Cause and effect may actually rule in the quantum realm, according to a fresh theoretical approach

There’s something about a provocative unproven theory in physics that appeals to me much more than a provocative unproven theory in religion, mysticism, and spirituality in general.

Probably it’s because physicists approach a deeper understanding of reality from a solid factual foundation, while that isn’t the case with religion, mysticism, and spirituality in general — unless the new idea there is completely based on natural, rather than supernatural, notions.

So when the November 29-December 5 issue of New Scientist appeared in my mailbox with the cover blaring The Quantum Leap — A surprising twist on cause and effect reveals how everything comes from nothing, I started reading that article even though I had several unread previous issues piled up.

I don’t claim to grasp all that physicist Ciarán Gilligan-Lee says in “A new understanding of causality could fix quantum theory’s fatal flaw.” Here’s a PDF file:
A new understanding of causality could fix quantum theory’s fatal flaw | New Scientist

The easiest part of his article to grasp is the beginning, where Gilligan-Lee discusses the fatal flaw. Basically, this is the traditional view of quantum theory as dealing with probabilities at the atomic and subatomic level that only become a single actuality when a measurement occurs. So how is it that our familiar world of cause and effect is underlaid by a quantum realm where probabilities hold sway?

The ball rolls across the floor because it was kicked, just as Earth orbits the sun because it is tugged by gravity. The connection between cause and effect is fundamental to how we understand the world – or at least, it is for the world we see, governed by classical physics.

Notoriously, everything gets murkier in the underlying realm of atoms and subatomic particles described by quantum theory. But, as a fundamental physicist who develops algorithms to extract cause and effect from correlations, I have long believed that causality could help us solve the mystery at the heart of quantum mechanics: the confounding notion that quantum systems like electrons exist in a state of uncertainty until an observer measures them.

This is why I am intrigued by a fresh attempt to rid quantum theory of this so-called observer problem. Building on insights from existing interpretations and recently developed models of quantum causality, this new take uses the precise mathematics of cause and effect to show how interactions between and within quantum systems can determine which of the many possible ways they could change over time actually happen, without any reference to the mysterious power of observers.

What it amounts to is a quietly radical rethink of reality. In this view, quantum causality is the most essential aspect of reality from which the cosmos springs. Remarkably, this view seems to resolve several quantum paradoxesin one fell swoop. Perhaps the biggest indication that it might be on the right track is that it could also provide a plausible route to the long-sought holy grail of physics: a theory that unifies quantum theory with Albert Einstein’s theory of general relativity.

Quantum mechanics is, without doubt, one of our most successful scientific theories. It describes the often counterintuitive behaviour of subatomic particles with incredible accuracy, precisely predicting the outcomes of countless experiments. It is also the source of endless confusion, however, because ever since it was first devised, it has resisted attempts to make sense of what it means for our understanding of reality.

Our frustrations boil down to the vagueness of the concept of “measurement” at the heart of the theory. Until we measure a particle, its properties are best described by the Schrödinger equation and its famous “wave function”, which paints those properties as a kind of fog of possible alternatives. This allows us to calculate the odds on which of them we will see when a particle is measured. But it can’t tell us the outcome of any measurement. Until we make one, all we have is probabilities.

The upshot is that the observer who performs the measurement is all-important. The gnawing problem is that it isn’t at all clear what qualifies as an observer. With no precise definition, quantum theory offers no answer to the key question of how and why the world we see – where particles have definite properties – emerges from the quantum fog.

That’s why many physicists view quantum theory as it is typically understood to be deeply unsatisfying. “The current situation with quantum mechanics is that it’s a theory that predicts very well and explains very badly,” says Nick Ormrod at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. We can’t just fall back on the phrase “because we measure it”, he says, particularly as many suspect that the vagueness of quantum theory is a big part of why physicists struggle to apply it in contexts where no observers are present, such as the very early universe or the fabric of space-time.

Quantum causality is an attempt to reframe quantum theory so the measurement problem and apparent role of consciousness in making an observation go away. This possible solution is considerably more difficult to grasp than Gilligan-Lee’s statement of the problem. It appeals to me, even as I fail to grasp more than the general tenor of it. What I like about it is how it allows quantum theory to be founded on the familiar notion of cause and effect rather than viewing goings-on in the quantum realm as being a mysterious interloper in our customary view of how reality operates.

Which is where cause and effect comes in. Now, you might think it seems odd to apply classical notions of how things influence each other to the quantum world, which doesn’t play by the same rules. But to me, and to some other physicists, it has always made sense given that causal reasoning possesses extraordinary explanatory power. “You can’t do physics without using cause and effect,” says Robin Lorenz, a researcher in causality and quantum computing at Quantinuum. “Causality is the bread and butter of the sciences.” What’s more – and this is vital – these days, we have a better understanding of how cause and effect operate in the quantum regime.

What Ormrod and Barrett realised is that we can marry the tantalising insights from the consistent histories interpretation and relational quantum mechanics, then overcome their flaws by underpinning them with recently developed models of quantum causality – and by elevating those causal structures to fundamental status.

In a paper released in 2024, they showed that if we consider quantum systems as a network of “causal bubbles” with specific mathematical rules for how subsystems within a bubble influence one another, the “correct” sequence of properties a given bubble has or had over time naturally emerges. In other words, the causal structure of the system determines how it evolves – that is, what properties it has at any given moment – in a way that matches what we would predict with standard quantum theory, but without needing to appeal to the mysterious powers of external observers. “By analysing a quantum causal structure, you can always derive a unique set of consistent histories,” says Ormrod.

Truly grasping the appeal of this model requires some advanced mathematical skills. To get a sense of what it amounts to, however, Ormrod suggests thinking of a spider’s web. The spider doesn’t begin with a set of points and connect them with threads. Rather, it begins with the threads, laying them down one after another – and where they interact, points form.

The key thing is that points in the spider’s web aren’t fundamental. They are by-products of how the threads are woven. “The points only exist because of the pieces of thread,” says Ormrod. “They emerge from the threads. The threads are actually the conceptually fundamental thing.”

In the same way, Ormrod and Barrett suggest, causation is the fundamental “thread” from which quantum reality emerges. The properties of particles are the points, the places where causal influences interact. But the causal structure – the threads – comes first. The properties of a quantum system – what we might call reality – emerge from causality, rather than from the mysterious and ill-defined process of measurement.

The spider web analogy, I can grasp. If this theory is correct, and we’re a long way from that happening, it fits with the pleasing perspective of an interconnected cosmos where everything is woven together and particles are just the knots or places where threads intersect. That’s still really difficult to comprehend, but at least it makes quantum theory part of the cause and effect patterns so familiar to us in everyday life.