The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time for free here.
Aren’t we living in interesting times for quantum physics? Over the last few years, global investments into quantum technologies have risen from . At the same time, Hollywood is feeding our imaginations by producing more and more “quantum movies”, such as the recent instalment of the MCU’s Ant-Man – Quantumania.
“Quantum-mania” seems to be an apt description of the hype cycle we are in, where it’s not exactly clear what is real and what is fiction. Part of this is just good old-fashioned storytelling, and part of it comes from confusion around quantum physics. But we, as humans, can’t be too hard on ourselves for that. Understanding what a quantum system actually looks like is pretty much impossible for us. The universe made it that way.
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In our everyday lives, we experience the universe through what we can see, touch, hear, taste and smell. This gives us a wonderful understanding and intuition for the things that we encounter. For instance, we don’t have to think about how to pick up a cup of water – we just do.
However, if we try to imagine the same cup of water falling into the black hole at the centre of the Milky Way, we are pretty much lost. Why is that? It’s because we have never actually seen it with our own eyes. Things and processes that we can perceive govern our intuition and help us form expectations about how the world should behave. This is what physicists call the “classical regime”.
Similarly to the behaviour of black holes in the cosmos, we have a hard time grasping how extremely small systems at very cold temperatures behave. For such situations, we often expect that things still act like they do in our everyday lives. Yet we have known for more than a century that this is the domain of quantum physics, where things become really weird.
The most famous – or rather, infamous – attempt to explain how the quantum world differs from our classical expectation is Schrödinger’s cat. In this thought experiment, a cat is placed into a sealed box along with a poisonous gas that will be released when struck by a particle from the radioactive decay of some element. Since we can’t predict exactly when this decay will happen, nobody outside the box knows if and when the cat will die. In other words, as long as the box hasn’t been opened, we can describe the cat as being in a “superposition” of both dead and alive.
While this experiment has been helpful in developing the subtleties of quantum theory, it has also had the misfortune of being totally misinterpreted in a lot of science fiction. It would be absurd for a living, breathing creature to be in some macabre state of death and life at the same time.
What Schrödinger really expressed is that an observer outside the box doesn’t know what may or may not have happened to the cat already. It is this observational ignorance that physicist Werner Heisenberg called the “indeterminacy principle”. The classical state of a physical object is indeterminate until it is measured by a classical observer. Quantum states, however, are perfectly able to exist in superpositions of many classical states.
But this raises questions. What is a classical observer? Why are we, as humans, classical observers? And why do we never see any quantum effects in our everyday lives? To understand this, we need to understand what “observing” actually entails.
Estimates suggest that most of us receive about . On the microscopic level, this means that we learn about our surroundings by intercepting photons – energy packets which carry momentum – that bounce off physical objects. Since we never see any quantum effects, the question we need to ask is why photons don’t seem to carry any information about objects’ quantum properties.
The answer comes down to how they interact with light. Physical objects are bombarded by an enormous number of photons every second. The photons arrive from many different angles, and this barrage eventually forces physical objects to settle into an equilibrium state in which they don’t move around. The only information light carries away from a given object pertains to the properties of its “stationary state”.
Does this mean that quantum states aren’t real, since we can see them? They are real – but their “quantumness” is quickly destroyed.
What makes quantum states really “quantum” is that they can be superpositions of classical states. The problem is, superpositions are fickle; they don’t take well to being kicked around by photons. Theoretical physicists have shown that only simple classical states can survive this process, which is called “decoherence”. Classical states are therefore the positions that hold up after all other quantum states are killed off (and which classical states survive depends on the distribution of different photons in the environment).
Have you ever wondered why both you and your buddy see the same things? This is because our eyes only intercept a tiny portion of scattered photons, and each fraction carries exactly the same information: an object’s stationary properties, which have survived being kicked around by light particles. Once a system achieves equilibrium with its environment, intercepting any fragment of the photons involved gives you exactly the same information. Hence, the observation becomes classically objective – you and your friend see the same thing.
This is the essence of “quantum Darwinism”, which explains why, to us, the universe appears to be classical and why we never see any quantum effects. For most of us, the term “Darwinism” might seem a little out of place in physics. We aren’t talking about the evolutionary adaptation of living beings to their environment, after all. However, the core tenet of Darwinism is the “survival of the fittest”, and, to a certain extent, this is exactly how stationary states emerge. All non-classical states are “not fit” to survive decoherence – only the classical states are adapted to survive.
Understanding what makes quantum systems lose their quantumness presents a clear roadmap for how to fully access such states. We just need to come up with ways to perfectly isolate quantum systems from their environment, so that they won’t be kicked around and subjected to decoherence.
Despite how convincing these arguments may sound, quantum Darwinism is still an active area of research. Its core ideas were developed some 20 years ago by , a theoretical physicist working at the Los Alamos National Laboratory. Nowadays, there is an active scientific community exploring further subtleties and proving some of these intuitive arguments in mathematically rigorous ways. If you want to read more about the topic, we published in honour of Zurek’s 70th birthday earlier this year.
For everyone else, what does this really mean? It can still be fun to watch Ant-Man and the Wasp beat up Kang in the Quantum Realm. However, we also need to recognise that the Quantum Realm is just a fun idea created for a movie. The real quantum universe is even weirder, but we will never be able to see it with our own eyes. The fact that simply “seeing” requires intercepting the photons that make a quantum system, well, no longer quantum will forever prevent us from truly experiencing anything but our own classical world.
