A Superfluid Universe

Transcription

1 A Superfluid Universe Lecture 2 Quantum field theory & superfluidity Kerson Huang MIT & IAS, NTU

2 Lecture 2. Quantum fields The dynamical vacuum Vacuumscalar field Superfluidity Ginsburg Landau theory BEC Quantized vortex Vortex dynamics Quantum turbulence 2

3 Quantum field: An operator attached to each point of space time: r,t Annihilates 1 particle at r,t Annihilates 1 particle, or creates 1 antiparticle, at r,t Nonrelativistic Relativistic N dr r,t, r,t, = (No. of particles) (No. of antiparticles) Time translation is represented by phase factor exp( iet). 2 E p2 2m E p 2 m 2 Non rel. : only positive frequencies Relativistic: Positive and negative f Antiparticle = Particle going backwards in time. Feynman 3

4 Quantization: initial condition at t=0: r L 3/2 k e ik r a k (Non-rel) L 3/2 ik r ik r k e r a k e r b k (Rel) k 2 L n Components of n 0, 1, Annihilation and creation operators: a k,a p kp ( + fermion boson ) b k,b p kp a k,b p a k,b p 0 Equations of motion (example) Relativistic scalar field: NonlinearKlein Klein Gordon equation 2 E p 2 m U t U V 4

5 Relativistic fields: Representations of Poincare group (inhomogeneous Lorentz group) Classified by spin. Spin 0: Scalar field Pi meson ½: Spinor field Electron, nucleon, quark 1: Vector field EM field, gauge fields 2: Tensor field Gravitational wave < Not yet observed Spin & statistics: integer fields obey Bose statistics, half integer fields obey Fermi statistics. 5

6 QED quantum electrodynamics Quantum field theory of interacting photons and electrons The most successful theory in physics Virtual processes vacuum fluctuations ti Cutoff renormalization Experimental manifestations of vacuum fluctuations Lamb shift Electron anomalous magnetic moment Casimir effect P.A.M. Dirac ( ) 6

7 The Lamb shift in H atom Electron orbit in H atom, in Fluctuating vacuum EM field 7

8 Electron anomalous magnetic moment 1 g expt 1 2 g theory 1 (a) 1928 (Dirac equation) /2 (b) 1949 (1 diagram) / 2 (c) 1958 (18 diagrams) / 026 / 3 (d) 1974 (72 diagrams) / 4 Non-QED (e) 2006 (891 diagrams)

9 Casimir effect Two metal plates in vacuum attract each other. Dyne/cm 2 H.B.G. Casimir ( ) L 0.10 L d (micron) a L Suppression of modes lowers vacuum energy. Mode counting requires cutoff. 9

10 Cutoff and renormalization Virtual processes in Feynman graphs Pure vacuum process p k Electron self energy m Photon self energy vacuum polarization Example: electron self energy m e 0 2 k m m 0 m d 4 k p k 2 k 2 e 0 2 ln Take m from experiments. 10

11 Vacuum scalar field spontaneous symmetry breaking Higgs mechanism generates mass in the standard model, through spontaneous breaking of local gauge invariance. 0 Fe i Peter Higgs (1929 ) classical field quantum field Spatial variation iti of the phase leads to superfluidity.

12 What is a superfluid? A macroscopic quantum system with a coherent quantum phase. superconductor liquid helium cold trapped atoms in Bose Einstein condensation (BEC) A complex scalar field can describe a superfluid as a phenomenlogical order parameter, because it can have a coherent phase field over space. Fe i v (Superfluid velocity)

13 Superconductivity Kamerlingh Onnes ( ) Liquified helium (1908) Discovered superconductivity (1911) after spending weeks, with his assistant, in unsuccessful attempt to fix a short circuit. 1 2 Insulating layer Josephson junction Sandwich of two superconductors, with different phases. SQUID 13

14 Ginsburg Landau theory of superconductivity order parameter. NLSE with magnetic field. Spontaneous symm. breaking generates photon mass. Meissner efffect: penetration depth (photon mass) 1 BCS : microscopic theory Order parameter = condensate wave function of Cooper pairs In Bose Einstein condensation Lev Landau ( ) Vitaly Ginzburg ( ) 14

15 Liquid helium Liquifaction at 3 K ( 269 C ) Superfluid phase at 2.18 K The lambda point: singular specific heat marks transition to superfluid Bose Einstein i condensation. Superfluid cannot be contained. 15

16 Bose Einstein condensation in atomic gases Trapping, & cooling Eric A. Cornell (1961 ) Phase coherence Wolfgang Ketterle (1957 ) Condensates collide, overlap Quantum interference pattern Carl E. Wieman (1951 ) 16

17 Quantized vortex line Vortex line Fe i v (Superfluid velocity) v ds ds 2 n 2 rv 2 n v n r 17

18 Vortex lines made visible Metallic powder sticks to surface of vortex core in liquid helium (University of Fribourg) p p (V. Lebedev et al, J. Low Temp Phys. DOI /s ) (a) Copper (b) gold Under electron microscope 18

19 Vortex dynamics Elementary structure is vortex ring Self induced vortex motion v 1 4 R ln R R 0 The smaller the radius of curvature R, the faster it moves normal to R. 19

20 Vortex reconnection Feynman s sketch Signature: cusps spring away from each other at very high speed (due to small radii), creating two jets of energy. Observed vortex reconnection in liquid helium a millisecond event. D. Lathrop, Physics Today, 3 June,

21 Change of topology due to reconnections Magnetic reconnections in sun s corona are responsible for solar flares. 21

22 Simulation of quantum turbulence [K.W. Schwarz, Phys. Rev. B38, 2398 (1988)] Fractal dimension = 1.6 [D. Kivotides, C.F. Barenghi, and D.C. Samuels. Phys. Rev. Lett. 87, (2001).] Number of reconnections: vortex tube density (length per unit spatial volume) A B 3/2 2 (Vinen s equation) Growth Decay 22