When there is no reality without observation

"This is exactly where Erwin Schrödinger's office used to be. His desk remained here for a long time, but eventually, we had to move it because we didn’t want it damaged during coffee breaks." With these words, quantum physicist Časlav Brukner begins our conversation at Boltzmanngasse 5 in the 9th district of Vienna, where the Faculty of Physics is located.

Schrödinger—or at least his cat—will, of course, come up in our discussion, but we will spend most of our time talking about the so-called 'things' that are "not there if you do not observe them." It will be a conversation which first denies the existence of an objective reality independent of observation and then questions what observation and reality are in the first place.

Reality is fundamentally probabilistic

"Historically, physics has aimed to describe the world as it 'truly' is—independent of our observations or an observer. But quantum physics seems to challenge, or even shatter, that ambition," states the professor of quantum foundations and quantum information theory. In quantum physics, he explains, it is fundamentally impossible to assume that the properties of objects exist prior to measurement—but even when we perform a measurement, we cannot be certain which property will be observed. 

"The decay of atoms, for example, can only be described in the language of quantum physics, which uses the language of probabilities—in other words, we are always and only talking about possibilities when we try to explain what we’re seeing," explains Brukner.

Any attempt to find "the hidden factors" behind these probabilities contradicts some fundamental principles, Brukner adds: "Whenever we assume that there are mechanistic causes behind the probabilities, this leads to theorems that are then disproved by experiments. That suggests to me that our idea of reality, as we know it from everyday life and classical physics, should be loosened." Reality itself, it seems, is fundamentally probabilistic.

How not to understand Schrödinger's cat

There is thus a very bizarre relationship between observation and the observed. Before we bring up Schrödinger's cat, we should first clear up a widespread misunderstanding of this famous thought experiment. 

It is not true that before being measured, the cat—or the atoms—is at the same time both dead (or decayed) and alive (or intact). In many science texts aimed at the general public, one often reads that a light particle can be in two places at the same time. Brukner prefers a more nuanced take on things: "In my opinion, that is an imprecise formulation that I've always tried to avoid." It is better to say that the concrete state is 'neither' one or the other or that it's simply 'undefined'—it does not exist. This "state of non-existence" is referred to by quantum physicists as "superposition."
 

Here's another misconception. Although quantum physics was developed to describe subatomic phenomena, Brukner says there is no proof that it only applies at this scale. "To do so, we need to be able to define the line where the very small ends and the very big begins," yet there is no principled way to define this boundary. "Nothing in the theory of quantum mechanics," he stresses, "prohibits the strangeness of quantum behaviour from occurring at larger scales." This means that the basic principles we believe in apply not just in a special area of physics, but is universally valid.

Quantum physics, for Brukner, is the language of modern physics. "Quantum physicists think of their theories as universal at all scales, so they try to reformulate all other physical theories in the terms of quantum physics, which can then be tested experimentally."
 

From thought experiments to reality: a second revolution in physics

Časlav Brukner works in the field of "quantum foundations." The primary aim, he explains, is to test the fundamental laws of quantum physics themselves. Thought experiments (Gedankenexperiments) are essential tools of the trade—Schrödinger’s cat is a famous one—as they allow us to build up a concrete world from the abstract theorems of the physical world, which can then be tested in real-world experimental set-ups. 

This is exactly what the scientists recognised by the 2022 Nobel Prize in Physics did. Running real-world experiments based on a famous thought experiment developed in 1935 by Einstein and his colleagues, they confirmed that the "hidden factors" we mentioned, which were used to explain away bizarre quantum behaviours, do not exist. These results have now ushered in a "second revolution" of physics with enormous leaps in groundbreaking quantum technologies. 

But these experiments also have a fundamental, existential implication. As Brukner explains, some of our fundamental theorems of how the universe works must go. "My own take," he states, "is that there is something wrong with the notion of realism—the idea that objects have independent properties regardless our observations."

Realism is the problem

"Let’s say you want to observe the colour and shape of an object," Brukner raises his hand, as if holding a big apple, "in classical physics and everyday life, we take it for granted that this apple, for instance, is both red and round, even when we’re not looking." Quantum mechanics tells us that it is impossible to simultaneously observe all properties at once. For instance, when you examine the position of an atom, you can’t also measure its momentum or velocity. 

Raising his other hand to block the imaginary apple, Brukner continues: "one way to think about this, the classical way, is that it’s like trying to observe an apple behind a piece of paper. You can prick a pinhole and see the colour, but then you wouldn’t know the shape. Or you could shine a light from behind and see a shadow of the shape, but then you wouldn’t know the colour." In any case, the apple is both red and round, we’re just limited by our methods. This is the idea of 'realism'—the world exists as is regardless of our observations.

Any conception of reality must include the role of observation

According to quantum mechanics, however, the property you’re not observing literally does not 'exist' before you measure it. It is in a superposition, or an indefinite state. And with that the cat is out of the bag: in Schrödinger’s thought experiment, a cat’s fate is tied to the decay of an atom. If the atom decays, a mechanism poisons the cat. However, since the state of the atom is not only unknown as long as it is not measured, but is actually undecided—in superposition—the life of the cat would also have to be in superposition. Not both dead and alive, but neither.

Over the past decades, this problem has given rise to a large number of interpretations of the question of how observation and reality are related. "You simply cannot talk about properties that you do not measure," concludes Brukner.
 

We can't even have a shared reality

But why should we care about the properties we’re not observing? "They exist or they don’t, why would it matter? If I want to observe these properties, I’ll just make a different measurement." "In fact," grins Brukner as he sets up the next thought experiment, “why not delegate the other observation to a friend?" If both friends are making independent observations, looking at different properties while leaving the other properties in superposition, can we at least combine these observations to fill out a common, complete reality?

"But no," shaking his head, Brukner argues that "quantum mechanics will also show that this scenario is impossible." Analysing the “Wigner’s friend" thought experiment, Brukner surprised even himself: "Because now we are no longer engaging in 'what-if' reasoning to think what another property would be when we're making a measurement—we have both measurements at the same time! Yet quantum mechanics still seems to say that no, you cannot assume that the observed outcomes coexist, even if they are measured separately."
 

Perhaps we can convince ourselves to accept the idea that observations are always relative to how they are observed by us, the subjects. We might have hoped, then, that we could at least combine observations across co-existing subjects to piece together a picture of the world. 

With this thought experiment, argues Brukner, that hope is now dashed.

What would a theory of reality look like in quantum physics?

Quantum physics is one of the most precise theories for describing reality. "However," says Brukner, “we do not fully understand what it means for our idea of reality." Why is this the case? "Quantum theory excels at telling us what is not the case; it does not, however, tell us what is." In other words, while quantum mechanics forces us to rethink our pre-existing idea of reality, it does not actually offer a concrete, alternative account as to how we should think about it instead. 

"So, this is my question for philosophers," says Brukner. "If quantum physics is right, how can we formulate a better theory of reality?" 

Taking this question with us, we now travel across the city to seek out University of Vienna philosopher Anne Sophie Meincke. As she is a metaphysician who also probes the fundamental nature of reality, we ask how she would respond to this intriguing question. 

The problem, Meincke suggests, is that "physics, aiming to describe the world as it 'truly' is, has traditionally assumed that reality is made up of things with intrinsic states and properties that exist independently of our observations or an observer." A rapidly developing area of philosophy, however, rejects this basic premise. It might thus offer us a way forward.

Can philosophy help?

As an expert in the theory of process metaphysics, she describes what reality fundamentally looks like from this philosophical viewpoint. Instead of looking at the world as made up of individual 'things,' "reality is most fundamentally made up of processes, which have the properties they have because they are continually changing and interacting with other processes." 

Process philosophy offers a vision of reality that is ultimately grounded in interactive processes. It is only through interaction, states Meincke, who is also a member of the Young Academy of the Austrian Academy of Science, that "individual processes can distinguish themselves from others for certain periods of time," which results in seemly static but ultimately transient states. Observations and measurements are also interactions, explains Meincke, that intervene in the course of things. 

Is process metaphysics the answer? This preliminary suggestion is still being explored at the cutting-edge of science and philosophy. Quantum mechanics is fundamentally grounded in uncertainty, but we know one thing for sure: the field has generated a wealth of conceptual, theoretical, empirical, and technological outcomes that will require interdisciplinary, collaborative efforts to move forward. 

This article is an expanded and updated version of the original article (in German) published on 23 January 2023 by Daniel Schenz.