Unsolved Problems in Theoretical Physics V. BASHIRY CYPRUS INTRNATIONAL UNIVERSITY 1
I am going to go through some of the major unsolved problems in theoretical physics. I mean the existing theories seem incapable of explaining a certain observed phenomenon or experimental result. 2
Unsolved problems by subfield 1. General physics/quantum physics 2. Cosmology and general relativity 3. Quantum gravity 4. High-energy physics/particle physics 5. Astronomy and astrophysics 6. Nuclear physics 7. Atomic, molecular and optical physics 8. Condensed matter physics 9. Biophysics 3
General physics/quantum physics Entropy Why did the universe have such low entropy in the past, resulting in the distinction between past and future? 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? Why does time have a direction? Is the present moment physically distinct from the past and future or is it merely an emergent property of consciousness? 4
Interpretations of quantum mechanics How does the quantum description of reality? Another way of stating this question regards the measurement problem. Quantum mechanics cannot be simultaneously not Local, Causal and Real, which of them must be scarified. 5
Grand Unification Theory ("Theory of everything") Is there a theory which explains the values of all fundamental physical constants? Is the theory string theory? Why observed spacetime has 3 spatial dimensions and 1 temporal dimension. Do "fundamental physical constants" vary over time? Are there fundamental particles that have not yet been observed. Are they really fundamental or composite? Are there unobserved fundamental forces? 6
Physical information The information is destroyed when wave function in quantum mechanics collapse(black holes). Can it be restored. If not then, how can quantum information allocated as a state of system. 7
Dimensionless physical constant All of dimensionless constants can not be calculated theoretically. How many of dimensionless constants are necessary to derive all other dimensionless physical constants. Are dimensionful physical constants necessary at all? Is Dirac large numbers hypothesis true? 8
Fine-tuned Universe What explains why the fundamental physical constants are set in the narrow range that is necessary to support carbonbased life? Is the Earth unique place to have carbon based life. 9
Cosmology and general relativity Problem of time How can time be reconciled with general relativity? Cosmic inflation Inflation theory can explain the homogeneity of the universe. Is it correct? What is the detail of inflation epoch? Is the universe heading towards a Big Freeze? Or is it part of an infinitely recurring cyclic model? 10
Size of universe Diameter of the observable universe is about 93 billion light-years, but what is the size of whole universe? Does a multiverse exist? Baryon asymmetry Why is there far more matter than antimatter in the observable universe? Cosmological constant problem The theoretical and experimental discrepancy ranges from 40 to more than 100 orders of magnitude. 11
Dark matter What is the identity of dark matter? Is it a particle? Is it the lightest superpartner (LSP)? Is dark matter a form of matter or an extension of gravity? 12
Dark Energy Dark energy helps us to understand the accelerated expansion 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? 13
Quantum gravity Why does the predicted mass of the quantum vacuum have little effect on the expansion of the universe? Can quantum mechanics and general relativity be realized as a fully consistent theory? Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a graviton? Are there deviations from the predictions of general relativity? 14
Black holes, black hole information paradox Do black holes produce thermal radiation, as expected? Does this radiation contain information about their inner structure, as suggested Hawking's? what happens to the information stored in them? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure? 15
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 observe evidence of higher spatial dimensions? 16
High-energy physics/particle Higgs mechanism physics Are the branching ratios of the Higgs boson decays consistent with the standard model? Is there only one type of Higgs boson? Magnetic monopoles Did particles that carry "magnetic charge" exist in some past, higher-energy epoch? If so, do any remain today? 17
Hierarchy problem Why is gravity such a weak force? It becomes strong for particles only at the Planck scale. 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? 18
Planck particle The Planck mass plays an important role in parts of mathematical physics. A series of researchers have suggested the existence of a fundamental particle with mass equal to or close to that of the Planck mass. The Planck mass is however enormous compared to any detected particle even compared to the Higgs particle. 19
Proton decay and spin crisis Is the proton fundamentally stable? D Does it decay with a finite lifetime as predicted by some extensions to the standard model? How do the quarks and gluons carry the spin of protons? 20
Supersymmetry Is spacetime supersymmetry realized at TeV scale? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest supersymmetric particle (LSP or Lightest Supersymmetric Particle) comprise dark matter? 21
Generations of matter Why are there three generations of quarks and leptons? Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles? 22
Neutrino mass What is the mass of neutrinos, whether they follow Dirac or Majorana statistics? Is mass hierarchy normal or inverted? Is the CP violating phase 0? 23
Colour confinement Why has there never been measured a free quark or gluon, but only objects that are built out of them, such as mesons and baryons? How does this phenomenon emerge from QCD? 24
Anomalous magnetic dipole moment Why is the experimentally measured value of the muon's anomalous magnetic dipole moment ("muon g 2") significantly different from the theoretically predicted value of that physical constant? 25
Pentaquarks and other exotic hadrons What combinations of quarks are possible? Why were pentaquarks so difficult to discover? Are they a tightly-bound system of five elementary particles, or a more weaklybound pairing of a baryon and a meson? 26
Quantum chromodynamics Do glueballs exist? Do gluons acquire mass dynamically despite having a zero rest mass, within hadrons? Does QCD truly lack CP-violations? Do gluons saturate when their occupation number is large? Do gluons form a dense system called Colour Glass Condensate? 27
Some of the solved problems in recent decades Existence of gravitational wave (1916 2016) On 11 February 2016, the Advanced LIGO team announced that they had directly detected gravitational waves from a pair of black holes merging, which was also the first detection of a stellar binary black hole. 28
Existence of pentaquarks (1964 2015) In July 2015, the LHCb collaboration at CERN identified pentaquarks in the Λ b J/ψK - p channel, which represents the decay of the bottom lambda baryon (Λ b ) into a J/ψ meson (J/ψ), a kaon (K - ) and a proton (p). The results showed that sometimes, instead of decaying directly into mesons and baryons, the Λ b decayed via intermediate pentaquark states. The two states, named P c (4380) and P c (4450), had individual statistical significances of 9 σ and 12 σ, respectively, and a combined significance of 15 σ enough to claim a formal discovery. 29
The two pentaquark states were both observed decaying strongly to J/ψp, hence must have a valence quark content of two up quarks, a down quark, a charm quark, and an anti-charm quark. c 30
Higgs boson and electroweak symmetry breaking (1963-2012) The mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z bosons, was solved with the discovery of the Higgs boson of the Standard Model, with the expected couplings to the weak bosons. No evidence of a strong dynamics solution, as proposed by technicolor, has been observed. 31
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