Unsolved problems in physics.html
 
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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 some 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

Notation

** Problems marked with two stars are considered by a significant number of physicists to be resolved, though there is still significant debate about them.

*** Problems marked with three stars are considered by some physicists to be outside the purview of physics, more properly philosophical in nature.

**** The existence of problems marked with four stars is disputed.

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 and cosmology

Quantum gravity
How can gravity and general relativity be realized as a fully consistent quantum field theory? Is string theory (M-theory) the correct approach? More pressing, how much experimental information can be extracted about physics near the Planck scale?
Black holes, Black hole information, 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 allow information to be destroyed) 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"? 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? ***

High energy physics

Higgs mechanism
Does the Higgs particle exist? What 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 atom?
Magnetic monopoles
Do particles that carry "magnetic charge" exist?
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?

Other problems

Consciousness
Is a complete understanding of particle physics sufficient to fully understand consciousness?***
Quantum mechanics in the correspondence limit
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, 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?1 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 high energy physics

Accelerating universe and the Cosmological constant
Why doesn't the zero-point energy of the vacuum cause a large cosmological constant? What cancels it out? Is a non-total cancellation of the cosmological constant responsible for the observed accelerated expansion (deSitter phase) of the Universe? If it is, why is the energy density of the cosmological constant 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? Or is the nature of the dark energy driving this acceleration differently?
Baryon asymmetry
Why is there far more matter than antimatter in the observable universe?
Dark matter
What is dark matter?2 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,3 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?4
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?
Mass/Weight ratio of elementary particles
According to General Relativity the ratio of mass to weight 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?

Astronomy

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?
Corona heating problem
Why is the Sun's Corona (atmosphere layer) so much hotter than the Sun's surface?****
Gamma ray bursts (short duration)
How do these short-duration high-intensity bursts originate?5
Observational anomalies
Hipparcos anomaly: How far away are the Pleiades, exactly?**
Pioneer anomaly2: What causes the apparent residual sunward acceleration of the Pioneer spacecraft?67 ****
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 ray2
Why is it that some cosmic rays appear to possess energies that are impossibly high (the so called Oh-My-God 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?89

Condensed matter physics

Amorphous solids
What is the nature of the phase transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses?10 **
Cold fusion
What is the explanation for the controversial reports of excess heat, radiation and transmutations?21112
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?

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.1516

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. ^ Open Questions, Particle Physics, item 12
  2. ^ a b c d 13 things that do not make sense newscientistspace, 19 March 2005, Michael Brooks
  3. ^ Open Questions, Particle Physics, item 6
  4. ^ Open Questions item 4
  5. ^ Open Questions, Cosmology and Astrophysics, item 11
  6. ^ Open Questions, Particle Physics, item 13
  7. ^ newscientistspace item 8
  8. ^ Open Questions, Cosmology and Astrophysics, item 12
  9. ^ newscientistspace item 3
  10. ^ Anahad O’Connor (Tuesday, August 12, 2008), The debate continues..., Deccan Herald, http://www.deccanherald.com/Content/Aug122008/snt2008081184001.asp 
  11. ^ John R. Vacca (2004), The World's 20 Greatest Unsolved Problems, Prentice Hall, ISBN 9780131426436, http://books.google.com/books?id=ouMmAAAACAAJ 
  12. ^ Feder, T.; John, O. (2004), Physics Today: 27, doi:10.1063/1.1881896, http://www.physicstoday.com/pt/vol-58/iss-1/PDF/vol58no1p31a.pdf 
  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?

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