The Stability of the Electron

Transcription

1 The Stability of the Electron F. J. Himpsel, Physics Dept., Univ. Wisconsin Madison 1. Coulomb explosion of the electron: a century- old problem 2. Exchange hole and exchange electron 3. Force density balance in the H atom (single particle) 4. Stability of the vacuum polarization (manybody) 5. Stability of an electron in the Dirac sea (the real deal) 6. Application to insulators and semiconductors

2 A really bad hair day Equal charges repel each other Coulomb explosion of the electron?

3 The stability of the electron: a century-old problem 1897 Discovery of the electron by J. J. Thomson, Phil. Mag. 44, H. Poincaré, Comptes Rendus 140, H. A. Lorentz, The Theory of Electrons, Columbia University Press 1922 E. Fermi, Z. Physik 23, P. A. M. Dirac, Proc. R. Soc. Lond. A 167, 148 (1938); A 268, 57 (1962) The self-energy of the electron 1934 V. F. Weisskopf, Zeits. f. Physik 89, 27; Phys. Rev. 56, 72 (1939)

4 Sommerfeld s successor, my Diplom thesis advisor

5 Considerations for a solution 1. Can magnetic attraction compensate electric repulsion? Requires a charge rotating with the speed of light at the reduced Compton wavelength, where classical physics loses its validity. 2. Can gravity compensate repulsion, forming a black hole? The Schwarzschild radius R S of the electron corresponds to an energy of GeV via the uncertainty relation p ħ/r S and E(p). That generates an astronomical number of extra e e + pairs. 3. Compensate Coulomb repulsion with exchange attraction? The self-coulomb and self-exchange terms cancel each other. An exchange hole forms around the electron due to the exclusion principle. The positive hole threatens to collapse. 4. What can prevent exchange collapse? a) Compressing the exchange hole generates a repulsive force. b) Adding an exchange electron to the hole preserves neutrality.

6 The Dirac sea The Dirac equation for electrons admits solutions with both positive and negative energy in order to satisfy special relativity (E 2 =p 2 +m 2 ). In the vacuum of quantum electrodynamics the states with negative energy are all occupied and those with positive energy are all empty. To satisfy particle-antiparticle symmetry and to cancel the infinite negative charge of the vacuum electrons one has to assign empty states to positrons (= holes) with negative energy. The energy diagram is similar to that of an insulator with a band gap of 2m. E el positrons (holes) (E pos ) electrons

7 Weisskopf s picture The point-like electron is compensated by a point-like hole. A spread-out electron is generated by the vacuum electrons. An electron added to the Dirac sea Phys. Rev. 56, 72 (1939)

8 A new picture The point-like electron is surrounded by a spread-out exchange hole. The hole is surrounded by an exchange electron (double exchange). sum exchange electron Vacuum electron density 4 r 2 exchange hole An electron inside the Dirac sea r (in reduced Compton wavelengths) arxiv: [quant-ph] (2017)

9 The exchange hole The exclusion principle forbids two electrons with the same spin to occupy the same location. As a result, nearby electrons with the same spin are pushed away from a reference electron, forming a positive hole with opposite spin. This exchange hole has been defined mathematically for an electron gas, such as the Fermi sea formed by the electrons in a metal (Slater 1951, Gunnarson and Lundqvist 1976). Weisskopf s work in 1934 can be viewed in retrospect as an attempt to define the exchange hole for the Dirac sea. Generalizing today s definition from the Fermi sea to the Dirac sea gives a slightly different picture, where the spread-out electron becomes a spread-out hole. But both are described by the same Bessel function: -1/2 2 K 1 (r)/r The extent of the exchange hole in the Fermi sea is determined by the Fermi wavelength F. For the Dirac sea it is determined by the reduced Compton wavelength C ( C =1/m e in units of h,c).

10 Start with a simple system: the H1s electron The Dirac wave function is equivalent to a classical field. The Lagrangian formalism defines the two force densities acting on : electrostatic attraction and confinement repulsion. They cancel each other automatically. Such a local balance between force densities goes beyond the usual stability criteria which rely on a global energy minimum. arxiv: [physics.atom-ph] (2015)

11 Adding the magnetic hyperfine interaction to the Coulomb potential leads beyond classical field theory. The magnetic field is generated by the quantum-mechanical angular momentum operator. The ground state wave function remains isotropic, since the proton spin has equal probability of pointing up or down in the entangled singlet spin wave function ( p e p e)/ 2. The effect of the hyperfine interaction is mainly electrostatic. The electron density becomes compressed near the proton and thereby enhances both electrostatic attraction and confinement repulsion. Force densities in the singlet ground state of H. The hyperfine interaction adds the contribution f E, to the electrostatic force density f E,C. Both are nearly balanced by the corresponding confinement force densities (not shown). The residual f is given with an amplification factor The electrostatic force density f VP acts on the vacuum polarization surrounding the proton. It is balanced by a confinement force density, too. arxiv: [physics.atom-ph] (2017)

12 Vacuum polarization as manybody system The charge induced in the Dirac sea by the Coulomb potential is attracted toward the inducing point charge. This attraction is compensated by the confinement repulsion (as in the H atom). This is shown explicitly using hydrogenic solutions for the vacuum electrons/positrons and summing their force densities over all radial and angular momenta (see the figure). The force balance remains valid for each filled shell in the sum (i.e., all m j for each j ), and thus for the complete sum. Vacuum electron density 4 r 2 r (in reduced Compton wavelengths) arxiv: [quant-ph] (2015)

13 The response of the Dirac sea to an electron: exchange hole + electron = exchange exciton An electron interacts with the Dirac sea mainly via exchange. Vacuum polarization is down by a factor of This situation is similar to to exchange vs. correlation in the Fermi sea of solid state physics. The exchange hole is defined via the pair correlation, i.e., the probability of finding an electron at r 2 if there is one at r 1. In contrast to the Fermi sea the electron displaced by the exchange hole cannot be removed to infinity in the Dirac sea. That would require the energy 2m. Instead, this exchange electron remains in the neighborhood of the hole. Together they form a neutral exchange exciton. A proper definition of the exchange exciton starts with the 3-fermion correlation, the probability of finding a hole at r 2 and an electron at r 3, if there is an electron at r 1. Such a definition created problems with normalizing the hole charge. Using two sequential pair correlations instead gave a sensible result similar to the negative positronium ion. arxiv: [quant-ph] (2017)

14 Force densities Electrostatic force densities are obtained easily from the products of charge densities and electric fields (see the plot below). But the proper definition of confinement force densities remains unsolved. Force density 4 r 2

15 Application to insulators and semiconductors The concept of an exchange exciton might have applications in solid state physics for characterizing the exchange interaction in insulators and semiconductors. The neutral exchange exciton is a better match for them, since the electron displaced by the exchange hole cannot delocalize. The exchange exciton defined via 3-fermion correlations works for the Fermi sea, and it contains the standard exchange hole. Electron density Exchange electron (-1) Exchange hole r (in Fermi wavelengths)

16 Can the fine structure constant be obtained from a force balance? (another century-old problem) There are two possible outcomes of a force balance: If the opposing forces scale the same way with, the force balance does not provide any information about.that is the case for the H atom and the vacuum polarization. If the opposing forces scale differently with, a balance can only be achieved for a particular value of. This might be the case for an electron in the Dirac sea.the electric force density scales like, but the confinement force density does not depend on,sinceit is determined by free-electron wave functions.

17 was die Welt im Innersten zusammenhält Goethe s Faust makes a pact with the devil to find out whatever holds the world together at its inner core.