Superposition raises the question of whether a system can truly be in several states at once or whether the mathematics is only a convenient shorthand. Entanglement, where two particles remain linked in ways that seem to defy distance, forces us to wonder whether reality itself is nonlocal in some deep sense. Each of these problems points to the fact that while the predictive rules of quantum theory are clear, their meaning is not.

Over the past century, physicists and philosophers have proposed many interpretations of quantum mechanics. The most traditional is often called the Copenhagen interpretation, illustrated by the Schrödinger’s cat thought experiment. In this view, the wavefunction is not real but only a computational tool. In many Copenhagen-style readings, the wavefunction is a device for organising expectations while measurement is taken as a primitive, irreducible step. The many-worlds interpretation offers a different view that denies the wavefunction ever collapses. Instead, all possible outcomes occur, each in its own branch of reality. When you measure the electron, there is one version of you that sees it on the left and another version that sees it on the right.

In Bohmian mechanics, particles always have definite positions guided by a pilot wave that’s represented by the wavefunction. In this view, the randomness of measurement outcomes arises because we can’t know the precise initial positions of the particles. There are also objective collapse theories that take the wavefunction as real but argue that it undergoes genuine, physical collapse triggered randomly or by specific conditions. Finally, an informational approach called QBism says the wavefunction isn’t about the world at all but about an observer’s expectations for experiences upon acting on the world.

Most interpretations reproduce the same experimental predictions (objective-collapse models predict small, testable deviations) but tell different stories about what the world is really like.

It’s natural to ask why interpretations are needed at all if they don’t change the predictions. Indeed, many physicists work happily without worrying about them. To build a transistor, calculate the energy of a molecule or design a quantum computer, the rules of standard quantum mechanics suffice. Yet interpretations matter for several reasons, but especially because they shape our philosophical understanding of what kind of universe we live in.

They also influence scientific creativity because some interpretations suggest directions for new experiments. For example, objective collapse theories predict small deviations from the usual quantum rules that can, at least in principle, be tested. Interpretations also matter in education. Students taught only the Copenhagen interpretation may come away thinking quantum physics is inherently mysterious and that reality only crystallises when it’s observed. Students introduced to many-worlds alone may instead think of the universe as an endlessly branching tree. The choice of interpretation moulds the intuition of future physicists. At the frontiers of physics, in efforts to unify quantum theory with gravity or to describe the universe as a whole, questions about what the wavefunction really is become unavoidable.

In research fields that apply quantum mechanics to practical problems, many physicists don’t think about interpretation at all. A condensed-matter physicist studying superconductors uses the standard formalism without worrying about whether electrons are splitting into multiple worlds. But at the edges of theory, interpretation plays a major role. In quantum cosmology, where there are no external observers to perform measurements, one needs to decide what the wavefunction of the universe means. How we interpret entanglement, i.e. as a real physical relation versus as a representational device, colours how technologists imagine the future of quantum computing. In quantum gravity, the question of whether spacetime itself can exist in superposition renders interpretation crucial.

Interpretations also matter in teaching. Instructors make choices, sometimes unconsciously, about how to present the theory. One professor may stick to the Copenhagen view and tell students that measurement collapses the wavefunction and that that’s the end of the story. Another may prefer many-worlds and suggest that collapse never occurs, only branching universes. A third may highlight information-based views, stressing that quantum mechanics is really about knowledge and prediction rather than about what exists independently. These different approaches shape the way students can understand quantum mechanics as a tool as well as as a worldview. For some, quantum physics will always appear mysterious and paradoxical. For others, it will seem strange but logical once its hidden assumptions are made clear.

Interpretations also play a role in experiment design. Objective collapse theories, for example, predict that superpositions of large objects should spontaneously collapse. Experimental physicists are now testing whether quantum superpositions survive for increasingly massive molecules or for diminutive mechanical devices, precisely to check whether collapse really happens. Interpretations have also motivated tests of Bell’s inequalities, an idea that shows no local theory with “hidden variables” can reproduce the correlations predicted by quantum mechanics. The scientists who conducted these experiments confirmed entanglement is a genuine feature of the world, not a residue of the mathematical tools we use to study it — and won the Nobel Prize for physics in 2022. Today, entanglement is exploited in technologies such as quantum cryptography. Without the interpretative debates that forced physicists to take these puzzles seriously, such developments may never have been pursued.

The fact that some physicists care deeply about interpretation while others don’t reflects different goals. Those who work on applied problems or who need to build devices don’t have to care much. The maths provides the answers they need. Those who are concerned with the foundations of physics, with the philosophy of science or with the unification of physical theories care very much, because interpretation guides their thinking about what’s possible and what’s not. Many physicists switch back and forth, ignoring interpretation when calculating in the lab but discussing many-worlds or informational views over chai.

Quantum mechanics is unique among physical theories in this way. Few chemists or engineers spend time worrying about the ‘interpretation’ of Newtonian mechanics or thermodynamics because these theories present straightforward pictures of the world. Quantum mechanics instead gives flawless predictions but an under-determined picture. The search for interpretation is the search for a coherent story that links the extraordinary success of the mathematics to a clear vision of what the world is like.

To interpret quantum physics is therefore to move beyond the bare equations and ask what they mean. Unlike classical theories, quantum mechanics doesn’t supply a single picture of reality along with its predictions. It leaves us with probabilities, superpositions, and entanglement, and it remains ambiguous about what these things really are. Some physicists insist interpretation is unnecessary; to others it’s essential. Some interpretations depict reality as a branching multiverse, others as a set of hidden particles, yet others as information alone. None has won final acceptance, but all try to close the gap between predictive success and conceptual clarity.

In daily practice, many physicists calculate without worrying, but in teaching, in probing the limits of the theory, and in searching for new physics, interpretations matter. They shape not only what we understand about the quantum world but also how we imagine the universe we live in.

In physics, the second law of thermodynamics says that a closed system tends to become more disordered over time. This disorder is captured in an entity called entropy. Many devices, especially clocks, are affected by this law because they need to tick regularly to measure time. But every tick creates a bit of disorder, i.e. increases the entropy, and physicists have believed for a long time now that this places a fundamental limit on how precise a clock can be. The more precise you want your clock, the more entropy (and thus more energy) you’ll have to expend.

A study published in Nature Physics on June 2 challenges this wisdom. In it, researchers from Austria, Malta, and Sweden asked if the second law of thermodynamics really set a limit on a clock’s precision and came away, surprisingly, with a design of a new kind of quantum clock that’s too precise scientists once believed possible for the amount of energy it spends to achieve that precision.

The researchers designed this clock using a spin chain. Imagine a ring made of several quantum sites, like minuscule cups. Each cup can hold an excitation — say, a marble that can hop from cup to cup. This excitation moves around the ring and every time it completes a full circle, the clock ticks once. A spin chain is, broadly speaking, a series of connected quantum systems (the sites) arranged in a ring and the excitation is a subatomic particle or packet of energy that moves from site to site.

In most clocks, every tick is accompanied by the dissipation of some energy and a small increase in entropy. But in the model in the new study, only the last link in the circle, where the last quantum system was linked to the first one, dissipated energy. Everywhere else, the excitation moved without losing energy, like a wave gliding smoothly around the ring. The movement of the excitation in this lossless way through most of the ring is called coherent transport.

The researchers used computer simulations to help them adjust the hopping rates — or how easily the excitation moved between sites — and thus to make the clock as precise as possible. They found that the best setup involved dividing the ring into three regions: (i) in the preparation ramp, the excitation was shaped into a wave packet; (ii) in the bulk propagation phase, the wave packet moved steadily through the ring; and (iii) in the boundary matching phase, the wave packet was reset for the next tick.

The team measured the clock’s precision as the number of ticks it completed before it was one tick ahead or behind a perfect clock. Likewise, team members defined the entropy per tick to be the amount of energy dissipated per tick. Finally, the team compared this quantum clock to classical clocks and other quantum models, which typically show a linear relationship between precision and entropy: e.g. if the precision doubled, the entropy doubled as well.

The researchers, however, found that the precision of their quantum clock grew exponentially with entropy. In other words, if the amount of entropy per tick increased only slightly, the precision increased by a big leap. It was proof that, at least in principle, it’s possible to build a clock to be arbitrarily precise while keeping the system’s entropy down, all without falling afoul of the second law.

That is, contrary to what many physicists thought, the second law of thermodynamics doesn’t strictly limit a clock precision, at least not for quantum clocks like this one. The clock’s design allowed it to sidestep the otherwise usual trade-off between precision and entropy.

During coherent transport, the process is governed only by the system’s Hamiltonian, i.e. the rules for how energy moves in a closed quantum system. In this regime, the excitation acts like a wave that spreads smoothly and reversibly, without losing any energy or creating any disorder. Imagine a ball rolling on a perfectly smooth, frictionless track. It keeps moving without slowing down or heating up the track. Such a thing is impossible in classical mechanics, like in the ball example, but it’s possible in quantum systems. The tradeoff of course is that the latter are very small and very fragile and thus harder to manipulate.

In the present study, the researchers have proved that it’s possible to build a quantum clock that takes advantage of coherent transport to tick while dissipating very little energy. Their model, the spin chain, uses a Hamiltonian that only allows the excitation to coherently hop to its nearest neighbour. The researchers engineered the couplings between the sites in the preparation ramp part of the ring to shape the excitation into a traveling wave packet that moves predominantly in the forward direction.

This tendency to move in only direction is further bolstered at the last link, where the last site is coupled to the first. Here, the researchers installed a thermal gradient — a small temperature difference that encouraged the wave to restart its journey rather than be reflected and move backwards through the ring. When the excitation crossed this thermodynamic bias, the clock ticked once and also dissipated some energy.

Three points here. First, remember that this is a quantum system. The researchers are dealing with energy (almost) at its barest, manipulating it directly without having to bother with an accoutrement of matter covering it. In the classical regime, such accoutrements are unavoidable. For example, if you have a series of cups and you want to make an excitation hop through it, you do so with a marble. But while the marble contains the (potential) energy that you want to move through the cups, it also has mass and it dissipates energy whenever it hops into a cup, e.g. it might bounce when it lands and it will release sound when it strikes the cup’s material. So while the marble metaphor earlier might have helped you visualise the quantum clock, remember that the metaphor has limitations.

Second, for the quantum clock to work as a clock, it needs to break time-reversal symmetry (a concept I recently discussed in the context of quasicrystals). Say you remove the thermodynamic bias at the last link of the ring and replace it with a regular link. In this case the excitation will move randomly — i.e. at each step it will randomly pick the cup to move to, forward or backward, and keep going. If you reversed time, the excitation’s path will still be random and just evolve in reverse.

However, the final thermodynamically biased link causes the excitation to acquire a preference for moving in one direction. The system thus breaks time-reversal symmetry because even if you reverse the flow of time, the system will encourage the excitation to move in one direction and one direction only. This in turn is essential for the quantum system to function like a clock. That is, the excitation needs to traverse a fixed number of cups in the spin chain and then start from the first cup. Only between these two stages will the system count off a ‘tick’. Breaking time-reversal symmetry thus turns the device into a clock.

Three, the thermodynamic bias ensures that the jump from the last site to the first is more likely than the reverse, and the entropy is the cost the system pays in order to ensure the jump. Equally, the greater the thermodynamic bias, the more likely the excitation is to move in one direction through spin chain as well as make the jump in the right direction at the final step. Thus, the greater the thermodynamic bias, the more precise the clock will be.

The new study excelled by creating a sufficiently precise clock while minimising the entropy cost.

According to the researchers, its design design could help build better quantum clocks, which are important for quantum computers, quantum communication, and to make ultra-precise precise measurements of the kind demanded by atomic clocks. The clock’s ticks could also be used to emit single photons at regular intervals — a technology increasingly in demand for its use in quantum networks of the sort China, the US, and India are trying to build.

But more fundamentally, the clock’s design — which confines energy dissipation to a single link and uses coherent transport everywhere else — and that design’s ability to evade the precision-entropy trade-off challenges a longstanding belief that the second law of thermodynamics strictly limits precision.

Featured image credit: Meier, F., Minoguchi, Y., Sundelin, S. et al. Nat. Phys. (2025).