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Unsolved problems in physics
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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 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.
problems are not considered significant enough by some physicists.
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Encyclopedia
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.
Notation
Some problems are not considered significant enough by some physicists. If so, they are marked as follows:
** 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: How can gravity and general relativity be realized as a fully consistent quantum field theory? Is string theory (M-theory) the correct approach?
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"? 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?
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? The non-existence of magnetic monopoles is the basis of Classical Electrodynamics and modern technology as we know it.
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 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? 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
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? 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, 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?
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?
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?
Observational anomalies:
- Hipparcos anomaly: How far away are the Pleiades, exactly?**
- Pioneer anomaly: What causes the apparent residual 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 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?
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? **
Cold fusion: What is the explanation for the controversial reports of excess heat, radiation and transmutations? Many scientists view this field as an example of pathological science. ****
High-temperature superconductors: What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 50 Kelvin?
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)? Also, under what conditions do smooth solutions to the Navier-Stokes equations exist? This probably happens to be 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 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.
External links
- Science journal special project for its 125th anniversary: top 25 questions and 100 more.
- A weekly physics news bulletin hosted by the American Institute of Physics.
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