The following problems are either fundamental theoretical problems, or theoretical ideas which lack experimental evidence and are in search of one, or both, as most of them are. Some of these problems are strongly interrelated. For example, extra dimensions or supersymmetry may solve the hierarchy problem. It is thought that a full theory of quantum gravity should be capable of answering most of these problems (other than the Island of stability problem).
Quantum gravity, cosmology, and general relativity
- Vacuum catastrophe
- Why does the predicted mass of the quantum vacuum have little effect on the expansion of the universe?
- Quantum gravity
- How can quantum mechanics and general relativity be realized as a fully consistent 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 theory?
- 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 (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?
- Extra dimensions
- Does nature have more than four spacetime dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally “see” evidence of higher spatial dimensions?
- Cosmic inflation
- Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflaton field giving rise to inflation? If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some impossibly distant place?
- Are there physical reasons to expect other universes that are fundamentally non-observable? For instance: Are there quantum mechanical “alternative histories” or “many worlds”? Are there “other” universes with physical laws resulting from alternate ways of breaking the apparent symmetries of physical forces at high energies, possibly incredibly far away due to cosmic inflation? Is the use of the anthropic principle to resolve global cosmological dilemmas justified?
- 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?
- Arrow of time
- What do the phenomena that differ going forward and backwards in time tell us about the nature of time? How does time differ from space? Why are CP violations observed in certain weak force decays, but not elsewhere? Are CP violations somehow a product of the Second Law of Thermodynamics, or are they a separate arrow of time? Are there exceptions to the principle of causality? Is there a single possible past?
- Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to transfers of information, or can energy and matter also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime? How does this relate to quantum entanglement? How does this elucidate the proper interpretation of the fundamental nature of quantum physics?
- Future of the universe
- Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch or a Big Bounce ? Is our universe part of an infinitely recurring cyclic model?
High energy physics/Particle physics
- Higgs mechanism
- Does the Higgs particle exist? What are the implications if it does not?
- Hierarchy problem
- Why is gravity such a weak force? It becomes strong for particles only at the Planck scale, around 1019 GeV, much above the electroweak scale (100 GeV, the energy scale dominating physics at low energies). Why are these scales so different from each other? What prevents quantities at the electroweak scale, such as the Higgs boson mass, from getting quantum corrections on the order of the Planck scale? Is the solution supersymmetry, extra dimensions, or just anthropic fine-tuning?
- Koide formula
- How can physicists explain or understand Yoshio Koide’s formula relating lepton masses?
- Magnetic monopoles
- Do particles that carry “magnetic charge” exist? The existence of magnetic monopoles would explain charge quantization.
- Proton decay and unification
- How do we unify the three different quantum mechanical fundamental interactions of quantum field theory? As the lightest baryon, are protons absolutely stable? If not, then what is the proton’s half-life?
- Is spacetime supersymmetry realized in nature? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest supersymmetric particle comprise dark matter?
- Generations of matter
- Are there more than three generations of quarks and leptons? Why are there generations at all? Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles?
- Fundamental symmetries and Neutrinos
- What is the nature of the neutrinos, what are their masses, and how have they shaped the evolution of the universe? Why is there now more detectable matter than antimatter in the universe? What are the unseen forces that were present at the dawn of the universe but disappeared from view as the universe evolved?
- Quantum chromodynamics
- What are the phases of strongly interacting matter, and what roles do they play in the cosmos? What is the internal landscape of the nucleons? What does QCD predict for the properties of strongly interacting matter? What governs the transition of quarks and gluons into pions and nucleons? What is the role of gluons and gluon self-interactions in nucleons and nuclei? What determines the key features of QCD, and what is their relation to the nature of gravity and spacetime?
- Nuclei and Nuclear astrophysics
- What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What is the origin of simple patterns in complex nuclei? What is the nature of neutron stars and dense nuclear matter? What is the origin of the elements in the cosmos? What are the nuclear reactions that drive stars and stellar explosions?
- Island of stability
- What is the heaviest possible stable or metastable nucleus?
- Quantum mechanics in the correspondence limit (sometimes called Quantum chaos)
- Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse or quantum decoherence, give rise to the reality we perceive? Another way of stating this is the Measurement problem – what constitutes a “measurement” which causes the wave function to collapse into a definite state?
- Physical information
- Are there physical phenomena, such as black holes or wave function collapse, which irrevocably destroy information about their prior states?
- Theory of everything
- Is there a theory which explains the values of all fundamental physical constants? Is there a theory which explains why the gauge groups of the standard model are as they are, why observed space-time has 3 + 1 dimensions, and why all laws of physics are as they are? Do “fundamental physical constants” vary over time? Are any of the particles in the standard model of particle physics actually composite particles too tightly bound to observe as such at current experimental energies? Are there fundamental particles that have not yet been observed and if so which ones are they and what are their properties? Are there unobserved fundamental forces implied by a theory that explains other unsolved problems in physics?
Empirical phenomena lacking clear scientific explanation
Cosmology and astronomy
- Baryon asymmetry
- Why is there far more matter than antimatter in the observable universe?
- Cosmological constant problem
- Why doesn’t the zero-point energy of the vacuum cause a large cosmological constant? What cancels it out?
- Dark energy
- What is the cause of the observed accelerated expansion (deSitter phase) of the Universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time? Is dark energy a pure cosmological constant, or are models of quintessence such as phantom energy applicable?
- Dark matter
- What is dark matter? Is it related to supersymmetry? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
- Entropy (arrow of time)
- Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics?
- Horizon problem
- Why is the distant universe so homogenous, when the Big Bang theory seems to predict measurable anisotropies of the night sky larger than those observed? Possible approaches to a solution are inflation and the variable speed of light hypothesis.
- Ecliptic alignment of CMB anisotropy
- Some large features of the microwave sky, at distances of over 13 billion light years, appear to be aligned with both the motion and orientation of the Solar System. Is this due to systematic errors in processing, contamination of results by local effects, or a violation of the Copernican principle?
- Shape of the Universe
- What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the Universe, informally called the “shape” of the Universe? Neither the curvature nor the topology is presently known, though the curvature is known to be “close” to zero on observable scales. The cosmic inflation hypothesis suggests that the shape of the Universe may be unmeasurable, but since 2003, Jean-Pierre Luminet et al. and other groups have suggested that the shape of the Universe may be the Poincaré dodecahedral space. Is the shape unmeasurable, the Poincaré space, or another 3-manifold?
High energy physics/Particle physics
- Electroweak symmetry breaking
- What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z bosons? Is it the simple Higgs mechanism of the Standard Model,or does nature make use of strong dynamics in breaking electroweak symmetry, as proposed by Technicolor?
- Neutrino mass
- What is the mechanism responsible for generating neutrino masses? Is the neutrino its own antiparticle? Or could it be an antiparticle that simply cannot join and annihilate with a normal particle because of its irregular state?
- Inertial mass/gravitational mass ratio of elementary particles
- According to the equivalence principle of general relativity, the ratio of inertial mass to gravitational mass of all elementary particles is the same. However, there is no experimental confirmation for many particles. In particular, we do not know what the weight of a macroscopic lump of antimatter of known mass would be.
- Proton spin crisis
- As initially measured by the European Muon Collaboration, the three main (“valence”) quarks of the proton account for about 12% of its total spin. Can the gluons that bind the quarks together, as well as the “sea” of quark pairs that are continually being created and annihilating, properly account for the rest of it?
- Quantum chromodynamics (QCD) in the non-perturbative regime
- The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei, and only mainly numerical approaches seem to begin to give answers at this limit. How does QCD give rise to the physics of nuclei and nuclear constituents?
- Strong CP problem and axions
- Why is the strong nuclear interaction invariant to parity and charge conjugation? Is Peccei-Quinn theory the solution to this problem?
- Hypothetical particles
- Which of the hypothetical particles predicted by supersymmetric theories and other fairly well-known theories actually occur in nature?
Astronomy and astrophysics
- Accretion disc jets
- Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes? Why are there Quasi-Periodic Oscillations in many accretion discs? Why does the period of these oscillations scale as the inverse of the mass of the central object? Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?
- Coronal heating problem
- Why is the Sun’s Corona (atmosphere layer) so much hotter than the Sun’s surface? Why is the magnetic reconnection effect many orders of magnitude faster than predicted by standard models?
- Gamma ray bursts
- How do these short-duration high-intensity bursts originate?
- Supermassive black holes
- What is the origin of the M-sigma relation between supermassive black hole mass and galaxy velocity dispersion?
- Observational anomalies
- Hipparcos anomaly: What is the actual distance to the Pleiades?
- Pioneer anomaly: What causes the small additional sunward acceleration of the Pioneer spacecraft?
- Flyby anomaly: Why is the observed energy of satellites flying by earth different by a minute amount from the value predicted by theory?
- Galaxy rotation problem: Is dark matter responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies, or is it something else?
- Supernovae: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
- Ultra-high-energy cosmic ray
- Why is it that some cosmic rays appear to possess energies that are impossibly high (the so called OMG particle), given that there are no sufficiently energetic cosmic ray sources near the Earth? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the Greisen-Zatsepin-Kuzmin limit?
- Pulsar Time Dilation
- Why do the emissions from pulsars at great cosmological distances fail to exhibit the predicted time dilation properties?
- Rotation rate of Saturn
- Why does the magnetosphere of Saturn exhibit a (slowly changing) periodicity close to that at which the planet’s clouds rotate? What is the true rotation rate of Saturn’s deep interior? 
Condensed matter physics
- Amorphous solids
- What is the nature of the glass transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties and the physics of glasses?
- Cryogenic electron emission
- Why does the electron emission in the absence of light increase as the temperature of a photomultiplier is decreased?
- High-temperature superconductors
- What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 50 kelvin?
- What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?
- Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)? Also, under what conditions do smooth solutions to the Navier-Stokes equations exist? This is probably the last unsolved problem in Classical or Newtonian Physics .
Biological problems approached with physics
These fields of research normally belong to biology, and traditionally were not included in physics but are included here because increasingly it is physicists who are researching them using methods and tools more popular in physics research than biology.
- Synaptic plasticity
- It is necessary for computational and physical models of the brain, but what causes it, and what role does it play in higher-order processing outside the hippocampus and visual cortex?
- Axon guidance
- How do axons branching out from neurons find their targets? This process is crucial to nervous system development, allowing the building up of the brain.
- Stochasticity and robustness to noise in gene expression
- How do genes govern our body, withstanding different external pressures and internal stochasticity? Certain models exist for genetic processes, but we are far from understanding the whole picture, in particular in development where gene expression must be tightly regulated.
- Quantitative study of the immune system
- What are the quantitative properties of immune responses? What are the basic building blocks of immune system networks? What roles are played by stochasticity?
Problems solved in recent decades
- Long-duration gamma ray bursts (2003)
- Long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar.
- Solar neutrino problem (2002)
- Solved by a new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics—specifically, neutrino oscillation.
- Age Crisis (1990s)
- The estimated age of the universe was around 3 to 8 billion years younger than estimates of the ages of the oldest stars in our galaxy. Better estimates for the distances to the stars and the addition of dark energy into the cosmological model reconciled the age estimates.
- Quasars (1980s)
- The nature of quasars was not understood for decades. They are now accepted as a type of active galaxy where the enormous energy output results from matter falling into a massive black hole in the center of the galaxy.
- The Eleventh Dimension–String Theory, Supersymmetry and Super Gravity (brighthub.com)
- Elementary Particles of the Standard Model (brighthub.com)
- Quantum Understanding wil Reveal Matter and Energy a Manifestation of Mind (grantlawrence.blogspot.com)
- An Experiment on Quantum Entanglement based on String Theory Predictions (nextbigfuture.com)
- The ‘Why?’ Questions, Chapter and Multiverse (online.wsj.com)
- A New View of Gravity (sciencenews.org)
- The 6 Smartest Ideas From Stephen Hawking’s New Book (thedailybeast.com)