Some aspects of life simply unfold without direct intervention. Desks accumulate dust and stray papers. Clothing reaches a point of being dirty, prompting a trip to the laundry basket. Untended flowerbeds are gradually overtaken by creeping weeds. In essence, things tend towards disarray unless actively managed.
Consider this intriguing notion: what if gravity itself operates in a similar fashion? This perspective offers a distinct approach to understanding the force that anchors us to the ground and orchestrates the celestial ballet of planets. While the majority of physicists classify gravity as a fundamental force of nature, akin to electromagnetism or the nuclear forces, physicist Erik Verlinde proposed in 2010 that it might not be a fundamental force at all. Instead, he suggested it could be an emergent phenomenon, a consequence of the universe’s inherent tendency towards increasing disorder.
At the time of his announcement, Verlinde famously stated, “For me, gravity doesn’t exist.” The initial reception to this idea was, to be diplomatic, varied. However, over the ensuing years, perspectives have evolved. We now possess a more refined understanding of how this concept, termed entropic gravity, might function. Early indications suggest it could offer explanations for profound cosmic mysteries, including the enigmatic nature of dark energy and dark matter. More recently, researchers have put forward specific proposals for how the entropic gravity hypothesis could be subjected to experimental scrutiny. These developments are prompting some physicists to reconsider a foundational question: is gravity truly a fundamental force?
The Historical Context of Entropy in Physics
Physics, a discipline typically associated with meticulous precision, has a notable and enduring fascination with the concept of disorder. This interest can be traced back to the transformative era of the Industrial Revolution, marked by steam engines and mechanical innovation. In the mid-19th century, physicist Rudolf Clausius confronted the inescapable reality that mechanical engines could never achieve perfect efficiency in converting heat into usable work. He attributed this limitation to a newly identified quantity he termed entropy.
Clausius observed that heat naturally dissipates from warmer areas to cooler ones. During this process, a portion of the heat invariably escapes into the surrounding environment. Clausius proposed that this pervasive loss of energy was so universal that it could be enshrined as a fundamental principle, a concept now widely recognized as the second law of thermodynamics.
A more precise definition of entropy emerged approximately a decade later, courtesy of Ludwig Boltzmann. Boltzmann conceptualized entropy in terms of the multitude of possible spatial arrangements for atoms and molecules within a given volume. His crucial insight was that certain configurations are inherently more ordered than others. For example, a scenario where all fast-moving molecules are confined to one side of a piston and all slow-moving molecules to the other represents a more ordered state than one where fast and slow molecules are intermingled. Boltzmann posited that entropy serves as a measure of this disorder, and that in any isolated system, entropy invariably increases over time.
From Black Holes to Spacetime: The Astronomical Leap of Entropy
Roughly a century after Boltzmann’s formulation, the study of entropy experienced a significant expansion into astronomical realms. Theorist Jacob Bekenstein, while at Princeton University, investigated the inherent disorder within black holes. His calculations indicated that a black hole’s entropy must be directly proportional to the surface area of its event horizon—the boundary beyond which no matter or energy can escape its immense gravitational pull.
Later, physicist Ted Jacobson demonstrated that under specific conditions, the fabric of spacetime itself exhibits behavior analogous to a thermodynamic system. This finding was remarkable. Gravity is understood as a force affecting all objects, even fundamental particles. Thermodynamics, conversely, typically emerges from systems composed of a vast number of smaller components. Despite its startling nature, Jacobson’s insight was often regarded as a curious coincidence rather than a profound assertion about the fundamental nature of reality.
Verlinde’s Eureka Moment: Gravity as an Emergent Phenomenon
The pivotal moment for Erik Verlinde occurred during a summer holiday in 2009. Stranded while on vacation following the theft of his passport, Verlinde found himself with ample time to contemplate Jacobson’s paper, which he had studied numerous times. The potential connection between entropy and gravity lingered in his thoughts. With little else to occupy him, he revisited this idea, ultimately arriving at an interpretation he would present in a remarkably concise paper the following year.
Verlinde’s central argument proposed that gravity is not a fundamental force but rather an emergent property arising from a deeper, more fundamental law: the universal tendency for entropy to increase. He stated that if one takes the laws of thermodynamics and black hole physics seriously, then the emergent perspective on gravity should also be seriously considered.
Emergent Gravity: A New Paradigm
The term “emergent” carries significant weight in this context. Physicists differentiate between fundamental constituents of reality, which cannot be further broken down into simpler components, and emergent phenomena. These emergent phenomena arise from the collective behavior of numerous simpler entities. Consider, for instance, the concept of air pressure within a tire. While pressure is a useful descriptive term, it is not considered fundamental. Instead, it is understood as the result of countless air particles colliding with the inner surface of the tire. Verlinde’s hypothesis posits that gravity is emergent in a comparable fashion.
His peers’ reaction was varied. Andrew Strominger, a theorist at Harvard University, noted at the time that some found the idea incorrect, while others believed it was correct but already understood. The primary challenge faced by Verlinde’s hypothesis stemmed from the well-established framework of physics. Our most comprehensive understanding of gravity is derived from Albert Einstein’s theory of general relativity. For decades, physicists have grappled with the difficult task of reconciling general relativity with quantum theory, two theoretical frameworks that are built upon fundamentally different assumptions.
String theory emerged as a leading contender for bridging this gap. It proposes that fundamental particles and spacetime itself are composed of one-dimensional entities called strings, which vibrate and exist in ten dimensions. Despite decades of intensive research, string theorists have yet to formulate a complete description of a universe that precisely matches our own, though their optimism remains robust.
While appearing distinct, entropic gravity shares surprising common ground with ideas within string theory. Verlinde’s initial work drew from the concept of holography. Holography suggests that it is possible, mathematically, to translate phenomena occurring in a higher-dimensional reality into an equivalent description in a lower-dimensional one. This implies that our perceived three-dimensional world might be analogous to a holographic projection, originating from a more fundamental, flatter two-dimensional reality.
Reimagining Reality: Entropic Gravity’s Approach
Applying holographic principles, Verlinde envisioned a scenario where a mass is positioned just outside a two-dimensional spherical screen, enclosing another mass. His calculations revealed that the external mass experiences an inward “push”—not from any direct physical attraction, but because the system’s total entropy increases as the two masses move closer together. This formed the core of Verlinde’s epiphany: gravity could be understood not as a force, but as a consequence of nature’s inherent drive towards greater entropy within a hidden, lower-dimensional realm.
The identity of this underlying realm remains a critical point of inquiry. Grant Remmen, a theorist at New York University who has researched entropic gravity, highlights the challenge of identifying the specific microscopic system responsible for this phenomenon and determining the appropriate entropic model to apply. Verlinde did not initially provide a definitive answer. However, subsequent developments in quantum gravity offered new avenues for exploration. Quantum physics describes phenomena like entanglement, where particles can influence each other instantaneously regardless of distance.
A growing perspective among some quantum gravity theorists is that an informational network, built upon entanglement, constitutes the true fundamental fabric of existence—that reality, at its deepest level, is comprised of information. For Verlinde, this information-centric view resonated deeply. He proposed that the fundamental aspect of reality undergoing increasing disorder is this entanglement information network. Intriguingly, his model offered potential solutions to long-standing cosmological puzzles.
As Verlinde worked through the equations, he discovered that the entanglement surrounding galaxies should be more perturbed than predicted by the visible distribution of matter alone. This greater disruption leads to increased entropy and what appears as “extra” gravity. Astonishingly, this approach provided a potential explanation for galactic rotation curves, a phenomenon where galaxies spin too rapidly for their visible mass, suggesting the presence of an unseen “dark matter.” Verlinde’s entropic gravity model accounted for this effect without invoking dark matter.
The Quest for Experimental Verification
Despite these theoretical advancements, many experimental physicists remained skeptical. Verlinde’s hypothesis lacked concrete, testable predictions that could be empirically verified. Dan Carney, a physicist at the Lawrence Berkeley National Laboratory, expressed the need for clear observable signatures. Like Verlinde, Carney was drawn to Jacobson’s early work connecting entropy and gravity. A particular passage in Jacobson’s paper suggested that gravity might be as fundamental as the seemingly transient phenomenon of a sound wave, dependent on the ebb and flow of air molecules, and thus equally ephemeral and reliant on simpler underlying processes.
Carney has pondered this idea for years, and recently identified a potential pathway for laboratory testing. His approach is elegantly straightforward. Rather than subscribing to Verlinde’s speculative “information is reality” stance, Carney and his colleagues propose a more generalized background system—a collection of microscopic entities whose precise nature is secondary. This background system, akin to the gas molecules inside a piston, operates thermodynamically, conserving energy while striving to maximize entropy.
Remarkably, their models demonstrate that when test masses interact with this background system, they exhibit gravitational attraction in accordance with Isaac Newton’s law of gravitation, even though the force of gravity itself is not explicitly incorporated into the model. This finding suggests alternative theoretical frameworks for understanding gravity. Carney’s team explored two distinct models. The first, a highly simplified model, predicted erratic gravitational behavior inconsistent with observations. The second model, however, incorporated quantum effects, such as the ability of fundamental components of reality to exist in multiple energy states simultaneously and to be entangled.
This second model yielded a more realistic depiction of gravity, though not an exact replica. Because the force arises from a background system governed by thermodynamic principles, it would inevitably exhibit subtle fluctuations. In other words, if this model accurately reflects reality, then minor irregularities should be detectable in the otherwise smooth gravitational attraction between objects.
These findings represent a significant step forward, potentially opening the door to experimental verification of entropic gravity. Researchers would need to detect these characteristic gravitational anomalies, or “blips.” Existing technologies could be adapted for this purpose. For example, highly sensitive instruments such as tiny, weighted levers could detect minute movements as a mass is brought into proximity. These devices are currently employed in research related to gravitational waves or the limits of quantum behavior. Repurposing them for entropic gravity detection would require time and dedicated effort. Carney and his colleagues are already designing an experiment involving a weight on a twisting pendulum alongside a cloud of atoms in a quantum state. Traditional gravity would produce predictable changes in the atoms’ quantum state as the pendulum oscillates. However, any random fluctuations stemming from entropic effects should be discernible.
Remmen finds these developments particularly compelling, especially the identification of an experimental signature. Nevertheless, he notes that Carney’s work currently only reproduces Newtonian gravity, not the more advanced predictions of general relativity. Verlinde, for his part, would have preferred a model incorporating holography, which he believes is essential for a truly emergent gravity framework. Despite this, he praises the development as “really nice” and values the prospect of experimental investigation. Verlinde emphasizes that both theorists and experimentalists benefit from such inspiration and that collaborative efforts, as exemplified by Carney’s work bridging these two worlds, are crucial.
Expanding the Scope: Dark Energy and the Universe’s Expansion
Meanwhile, other researchers are finding entropic gravity increasingly compelling. Kazem Rezazadeh, at the Institute for Research in Fundamental Sciences in Iran, drew inspiration from Verlinde’s original paper to refine the description of entities on the putative two-dimensional holographic screen responsible for gravity through increasing disorder. In thermodynamics, it is recognized that entropy does not always scale directly with the energy of a system’s microscopic components.
By applying appropriate adjustments to Verlinde’s entropy equations, Rezazadeh recently determined that entropic gravity, operating on the largest cosmological scales, should manifest as an accelerated expansion of spacetime. This phenomenon, known as dark energy, has perplexed cosmologists for nearly three decades. Amazingly, Rezazadeh’s entropic gravity model aligns better with observational data indicating dark energy than the current standard cosmological model.
This suggests that another of physics’ grand enigmas could potentially be an illusion generated by entropic effects. Rezazadeh acknowledges the significance of this finding, but stresses the need for more precise observational data to definitively assess its validity. The fundamental question of what entropic gravity truly signifies remains ongoing. No one can definitively identify the disordered microscopic entities proposed to generate our perception of gravity, nor convincingly explain whether their presumed world—a spaceless, two-dimensional screen—is inherently more “real” than our own.
However, for some physicists, these questions may not be the most pressing. If one adopts the philosophical stance that all reality ultimately comprises information, then debates about the underlying nature of that information become less critical. Verlinde views this perspective as fitting for our current era, noting that we naturally employ the language of information because it reflects the technological age in which we live. This might appear a rather sociological observation for a theoretical physicist. Yet, history suggests that scientific advancements are often influenced by the prevailing paradigms of their time. Carney draws a parallel to Albert Einstein’s deep engagement with relativity, suggesting it was partly driven by the widespread problem of synchronizing train schedules across distant cities in his era. In essence, as Carney concludes, individuals endeavor to find answers using the conceptual tools and frameworks available in their present-day world.