Can We Feel What It’s Like to Be Quantum?

Part of the magic of physics is that you can ask really big, wild questions about the meaning of everything, and reduce the answer down to a solid, concrete experiment. What does our sense of ourselves, of existing, have to do with physics? What is really going on in our brains? Can we feel what it’s like to be quantum? After decades (or longer) of asking these questions, researchers may be at the point of finding some answers.

"Consciousness" has a slippery meaning, but it means something like the experience of being aware, aware of existing, aware of…being conscious. "Consciousness, meaning subjective experience or ’what it is like’ to be, used to be a taboo topic in science," says Larissa Albantakis, a computational neuroscientist at the University of Wisconsin-Madison. Traditionally, scientists tended to avoid topics that brought in a personal or subjective ingredient, but, Albantakis says, over the past few decades, more and more have turned toward it. "Ultimately, it should be possible to understand scientifically, why consciousness is present or absent and why specific experiences feel the way they do," she says. "If we don’t, our understanding of nature is incomplete."

These issues seem to sit firmly in the domain of the biological sciences. But within physics, a major puzzle has been how consciousness plays with quantum mechanics, our theory of the very small. The problem is, quantum mechanics has some notoriously fuzzy features, like an uncertainty principle that prevents you knowing everything about a particle at the same time. These seem to put it at odds with our everyday experience of a concrete, definite reality. But for some this bug is a feature: they argue that the fuzziness is exactly what causes the conscious experience.

At the heart of this conundrum is what actually happens to quantum stuff when we do an experiment on it. According to quantum physics, particles have wave-like features, allowing them to be in more than one place at once, at least until you look at them, when they snap into a set location. In physics language, quantum particles are described mathematically by a wavefunction. So, what does the act of making a measurement do to the quantum wavefunction?

Wavefunctions enable physicists to calculate the chances of different results of our measurements—the odds of finding a particle here or there, when we look, say. The theory tells us how the wavefunction, and thus the chances, change over time. Except, not totally: the most basic form of quantum mechanics does not tell us what actually happens to the wavefunction at the moment of a measurement.

The theory does predict that the odds change quickly, maybe instantly, to reflect the impact of a test. Physicists say the wavefunction "collapses" from a state where the particle might be found here OR over there when measured, to one where we have definitely seen it right here. But standard quantum theory doesn’t tell us how that collapse happens or what exactly triggers it. Physicists also do not have a solid grasp of what counts as a measurement, or even whether the collapse takes some time to happen or is totally instantaneous. Although quantum mechanics makes wonderfully accurate, experimentally confirmed predictions about the world, it does not tell us everything about how things happen.

When we step back to think about consciousness, we have to confront even more bizarre questions: What does being quantum feel like? When the wavefunction is in that here-or-there state, can we feel that? And what about when the wavefunction collapses? We don’t seem to experience that kind of jolt in our everyday lives—or do we? Perhaps that jolt is exactly what consciousness is itself.

Collapse to Consciousness

In the 1990s, physicist Roger Penrose and anesthesiologist Stuart Hameroff proposed that wavefunction collapse was a central part of consciousness. The theory identified structures called microtubules within the brain’s neurons, as possibly having the right size and shape to allow the growth of the here-or-there kind of quantum state. They predicted that these states would collapse under the influence of the brain’s complexity. Their Orchestrated Objective Reduction theory, or Orch OR, is controversial. But it is perhaps the only widely known theory that connects consciousness and quantum mechanics; and, a key benefit of the idea is that it is potentially testable in a lab.

The testability of Orch OR grew out of work that offered details of how and why a wavefunction collapses. "Collapse theories" developed largely in the 1980s by several researchers, including American physicist Philip Pearle, a team of Italian physicists Giancarlo Ghirardi, Alberto Rimini and Tullio Weber, and Lajos Diósi, now an emeritus researcher at the Wigner Research Center for Physics in Budapest, Hungary. They propose that collapse is spontaneous and random, but the chances of collapse can depend on details of the quantum system or what is happening around it. Typically, as the size or complexity of the system and its connections go up, the chances of collapse increase. There are many possible ways to imagine how those odds of collapse could evolve, but because the theory adds to the basic quantum laws, they lead to testable effects.

Diósi, in particular, studied a type of collapse theory that would later show up in the Orch OR idea. This version connected the odds of collapse to gravity and the amount of mass in a system. As the amount of mass goes up, the idea goes, the uncertainty of the location in space of all the mass increases, and so the chances of wavefunction collapse are higher. This model can explain why we don’t see obvious quantum effects in large, heavy objects. Penrose approached this idea from a different angle a few years later, and physicists now call the theory the Diósi-Penrose model. Penrose then used this theory as the centerpiece of his consciousness plan, connecting the complexity and mass of the brain to the collapse.

Diósi will now explore the connection of collapse and consciousness through a grant of $78,000 from FQXi that teams him with experimental physicist Catalina Curceanu, at INFN in Frascati, Italy, and theoretical physicist Maaneli Derakhshani, at Rutgers University, in New Jersey. Curceanu’s interest in the connections between collapse models and consciousness stems from a conversation with Penrose over a decade ago. She realized that her expertise enabled her to put his idea to the test, and connected with Diósi. The team has a clear division of labor: Diósi and Derakhshani can focus on the theory, while Curceanu and her team of 10 researchers can focus on the experimental tests.

The key to the team’s test of collapse theories is that, in theory, spontaneous collapses should affect the way particles move. The impact would nudge particles in a seemingly random way, and this nudging should then cause the particles to emit extra radiation beyond that predicted by basic quantum mechanics.

In Curceanu’s lab, located underground beneath the Gran Sasso mountain in Italy, highly sensitive detectors sit patiently around a simple block of germanium, watching for this spontaneous radiation. So far, results from the lab look just like you would expect from quantum mechanics without the extra collapse rules. In 2020, this allowed them to rule out the simplest version of the Diósi-Penrose model (Donadi, S. et al.Nat. Phys. 17, 74–78 (2021)).

But the team’s negative results do not yet spell the end of collapse theories. Different ways the collapse could happen should lead to smaller, harder to detect signals. Diósi and Derakhshani are busy looking for more complicated variations to the theory that have not been ruled out by the experimental tests so far, and still predict some detectable sign that might be discovered in the future. The team are also considering how best to analyse experimental data to avoid missing such potentially small signals.

Diósi says that they have already made huge progress just by doing their first experiments. "It is a miracle that after hopeless decades, we became able to test the theory," he says, referring both to Curceanu’s experiment and the growing number of independent tests of collapse theories now being done by groups around the world.

Derakhshani brings a philosophical tilt to the project. He is not invested in any particular model, and wants to investigate if the Orch OR theory really makes sense when it comes to describing consciousness as we experience it. "How does this help explain the emergence of consciousness?" he says. "That’s not entirely clear."

Complicated Brains

There’s another way to probe whether Orch OR is feasible, without thinking directly about consciousness. Physicists can ask whether the brain can sustain the necessary mix of quantum effects and collapse suggested by Orch OR—an approach being taken by Angelo Bassi, a physicist at the University of Trieste in Italy, who has been awarded an FQXi grant of almost $75,000. Physicists working in the lab know that quantum states are fragile and easily disrupted, even in pristine isolated conditions. A brain, by contrast, is a very complicated place with lots of particles, after all—a potentially tough environment for a quantum state to survive. "The essence of collapse models is that if you have a few particles, nothing happens: it’s quantum," says Bassi. "But when you have enough particles interacting with each other, these collapses become strong enough."

Bassi’s interest in collapse theories began as a student when he interacted with Ghirardi. "I was lucky that I was exposed to the thing that really interested me," Bassi says. Like Curceanu, the attraction was the theory’s testability and the way collapse models "bring together these two worlds of theoretical and experimental physics," he says.

Bassi and his team of theorists will study models that contain just a few atoms. This is not as many as a full brain but enough to start to see how collapses behave in a complicated system. They will investigate, for instance, whether collapse moves sequentially down a line of particles. Their models do not specifically involve gravity as the trigger for collapse, the way the Diósi-Penrose theory does, but, since their models should also generate detectable signatures, their results will be impacted by what Curceanu’s team finds in the lab.

"Wavefunction collapse theories are an elegant, mathematically consistent way of explaining quantum measurements as a physical process," says Stephen Adler, a physicist at the Institute for Advanced Study in Princeton, New Jersey, who has studied collapse models for many years (but is not involved with these FQXi-supported projects). "Experimental physicists are now rising to the challenge," trying to verify the tell-tale signs of the theories, he says, adding: "This is a task of utmost importance for our understanding of quantum mechanics." However, Adler cautions that even if tests turn up signs that look like a collapse theory at work, the question might remain how to definitively pin it on collapse theories, versus other mechanisms.

The negative results coming from Curceanu’s lab, showing no sign as yet to support collapse theories, do not discourage Bassi. He notes that more time is needed to refine the tests to search for more subtle signals predicted by more complicated collapse theories. In a recent paper for Nature Physics, he reviewed the status of tests for collapse models (Carlesso, M. et al. Nat. Phys. 18, 243–250 (2022)). "I know it will take longer," Bassi says. "It needs patience, a lot of patience."

"The fact that we are struggling for 100 years to try to understand the basic features, or basic whys of quantum mechanics, might point to something which is still to be uncovered," adds Curcenau. She feels inspired by the test so far. "This is by itself somehow extremely, extremely exciting to be part of this adventure," she says. "It’s really a lot of fun."