Some of the major unsolved problems in physics are theoretical, meaning that existing theories are currently unable to explain certain observed phenomena or experimental results. Others are experimental, involving challenges in creating experiments to test proposed theories or to investigate specific phenomena in greater detail.
A number of important questions remain open in the area of physics beyond the Standard Model, such as the strong CP problem, determining the absolute mass of neutrinos, understanding matter–antimatter asymmetry, and identifying the nature of dark matter and dark energy.
Another significant problem lies within the mathematical framework of the Standard Model itself, which remains inconsistent with general relativity. This incompatibility causes both theories to break down under extreme conditions, such as within known spacetime gravitational singularities like those at the Big Bang and at the centers of black holes beyond their event horizons.
Historical problems and lists Isaac Newton in his book Opticks (first edition 1704), published a list of 31 queries about light and matter.
In 1900, Lord Kelvin in a speech identified two main issues in classical physics that represented "two clouds" for the physics of 20th century: the lack of evidence for the existence of the luminiferous aether and the failures of the equipartition theorem in predicting the heat capacity of solids. Both problems were solved within a decade with the development of special relativity and quantum mechanics respectively.
In 1982, Roger Penrose proposed a list of mathematical physics problems related to general relativity, about 14 problems remain unsolved as of 2017. A similar list of 53 problems was made by Robert Bartnik in 1988.
In 1984, mathematical physicist Barry Simon proposed a list of 15 open problems on quantum operators (about 5 already solved in 2000). A second list of 15 Simon problems was released in 2000 (at least 3 problems solved).
Nobel laureate Vitaly Ginzburg is known for having compiled from 1971 to 2005, a list of open problems in physics on "the verge of the 21st century", the final list includes 30 problems. Ginzburg also included three "great problems": the problem of the arrow of time, the interpretations of quantum mechanics and life arising from physics alone.
During the Strings 2000 conference a list of 10 millennium problems in theoretical physics was proposed by its members, none has been conclusively solved as of 2025.
Mathematical lists Some mathematical physics problems are included in notable lists of unsolved problems in mathematics, these include: The sixth problem of 1900 Hilbert's problems about the axiomatization of physics, the problem is either ill-defined or partially solved. The Navier–Stokes existence and smoothness and the Yang–Mills existence and mass gap problems of the Millennium Prize Problems, both remain unsolved. Navier–Stokes smoothness is also the 15th Smale's problem from 1998. United States DARPA issued 23 mathematical challenges in 2008. Physics problems included: advancing techniques in modelling fluid dynamics and soft matter, developing mathematics for quantum computing, applying quantum field theory and action principles to biology, and finding a relation between the geometric Langlands program and quantum physics.
List by topic
General physics Theory of everything: Is there a singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe?"The Unknown Universe: The Origin of the Universe, Quantum Gravity, Wormholes, and Other Things Science Still Can't Explain." Proceedings of the Royal Society of London, Series A, 1 May 2008. Dimensionless physical constants: At the present time, the values of various dimensionless physical constants cannot be calculated; they can be determined only by physical measurement. What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived? Are dimensional physical constants necessary at all?Alcohol constrains physical constant in the early universe." Science, 13 December 2012.
Quantum gravity Quantum gravity: Can quantum mechanics and general relativity be realized as a fully consistent theory (perhaps as a quantum field theory)? Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical graviton, or be a product of a discrete structure of spacetime itself (as in loop quantum gravity)? Are there deviations from the predictions of general relativity at very small or very large scales or in other extreme circumstances that flow from a quantum gravity mechanism? Black holes, black hole information paradox, and black hole radiation: Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by gauge–gravity duality, or not, as implied by Hawking's original calculation? If not, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point, leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure even exists? The cosmic censorship hypothesis and the chronology protection conjecture: Can singularities not hidden behind an event horizon, known as "naked singularities", arise from realistic initial conditions, or is it possible to prove some version of the "cosmic censorship hypothesis" of Roger Penrose which proposes that this is impossible? Similarly, will the closed timelike curves which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards time travel) be ruled out by a theory of quantum gravity which unites general relativity with quantum mechanics, as suggested by the "chronology protection conjecture" of Stephen Hawking? Holographic principle: Is it true that quantum gravity admits a lower-dimensional description that does not contain gravity? A well-understood example of holography is the AdS/CFT correspondence in string theory. Similarly, can quantum gravity in a de Sitter space be understood using dS/CFT correspondence? Can the AdS/CFT correspondence be vastly generalized to the gauge–gravity duality for arbitrary asymptotic spacetime backgrounds? Are there other theories of quantum gravity other than string theory that admit a holographic description? Quantum spacetime or the emergence of spacetime: Is the nature of spacetime at the Planck scale very different from the continuous classical dynamical spacetime that exists in general relativity? In loop quantum gravity, the spacetime is postulated to be discrete from the beginning. In string theory, although originally spacetime was considered just like in general relativity (with the only difference being supersymmetry), recent research building upon the Ryu–Takayanagi conjecture has taught that spacetime in string theory is emergent by using quantum information theoretic concepts such as entanglement entropy in the AdS/CFT correspondence. However, how exactly the familiar classical spacetime emerges within string theory or the AdS/CFT correspondence is still not well understood. Problem of time: In quantum mechanics, time is a classical background parameter, and the flow of time is universal and absolute. In general relativity, time is one component of four-dimensional spacetime, and the flow of time changes depending on the curvature of spacetime and the spacetime trajectory of the observer. How can these two concepts of time be reconciled?
Quantum physics Yang–Mills theory: Given an arbitrary compact gauge group, does a non-trivial quantum Yang–Mills theory with a finite mass gap exist? (This problem is also listed as one of the Millennium Prize Problems in mathematics.) Quantum field theory (this is a generalization of the previous problem): Is it possible to construct, in a mathematically rigorous way, a quantum field theory in 4-dimensional spacetime that includes interactions and does not resort to perturbative methods?
Cosmology and general relativity Cosmic inflation: Is the theory of cosmic inflation in the very early universe correct, and, if so, what are the details of this epoch? What is the hypothetical scalar field that gave rise to this cosmic inflation? If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some extremely distant place? Cosmic coincidence problem: the energy densities of dark energy and dark matter are of the same order of magnitude in the Universe. Dark matter energy density decreased during the evolution of the universe, so it must have been very large in the past. Is there any relation between the two energy densities? Horizon problem: Why is the distant universe so homogeneous when the Big Bang theory seems to predict larger measurable anisotropies of the night sky than those observed? Cosmological inflation is generally accepted as the solution, but are other possible explanations such as a variable speed of light more appropriate? Origin and future of the universe: How did the conditions for anything to exist arise? Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce? Size of universe: The diameter of the observable universe is about 93 billion light-years (or equivalently a radius of about 14 Gpc), but what is the size of the whole universe? Is the universe infinite? Matter–antimatter asymmetry: Theoretical models suggest that the early universe should have produced equal amounts of matter and antimatter. However, observations indicate no significant primordial antimatter. Understanding the mechanisms that led to this asymmetry is a major unsolved problem in physics. Cosmological principle: Is the universe homogeneous and isotropic at large enough scales, as claimed by the cosmological principle and assumed by all models that use the Friedmann–Lemaître–Robertson–Walker metric, including the current version of the ΛCDM model, or is the universe inhomogeneous or anisotropic? Is the CMB dipole purely kinematic, or does it signal anisotropy of the universe, resulting in the breakdown of the FLRW metric and the cosmological principle? Is the Hubble tension evidence that the cosmological principle is false? Even if the cosmological principle is correct, is the Friedmann–Lemaître–Robertson–Walker metric the right metric to use for our universe? Are the observations usually interpreted as the accelerating expansion of the universe rightly interpreted, or are they instead evidence that the cosmological principle is false? Cosmological constant problem: Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out?
Dark matter: What is the identity of dark matter? Is it a particle? If so, is it a WIMP, axion, the lightest superpartner (LSP), or some other particle? Or, are the phenomena attributed to dark matter the result of an alternate theory of gravity separate from general relativity altogether? Despite extensive research, the exact composition of dark matter remains unknown. It is inferred from gravitational effects on visible matter, radiation, and the universe's large-scale structure. Understanding its properties is crucial for a comprehensive understanding of the universe.