Informational Turn
Informational Crucible
By the late twentieth century, the Standard Model had successfully reduced the physical universe to continuous, interacting fields. Yet, beneath this undeniable experimental triumph, the foundational crisis provoked by quantum mechanics in the 1920s had never been resolved. The ontological spotlight was slowly shifting away from the fields themselves and onto the abstract information encoded within their correlations.
The first physical link between the abstract "Bit" and the concrete "Atom" had been forged decades earlier, almost unnoticed, by a Hungarian physicist fleeing the collapse of the Austro-Hungarian Empire. In 1929, Leo Szilard published a paper addressing the paradox of Maxwell's Demon, a hypothetical, microscopic being proposed by James Clerk Maxwell in 1867 who sorts fast molecules from slow ones to create a temperature difference, seemingly violating the Second Law of Thermodynamics without expending any energy.
Szilard realized that for the Demon to sort the molecules, it must first measure them. It must acquire information about their velocity and store this result in a memory. Closing the cycle, which requires resetting the Demon's memory so it can measure again, carries an unavoidable thermodynamic cost. Szilard mathematically proved that the acquisition or erasure of one bit of information corresponds directly to an entropy increase of
This anticipated Claude Shannon's Information Theory by two decades and established a radical new equivalence: information is profoundly physical. The "It," represented by entropy and energy, and the "Bit," represented by information, were convertible currencies. One could not talk about the objective state of a gas without explicitly accounting for the information stored in the observer's memory. The observer, on this account, is a thermodynamic engine physically entangled with the system it measures, not a ghost watching from outside.
Observation, on this account, is a physical entanglement. That raises the question of what exactly happens to the Schrödinger equation when an observer looks at a quantum superposition. The orthodox Copenhagen interpretation demanded an arbitrary "collapse" of the wave function, a mathematical discontinuity that Niels Bohr philosophically justified but never physically explained. In 1957, a Princeton graduate student named Hugh Everett III proposed a solution that required abandoning the last, most deeply held vestige of classical realism: the uniqueness of history.
Everett modeled the observer as a physical system, specifically a mechanical automaton with a memory register, governed entirely by the continuous, deterministic evolution of Schrödinger's equation. He showed that if this observer interacts with a superposed quantum system, the observer themselves enters a superposition. There is no mystical "collapse." Instead, reality is defined by the correlation between the system and the memory. Relative to the memory state "I saw the spin pointing UP," the electron is UP. Relative to the memory state "I saw the spin pointing DOWN," it is DOWN. Both branches exist simultaneously and orthogonally in a single, universal wave function.
Everett's "Relative State Formulation," later popularized as the Many-Worlds Interpretation, represented the ultimate triumph of Information over Substance. The universal wave function was not a physical wave vibrating in space; it was a vast, deterministic catalog of all possible informational correlations.
This radical proliferation of histories raised a terrifying question: if the universe does not collapse, is everything permanently and inextricably bound together? An Irish physicist at CERN named John Stewart Bell stepped forward in 1964 to tear down the last theoretical defenses of local realism, proving that this deep quantum interconnectedness was an unavoidable reality. He found a way to take the philosophical dispute between Albert Einstein and Niels Bohr out of the realm of metaphysics and make it experimentally decidable in a laboratory.
Bell took Einstein's fundamental assumptions of local realism seriously: he assumed that particles have definite properties before they are measured, which is the baseline of realism, and he assumed that nothing can coordinate distant outcomes faster than the speed of light, which is the constraint of locality. Using just these two assumptions, Bell proved a rigorous mathematical inequality; any physical theory built on local realism places a strict, numerical ceiling on how strongly the measurements of two distant particles can be correlated.
Quantum mechanics, however, predicted correlations that broke that ceiling. In 1982, Alain Aspect and his team in Paris ran a landmark experiment, switching detector settings while entangled photons were already in flight. This configuration addressed the loophole that the detectors could somehow secretly communicate, though the switching itself followed a predictable, quasi-periodic pattern rather than a genuinely random one, and the detection loophole remained open. The inequality was violated by several standard deviations, a result physicists at the time found decisive, even though a fully loophole free version of the experiment, closing locality, detection, and memory loopholes all at once, would not arrive until three independent teams achieved it in 2015.
Local realism was dead. Bell's theorem proved mathematically that reality could not be cleanly separated into independent, isolated parts. Einstein's hope that the universe was made of independent, localized stuff possessing its own private properties was false. Entanglement was real, and it was unavoidably non-local. The entire arrangement, including the source, the particles, and the distant detectors, constituted a single, unanalyzable phenomenon. Schrödinger's concept of entanglement was vindicated; when two systems interact, they lose their individual wave functions, and information is stored entirely within the abstract correlations between them.
This total capitulation of local realism left physics trapped in a non-local maze. If the universe at its base is a sprawling, undivided web of non-local entanglement where every history branches endlessly, how does a clean, localized classical reality emerge for human experience? Why do everyday objects appear solid and singular rather than smeared out across multiple places at once?
H. Dieter Zeh provided the missing physical insight in 1970. He noted that no macroscopic object is ever perfectly isolated; it is constantly struck by photons, air molecules, and cosmic rays. Zeh argued that once a fragile quantum system becomes entangled with this chaotic environment, the delicate phase interference between its possible states is effectively scrambled, leaking out into the vast, untrackable degrees of freedom of the surroundings.
Wojciech Zurek gave this idea mathematical teeth in 1981, developing the full theory of Decoherence. Zurek asked a specific question: out of all the possible, infinite quantum superpositions, why does the environment only allow us to observe definite positions and states? His answer was that the environment acts as a relentless, blind interrogator. It interacts with a system through specific physical channels, singling out a very small set of robust "pointer states" that survive being continuously monitored.
Zurek called this process einselection, meaning environment-induced superselection. The everyday world looks solid and classical not because of a mysterious, instantaneous collapse triggered by human consciousness, but because macroscopic objects are relentlessly hemorrhaging information into their surroundings. The classical "It" is simply the robust informational consensus that survives this environmental scrambling.
Through Szilard’s thermodynamics, Everett’s relative states, Bell’s non-locality, and Zurek’s decoherence, the foundational scaffolding of reality had been entirely rewritten. The ultimate reality was no longer a discrete particle, nor was it a continuous geometric field. The true foundation of reality was the abstract web of informational correlations connecting them. The stage was set for a new generation of physicists to ask an even more radical question: what happens when you build space and time themselves out of pure information?
Participatory Stage
If Albert Einstein established the dynamic geometric stage of the twentieth century, it was John Archibald Wheeler who attempted to dismantle it to find what lay beneath. Wheeler, a physicist of immense imagination who worked alongside both Niels Bohr and Einstein, serves as the great transitional grandfather of modern physics. His intellectual journey, sweeping from "Everything is Geometry" to "Everything is Information," represents the definitive pivot of the modern era, mirroring the larger collapse of the continuum.
In the 1950s, Wheeler was initially obsessed with a radical unification program known as Geometrodynamics. His ambition was to eliminate matter entirely and fulfill the ultimate Cartesian dream of a pure, continuous continuum. He proposed that particles, such as electrons, protons, and neutrons, were not foreign, discrete objects dropped onto the stage of spacetime; instead, they were intense, localized knots of curvature in spacetime itself, structures he termed "geons" or gravitational electromagnetic entities. A charge was not a physical particle; it was simply a wormhole mouth trapping lines of force. In this monistic view, there was no "It" separate from the geometry: there was only empty, curved space.
But this dream of a pure, continuous geometric ontology collapsed under the weight of quantum reality. Wheeler realized that as one zooms in on the smooth manifold of Einstein, approaching the Planck scale at roughly centimeters, the continuum must violently break down. It shatters into a churning, turbulent quantum foam where topology fluctuates violently, spontaneously creating and destroying microscopic wormholes. One cannot build a stable, deterministic reality on an ocean of singularities. The "It" refused to be reduced to pure geometry, and spacetime points, the foundational building blocks of the classical continuum, had to be abandoned.
The failure of Geometrodynamics drove Wheeler toward a profound philosophical pivot. Geometry, evidently, was not the bottom of reality. The question was what could be. The spark for this revolution arrived in the early 1970s via a teacup, a black hole, and his brilliant PhD student, Jacob Bekenstein.
Wheeler challenged Bekenstein with a thought experiment. Wheeler joked about committing a crime against the Second Law of Thermodynamics: if he mixed hot tea with cold tea, he increased the entropy of the universe without doing work. But what if he threw the teacup into a black hole? The entropy would seemingly vanish from the observable universe, violating the Second Law. According to classical General Relativity, a black hole was a featureless, continuous pit possessing no external characteristics. If it had no internal features, it could have no entropy.
Bekenstein’s solution was radical: the black hole itself must possess entropy. Crucially, he proved that this entropy was proportional not to the black hole's three-dimensional volume, as one would expect for a container of gas, but to the two-dimensional surface area of its event horizon.
This was the first fatal crack in the geometric facade. It implied that the absolute limit of information a region of space could hold was bounded by its surface, not its volume. The "It," representing the matter inside the hole, was perfectly and exclusively encoded on the "Bit" of the surface area. Wheeler championed Bekenstein's result, realizing instantly that thermodynamics and information were operating at a deeper, more fundamental level of reality than geometry.
By 1989, Wheeler had fully transitioned to an information-first perspective. In his seminal manifesto, he coined the phrase that would define the next era of physics: "It from Bit."
Wheeler argued that the physical world is not a pre-existing continuous machine, but a discrete construct built from binary choices. Drawing directly on Bohr’s insistence that observation creates phenomena, Wheeler declared that every particle, every field of force, and even the spacetime continuum itself derives its function, its meaning, and its very existence entirely from the apparatus-elicited answers to yes-or-no questions, binary choices, or bits.
He illustrated this with a modified game of Twenty Questions. In the classical version, an object already exists in the room, such as a cat, and the player asks questions to uncover it. This is classical physics: reality is out there, and we merely measure it. In Wheeler's quantum version, the room is empty. The players, representing nature, only decide that their answers must remain logically consistent with previous answers. If the first answer is "animal," the second cannot be "mineral." The object emerges only at the end of the questioning process.
This view, termed the Participatory Universe, posits that the observer is not a passive spectator but a co-creator of reality. Spacetime is not a fundamental container; it is a secondary, macroscopic illusion synthesized from the aggregation of billions of discrete, binary quantum events. Wheeler pushed this logic to its extreme with the concept of "Law without Law," suggesting that the laws of physics themselves were not eternal, continuous decrees, but simply the frozen habits of the universe, stabilized over eons of quantum information processing.
Yet, as Wheeler declared the spacetime container an illusion synthesized from binary quantum events, a parallel architectural front emerged that reached the exact same conclusion through an entirely different structural path. Wheeler had looked toward the discrete pixel to replace the smooth stage. Roger Penrose took a different route entirely, proposing a radical alternative in 1967 known as Twistor Algebra. Penrose argued that the fundamental arena of physics is not built from spacetime points at all, but from an abstract complex space whose primitive elements are light rays, specifically null geodesics.
In twistor theory, a light ray is not a path derived by connecting physical points; the light ray itself is the primary, indivisible entity. A spacetime point is reduced to a secondary, derived phenomenon, emerging only as the intersection where a family of abstract light rays happen to cross. It literally takes two twistors to pin down a single spacetime event.
This instinct to abandon the spacetime point reached its ultimate mathematical extreme in 2013, when Nima Arkani-Hamed and Jaroslav Trnka discovered the Amplituhedron. This is a single, multifaceted geometric object existing in an abstract, high-dimensional mathematical space. Crucially, the amplituhedron calculates particle scattering amplitudes directly through its volume, completely bypassing the traditional requirements of spacetime coordinates or local quantum probabilities.
Unitarity and locality, the twin pillars of standard quantum field theory, are not built into the amplituhedron as foundational axioms; they emerge as mere accidental consequences of its geometric properties. Spacetime and probability itself are revealed to be secondary, decorative footnotes projected from a deeper, purely geometric primitive.
But while mathematical physicists were trying to replace spacetime points with abstract geometries of light and high-dimensional volumes, a far more practical, hardware-driven revolution was asking an even simpler question: if the universe is fundamentally information, how does it actually run the code? Information sitting still is just a ledger. How does a binary "Yes/No" generate the infinite, fluid complexity of the physical world?
The answer came from the dawn of computer science. Back in the 1940s, John von Neumann and Stanislaw Ulam had developed cellular automata: a grid of simple cells, each existing in a basic state, updated in discrete steps by a rule depending only on its immediate neighbors. In 1970, mathematician John Conway showed what such a rule could do with his Game of Life. Using just four elementary, binary conditions governing birth and death on a two-dimensional grid, Conway generated structures that could move, self-replicate, and perform universal computation. The lesson was profound: complexity does not require a complicated, continuous law; it requires a simple, discrete rule, executed enough times.
Physicists and computer scientists began to take this literally. Konrad Zuse, who built some of the earliest programmable computers, proposed in his 1969 book Rechnender Raum that the universe itself is being computed in real time on a discrete computational grid. Edward Fredkin coined the term Digital Physics, arguing that the universe is a reversible, information-conserving cellular automaton. Stephen Wolfram later cataloged these systems, demonstrating that simple rules generate computational irreducibility: patterns so complex that the only way to know what the universe will do next is to actually run the code. Time itself, in this view, is not a continuous geometric dimension; time is just the computation running forward.
To measure this new reality, Ray Solomonoff, Andrey Kolmogorov, and Gregory Chaitin developed Algorithmic Information Theory, arriving at the same core idea independently and from different motivations: Solomonoff from the problem of induction in 1960, Kolmogorov from information theory in 1965, and Chaitin from randomness later in the decade. Between them, they declared that an object's true complexity is not defined by its physical mass or energy, but by the length of the shortest computer program capable of generating it.
The transition from Version 1.0 of physics to Version 2.0 was complete. The deepest question about the cosmos had ceased to be "what is it made of?" and had become exactly what Gottfried Wilhelm Leibniz prophesied three centuries earlier with his binary alphabet of human thought: what is the shortest binary program whose output is everything we see?
Relational Network
If the universe is fundamentally built from discrete bits of information rather than continuous, deterministic fields, the most profound casualty is the smooth, geometric stage of spacetime itself. The most direct heirs to the idea that the continuum is merely a macroscopic illusion are Causal Set Theory and Relational Quantum Mechanics. These frameworks attempt to do to Einstein's spacetime what Dalton and the kinetic theory of gases did to continuous fluids: break them down into discrete, fundamental units.
Causal Set Theory (CST), championed by physicists like Raphael Sorkin and Fay Dowker, argues that if one takes the informational turn seriously, one must abandon the notion of continuous space at the Planck scale. Dowker points out that General Relativity famously predicts its own demise through singularities; these are structural points, such as the center of a black hole or the initial Big Bang itself, where mathematical curvature becomes infinite and the continuous laws of physics break down entirely. To the causal set theorist, these singularities are not mathematical errors to be glossed over; they are physical signals. They indicate that the continuum assumption is merely a low-resolution approximation, much like fluid mechanics operates as an approximation of discrete, bumping atoms. Just as water appears perfectly smooth and continuous to the eye but is composed of discrete molecules, spacetime appears smooth but is built from discrete atoms of causality.
In the CST framework, the universe does not exist in space or time; rather, space and time are emergent properties of an underlying network. The fundamental reality is a partially ordered set, known as a poset. The bit here is not a particle of matter, nor is it a coordinate in a void; it is a Causal Link, defined by the relation that Event A causes Event B.
This approach radically reinterprets the passage of time. In the Einsteinian Block Universe, all of spacetime, including past, present, and future, is laid out simultaneously in a four-dimensional manifold; the human sensation of "now" is treated as a subjective, psychological illusion. In Causal Set Theory, the universe is a growing set. New atoms of spacetime are born sequentially, adhering to probabilistic rules. This model deliberately reintroduces a genuine, objective becoming into physics. The universe is not a static geometric block, but an active, accretive process. The "now" is simply the active edge of the causal set where new events are being added, operating as a digital tick of a cosmic clock that expands the universe one discrete atom of relation at a time.
Yet, this informational hollowing out of space split the modern vanguard into two fiercely incompatible camps. The causal set theorists had taken a knife to the continuum specifically to save the objective reality of time, making sequential becoming the fundamental law of the cosmos. The rival camp pursued the exact opposite goal. Carlo Rovelli and the architects of Relational Quantum Mechanics sought to dissolve the container entirely, arguing that time itself is the ultimate macroscopic illusion.
Introduced by Rovelli in 1994, Relational Quantum Mechanics (RQM) operates as an interpretation derived from the realization that quantum mechanics acts much like special relativity. In Einstein's relativity, velocity is not an absolute, intrinsic property of an object; an object possesses a velocity only relative to a specific observer. Rovelli extended this exact logic to all physical states. An electron does not have a position or a spin in an abstract void; it possesses a state only relative to a specific physical system interacting with it.
As Rovelli famously formulated, the universe is not just simply the position of all its Democritean atoms; it is also the net of information systems have about other systems. In this view, there is no View from Nowhere or God's Eye View of the universe. There are only localized, relative descriptions. The Relational Bit, therefore, is the interaction between two physical systems, while the corresponding object or state is the resulting mathematical correlation established between them.
When this relational perspective is applied to the problem of gravity, it leads directly to Loop Quantum Gravity (LQG). In LQG, the continuous metric of Einstein's general relativity is replaced by Spin Networks: mathematical graphs where nodes represent discrete, quantized chunks of volume, measuring roughly cubic centimeters, and the links connecting them represent surfaces of area. Crucially, these spin networks do not exist in space; they are space. Geometry is nothing more than the graph of their connectivity.
The most radical consequence of Rovelli's framework is the Thermal Time Hypothesis. In the fundamental, microscopic equations of Loop Quantum Gravity, the variable representing time disappears entirely. The theory describes how physical variables change with respect to one another, tracing, for example, how a pendulum swings relative to the position of a clock hand, but there is no external, overarching time parameter governing the whole system.
If time is not a fundamental ingredient of the universe, why do we experience it so viscerally? Rovelli argues that time is a macroscopic, statistical phenomenon, akin to heat. Just as temperature is not a property of a single molecule but a statistical average of billions of them, the sensation of time emerges only when we step back and statistically average over the microscopic informational states we cannot track. Time is the physical expression of our thermodynamic ignorance. In a theoretical universe of perfect, granular information, there would be no time and no flow, leaving only a frozen, complex network of relations.
Whether it is the growing, sequential network of Causal Sets or the frozen, emergent correlations of Relational Quantum Mechanics, the mechanical stage of the universe had been thoroughly dismantled. Space and time were no longer the absolute, divine containers of reality envisioned by Newton, nor the smooth, flexible rubber sheet of Einstein. Both were low-resolution outputs of a discrete informational network running underneath.
Holography and "It from Qubit"
The final, and perhaps most dominant, paradigm of modern quantum gravity fuses John Archibald Wheeler's informational vision with the high-energy physics of string theory. This movement, unified under the contemporary slogan "It from Qubit," provides the ultimate, devastating subversion of the geometric stage. It posits that space, time, and gravity are holographic projections, constructed entirely out of boundary quantum entanglement.
This radical conclusion did not appear from nowhere; it was discovered inside a theoretical framework that had already abandoned the Democritean point particle decades earlier. String theory’s origin was almost accidental. In 1968, a young Italian physicist named Gabriele Veneziano noticed that the Euler beta function, a two-hundred-year-old piece of pure mathematics, reproduced the scattering behavior of strongly interacting particles with striking accuracy. Physicists Yoichiro Nambu, Holger Bech Nielsen, and Leonard Susskind soon realized the physical reality underlying Veneziano's formula: the mathematical behavior was exactly what one would get if the fundamental, elementary particles were not dimensionless, zero-volume points, but tiny, vibrating, one-dimensional strings. Different particle types, masses, and forces were simply different vibrational modes of the same underlying string, much like the overtones of a violin string.
Through the superstring revolutions of 1984 and 1995, physicists realized this was not merely a theory of the strong nuclear force, but a formidable candidate for a unified theory of quantum gravity. Edward Witten demonstrated that the five competing string theories were all limits of a single, eleven-dimensional framework, M-theory, whose fundamental objects included higher-dimensional membranes, or D-branes. The discrete point particle that Roger Joseph Boscovich, John Dalton, Lord Kelvin, and Paul Dirac had fought over was, on string theory's own account, an illusion; it was simply a string, seen from too far away to notice its physical extension.
But string theory was about to undergo an even more profound ontological shift. Building directly on Jacob Bekenstein's discovery that a black hole's entropy scales with its surface area, Gerard 't Hooft proposed in 1993 that this was not a bizarre peculiarity of black holes, but a universal, general property of any region of space that includes gravity. The absolute limit of information that can be stuffed into any three-dimensional volume is bounded not by the volume itself, but by the two-dimensional surface area of its boundary.
Leonard Susskind pushed this further in 1995 with his manifesto "The World as a Hologram." He suggested that our three-dimensional universe could be fully and rigorously described by data living on a distant, flat boundary, exactly the way a flat, two-dimensional holographic plate encodes a fully three-dimensional image. But neither 't Hooft nor Susskind possessed a working mathematical example to prove this outrageous claim.
That proof arrived in late 1997, when a young Argentine physicist named Juan Maldacena made a discovery that shook the foundations of theoretical physics: the AdS/CFT correspondence. Using the mathematics of D-branes, Maldacena showed that a theory of quantum gravity in a bulk, saddle-shaped, five-dimensional spacetime, known as Anti-de Sitter space or AdS, is mathematically and exactly equivalent to an ordinary quantum field theory, called Conformal Field Theory or CFT, living on its flat, four-dimensional boundary, completely free of gravity.
This was the rigorous mathematical realization of the Holographic Principle. It proved that everything happening inside the bulk of the universe, including gravity, stars, black holes, and the falling apple, is a holographic projection generated by the complex, non-gravitational interactions of quantum particles on the boundary. The "It," representing the higher-dimensional spacetime bulk, is literally generated by the "Bit" of the boundary quantum data.
But what, exactly, was the physical glue projecting the boundary data into the bulk? Maldacena and his colleagues soon realized that the geometry of spacetime is stitched together by quantum entanglement. The slogan was explicitly updated from "It from Bit" to "It from Qubit."
The connection between entanglement and geometry was solidified by Shinsei Ryu and Tadashi Takayanagi in 2006. The Ryu-Takayanagi formula mathematically equates the quantum entanglement of a region on the boundary to the physical area of a minimal surface dipping deep into the bulk spacetime. This suggested that the area of space is literally a measure of quantum entanglement. If you have two regions of space, the physical distance between them is determined by how entangled their boundary states are.
In 2013, Maldacena and Susskind proposed the staggering ER=EPR conjecture. They linked two concepts Albert Einstein had introduced in separate papers in 1935, thinking them entirely unrelated: the Einstein-Rosen bridge, which is a wormhole in spacetime, and the Einstein-Podolsky-Rosen paradox, which describes quantum entanglement. Maldacena and Susskind declared they are the exact same phenomenon. A single pair of entangled particles is connected by a Planck-scale wormhole. If you entangle two macroscopic black holes, they are connected by a massive, traversable geometric wormhole.
The smooth, continuous connectivity of spacetime is an emergent property arising from the dense entanglement of quantum bits. If one were to theoretically break the entanglement between two regions of space, the space between them would pinch off and physically separate. Geometry is nothing but the result of the information processing of the boundary qubits.
A further question remained: what physically holds this holographic projection together, protecting it from being disrupted by quantum noise? The answer arrived from computer science. In 2009, Brian Swingle noticed that the mathematics of AdS/CFT closely mirrored tensor networks, which are computational tools developed by condensed matter physicists to organize how local qubits entangle with their neighbors to build larger quantum systems. Swingle proposed that the bulk of an AdS spacetime literally is one of these networks. The geometric distance between two points is not a given, primitive physical fact; it is a measure of how much entanglement must be crossed to connect them. Spacetime, on this reading, is what quantum information compression looks like from the inside.
In 2015, Ahmed Almheiri, Xi Dong, and Daniel Harlow pushed this analogy to its ultimate, logical conclusion: the AdS/CFT correspondence behaves exactly like a quantum error-correcting code.
In a quantum computer, ordinary qubits are incredibly fragile, easily corrupted by environmental noise. To protect the information, the computer uses a quantum error-correcting code, spreading a single logical qubit redundantly across many highly entangled physical qubits. If a few physical qubits are lost or corrupted, the logical information remains perfectly intact. Almheiri, Dong, and Harlow showed that the interior spacetime bulk and everything in it plays the role of the protected, highly robust logical information, while the boundary plays the role of the redundant, highly entangled physical qubits encoding it.
This is a genuine, paradigm-shifting answer to the deepest question of the modern era: If the universe is built from fragile, non-local quantum entanglement, why does the classical world look so solid, local, and robust?
The answer is that classical spacetime is what quantum error correction looks like once you are too far away to see the code underneath. Newton's rigid stage and Einstein's geometric one are both gone; what's left running is a computation.