Most physicists operate under the assumption that there is a world out there that is entirely independent of us, an objective reality in which more-or-less well-defined things behave according to immutable physical laws. Yet over the past century, ever since the development of quantum theory, there have been discombobulating questions about the role of observers – not least ourselves – in the makings of reality.
These questions are often brushed under the carpet, but , a theorist at the Complutense University of Madrid, prefers to confront them. Arguably, he has been pushed to do so by his work on one of the foremost challenges in modern physics: creating a quantum theory of gravity. The difficulty here is reconciling the inherently smooth picture of space-time in general relativity with quantum theory, which is written in contradictory mathematical language. Getting the two to play nicely together has forced Oriti to think deeply about the subtleties of physical laws – not least the fact that space-time is a shaky foundation on which to build them. His verdict? That physical laws can’t exist independently of us, as something that we can all agree on, but instead reside within us somehow.
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Oriti spoke to 91av about how he came to such a startling conclusion, why physicists need to be more aware of the complex relations between the world, scientific models and observers, and how appreciating the true nature of physical laws might yield fresh breakthroughs.
Thomas Lewton: What do people get wrong about the nature of reality?
Daniele Oriti: At the risk of seeming provocative, most scientists – and anybody who hasn’t really thought about the issue – maintain a position that philosophers call “naive realism”. This is the idea that there is a world out there that is entirely independent from us, not just in its existence but also in its properties: independent from the minds apprehending it, or from our theories about it. It’s made of things that are similarly independently defined, with intrinsic properties, which follow patterns that are also independent of us – even if we may not know about them.
I have been guilty of that position too. As a physics student, you want to understand the world. You build models, you revise them and you think that you are getting closer to the actual story. That’s the picture, and it’s very naive.
Why is that kind of thinking so naive?
First of all, it is naive on conceptual, philosophical grounds. But I would also say that modern science, particularly quantum mechanics, blurs this picture. One of the main lessons of quantum mechanics is that the distinction between us and the world isn’t really there, not sharply anyway. It tells us the properties of a particle are encoded in a probabilistic entity we call a wave function, which tells us the likelihood of it appearing here or there, for instance – but that the particle cannot be attributed definite properties until it is observed. It’s screaming at us that observers really matter.
How should we think about these observers?
There are many different interpretations of these experimental findings from quantum mechanics, which can be divided into two broad classes. In one set of interpretations, the quantum state, or the wave function, is “ontic”, meaning it is a real property, such as a particle’s position or velocity. In these interpretations, one can maintain the simple notion of realism, and just add quantum states to the intrinsic properties of systems.
Another class of interpretation says that the wave function isn’t a property of the system being studied, but rather a property that the observer – a second system – attributes to it. It is a way of accounting for the knowledge or the belief that the second system has about the first. In this case, the wave function is said to be “epistemic”.
Some physicists take this epistemic route, but maintain that there is still some hidden reality of the standard kind, it is just that quantum states do not capture it. However, a number of epistemic interpretations suggest something more radical: there is no hidden reality. This means that quantum states are all there is and reality can only be defined in the relations between observers and what they observe.
Perhaps the most extreme example is QBism, which holds that the wave function is merely a tool we use to organise our uncertainty about the world. But there are other interpretations, like relational quantum mechanics, which says that the core of reality lies not in objects themselves, but in their interactions.
How has your research into quantum gravity changed how you think about physical laws?
It has made it more complicated to characterise what laws are, even assuming they are somehow “out there”, independently of us.
One basic philosophical notion of laws starts with the idea that there are facts out there in the world and there are patterns among these facts. Physical laws go beyond patterns, they are said to “supervene” on facts – but both facts and laws are considered real.
The problem is that space and time are crucial for first identifying and then gluing together these facts about the world. If you check the main proposals for what laws are from philosophers, they all rely on space and time to some extent. But quantum gravity strongly suggests are not actually fundamental. Instead, we suppose that space-time emerges only in some approximation, rather than being built in at the fundamental level. How does that impact our understanding of laws? Well, we have to replace space and time with an alternative grounding for laws.
What do you mean when you say that space-time emerges?
We don’t yet have a working theory of quantum gravity, but every approach so far tells us that classical space-time – which functions as a gravitational field and is smooth and has a definite geometry – is not going to be there. That’s because the minimal step we have to take is to turn these classical fields into quantum fields, which means they have quantum properties, like superpositions and quantum fluctuations and so on. So already, you cannot rely on a space-time with a clear geometry and causal properties. Some approaches to quantum gravity go further and replace the classical fields with something else.
What could replace something as seemingly fundamental as space and time?
Modern approaches to quantum gravity say that space-time emerges from something deeper – and this could offer a new foundation for physical laws. These approaches suggest that space-time in general relativity isn’t just replaced by quantum fields, but they are themselves approximate notions to something that is more fundamental – some other conceptual ingredients that we are yet to identify. They’re suggesting that there is a microstructure beneath space-time.
What would this microstructure be like?
I work with quantum gravity models in which you don’t start with a space-time geometry, but from more abstract “atomic” objects described in purely mathematical language. I treat the universe as a peculiar quantum system made out of these building blocks and I apply techniques and concepts from condensed matter physics, which is typically concerned with liquids and solid matter. If you think of the universe like this, as if it were a strange kind of fluid, you can show that its usual space-time description can emerge in the coarsest approximation from those non-material microscopic “atoms”.
We may also need another ingredient in order to make this foundation solid enough to support laws of nature. One possibility here is that a key role is played by quantum entanglement, which is a subtle connection between two quantum objects. There is a suggestion that the classical geometry of space-time can be traded for the relations between those entangled “atoms” of space-time. There are interesting theoretical results where people relate measures of entanglement to the geometric properties of the same systems. For example, you compute the degree of entanglement between two separate quantum systems across a range of distances and you find that it scales with the distance. Then people reverse the usual thinking and say, well, instead of distance defining entanglement, perhaps the amount of entanglement defines the distance. That can provide a primitive notion of relations in space because it can be applied before the usual space-time emerges.
The notion of objective laws is like the scientific version of God
But if we can find an alternative background on which to build laws, doesn’t this mean that physical laws do exist after all?
Yes, these approaches to quantum gravity have been developed without questioning the reality of laws. And they help us to keep some of the traditional, ontic view of physical laws. As for me, though, I’m still inclined to think that physical laws are really epistemic in nature, so something that exists primarily in our minds.
That’s because, from a philosophical standpoint, what we identify as a “law of nature” always has some component of our models of the world, selected because of some epistemic virtue we favour. The further claim that the law is somehow “out there” seems gratuitous to me. And, as I’ve already said, I think quantum mechanics challenges the idea of a separation between the “world” and “us”.
So what would reality look like without any physical laws?
The picture I have in mind is like an infinitely complex, amorphous reality to which we give shape and meaning by virtue of the models we construct. In a sense, models and laws are in our head, but there isn’t such a sharp distinction between our thoughts and reality. Reality is made by our models, not just described by them. The models we create, the concepts, mathematics and language we use – these all depend on our interactions with the world, our cognitive structures, our history. The same is true in broad terms for any agent making sense of the external world. My cat, for example.
Your cat?
Of course, his cognitive structures and sensory experiences are different, as is the way he makes sense of observations. So the models of reality he constructs are not like mine. Therefore, from an epistemic perspective, the laws of nature, according to him, are different.
What are we supposed to do then, if cats and people and other agents, whatever they are, have differing perceptions of reality?
In the naive realist picture, that is totally trivial: there is just the world out there and models that get closer to it. But the moment we add the complex relations between models and the world, and we realise that what is real is somehow at the interface between the two, then it’s much less trivial.
The key issue – and it’s difficult – is to reflect upon the relations between the different, equally valid perspectives of different epistemic agents. What makes their perspectives somehow coherent with one another? Well, we communicate. We aren’t just epistemic agents, but also social epistemic agents. Social agents construct a world that is dependent on the interactions between agents. The best models in science are not out of the blue or simply dictated by nature. They are agreed upon by the community of epistemic agents we call scientists, considering all the other constraints coming from observations, logic and so on.
We have to embrace the fact that we make reality
What does this mean for the idea of there being an objective world out there?
It means that the strongest possible form of objectivity is naive. So what is true or false, what is real or not, cannot be perspective-independent. The best we can hope for is that there is a way to translate from the one perspective to another, and that, for any two observers, the translation doesn’t leave anything behind.
So it’s not that we necessarily agree on everything. But I can perfectly translate from my perspective to yours. I can put myself in your shoes and know precisely and comprehensively what you see of the universe. I have to confess that I’m not even convinced that this is actually something that we can achieve exactly.
How does all this change our conception of what a physical theory is in relation to reality?
It forces us to think more seriously about the relation between our theories and reality. And it suggests a direction for research in which we revise what we mean by what is real, where these epistemic components are far more relevant. To a certain extent, it also gives us much more responsibility. The notion of objective, exact laws governing the world is the scientific version of God. Without it, we are like a ship in a storm; we have to navigate this world alone.
In the old notion of reality, we have no responsibility for what things are – we just happen to be here, we don’t participate in the world. I would say that the epistemic perspective forces a more participatory understanding of what is real, with many more philosophical responsibilities as scientists to reflect on what we do. But it’s also much more exciting, because we are not just witnessing something we have no role in. We have to embrace the fact that we make reality.
