ONE snowy day last year, I trekked out of Vienna, Austria, winding my way to one of the city’s graveyards to visit the final resting place of a giant of 19th-century physics. Ludwig Boltzmann’s tombstone features an imposing bust of the man, frowning down with a severe expression. And there above him, in gold lettering, is his formula for entropy. It must be one of only a few tombstones in the world adorned with an equation.
I had come for a spot of contemplation because I think Boltzmann’s century-old ideas could help solve one of the trickiest problems in physics right now: how quantum particles, which exist in a fuzzy cloud of possible states, give rise to the solid, well-defined world of snow, leaves, tombstones and everything else around us.
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There have been many attempts to explain this over the years, including the outlandish idea that the other quantum possibilities play out in many other parallel universes or that they simply vanish. But my colleagues and I suspect the answer could lie with Boltzmann.
The theory he worked, called thermodynamics, is centred on entropy, a measure of how disordered things are. It explains how things cool down, power up and, crucially for our purposes, mix. It covers everyday goings-on, like milk mixing into coffee. But if we are right, its powers also extend to the quantum realm. We think those quantum possibilities are never lost at all: instead, they are just mixed so thoroughly into the cracks of reality that we can’t see them. It this is true, it would profoundly change our understanding of reality.
At a fundamental level, the world lives by the rules of quantum mechanics. The theory was first developed in the early 1900s to explain why things like light and matter sometimes behave as waves and sometimes particles. Then, in 1926, Erwin Schrödinger devised a way to treat them as both, in a mathematical term called a wave function.
Quantum theory describes the microscopic world with unprecedented accuracy. But its laws are strange: they allow a particle to exist in multiple places at once, for instance. We never see these odd effects in the classical, everyday world. So, what is happening?
When physicists considered this question over the years, they often thought about measurements. No matter how many places an electron was in before being detected, once it is measured, we only ever see it in one place. Somehow, the act of measurement snaps the wave-like cloud of possibilities into a point-like reality with a defined position. This has been shown time and again in experiments. The process seems to be random and instantaneous, but physicists like me aren’t completely satisfied with that, since nothing else acts this way.
Wave function collapse
Debate around how to interpret this weirdness has been raging for more than 100 years. In the 1920s, great thinkers like John von Neumann settled on the idea that, when a measurement is made, the wave function “collapses” into a single outcome, deleting the other possibilities. But this explanation, which came to be known as the Copenhagen interpretation, is far from the only reading of the situation. The many-worlds interpretation says that every possible outcome of a measurement happens, in other worlds we can’t access. And physicist David Bohm’s interpretation says the other possibilities never existed – they were only illusions created by our lack of information.
To make things more complicated, it has been clear since experiments in the 1970s that measurements don’t just happen on lab benches. Even stray air molecules hitting electrons can “measure” them and destroy their quantumness. We call this process decoherence, and it explains why we don’t see quantum effects at everyday scales: once something gets big enough, there are too many other objects flying around that can “measure” it and upset its delicate quantum properties. But the same question still applies: how precisely does this process happen?
In the 2000s, physicists , then at the California Institute of Technology, and at Los Alamos National Laboratory in New Mexico took the idea of decoherence one step further. They argued that during this process, all the information in a system, including the quantum kind, spreads into the surrounding environment. This quantum information includes the system’s “two-places-at-once” property, or its superposition. But it also accounts for other intrinsically quantum features, like the bizarre, long-range “entanglement” that appears to allow instantaneous interaction between two quantum objects.
The pair’s claim is that only certain types of information are easy to access after this spreading process: namely, the classical variety. The quantum information is there; it is just practically impossible to see. They named this idea quantum Darwinism, in analogy to the evolution of living things. In this reckoning, the environment around a quantum object “selects” for the classical information, akin to how an environment – in a very different meaning of the word – selects for, say, long necks in giraffes.
It is a great idea, even though, in my opinion, the name doesn’t quite fit. After all, it doesn’t incorporate any notion of mutation and it doesn’t iterate over generations. Nevertheless, this framework is a powerful way of describing the interactions between a quantum system and its environment. Until recently, however, it has lacked a rigorous description of the physical process that makes the selection happen. This is where our group comes in.
Quantum thermodynamics
Our idea is that every step in the process can be explained using thermodynamics. And we are interested in what happens to the apparently vanishing quantum states, something that tends not to be focused on in quantum Darwinism. In a nutshell, we think this quantum information gets spread out between the object and the detector. And we think this spreading-out process mirrors the way things mix according to thermodynamics.
Now, historically, thermodynamics and quantum mechanics don’t go well together. In fact, the conventional idea of quantum measurements . These laws, which are sacrosanct to physicists, say that energy can’t be created or destroyed and that the universe becomes more disordered over time. The textbook description of a measurement seems to violate all of this. What’s more, it involves deleting information: when the particle goes from being in two places at once to only one, . This violates the conservation of information, a principle upheld by every other law of physics.
These problems were easy to sweep under the rug for decades, since we couldn’t probe in detail the exact interactions between quantum objects and the thing doing the measuring. It was easy to imagine that the problem was a by-product of inaccurate modelling of the measuring “device”. As experiments have improved, though, the discrepancies have become harder to hide.
The strength of our idea is that it can’t help but obey the rules of thermodynamics, since these are built in from the start. “Any model of measurement should be in keeping with the rest of the laws of physics,” says , who is part of our team and works between the University of Bristol and Heriot-Watt University, UK.
At the heart of our idea is a thermodynamic process that Boltzmann studied called equilibration. Our group loves coffee, so we like to imagine this process by picturing a splash of milk being poured into a cup of coffee. At first, the milk is a separate blob, but as the various particles randomly move around, it rapidly spreads out and mixes with the coffee. Once the milk and coffee particles are fully mixed, it is extremely unlikely that all the milk particles will spontaneously gather up into a blob again. Eventually, the coffee-milk mixture settles into an equilibrium – we say that it equilibrates.
The laws of thermodynamics, however, say that, given long enough, the milk and coffee will spontaneously separate back into the original, unmixed state. We would probably never see this happen because it would take far longer than the age of the universe. But we do see it happen in much simpler set-ups.
We recently learned that something similar happens in the quantum world, too. In a 2018 study, (TU Wien) in Austria and his colleagues showed that quantum decoherence can also undo itself. They observed a few thousand ultracold atoms in a box and saw how the atoms’ positions . The amount of correlation eventually reached a low, “equilibrium” value. But, after a few milliseconds, the correlation went back to almost its initial value.
This was a brilliant result. Decoherence should destroy these sorts of correlations, so seeing them spontaneously reappear indicates that it isn’t deleting information, just scrambling or hiding it. It is findings like this that inspired my colleagues and , also at TU Wien, to wonder if equilibration could also underpin quantum measurements. Along with Emanuel Schwarzhans, formerly at TU Wien, and Felix Binder at Trinity College Dublin in Ireland, they collected their ideas in a framework they dubbed the .
MEH describes measurement as a process where a quantum system interacts with a measuring device. A “device” could be anything that interacts with the quantum object, not just what we would typically think of as a measuring device. This spreads information into the device, until an equilibrium of information is reached between the system and the device. The bigger the device, the more places there are for the quantum information to hide, making it harder to get that information back – but never impossible.
How would this work in practice? Let’s take the simple example of a particle in a cloud of many different locations at once. Before a detector measures that particle’s position, there is information about all of the potential places it could have been detected. When the detector comes into contact with the particle, these pieces of information mix into the particles of the detector. We think this spreading process somehow “broadcasts” information from the system, making the information about its classical position available to read but its “two-places-at-once” information harder to spot.
What are the implications?
The mathematics behind this process is complicated, so the first two papers on the framework, still in peer review, are heavy on calculations. In the first, my colleagues showed that between a quantum system and a detector can make the system look classical, while only hiding the quantum behaviour, not destroying it. But a small enough detector would still allow quantum effects to peek through. The next paper, which was led by Engineer and was the first to which I contributed, takes the first step towards connecting this to experiments, by looking at how best to extract information here.
Eventually, we would like to test our ideas in a lab, and thankfully Schmiedmayer is keen to work with our group to make this happen. With a set-up like Schmiedmayer’s 2018 experiment, we could potentially watch the measurement process happen in a small system, then watch it un-happen, Lock reckons. “We could then maybe show that, as the system gets larger, the ‘un-happening’ gets less likely,” he says. If we saw this, it would be evidence that MEH is on the right track. “That would be an extremely happy day,” says Lock.
So far, we have remained agnostic about what this idea means for any of the philosophical interpretations of quantum mechanics. But our ideas do brush up against these concepts. For instance, MEH explains what happens to all the measurement outcomes you don’t see – the other “worlds” of the many-worlds idea. They are all still here in our world; we just can’t control the quantum system finely enough to observe them. “If we could grab hold of every single electron and control them in whatever manner we wanted, we wouldn’t be asking ourselves why the particle went left or right,” says Lock. “The idea of measurement becomes moot.”
This would remove much of the supposed mystique from wave function collapse, since measurement only seems mysterious when we overlook just how difficult it is in practice. As Lock puts it, it is about asking: “How do I, an inaccurate, ape-sized lump-thing, try to access something as finely detailed as the spin of an electron?”
It would also rule out the idea that collapse is a physical process that deletes information, and that there is some harsh transition between classical and quantum realities. “Nobody forces you to make the classical world different from the quantum one,” says Schmiedmayer. “All you can say is that, in the classical world, the complexity is too big. I just can’t see the quantum part.”
Botlzmann brains
There is another possible implication of our ideas. If you take Boltzmann’s ideas about equilibration to their extremes, you can imagine the whole universe equilibrating. If this is the case, some have speculated that, in the ludicrously distant future, long after the last stars die out, random fluctuations away from equilibrium will happen, resulting in sentient beings spontaneously – and very briefly – flickering into existence. This thought experiment, known as the “Boltzmann brain”, suggests that equilibration isn’t the end of the story for a big, dynamic system.
What happens if we push our idea to such an extreme? Well, if we could could keep track of every single subatomic particle in a detector and its environment, MEH says that you could, in principle, find all the hidden quantum information. This would almost be akin to seeing a particle in two places at once again, even after it has been measured. It is certainly an outlandish idea and I sometimes can’t help wondering what Boltzmann would make of it if he were still around.
Tom Rivlin is a postdoctoral researcher at the Vienna University of Technology in Austria
