Unsolved Problems in Physics

This is a list of some of the major unsolved problems in physics. Some of these problems are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail.

Contents 1 Theoretical problems 1.1 Quantum gravity, cosmology, and general relativity 1.2 High energy physics 1.3 Nuclear physics 1.4 Other problems 2 Empirical phenomena lacking clear scientific explanation 2.1 Cosmology and astronomy 2.2 Astronomy and astrophysics 2.3 Condensed matter physics 2.4 Biological problems approached with physics 3 Problems recently solved 4 References 5 External link

 Theoretical problems

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?^[1]

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?

Multiple universes

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?^[2] 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?

 High energy 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 of order of the Planck scale? Is the solution supersymmetry, extra dimensions, or just anthropic fine-tuning?

Island of stability

What is the heaviest possible stable or metastable nucleus?

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?

Supersymmetry

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?

 Nuclear physics

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?

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 visible 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?

 Other problems

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?

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?^[3] Do "fundamental physical constants" vary over time? 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?

 Empirical phenomena lacking clear scientific explanation

Cosmology and astronomy

Cosmological constant problem

Why doesn't the zero-point energy of the vacuum cause a large cosmological constant? What cancels it out?

Baryon asymmetry

Why is there far more matter than antimatter in the observable universe?

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?^[4] 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?

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,^[5] or does nature make use of strong dynamics in breaking electroweak symmetry, as proposed by Technicolor?

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?^[6]

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?

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?

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?

 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?^[7]

Observational anomalies

Hipparcos anomaly: What is the actual distance to the Pleiades?

Pioneer anomaly^[4]: What causes the apparent residual sunward acceleration of the Pioneer spacecraft?^[8][9]

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^[4]

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?^[10][11]

 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?^[12]

High-temperature superconductors

What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 50 kelvin?^[13]

Sonoluminescence

What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?

Turbulence

Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)?^[14] 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.^[15][16]

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 recently solved

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.

References

1. ^ Alan Sokal (July 22, 1996), Don't Pull the String Yet on Superstring Theory, New York Times, http://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260 
2. ^ Joshi, Pankaj S. (January 2009), "Do Naked Singularities Break the Rules of Physics?", Scientific American, http://www.sciam.com/article.cfm?id=naked-singularities 
3. ^ Open Questions, Particle Physics, item 12
4. ^ ^{a b c} 13 things that do not make sense newscientistspace, 19 March 2005, Michael Brooks
5. ^ Open Questions, Particle Physics, item 6
6. ^ Open Questions item 4
7. ^ Open Questions, Cosmology and Astrophysics, item 11
8. ^ Open Questions, Particle Physics, item 13
9. ^ newscientistspace item 8
10. ^ Open Questions, Cosmology and Astrophysics, item 12
11. ^ newscientistspace item 3
12. ^ Anahad O’Connor (Tuesday, August 12, 2008), The debate continues..., Deccan Herald, http://www.deccanherald.com/Content/Aug122008/snt2008081184001.asp 
13. ^ Open Questions, Condensed Matter and Nonlinear Dynamics, item 2
14. ^ Open Questions, Condensed Matter and Nonlinear Dynamics
15. ^ The Nobel Prizes in Physics 1901-2000
16. ^ The Office of Science - What is Physics?

 External links

• What don't we know? Science journal special project for its 125th anniversary: top 25 questions and 100 more.
• Physics News Update A weekly physics news bulletin hosted by the American Institute of Physics.
• Open Questions in Physics
• New Scientist: 13 things that do not make sense.
• List of links to unsolved problems in physics, prizes and research.
• Ideas Based On What We’d Like To Achieve
• 2004 SLAC Summer Institute: Nature's Greatest Puzzles
• http://technology.newscientist.com/article/dn14179-glasss-dual-personality-explained-at-last.html

So it looks like there's work to be done.

Time to call upon the services of Englishman/German/Madman Caspar von Biergaaarten, below. He looks Mad Scientist enough.

 

Actually, that's Andrew Thomas of Swansea, Wales, author of the excellent "What is Reality?" website.  

Apparently, Science has lost his services for the moment. Look out Music world, here comes Caspar!