We can be conscious of more than one concept at the same time. You are holding multiple words in your mind while reading this sentence. It is so obvious, it hardly seems worth explaining. However, there are only two routes to an explanation and classical physics fails at both. The first is that this ‘simultaneity’ exists in experience only, with its information content mapping to a chain of physical activity over time. But the core equations of classical physics are Markovian: they depend only on current states. The second is that ‘phenomenal simultaneity’ maps to ‘physical simultaneity’ operating on data distributed across space. But classical physics permits only local interactions that take time to propagate. Quantum physics rejects the first route, but provides options under the second. Candidate mechanisms – from either classical or quantum physics – can be explored empirically.
1. Introduction
1.1. Classical mechanics can likely account for complex cognition
Reading this essay involves many complicated acts: analysing the pixels of your visual input, processing language, and reasoning, all while your brain continuously monitors itself and its environment to keep everything working.
For centuries, this incredible cognitive complexity convinced many that the brain could not simply be a machine in a traditional sense. No matter how many cogs and levers you put together – or how sophisticated the mechanical rule-sets by which they interact – a machine would not be able to read and reason about this essay.
This once-plausible story is no longer true. Explaining these acts in full is a live challenge, but the progress in artificial neural networks in computational neuroscience and large language models makes it likely that the mechanisms of classical physics suffice for an answer. Of course, the human system might use nonclassical mechanisms in practice, but we should expect that functionally equivalent classical alternatives are possible.
1.2. 'Simultaneous phenomenal complexity' is something different
However, reading this essay also involves something else. Something so simple we typically ignore it day to day. Something that we might naturally assume just comes along with cognitive complexity, with no further explanation required. As you read, you hold multiple words and multiple concepts in your mind at the same time. As you look at the screen, there are multiple shapes present simultaneously in your field of vision. You could not even perceive motion or any kind of change at all unless you were able to compare two values: which demands that the two be considered together.
The fact that you can observe yourself observing multiple objects/concepts is itself an observation – and observations demand an explanation. Let's call this observation 'simultaneous phenomenal complexity', where phenomenal refers to the nature of mental experiences in philosophical jargon and simultaneous complexity refers to the fact that those experiences can contain multiple units of information at what feels like the same time.
You might instinctively feel that explaining this observation is straightforward. The purpose of this essay is to challenge that instinct.
Before the formal argument, consider how mechanical logic exploited in modern AI can 'cognitively' read this essay without necessitating any simultaneous complexity. Put aside questions of whether machines can be conscious or not, this is simply about sequential vs simultaneous information processing.
1.3. Classical vs quantum physics
The frameworks of classical and quantum physics are often formally distinguished by the mathematical principles governing their equations [a; see endnotes]. In plain language, these principles mean that every measurable property in classical physics - like position, velocity, or energy - has a definite, knowable value at any given moment; all these properties together form one complete description of the system. In quantum physics, this is no longer the case: not all measurable properties can have simultaneous definite values [1; see references].
Our working characterisation is based on specifying which equations exemplify each framework.
The primary fundamental equations of contemporary classical physics include Maxwell’s equations for electromagnetism, Einstein’s equations for relativity, and the broader framework of classical field theory. These equations generalise the intuitions of earlier theories, such as Newtonian mechanics, in which forces push on objects. Large-scale systems can also be described by thermodynamics and statistical mechanics, but the underlying deterministic field equations are treated as the ground truth.
Meanwhile, for quantum physics, the leading approach is Quantum Field Theory. Its key equations - such as the Klein-Gordon or Dirac equations, or other relativistic generalisations of the Schrödinger equation - govern how quantum states, represented by wave functions or fields, evolve in time and space. In quantum physics these local and deterministic evolution equations are complemented by an indeterministic ‘measurement’ dynamic when a quantum system changes suddenly and holistically into a new state. This is a fundamental difference between classical and quantum evolution. The nature of these measurement events remains unresolved in physics, but every interpretation requires them in some form, as the point when the state evolving continuously according to a Schrödinger-like equation is transformed into a probability amplitude and squared to produce meaningful experimental predictions.
The core equations of classical physics can be derived as approximations or emergent limits of quantum physics, regarded therefore as the deeper underlying framework of reality. There are no classical analogues of the irreducible holism and indeterminacy in quantum physics. These result in nonclassical phenomena, such as superconductivity, in which electrons coordinate to pass through solid metal with zero resistance. Nonetheless, a full quantum theory that consistently includes gravity remains out of reach, so both the classical and quantum frameworks are known to be incomplete.
1.4. Essay structure
Simultaneous complexity must either be an illusion or not; the physical phenomena it corresponds either are not themselves simultaneous or they are. Section two shows that the equations of classical physics cannot support either route, because they depend only on the current state (appropriately defined) and permit only local interactions that take time to propagate. The former prevents any grounding via a historical chain across time. The latter prevents any simultaneous information connection across space.
Section three explores candidate solutions via quantum physics: particle entanglement, wave function collapse, and the vacuum entanglement of quantum field theory. While more work is needed, these theories have potential to identify specific ‘phenomenal binding mechanisms’ that provide the glue for simultaneous complexity. Section four discusses how empirical research can be brought to bear on candidate mechanisms, with gold standard tests viable within a few decades – or faster with the right investment and attention. We conclude by considering alternative solutions outside the laws of either classical or quantum physics.
2. Classical physics fails two routes to ‘simultaneous phenomenal complexity’
We want to explain how you can experience multiple units worth of information at the same time. There are two routes for doing so: the first allows the physical basis for that information to be non-simultaneous, while the second sets the physical basis to be simultaneous as well.
2.1. ‘Simultaneous phenomenal complexity’ without physical simultaneity
The first route suggests the impression of simultaneous complexity is an illusion.
The multiple units perceived as simultaneous are in fact distributed across physical acts that are not simultaneous. The classical example in neural networks is to think of the information encoded in a neuron activating (“1”) or not activating (“0”) at a given point in time. Illusory simultaneity occurs via a chain of neuronal activations, perhaps a temporarily self-perpetuating loop. As each neuron activates in turn, an additional binary unit of information is added into the overall experience. In line with standard neuroscience, the complexity we want to explain is more than can be accounted for by a single neuron and its immediate interactions: complex neural ensembles of multiple neurons are required [2].
For classical physics to explain this illusory simultaneity, we need an equation in which entities in the present state are directly defined by multiple states at different points in the past. None of the fundamental equations do this.
In the intuitions of classical mechanics, all past states feed only their immediate successor states. Something from two minutes ago does not ‘reach through time’ to directly influence the present. Rather, each moment simply shapes its successor, which shapes the next and so on. At most, you only need the prior state from an infinitesimal moment ago: no good for capturing multiple points across the past – and certainly not a loop of neurons activating over multiple milliseconds. Moreover, contemporary classical physics does not even require that reliance on the immediate past. Properly formulated, the current state includes any necessary time derivative values, based only on the time-stamp in the current state. We have to specify a particle’s velocity, i.e. its derivative of location with respect to time, but that value can be wholly specified in the current state [b].
The current state is fully defined by values in the current state alone. Future states are determined by those values and the equations that govern their evolution. Classical physics provides no fundamental equations that define a present state based on values from multiple states in the past. Non-Markovian equations emerge only as coarse-grained simplifications. Such references to past events are only an ‘epistemic’ form of memory, where we, as modelers, need to keep track of history because we’ve averaged out microstates. There is no true ontological memory, where the universe itself remembers past states.
Perhaps we could construct a new equation with the desired structure. It would need careful fine-tuning, to prevent the natural causal regress in neuronal activations which would connect conscious with unconscious information processing, combining them into a single experience. Either way, it lies outside of classical physics.
2.2. ‘Simultaneous phenomenal complexity’ via physical simultaneity
The second route looks for informational complexity within the current state, rather than by chaining complexity across a succession of previous states. A classic example from neuroscience would be the idea of synchronous firing of neurons [3]. If a set of neurons all fire at the same time (or fire repeatedly in phase), we assume their information content is experienced simultaneously [c].
Initially, this feels more promising. The equations of classical physics do describe a rich current state. There is more to work with.
Unfortunately, all the fundamental equations of classical physics have something in common. All causal interactions are local in spacetime. The only truly nonlocal interactions in the classical canon were the early Newtonian and Coulomb ‘action-at-a-distance’ laws, now reinterpreted as low-speed approximations of deeper field theories which do preserve locality.
What does it mean that all interactions are local in spacetime? It means that the impact of any distant event on a target particle must be transmitted via consecutive local entities before reaching the target particle. Each particle en route only knows about its immediate interactions. Crucially, each step also takes some finite, non-zero time to propagate. The exact speed and time interval will depend on an observer’s frame of reference, but is non-zero for any target system being causally affected [4].
The result is that no individual particle/entity in the current state interacts instantly with any other. If some of those particles carry micro-units of information, any interactions they have with each other to ‘pool their information’ take time. There is no solution here [d].
3. Potential options via nonclassical physics
Quantum physics remains a work in progress, with known gaps and considerable controversy over ontological interpretations of its behavioural equations [5]. Nonetheless, working with current knowledge, we can rule out the ‘illusory’ route here as well: quantum physics equations also depend only on the current state. Options are present, however, under the second route: particle entanglement, wave function collapse, and vacuum entanglement are outlined here. Nonetheless, there remains much to learn: perhaps none of these options work and deeper theory is required.
3.1. Particle entanglement
In entangled states of matter, there are correlations across particles such that we need a joint wave function to describe their joint evolution [6]. In this sense, information about individual particle properties is distributed across whole system. The holism is irreducible. If one attempts to break the system into locally interacting subsystems, the theory no longer matches experimental results. Irreducible nonlocality implies spatially extended but ontologically unified physical states. If it is irreducible as a theoretical object, one viable interpretation is taking that object as a single physical whole. In other words, perhaps entangled particles are no longer individual particles in a complex relationship, but rather a single entity.
By encompassing multiple units of information within itself, we might be able to ground simultaneous complexity from the entity’s perspective as a fundamental object. Atai Barkai has developed a theory of consciousness where entangled states correspond to ‘qualia clusters’, as a solution for phenomenal binding [7].
This perspective relies on the contestable interpretation of the entangled system as a single entity, without which it is unclear that any kind of instantaneous information exchange is possible within the system. Experimental work is also warranted to identify the quantum biology at play within the decoherence timescales for different parts of the brain, as well as theoretical work to identify the single cluster corresponding to our ordinary ‘unitary’ self.
3.2. Wave function collapse
If we doubt the ‘single entity’ perspective on entangled states or want clearer interactions between spatially distributed particles, then we might build on the entangled system concept of §3.1, but look to the point where it collapses into a state representing a single determinate outcome. Following a sufficient measurement or environment interaction, the information across these spatially distributed particles is jointly operated upon to define the new state.
Michael Wiest applies this solution to phenomenal binding for the Orch-OR theory of consciousness, based on coordinated quantum activity in neuron microtubules [8]. He argues this quantum model can confer evolutionary advantages in a way that is not open to classical models, despite a sense in which the model might appear to have no physical effects.
One challenge for this approach is whether phenomenal simultaneity is possible without formal information exchange, given that classical signalling between entangled subsystems is ruled out by the No Communication Theorem. However, one viable interpretation describes an ‘instantaneous mutual influence’ between entangled parts of a system when any one of them is subjected to a measurement, even when this provides no faster-than-light insights for third party observers. There also remains uncertainty over the nature of the ‘collapse’ event and what types of collapse might be sufficient to support phenomenal simultaneity.
3.3. Vacuum entanglement of quantum field theory
Quantum field theory has a deeper view of entanglement than the particle entanglement of §3.1, which exists even in non-relativistic quantum mechanics. This view describes the universal vacuum state – i.e. a state even without any particles – as an entangled state across all of space [9]. This vacuum entanglement persists when particles are added: excitations creating additional, structured correlations on top of the ever-present background entanglement. Fields at different points in space manifest nonclassical, nonlocal correlations. The correlations decay rapidly with spatial separation – but never decay to zero. The vacuum’s correlations are a continuous, omnipresent version of entanglement.
Perhaps the ontological unity of the universal state can be drawn on to provide phenomenal unity from the perspective of the state, similar to the ideas in §3.1 [e]. The universal state still needs to be ‘carved’ up into human-sized chunks of phenomenal simultaneity (as well as any other conscious entities’ experiences), but there are avenues to pursue, such as analysing the existence of separate conscious subjects in terms of the topological structure of entanglement or interpreting the conscious inaccessibility of distant galaxies (despite ubiquitous entanglement) in terms of the relevance of the background of nonlocal connections to a particular local organism.
Alternatively, perhaps some coordinated interaction with the universal state might drive phenomenal simultaneity, similar to the collapse ideas in §3.2. Joachim Keppler explores a related option in his theory of consciousness invoking neuronal microcolumns coupling with phase modes in the zero point field, being the universal vacuum state corresponding to quantised electromagnetic fields in Quantum Field Theory [10].
4.Empirical testing of candidate mechanisms
We have seen theories of consciousness taking the ideas in §3 and begun defining solutions to simultaneous phenomenal complexity, i.e. candidate ‘phenomenal binding mechanisms’. As these candidate binding mechanisms are developed further, it becomes possible to test their predictions empirically. For those unconvinced by this essay, various mechanisms grounded in classical physics can also be tested, such as the neuronal loop method of §2.1 and the synchronised firing method of §2.2.
As with all empirical research on consciousness, we are most well grounded when drawing on human self-report – or on no-report indicators that have been mapped to self-report insights in reliable settings [11].
The challenge is that self-report can be deceptive, particularly if we start to manipulate or augment the human brain outside of its natural state [12]. For instance, we might start to connect up various computer modules to the brain or replace the corpus callosum with a mechanical signalling device. In such experiments, even if a human voice still reports ordinary consciousness, we have to interpret these reports with increasing uncertainty. Maybe we lost something crucial in the experience, but the function of language still operates on sensory inputs, disconnected from any supervening subjectivity.
In the extreme case, an input/output look-up table could capture every behaviour a human might do within a restricted setting, including self-report of conscious experiences. Such look-up tables start to blur the lines with simple recorded speech or mechanical robots declaring consciousness: typically not taken as strong evidence of consciousness. For instance, GPT-4.5 was able to pass a five minute webchat conversation Turing test [13] – in that humans identified it as human more often than actual humans – and yet most experts suspect that GPT-4.5 is not conscious [14], and even if it is, a Turing test is only weak evidence either way.
With that caveat in place, working with self-report in naturalised settings is still our best empirical approach. We’ll provide an outline of the gold standard experiment to aim for, emphasising that many indirect experiments may be possible in the meantime, exploiting specific techniques or restricted settings.
Indirect experiments are particularly important to pursue, because the gold standard requires technological progress before we are ready to run it. Analysis by Igarashi suggests 20-40 years might get us to sufficient neuron-level measurement/simulation for the whole human brain, with deeper mapping likely to require further decades of progress [15]. But recent progress has been incredible: from mapping 300-400 neurons in a nematode worm [16] in the 1980s through to 200,000 cells in the mouse cortex [17] in the 2020s. The next decade is forecast to be even more remarkable, especially if society makes it a priority and AI delivers on its promise [18].
The gold standard approach leverages a combination of direct brain measurement and simulation to assemble a large database of what information is being consciously perceived in different moments alongside what the brain is doing in each moment, measured across many individuals in diverse settings. We then calculate which different units of information are proposed to be integrated together in each moment, according to each candidate binding mechanism (differentiating multiple experiencing entities where needed).
Some mechanisms can be captured in neuronal maps alone and will likely be viable for research sooner. Others will require the analysis of quantum activity within the brain. However, the latter is not necessarily more complicated than a full synaptic connectome. Where sophisticated quantum states are required, simulations might demonstrate that such states can only persist across certain small distances and time intervals in specific regions of the brain, allowing simulations to narrow in on these regions.
Once we have the database, it is the challenging matter of comparing predictions with self-reports. Even where exact content proves hard to map, we may be able to correlate approximate levels of complexity or type of content being experienced – sufficient at least to falsify some of the predictions.
Now is the time to be ambitious in the experiments we design, knowing that powerful designs can unlock large budgets, as particle physics has shown with ever-larger supercolliders.
Conclusion
In reading this essay, you repeatedly experience multiple items of information – multiple concepts/visual objects – at the same time. The models of classical physics cannot account for this simultaneous phenomenal complexity. Its equations depend only on the current state, so information from multiple stages in the past cannot be integrated directly into the present. Meanwhile, its current state entities only interact with their immediate neighbours; there is no faster than light travel and no instantaneous communication across distance. Under the mild assumption that your experiences are more complex than contained in a single local entity (e.g. a single particle, a single point), we need some nonclassical solution, with quantum candidates including particle entanglement, wave function collapse, and universal vacuum entanglement.
Our essay synthesises a long history of related ideas, from William James and Gottfried Leibniz through to contemporary thinkers, distilling out our two route argument and empirical testing vision [19-21]. If our argument holds, then mechanisms whose information processing is defined by classical physics cannot map to unified content in complex experiences. In other words, classical artificial neural network models cannot ground complex experiences, neither explaining it in humans nor generating it within digital computers. But there is an alternative to our argument.
If neither classical nor quantum physics provide the necessary mechanism for phenomenal binding, then any alternative solution must lie outside of current notions of physics. We might look to various well-trodden dualist theories in philosophy of mind, allowing there to be something beyond the physical: a mental aspect or soul-stuff that relates somehow to the physical objects we observe but obeys additional rules [22]. Similarly we might look to strong emergence, where something new happens at scale that contradicts the interaction of fundamental entities in accordance with quantum physics [23]. These additional rules create space for laws that have one of the necessary features – connecting current state entities directly to multiple points in the past or elsewhere in the current state – in such a way that exactly lines up with human experience.
There is a price to pay for this alternative. Either these new dualist or emergentist laws do affect the equations of physics or they do not. In the former, the onus is on dualists to identify and test their new empirical predictions. In the latter, consciousness is merely a spirit along for the ride – it cannot have any causal influence on the physical world. Our consciousness – everything that feels like ‘us’ on the inside – does not actually decide anything we say or shape any interaction we have with other people. In this context, it’s challenging to explain why natural selection appears to have put such effort into recruiting and refining complex conscious experiences – and why consciousness feels so agentic, so effective.
We suggest that such ‘epiphenomenal’ alternatives should be considered a fall-back position. Only once explanations using physical laws have failed should we revert to non-physical explanations. If there is a God in this particular story, let Them be the God of the Gaps. It is, however, too early to retreat to these positions – there are experiments to be run. Quantum biology is our leading contender and should be given a chance first.
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