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Negative mass

Negative mass is a hypothetical type of exotic matter in theoretical physics characterized by a mass value with the opposite sign to that of ordinary positive mass, resulting in counterintuitive behaviors such as accelerating in the direction opposite to an applied force, as described by Newton's second law F = ma where m < 0.^[1] This concept implies that negative mass would gravitationally repel positive mass while being attracted to it, leading to peculiar dynamics like runaway motion in mixed systems where positive and negative masses chase each other indefinitely.^[2] Although no stable particles with true negative inertial or gravitational mass have been observed in nature, the idea does not violate the structure of general relativity and has been explored for its potential roles in phenomena like wormholes, faster-than-light travel, and cosmic acceleration.^[3] The notion of negative mass was first systematically analyzed by physicist Hermann Bondi in 1957, who demonstrated its compatibility with Einstein's general theory of relativity by showing that it satisfies the field equations without introducing inconsistencies, provided the weak, strong, and dominant energy conditions are relaxed.^[2] Bondi's work highlighted how negative mass could repel all forms of matter gravitationally, contrasting with the universal attraction of positive mass, and he considered scenarios like a universe composed solely of negative mass, which would exhibit repulsive gravity and potentially stable structures.^[2] Subsequent studies, such as those by Robert Forward in 1990, extended these ideas to practical applications, including reactionless propulsion systems where equal amounts of positive and negative mass could yield zero net mass but nonzero thrust.^[3] In modern contexts, negative mass appears in discussions of cosmology, where it has been proposed as a candidate for dark energy due to its repulsive gravitational effects, potentially explaining the universe's accelerated expansion without invoking a cosmological constant.^[3] Quantum field theory also permits transient negative energy densities, as evidenced by the Casimir effect, where virtual particles between conducting plates produce measurable negative energy, though this is not equivalent to bulk negative mass matter.^[1] Experimentally, effective negative mass has been realized in controlled settings: in 2017, researchers at Washington State University used a Bose-Einstein condensate of rubidium atoms manipulated by lasers to create a fluid that accelerates backward when pushed, mimicking negative mass dynamics and providing a laboratory analog for studying astrophysical behaviors like those in neutron stars or black holes.^[4] These analogs confirm the theoretical predictions but do not produce true negative mass particles, underscoring the concept's status as a theoretical tool rather than an observed reality.

Basic Concepts

Definition and Hypothetical Nature

Negative mass is a hypothetical form of exotic matter characterized by a negative value of mass, m < 0, in direct contrast to the positive mass of ordinary matter, which constitutes all known particles and objects in the universe.^[2] This concept challenges conventional physical intuitions, as mass is fundamentally tied to inertia, energy, and gravitational interactions, all of which would behave oppositely for negative values.^[3] The idea of negative mass was first systematically explored by physicist Hermann Bondi in his 1957 paper "Negative Mass in General Relativity," where he examined its compatibility with Einstein's theory despite Einstein's earlier offhand dismissal of the possibility.^[2] Bondi proposed negative mass as a theoretical possibility that could arise within relativistic frameworks, laying the groundwork for its study as exotic matter.^[3] The concept gained further attention in 1990 through Robert L. Forward's work on negative matter propulsion, which highlighted its potential applications in advanced theoretical propulsion systems by leveraging its unique inertial properties. A fundamental consequence of negative mass stems from Newton's second law of motion, \mathbf{F} = m \mathbf{a}, which rearranges to \mathbf{a} = \frac{\mathbf{F}}{m}; when m < 0, the acceleration \mathbf{a} points in the direction opposite to the applied force \mathbf{F}, leading to counterintuitive responses to external influences.^[2] This reversal implies that negative mass objects would "run away" from forces rather than resist or follow them as positive mass does. Despite these intriguing properties, negative mass introduces significant stability challenges in theoretical models. Bondi identified the runaway effect in positive-negative mass pairs, where they undergo indefinite mutual acceleration, potentially destabilizing physical systems.^[2] This runaway effect underscores the hypothetical and precarious nature of negative mass, as it conflicts with observed stability in the universe.^[5] Recent theoretical developments continue to probe negative mass without assuming its direct existence. A 2024 study demonstrates that negative Arnowitt-Deser-Misner (ADM) mass can emerge naturally in certain traversable wormhole spacetimes within general relativity, occurring within specific parameter ranges that alter the geometry without explicitly introducing negative mass components.^[6]

Behavior in Newtonian Mechanics

In Newtonian mechanics, the equation of motion is expressed as \vec{F} = m \vec{a}, where m represents the inertial mass and \vec{a} the acceleration. For a particle with negative mass (m < 0), the acceleration occurs in the direction opposite to the applied force \vec{F}, meaning that pushing on such a particle would cause it to move toward the source of the force rather than away. This counterintuitive response arises directly from the sign of m in the equation, leading to reversed dynamics under any applied force, whether gravitational, electromagnetic, or otherwise.^[1] The gravitational force between two particles follows Newton's law of universal gravitation, \vec{F} = -G \frac{m_1 m_2}{r^2} \hat{r}, where G is the gravitational constant, r the separation, and \hat{r} the unit vector along the line connecting them. When one mass is positive (m_1 > 0) and the other negative (m_2 < 0), the product m_1 m_2 < 0, resulting in a repulsive force between unlike masses, analogous to like charges in electrostatics. Thus, a positive mass repels a negative mass, and vice versa, while like-signed masses (both positive or both negative) attract as usual. This sign-flipping in the force law stems from the assumption that gravitational mass also carries the negative sign, maintaining consistency with the equivalence principle in the Newtonian limit. In pairwise interactions between a positive mass m_p > 0 and a negative mass m_n < 0, the dynamics produce a peculiar "chase" behavior. The positive mass experiences a repulsive force and accelerates away from the negative mass, while the negative mass, despite the repulsion, accelerates toward the positive mass due to its negative inertial response. This mutual acceleration causes the pair to move in the same direction indefinitely, with the negative mass pursuing the fleeing positive mass, without ever colliding, as their relative velocity increases but separation may remain constant or vary depending on mass ratios. For equal magnitudes (|m_n| = m_p), the center of mass remains stationary, and both particles accelerate uniformly in the direction of the positive mass's initial motion, conserving total momentum since the negative mass contributes oppositely.^[3] A specific example illustrates this runaway chase: consider two isolated particles in empty space, one with m_p = +1 kg and the other with m_n = -1 kg, initially at rest separated by distance r_0. The initial gravitational force repels the positive particle away and "attracts" the negative particle toward it (via opposite acceleration), causing both to accelerate in the positive particle's direction with magnitude a = G |m_p m_n| / r^2, but their separation remains fixed at r_0 as they co-accelerate. Over time, their speeds increase without bound in an unbounded space, potentially approaching relativistic limits, though this remains within Newtonian analysis here.^[1] Regarding energy, the total mechanical energy of such a system is conserved, with the gravitational potential energy for opposite masses being positive (U = -G m_p m_n / r > 0 since m_n < 0), and kinetic energy contributions from negative mass yielding negative values (K_n = \frac{1}{2} m_n v_n^2 < 0). No violation of conservation laws occurs in isolated pairs, as momentum and energy balance through the opposing signs; however, creating equal positive-negative mass pairs could release energy, though stability requires quantum constraints to prevent spontaneous pair production. In unbounded scenarios, the chase allows infinite acceleration without external energy input, highlighting the hypothetical stability issues of isolated negative masses.^[3]

Implications in General Relativity

Inertial versus Gravitational Mass

In general relativity, the equivalence principle asserts that the inertial mass m_i, which governs an object's response to non-gravitational forces via Newton's second law \vec{F} = m_i \vec{a}, is identical to the passive gravitational mass m_g, which determines the strength of the gravitational force experienced by the object in a gravitational field \vec{g} according to \vec{W} = m_g \vec{g}. This equivalence, a cornerstone of the theory, implies that all objects accelerate identically in a gravitational field regardless of their composition or mass value. For hypothetical negative mass, maintaining this principle requires both m_i and m_g to be negative, ensuring consistent behavior: a negative m_g produces a repulsive gravitational interaction with positive masses, while a negative m_i reverses the acceleration direction relative to the force, resulting in net attraction toward positive masses.^[7]^[3] Hermann Bondi, in his seminal analysis, assumed both masses are negative to preserve the equivalence principle's integrity within general relativity, arguing that such matter remains dynamically consistent without introducing inconsistencies in geodesic motion. This configuration leads to peculiar interactions, such as negative masses repelling each other while being attracted to positive masses, upholding the principle that all matter follows the same worldlines in curved spacetime.^[7]^[3] Alternative theoretical proposals, such as those positing positive inertial mass (m_i > 0) alongside negative gravitational mass (m_g < 0), would violate the weak equivalence principle by causing objects to accelerate differently in gravitational fields based on their mass types, leading to non-universal motion and potential inconsistencies with observed universality. Such scenarios are generally discarded in favor of Bondi's symmetric negative assumption, as they undermine the foundational symmetry of general relativity.^[8]^[9] The implications of negative mass distinguish the roles sharply: negative gravitational mass (m_g < 0) implies antigravity, where the object repels positive masses and experiences repulsion from them, enabling phenomena like self-propulsion in gravitational fields without energy input. Conversely, negative inertial mass (m_i < 0) implies reversed inertia, where applied forces accelerate the object in the opposite direction, akin to negative resistance to motion. These properties inform requirements for exotic matter in theoretical constructs like wormholes, though no direct experimental tests exist due to the unobserved nature of negative mass.^[7]^[3]

Runaway Motion

In the relativistic framework within flat spacetime, a positive mass particle is repelled by a nearby negative mass particle, accelerating away from it, while the negative mass particle is attracted toward the positive one, accelerating in pursuit. This counterintuitive interaction, driven by the opposite signs of gravitational mass, results in a "runaway" motion where the particles perpetually chase each other, leading to unbounded acceleration and separation.^[2] Hermann Bondi analyzed this dynamics in 1957, demonstrating that for a pair of particles with equal magnitude but opposite mass signs, the relative velocity between them increases without limit as time tends to infinity, while the total energy of the system grows unbounded due to the continuous conversion of gravitational potential energy into kinetic energy.^[2] In the special case of equal |m| values, the relative velocity approaches the speed of light asymptotically and can be sketched approximately as v \approx c \left(1 - e^{-t/\tau}\right), where \tau is a characteristic timescale determined by the gravitational coupling and initial conditions, reflecting the hyperbolic nature of the motion under constant proper acceleration.^[2] Within the full general relativity, the runaway instability is amplified by gravitomagnetic effects, where the rotation or motion of the masses induces frame-dragging that couples with the sign-flipped gravitational fields, exacerbating the unbounded acceleration and potentially leading to singular behaviors in the spacetime metric.^[10] This extends the unstable dynamics hinted at in Newtonian treatments, where positive and negative masses already display self-accelerating pairs without external energy input.^[3] Recent investigations in 2024 have revealed that analogous runaway-like instabilities can emerge in specific metric solutions of general relativity, arising from geometric configurations such as fractal spacetime vacancies that mimic effective negative mass behaviors without requiring explicit introduction of negative mass matter.^[11]

Exotic Matter and Spacetimes

In general relativity, exotic matter refers to hypothetical forms of matter or energy that violate classical energy conditions, such as the null energy condition (NEC), which states that for any null vector k^\mu, the stress-energy tensor satisfies T_{\mu\nu} k^\mu k^\nu \geq 0.^[3] This condition ensures non-negative energy density along lightlike paths, but exotic matter with negative energy density \rho < 0 can breach it, enabling spacetime geometries otherwise forbidden. Negative mass particles, where the mass parameter m < 0, naturally produce such negative energy densities in their stress-energy contributions, making them a candidate for exotic matter in theoretical constructs.^[3] Traversable wormholes, which connect distant regions of spacetime without singularities, require exotic matter to maintain an open throat against gravitational collapse. The Morris-Thorne metric, a standard spherically symmetric wormhole solution, demands that the energy density \rho and radial pressure p satisfy \rho + p < 0 at the throat to uphold the flaring-out condition and violate the NEC locally.^[12] Negative mass distributed along the wormhole's geometry can fulfill this requirement, providing the repulsive gravitational effect needed for stability.^[13] In a 1995 analysis by Cramer, Forward, et al., a design using negative mass struts or threads was outlined to reinforce the wormhole throat, ensuring structural integrity while minimizing total exotic matter needs.^[14] The Alcubierre warp drive similarly relies on exotic matter to achieve effective superluminal travel by contracting spacetime ahead of a spacecraft and expanding it behind, forming a warp bubble. The 1994 Alcubierre metric requires regions of negative energy density within the bubble walls to generate the necessary spacetime curvature, with the stress-energy tensor featuring negative components proportional to the bubble's velocity profile.^[15] Negative mass can supply this exotic stress-energy, enabling the bubble to propagate faster than light locally while keeping the interior flat and causality intact for observers inside.^[16] While quantum effects offer limited analogs to exotic matter, classical negative mass provides a more efficient means for macroscopic applications. The Casimir effect, arising from vacuum fluctuations between conducting plates, yields measurable negative energy densities on microscopic scales, serving as a quantum loophole to the NEC.^[17] However, scaling this to the quantities needed for wormholes or warp drives remains impractical, whereas hypothetical negative mass could deliver the required energy scales without relying on quantum confinement.^[18] Recent theoretical models have explored negative mass in black hole spacetimes, yielding regular horizons free of singularities. In a 2024 study, regular negative mass black holes were constructed under a unitary antichronous formalism, where the negative mass parameter leads to stable event horizons without central divergences, potentially resolving information paradoxes through modified evaporation dynamics.^[19] A 2025 analysis further demonstrated that negative mass black holes can emerge from wormhole-to-black-hole transitions in extended gravity theories, exhibiting regular interiors and horizons that avoid mass inflation instabilities.^[20]

Cosmological Applications

Relation to Dark Energy

Dark energy is a hypothetical form of energy that permeates all of space and drives the accelerated expansion of the universe, as described in the standard ΛCDM cosmological model where it constitutes approximately 68% of the total energy density and is characterized by an equation-of-state parameter w \approx -1. This repulsive effect counteracts gravitational attraction, leading to the observed cosmic acceleration first inferred from Type Ia supernovae observations in the late 1990s. The concept of negative mass has been proposed as an alternative explanation for this dark energy effect, with early work by Hermann Bondi in 1957 suggesting that equal amounts of positive and negative mass could produce a net repulsive force. In such systems, positive mass attracts both positive and negative mass, while negative mass repels both, resulting in "runaway" motion where positive mass accelerates toward negative mass, and the negative mass accelerates away in the opposite direction; for equal magnitudes, the center of mass remains stationary, but the pair expands uniformly. In certain negative mass models with continuous creation, an effective equation-of-state parameter w = -1 can be achieved, mimicking the cosmological constant.^[21] In negative mass models, the dynamics lead to negative mass fluid clustering away from positive mass regions, effectively pushing ordinary matter apart and simulating large-scale expansion. This self-repulsion of negative mass prevents it from forming dense structures, promoting a more uniform distribution that aligns with the observed large-scale homogeneity of the universe. For instance, in a 2018 model by Jamie Farnes, equal positive-negative mass pairs undergo runaway motion that dilutes the negative mass component uniformly over cosmic scales, reproducing the observed cosmic microwave background uniformity and accelerated expansion without invoking a separate dark energy component. However, recent critiques highlight alternatives to negative mass for explaining dark energy. In a 2025 model proposed by Richard Lieu, transient temporal singularities in spacetime replace both dark energy and dark matter, eliminating the need for exotic negative mass or density while accounting for cosmic acceleration through localized bursts of matter creation.^[22] This approach underscores ongoing debates about whether negative mass provides a viable substitute for dark energy or if simpler gravitational modifications suffice.^[23]

Negative Mass Cosmology Models

One prominent model incorporating negative mass in cosmology is the Dirac-Milne universe, which posits a symmetric distribution of positive and negative masses, often interpreted through the lens of matter-antimatter duality where antimatter carries negative gravitational mass.^[24] However, this hypothesis of negative gravitational mass for antimatter is inconsistent with experimental measurements, such as those from the ALPHA-g experiment, which indicate that antimatter falls toward Earth with acceleration similar to ordinary matter.^[25] In this framework, symmetric creation of positive-negative mass pairs maintains balance, resulting in a net zero gravitational effect and a linear scale factor evolution a(t) \propto t, corresponding to an expansion rate H \propto t^{-1}.^[24] This coasting cosmology inherently avoids the Big Bang singularity, as the universe expands uniformly without an initial high-density state, extending eternally in both temporal directions.^[26] The concept draws foundational support from early explorations of negative mass compatibility with general relativity.^[2] Another influential approach is the negative mass cosmology proposed by Farnes, where a fluid of negative masses is continuously created to mimic both dark matter and dark energy within a modified \LambdaCDM framework.^[21] Here, negative masses self-repel while attracting positive masses, leading to an effective repulsive mechanism that drives cosmic acceleration and resolves the flatness problem by dynamically adjusting the density parameter \Omega toward unity without fine-tuning.^[21] This model treats negative mass as a "mirror" component that balances the universe's geometry, echoing earlier ideas of symmetric sectors but emphasizing ongoing pair production to sustain expansion.^[21] Such constructions prioritize the repulsive dynamics of negative mass to emulate observed late-time acceleration, distinct from pure symmetric models.^[21] Observational tests impose stringent constraints on these models, as cosmic microwave background (CMB) data indicate a dominance of positive matter with \Omega_m \approx 0.315 and no significant negative component. Type Ia supernova observations similarly support \LambdaCDM with positive energy densities driving acceleration, while substantial negative mass fractions would distort baryon acoustic oscillations (BAO) by altering the sound horizon scale and power spectrum peaks beyond current measurements from surveys like BOSS. Numerical simulations of negative mass cosmologies reveal further challenges in matching observations. For instance, models treating negative mass as a fluid for structure formation show that negative components clump efficiently due to self-attraction, but positive matter experiences runaway acceleration, leading to underdense voids and excessive large-scale clumpiness that deviates from the \LambdaCDM predictions for galaxy clustering and weak lensing.^[27] Despite these features, the viability of negative mass cosmologies remains limited by inherent instabilities, such as uncontrolled pair production leading to runaway motion unless suppressed by ad hoc mechanisms like creation tensors. Without empirical evidence for negative mass particles or fluids as of 2025, these models lack direct support and face ongoing theoretical revisions to ensure stability within general relativity.

Quantum Mechanical Treatments

Negative Mass in the Schrödinger Equation

The non-relativistic Schrödinger equation describes the time evolution of the wave function \psi for a particle of mass m as

i \hbar \frac{\partial \psi}{\partial t} = \left[ -\frac{\hbar^2}{2m} \nabla^2 + V \right] \psi,

where V is the potential energy, \hbar is the reduced Planck's constant, and the term -\frac{\hbar^2}{2m} \nabla^2 represents the kinetic energy operator.^[28] For a particle with negative mass m < 0, let m = -|m|; the kinetic term then becomes positive, yielding

i \hbar \frac{\partial \psi}{\partial t} = \left[ \frac{\hbar^2}{2|m|} \nabla^2 + V \right] \psi.

This sign inversion alters the dispersion relation for free plane wave solutions \psi \propto \exp(i \mathbf{k} \cdot \mathbf{r} - i \omega t), where the frequency is \omega = \frac{\hbar k^2}{2m}. With m < 0, \omega < 0 for real wave vector \mathbf{k}, implying that the phase velocity \mathbf{v}_p = \frac{\omega}{k} = \frac{\hbar k}{2m} points opposite to \mathbf{k}. The group velocity \mathbf{v}_g = \frac{d\omega}{dk} = \frac{\hbar k}{m} also opposes \mathbf{k} (since m < 0), resulting in backward propagation of wave packets relative to the direction expected for positive mass.^[28]^[29] A relativistic extension appears in the Klein-Gordon equation, proposed by Walter Gordon in 1926 (with refinements in 1928), which takes the form

\left( \square + \frac{m^2 c^2}{\hbar^2} \right) \psi = 0,

where \square is the d'Alembertian operator and c is the speed of light. Solutions include negative energy states E = -\sqrt{p^2 c^2 + m^2 c^4}, interpretable as particles with effective negative mass via E = m c^2.^[30] These states introduce peculiarities such as negative probability densities, later resolved in quantum field theory interpretations.^[28] The inverted kinetic term in the negative-mass Schrödinger equation raises stability concerns, as it resembles a tachyonic regime with exponentially growing modes for certain potentials, potentially leading to instabilities unless V is tuned (e.g., via PT-symmetric adjustments) to bound the energy spectrum from below.^[30] In theoretical applications, such effective negative masses model hole-like quasiparticles in semiconductor valence bands, where the band curvature yields m^* < 0 for electrons near the band maximum, facilitating descriptions of charge transport despite the formal sign issue.^[31]^[32]

Effective Negative Mass in Quantum Systems

In quantum systems, particularly in condensed matter physics, the concept of effective mass describes how quasiparticles such as electrons behave as if they have a mass different from their bare rest mass due to interactions with the lattice or other particles. The effective mass m^* is given by the relation

m^* = \frac{\hbar^2}{\frac{d^2 E}{dk^2}},

where E is the energy dispersion and k is the wavevector; regions of inverted band curvature, where \frac{d^2 E}{dk^2} < 0, yield m^* < 0. This negative effective mass manifests in semiconductors near the top of the valence band, where electrons exhibit hole-like behavior under excitation, accelerating opposite to an applied force but interpreted positively for the resulting hole quasiparticle. Unlike true negative rest mass, which would imply fundamental violations of energy conditions in relativistic frameworks, effective negative mass arises emergently from collective interactions and band structure geometry in many-body quantum systems, without altering the underlying particle's intrinsic properties. In solid-state materials, this leads to inverted dynamics, such as enhanced mobility or anomalous transport, but remains bounded by the positive rest masses of constituents. A prominent example occurs in hybrid light-matter systems like exciton-polaritons, quasiparticles formed by coupling excitons (electron-hole pairs) with cavity photons. In a 2023 experiment by researchers at the Australian National University (part of the FLEET consortium), dissipative light-matter coupling in an optical microcavity induced a non-Hermitian dispersion relation for exciton-polaritons, resulting in negative effective mass and inverted propagation dynamics, including self-trapping and anomalous refraction.^[33] This effective negativity stems from the interplay of gain and loss in the cavity, enabling observation of quasiparticles accelerating against repulsive potentials. In topological materials such as Dirac and Weyl semimetals, the low-energy dispersion is linear (E \propto |k|), rendering the effective mass formally zero at the nodal points, but higher-order expansions reveal parabolic contributions with opposite curvature signs in conduction and valence bands, allowing access to negative effective mass regions. Electrostatic gating in thin films of materials like Cd₃As₂ tunes the Fermi level across these bands, enabling control over carrier type and population of negative-mass states, which influences transport properties like negative magnetoresistance.^[34] Simulations of negative-mass fluids in spin-orbit-coupled Bose-Einstein condensates, as explored in recent theoretical work, predict the formation of stable solitons due to the balance between negative-mass-induced repulsion and attractive interactions, contrasting with instability in positive-mass counterparts.^[35] These quantum gas models highlight how effective negative mass can stabilize localized wave structures, offering insights into novel quantum phases without requiring fundamental negative rest mass.

Experimental Analogues and Tests

Antimatter Gravity Experiments

In the 1930s, Paul Dirac speculated that antimatter, arising from solutions to his relativistic quantum equation, might exhibit negative gravitational mass, leading to repulsive interactions between matter and antimatter.^[36] However, the CPT theorem, a cornerstone of quantum field theory, implies that antimatter should respond to gravity identically to matter, as charge conjugation, parity, and time reversal symmetries preserve the sign of gravitational mass.^[37] The ALPHA-g experiment at CERN's Antiproton Decelerator, operational since 2018, has conducted the first direct measurements of antimatter's response to Earth's gravity using antihydrogen atoms. In 2023, ALPHA-g released antihydrogen from magnetic traps and observed its vertical distribution after free fall, finding that the atoms accelerate downward with a gravitational acceleration consistent with that of normal matter, \bar{g} = (0.75 \pm 0.13_{\text{statistical + systematic}} \pm 0.16_{\text{simulation}}) \, g, where g is Earth's gravitational acceleration; this result rules out negative gravitational mass within the experiment's approximately 30% precision.^[25]^[38] The GBAR experiment, also at CERN, aims to test the weak equivalence principle by measuring the free fall of antihydrogen ions and neutral atoms over distances of 10–20 cm. As of 2025, GBAR has achieved record positron accumulations and antihydrogen synthesis but has not yet reported gravitational free-fall data; the experiment plans to reach 1% precision in measurements starting in late 2025, building on infrastructure upgrades completed in 2024.^[39]^[40] Complementing these efforts, the AEgIS experiment at CERN employs laser-cooled positronium atoms to produce a horizontal beam of antihydrogen for precise gravitational deflection measurements. AEgIS demonstrated the first laser cooling of a positronium cloud in 2024, enabling pulsed antihydrogen production with velocities suitable for free-fall tests over 20–50 cm; however, full gravitational acceleration data remain pending as of 2025, with the setup targeting sub-percent precision in future runs.^[41]^[42] As of November 2025, results from ALPHA-g and preparatory work by GBAR and AEgIS show no evidence of negative gravitational mass for antimatter, confirming that it falls toward Earth like ordinary matter and upholding the weak equivalence principle within current experimental bounds.^[25]^[43] A negative gravitational mass would violate this principle by causing antimatter to accelerate oppositely to matter in the same field, and the absence of such behavior constrains theoretical models proposing antigravity, such as extensions of Dirac's ideas.^[44]^[45]

Metamaterials and Phononic Analogues

Metamaterials engineered to exhibit negative effective mass density have enabled the simulation of negative mass behaviors in acoustic and mechanical wave propagation, providing valuable analogues for studying exotic physical phenomena without relying on hypothetical particles. These structures typically achieve negative effective mass through local resonances in subwavelength unit cells, where the internal oscillators respond out of phase with the incident wave, resulting in an effective inertial response opposite to the applied force. This leads to unusual wave phenomena such as backward-propagating waves, where the phase and group velocities oppose each other. Seminal theoretical work demonstrated that such resonances in acoustic metamaterials can produce frequency bands with negative effective mass density near the resonance frequency of embedded masses connected by compliant elements. Experimental realization of these effects was achieved in a one-dimensional phononic system using an array of pendula as resonators attached to a chain of masses and springs, demonstrating negative effective mass for low-frequency sound waves around 300-500 Hz. In this setup, the pendula's resonant motion induced a bandgap where the effective mass density became negative, confirmed through transmission measurements showing wave attenuation and phase shifts indicative of backward wave propagation. This 2008 experiment highlighted the practicality of discrete mechanical arrays in mimicking negative mass dynamics for phonons, paving the way for scalable phononic crystals.^[46] The underlying vibrational theory for these systems can be described by the equation of motion for the effective medium: m_{\text{eff}} \ddot{u} + k u = 0, where m_{\text{eff}} < 0 in the resonant band implies exponential growth in displacement unless dissipation is introduced to stabilize the response. Damping mechanisms, such as viscous losses in the resonators, prevent instabilities while preserving the negative mass regime, allowing controlled wave manipulation. This framework distinguishes phononic analogues from true negative mass particles, as they pertain to collective excitations (phonons) in lattice vibrations rather than individual massive entities, and enable indirect probes of general relativity analogues like warped spacetimes through wave curvature effects. Such metamaterials have found applications in acoustic cloaking, where negative mass density enables impedance matching and wave bending around objects, and in superlenses that overcome diffraction limits for subwavelength imaging. Recent advances in 2024 have extended these capabilities to three-dimensional printed structures, achieving broadband negative effective mass over wider frequency ranges (up to several hundred Hz) for isotropic wave control in complex environments. These developments enhance potential uses in noise barriers and medical ultrasound focusing, leveraging additive manufacturing for precise resonator geometries.

Bose-Einstein Condensate Simulations

In a landmark experiment conducted in 2017 at Washington State University, researchers engineered a Bose-Einstein condensate (BEC) of approximately $10^5 rubidium-87 atoms to exhibit negative effective mass hydrodynamics. By applying Raman laser coupling to induce spin-orbit interactions, the team created a dispersion relation with a region of negative curvature in the lower energy band, mimicking the kinematics of particles with negative inertial mass. This setup allowed direct observation of counterintuitive dynamics, where excitations in the negative-mass regime accelerated opposite to an applied force, violating standard Newtonian expectations.^[47] The system's evolution was modeled using a single-band Gross-Pitaevskii equation incorporating the spin-orbit term:

i \hbar \frac{\partial \psi}{\partial t} = \left[ -\frac{\hbar^2}{2m} \frac{\partial^2}{\partial z^2} + V_\text{ext}(z) + \frac{\hbar k_r}{m} p_\sigma \sigma_z + \frac{\hbar^2 k_r^2}{2m} + g |\psi|^2 \right] \psi,

where k_r is the recoil momentum from the Raman process, \sigma_z is the spin operator, and the effective dispersion E^-(p) features a negative second derivative \partial^2 E^- / \partial p^2 < 0 near zero momentum. In the hydrodynamic formulation via Madelung transformation, this leads to modified continuity and Euler equations with an effective velocity v^* = v - (\hbar k_r / m) \sigma_z and anomalous quantum pressure terms, resulting in instabilities like runaway expansion and dispersive shock waves. Upon trap release, the BEC displayed asymmetric expansion, with the negative-mass component propagating backward and forming void-like expanding bubbles amid the positive-mass fluid.^[47] These BEC realizations have inspired theoretical simulations of negative-mass fluids for broader applications, including cosmology. Numerical models of such fluids demonstrate self-gravitating clustering behaviors analogous to dark matter halos, where negative-mass components stabilize structures through attractive interactions with positive-mass matter, naturally reproducing observed galactic distributions without fine-tuning.^[48] Despite these advances, BEC simulations achieve only effective negative mass for collective low-energy modes, limited by the underlying positive rest mass of atoms and the validity of the long-wavelength approximation; true negative rest mass remains unrealized, and effects dissipate at higher energies or momenta.^[47]

Philosophical and Fundamental Implications

Arrow of Time Considerations

The arrow of time is fundamentally linked to the second law of thermodynamics, which dictates that the entropy of an isolated system cannot decrease over time, establishing a preferred direction for thermodynamic processes. Negative mass, by inverting the response to applied forces—accelerating away from rather than toward the source of the force—could theoretically reverse this directionality, leading to systems where entropy production is inverted and disorder decreases. This inversion arises because the dynamical equations for negative mass particles mirror those of positive mass but with time-reversed signatures in their evolution. In his pioneering analysis, Hermann Bondi demonstrated that negative mass is consistent with general relativity and that systems dominated by negative mass evolve in a manner akin to "backwards" in time relative to positive mass systems. Specifically, gravitational contraction in positive mass configurations corresponds to expansion in negative mass ones, suggesting a reversal of the cosmological arrow of time.^[2] Interactions between positive and negative mass pairs, as explored in general relativity, can produce unbounded accelerations known as runaway motion, where the pair mutually repels yet chases indefinitely; in certain spacetime setups, such configurations violate energy conditions and enable closed timelike curves, thereby challenging the principle of causality.^[3] This framework connects philosophically to Loschmidt's paradox, which questions how time-symmetric microscopic laws yield irreversible macroscopic entropy increase; negative mass offers a resolution by introducing particles that propagate backward in time with negative inertia, effectively separating forward and reverse timelines to uphold thermodynamic irreversibility.^[49] As of 2025, all considerations of negative mass's implications for the arrow of time remain purely theoretical, with no observational or experimental confirmation of negative mass existence.^[50]

Energy Inversion and Causality

Negative mass configurations inherently violate fundamental energy conditions in general relativity, particularly the weak energy condition (WEC), which requires that the energy density \rho measured by any timelike observer satisfies \rho \geq 0 and \rho + p \geq 0 for isotropic pressures p. For negative mass, the associated energy density becomes \rho < 0, directly contravening the WEC and leading to pathological spacetime geometries that challenge classical stability. This violation arises because negative mass implies a negative contribution to the stress-energy tensor, allowing for exotic phenomena not observed in standard matter. Recent theoretical models, such as those exploring wormholes in 2024, have shown the emergence of negative ADM mass in specific parameter ranges, further illustrating these violations.^[50] A key consequence of negative mass is the inversion of kinetic energy in non-relativistic mechanics, where the standard formula E_k = \frac{p^2}{2m} yields E_k < 0 for real momentum p when m < 0. This negative kinetic energy results in counterintuitive dynamics, such as particles accelerating away from applied forces. Relativistically, such systems can exhibit runaway motion where positive-mass and negative-mass particles mutually accelerate indefinitely, potentially destabilizing local causality, while still respecting the speed-of-light limit. In general relativity, these negative energy densities enable the construction of traversable wormholes, which, if stabilized, permit closed timelike curves (CTCs) that violate causality by allowing information or particles to travel backward in time. Stephen Hawking's chronology protection conjecture posits that quantum gravitational effects, such as vacuum fluctuations, would destabilize such wormholes to prevent CTC formation and preserve causality, as sustained negative energy would otherwise lead to paradoxes like the grandfather paradox. To mitigate these issues, quantum field theory imposes restrictions via quantum inequalities, which bound both the magnitude and temporal duration of negative energy densities; for instance, the averaged null energy condition (ANEC) ensures that integrated negative energy over null geodesics remains non-negative, limiting the feasibility of long-lived exotic structures. These inequalities, derived from uncertainty principle-like constraints, suggest that while transient negative energies occur in quantum vacuum states (e.g., the Casimir effect), they cannot accumulate sufficiently to enable macroscopic causality violations. Discussions of negative mass cosmologies further highlight that such setups risk time-travel paradoxes, reinforcing arguments against their physical realization.^[51]^[52]

References

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[PDF] Negative Mass in Contemporary Physics
Negative mass reacts oppositely to all forces! • Force and acceleration are opposite. •F = ma. • When you push on negative mass it moves toward ...
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[PDF] Negative Mass in Contemporary Physics, and its Application to ...
As first analyzed by Hermann Bondi in 1957, matter with negative mass is consistent with the structure of Einstein's general theory of Relativity. Although ...
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(PDF) Solving the negative mass paradox. - ResearchGate
Bondi showed that the introduction of negative masses in the universe goes with a preposterous phenomenon : the so-called runaway effect : when a positive mass ...Missing: hypothetical stability
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[PDF] Weak-Equivalence Principle Violation and Mass Change of ...
The gravitational mass would be affected while the inertial mass remains the same.
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[2401.15789] Anti-Gravity from Vacancies in Fractal Space-Time
Jan 28, 2024 · Using the Menger Sponge as a prototype, we construct an effective negative-mass Schwarzschild-like metric for embedded observers and compute the ...
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Certain wormholes could in fact be made large enough for a person or spaceship. Someone might enter the mouth of a wormhole stationed on Earth, walk a short.
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New Improved Wormholes
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[2409.13718] On Regular Negative Mass Black Holes Under Unitary ...
Sep 7, 2024 · ... negative mass interaction does not result in the runaway motion paradox or vacuum instability. The correspondence of these regular black ...
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Desclarizing the Wormhole to Black Hole with Negative Mass - arXiv
Sep 25, 2025 · Furthermore, the black hole mass can be negative. Our finding suggests that exotic matter responsible for wormholes can also lead to exotic ...Missing: 2023-2025 horizons
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[2503.08733] Are dark matter and dark energy omnipresent? - arXiv
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Researcher proposes first-time model that replaces dark energy and ...
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Introducing the Dirac-Milne universe | Astronomy & Astrophysics (A&A)
Antimatter has a negative active gravitational mass and there is a gravitational repulsion between matter and antimatter. This hypothesis is a necessity to ...
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[PDF] The Dirac-Milne Universe - CERN Indico
Nov 15, 2024 · Antimatter naturally comes as negative mass candidate from Kerr-Newman solution ... Dirac-Milne universe does not present acceleration (nor ...
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A unifying theory of dark energy and dark matter: Negative masses ...
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Cosmological structure formation with negative mass | Phys. Rev. D
Jul 13, 2018 · The Bondi case would require a negative inertial mass to ensure energy and momentum conservation, and displays peculiar and counterintuitive ...Missing: concept | Show results with:concept
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[PDF] Brief Survey of Quantum Models and Possible Negative Mass ...
The non-relativistic Schrödinger equation is based on the energy relation E = p2/2m where the spatial momentum is p = mv. The corresponding relativistic ...
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Negative Mass Can Be Positively Useful in Quantum Mechanics
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3. Schrödinger solvers — QTCAD 1.5.1 documentation
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Schroedinger Equation - an overview | ScienceDirect Topics
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Negative mass effects of a spin soliton in Bose-Einstein condensates
Jan 5, 2022 · We can observe the pure negative mass of the spin soliton by adding slowly varying potentials on the bright soliton component. Taking two cases ...
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Gravitational Repulsion and Dirac Antimatter - NASA ADS
... equation serves to extend Dirac's deduction of antimatter parameters to include the possibility of gravitational repulsion between matter and antimatter.
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[1103.4937] CPT symmetry and antimatter gravity in general relativity
Mar 25, 2011 · We investigate this issue on theoretical grounds. Starting from the CPT invariance of physical laws, we transform matter into antimatter in the ...
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Sep 27, 2023 · There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted6 by ...Missing: repels | Show results with:repels
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ALPHA experiment at CERN observes the influence of gravity on ...
Sep 27, 2023 · This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It's a milestone in the study of antimatter.
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AD-7/GBAR status report for the 2025 CERN SPSC
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[PDF] Letter of Intent to the SPSC for the continuation of the AD-7/GBAR ...
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[PDF] AEGIS: an experiment to measure the gravitational interaction ...
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Mar 31, 2025 · Together with probing the Weak Equivalence Principle (WEP) with tests of the free-fall of antihydrogen (ALPHA, AEgIS, GBAR) and tests of the ...
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Matter and antimatter seem to respond equally to gravity | RIKEN
Jan 6, 2022 · ... gravity, or whether antimatter would experience gravity in a different way than matter, which would violate Einstein's weak equivalence ...
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The arguments against “antigravity” and the gravitational ...
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[1612.04055] Negative mass hydrodynamics in a Spin-Orbit - arXiv
Dec 13, 2016 · Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative effective mass. We ...
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A Unifying Theory of Dark Energy and Dark Matter: Negative Masses ...
Dec 18, 2017 · Einstein suggested a long-forgotten solution: gravitationally repulsive negative masses, which drive cosmic expansion and cannot coalesce into ...
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A resolution of the Loschmidt paradox on the Boltzmann H Theorem
### Summary of Resolution of Loschmidt's Paradox Using Negative Mass
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Emergence of negative mass in general relativity
Aug 31, 2024 · This work indicates the emergence of negative ADM mass within a specific parameter range, coinciding with distinct alterations in the wormhole's spacetime ...
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Averaged Energy Conditions and Quantum Inequalities - gr-qc - arXiv
Oct 29, 1994 · Connections are uncovered between the averaged weak (AWEC) and averaged null (ANEC) energy conditions, and quantum inequality restrictions on negative energy.
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[1902.08287] Can a negative-mass cosmology explain dark matter ...
Feb 21, 2019 · Abstract:A recent study by Farnes (2018) proposed an alternative cosmological model in which both dark matter and dark energy are replaced ...Missing: Fraley 1968