Gravity is one of the most familiar features of the universe. It keeps planets in orbit around the Sun, holds galaxies together, and makes objects fall toward Earth. Yet despite its importance, gravity remains one of the greatest puzzles in modern physics. Scientists understand remarkably well how gravity behaves, but they do not fully understand what it is at the deepest level.
One intriguing possibility is that gravity may not be a fundamental force at all. Instead, it could be an emergent phenomenon: something that arises from the collective behavior of more basic ingredients, much as temperature emerges from the motion of countless atoms or the pressure of a gas emerges from particles colliding.
This idea challenges the traditional picture of gravity as one of nature’s basic interactions. It also offers a possible path toward understanding why gravity is so difficult to reconcile with quantum mechanics. Although emergent gravity remains a developing area of theoretical research rather than an established description of nature, it raises an important question: Could the force that shapes the universe be a large-scale consequence of something more fundamental?
What physicists mean by fundamental and emergent
In physics, a fundamental interaction is generally treated as a basic part of the universe’s underlying structure. It does not need to be explained as the collective result of other, more elementary processes within the theory.
The four recognized fundamental interactions are gravity, electromagnetism, the strong nuclear interaction, and the weak nuclear interaction. Each plays a distinct role. Electromagnetism governs interactions involving electric charge, the strong interaction binds quarks into particles such as protons and neutrons, and the weak interaction is responsible for certain forms of radioactive decay. Gravity governs the attraction between energy and matter and influences the structure and evolution of spacetime.
An emergent phenomenon works differently. It appears when many simpler components interact, producing behavior that cannot be understood by examining just one component in isolation.
Temperature provides a familiar example. A single atom does not have a temperature in the ordinary thermodynamic sense. Temperature describes the collective statistical behavior of an enormous number of particles. Likewise, a wave moving across the surface of water is not a separate fundamental substance. It is a pattern produced by the coordinated motion of water molecules.
Emergence does not mean that a phenomenon is imaginary or unimportant. Temperature is real, measurable, and essential to understanding physical systems, even though it is not a basic property of an individual atom.
If gravity is emergent, it could be similarly real without being fundamental. The gravitational behavior we observe might arise from deeper relationships among quantum information, matter, energy, and the microscopic structure of spacetime.
Einstein’s theory changed how we understand gravity
The possibility of emergent gravity becomes easier to understand when viewed against the history of gravitational physics.
In the 17th century, Isaac Newton described gravity as a universal attraction between objects with mass. His theory explained falling bodies, planetary orbits, and many other phenomena with extraordinary accuracy. It treated gravity as a force acting between masses, although Newton himself did not provide a physical explanation for how that attraction was transmitted across empty space.
More than two centuries later, Albert Einstein developed general relativity, fundamentally changing the picture.
According to general relativity, gravity is not simply a conventional force pulling objects toward one another. Matter and energy influence the geometry of spacetime, and objects move along paths shaped by that geometry.
Spacetime combines the three dimensions of space with time into a single four-dimensional structure. A planet orbiting a star, for example, follows a path through curved spacetime. What appears to us as gravitational attraction is, in this description, the result of motion through that geometry.
This framework explains phenomena that Newton’s theory cannot fully describe, including the bending of light by gravity, the precise behavior of Mercury’s orbit, gravitational waves, and the dynamics of black holes.
General relativity has passed extensive experimental tests. It remains the most successful description of gravity on large scales.
However, describing gravity as the curvature of spacetime does not necessarily explain why spacetime has the properties it does. Einstein’s equations tell physicists how geometry responds to matter and energy, but they do not provide a complete microscopic account of spacetime itself.
That gap leaves room for the possibility that spacetime, and perhaps gravity along with it, could emerge from something more fundamental.
Why gravity presents a challenge for quantum physics
The strongest motivation for exploring emergent gravity comes from a major problem in modern theoretical physics: general relativity and quantum mechanics do not fit together comfortably.
Quantum mechanics describes the behavior of matter and energy at very small scales. It successfully explains atoms, elementary particles, and the interactions responsible for much of the physical world.
General relativity describes gravity and spacetime on large scales. It works exceptionally well for planets, stars, galaxies, and the universe as a whole.
Both theories are extraordinarily successful within their respective domains. The difficulty arises when physicists try to apply them simultaneously under extreme conditions.
The interior of a black hole and the earliest moments of the universe are examples of environments where both quantum effects and strong gravitational effects are expected to matter. In these situations, treating spacetime as a smooth, continuous background may no longer be adequate.
Physicists have attempted to construct a quantum theory of gravity, a framework that would explain how gravity behaves at the quantum level. One approach is to treat gravity as an interaction mediated by a hypothetical quantum particle called the graviton.
The graviton would be the quantum counterpart of the electromagnetic field’s photon. It is expected to have no mass and a spin of two. However, no graviton has been experimentally detected.
A direct quantum description of gravity also encounters mathematical difficulties when approached using the same straightforward methods applied to other fundamental interactions. Ordinary perturbative quantum gravity does not produce a theory that remains predictive at arbitrarily high energies without additional ingredients.
This does not prove that gravity cannot be fundamental. It does, however, encourage researchers to investigate whether gravity should be understood differently.
If gravity is emergent, physicists might not need to quantize spacetime in exactly the same way they quantize other fundamental fields. Instead, they could seek a deeper quantum description from which familiar gravitational behavior emerges.
How gravity might emerge from quantum information
One influential direction in emergent gravity research explores the relationship between gravity and quantum information.
Quantum information describes the information stored and processed by quantum systems. Unlike ordinary classical information, quantum information can involve superposition and entanglement.
Entanglement is a quantum relationship in which two or more systems share correlations that cannot be fully explained by treating them as independent objects. Measuring one part of an entangled system can reveal information about another, even when the systems are separated.
This relationship has attracted considerable attention because of its possible connection to the structure of spacetime.
In certain theoretical models, especially those involving holography, researchers have found mathematical connections between quantum entanglement and the geometry of spacetime.
The holographic principle suggests that information describing a region of space can, under appropriate conditions, be represented by a theory defined on a lower-dimensional boundary. This is a surprising idea because it implies that the information associated with a volume of space may be encoded in a different mathematical description involving fewer spatial dimensions.
A particularly important example is the anti-de Sitter/conformal field theory correspondence, commonly called AdS/CFT. It connects a theory of gravity in a higher-dimensional spacetime with a quantum theory without gravity on its boundary.
The correspondence provides a mathematical framework in which certain gravitational systems can be studied using quantum field theory. In this setting, quantum entanglement appears closely related to the geometry of the gravitational spacetime.
For example, the Ryu-Takayanagi formula connects the entanglement entropy of a region in the boundary theory to the area of a corresponding minimal surface in the gravitational description.
Entanglement entropy measures how much information about one part of a quantum system is unavailable when considering that part alone.
These results suggest that spacetime geometry may be connected to the pattern of quantum entanglement within an underlying system.
If those connections reflect a more general principle, the geometry we experience as space could arise from relationships among quantum degrees of freedom rather than existing as a fundamental structure from the beginning.
Gravity might then be understood as part of the large-scale behavior of this emergent geometry.
However, these ideas have been established mathematically in particular theoretical settings, not demonstrated as a complete explanation of the actual universe. Connecting them to the spacetime we observe remains a substantial challenge.
The connection between gravity and thermodynamics
Another important line of research explores the possibility that gravity behaves like a thermodynamic phenomenon.
Thermodynamics describes large-scale properties of physical systems, including temperature, entropy, and energy flow. These properties emerge from the collective behavior of microscopic components.
The connection between thermodynamics and gravity became especially interesting through the study of black holes.
Black holes are regions of spacetime where gravity is so strong that nothing inside a boundary called the event horizon can escape to the outside. Despite their extreme nature, black holes appear to obey laws that closely resemble the laws of thermodynamics.
In the 1970s, physicists Jacob Bekenstein and Stephen Hawking established that black holes have entropy and temperature.
The entropy of a black hole is proportional to the area of its event horizon, rather than the volume enclosed by it. This is a remarkable result because ordinary thermodynamic entropy is usually associated with the number of microscopic configurations available within a system.
Hawking’s theoretical work also showed that black holes should emit thermal radiation because of quantum effects near their event horizons. This radiation, now known as Hawking radiation, means that black holes are not perfectly black in a quantum description.
These discoveries revealed a deep connection among gravity, quantum theory, and thermodynamics.
They also raised a provocative possibility: perhaps the equations describing gravity resemble thermodynamic laws because gravity itself is a large-scale, statistical phenomenon.
In 1995, physicist Ted Jacobson demonstrated that Einstein’s field equations could be derived from a thermodynamic relationship involving horizon entropy and energy flow, under specific assumptions.
The result suggested that general relativity might be understood as an equation of state, much as the laws governing gases emerge from the statistical behavior of their constituent molecules.
An equation of state describes how macroscopic properties of a physical system are related. For example, the pressure, volume, and temperature of a gas are connected even though each property reflects the collective behavior of many particles.
In this interpretation, Einstein’s equations might describe the large-scale response of spacetime rather than its ultimate microscopic structure.
This is an important theoretical insight, but it does not establish that gravity is definitely emergent. Deriving gravitational equations from thermodynamic principles shows that such a relationship is possible; it does not, by itself, identify the underlying microscopic ingredients or prove that they exist in nature.
What would emergent gravity mean for the universe?
If gravity emerges from deeper physical processes, the implications could extend far beyond how scientists describe falling objects or planetary motion.
One major consequence would be a different understanding of spacetime. Rather than treating space and time as the fundamental stage on which physical events occur, physicists might need to regard them as large-scale properties of a more basic quantum system.
Space could emerge from relationships among microscopic degrees of freedom. Time might also have an emergent description, although explaining the familiar passage of time presents additional conceptual difficulties.
The idea could also change how physicists approach black holes. Their unusual thermodynamic properties might not be isolated curiosities but clues to the microscopic structure of spacetime.
Another possible consequence concerns the nature of gravity at extremely small distances or very high energies.
If gravity is emergent, the familiar equations of general relativity might work in much the same way that the laws of fluid motion work. They would accurately describe large-scale behavior while failing to reveal every detail of the underlying microscopic system.
At ordinary scales, water can be treated as a continuous fluid. But at sufficiently small scales, its molecular structure becomes important. Similarly, spacetime might appear smooth and continuous under everyday conditions while possessing a more complicated microscopic organization.
This analogy is useful, although the underlying physics of emergent spacetime could be very different from that of an ordinary material.
How emergent gravity differs from other approaches
Emergent gravity is not the only attempt to understand gravity at a deeper level.
String theory, for example, proposes that the fundamental constituents of nature are tiny vibrating strings rather than point-like particles. In consistent versions of the theory, a graviton-like excitation naturally appears, providing a quantum description of gravitational interactions.
Loop quantum gravity takes a different approach. It attempts to quantize spacetime geometry itself, suggesting that areas and volumes may have discrete quantum properties.
These approaches are not interchangeable, and they do not all make the same assumptions about what is fundamental.
Emergent gravity differs most clearly in its starting question. Instead of assuming that gravity must be one of the basic interactions requiring direct quantization, it asks whether the gravitational description itself could arise from more fundamental non-gravitational degrees of freedom.
Some approaches to quantum gravity incorporate emergent ideas without treating them as alternatives to every other framework. For instance, holographic theories can contain a conventional gravitational description in one mathematical formulation while describing the same physics through a quantum theory without gravity in another.
This illustrates why the distinction between fundamental and emergent can be subtle. A phenomenon may be fundamental in one effective description while emerging from a deeper description.
What evidence would establish that gravity is emergent?
The biggest challenge for emergent gravity is not showing that gravitational equations can arise from other mathematical principles. Researchers have already demonstrated several ways in which this can happen.
The harder task is establishing that one of these mechanisms actually describes our universe.
A successful theory must reproduce the predictions of general relativity wherever those predictions have been tested. It must explain the motion of planets, the behavior of gravitational waves, the dynamics of black holes, and the evolution of the universe.
It must also remain consistent with quantum mechanics and explain how the familiar classical spacetime description emerges at larger scales.
Most importantly, it should ideally produce testable predictions that distinguish it from existing theories.
Such predictions might involve subtle departures from general relativity under extreme conditions, new relationships between quantum information and gravitational geometry, or observable consequences of the microscopic structure of spacetime.
However, identifying such signals is difficult. Many proposed effects would occur at energy scales far beyond those accessible to current experiments.
There is also an important distinction between a theory being mathematically consistent and being experimentally confirmed. A compelling mathematical connection can suggest a promising direction without demonstrating that nature actually works that way.
At present, no experimental result has established that gravity is an emergent phenomenon.
Why the question remains open
Gravity occupies a unique position in modern physics. It is essential to the structure of the universe, yet its relationship with quantum mechanics remains unresolved.
General relativity describes gravity with extraordinary precision on large scales. Quantum mechanics successfully explains the microscopic world. But neither framework, in its standard form, provides a complete account of the conditions where quantum effects and strong gravity become inseparable.
Emergent gravity offers one possible way forward. It suggests that spacetime and gravitational behavior could arise from more fundamental relationships involving quantum information, entanglement, and microscopic degrees of freedom.
The idea is scientifically interesting because it may explain why gravity behaves so differently from the other interactions and why spacetime appears smooth at the scales we can observe.
But it remains an open research question, not an established scientific conclusion. Physicists have identified important mathematical connections between gravity, thermodynamics, and quantum information, yet a complete microscopic theory with clear experimental confirmation has not emerged.
The deeper lesson is that something does not have to be fundamental to be one of the most important features of reality. Temperature, pressure, and fluid motion are all emergent, but they govern countless physical processes.
Gravity could conceivably belong to the same category. If so, the force that shapes planets, stars, galaxies, and the evolution of the cosmos may ultimately be a consequence of something even more fundamental: the underlying quantum structure of the universe.