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21/01/2014 Physics or Philosophy? Quantum Interpreta5ons in the Undergraduate Curriculum AAPT Summer Mee5ng July 17, 2013 Charles Baily Develop mathemaPcal tools first, quantum intuiPon later?? Develop quantum intuiPon first, mathemaPcal tools later!! University of Colorado Boulder!
University of St Andrews!
[email protected] per.colorado.edu/baily InstrucPonal choices impact student thinking “Don’t worry about what [ Ψ
] means, you’ll get used to it.” InterpretaPon is about sense-­‐making -­‐ Willis E. Lamb, Jr. (1913-­‐2008) Thought Experiment with Single Photons 50% MA LIGHT SOURCE BS MA DA 50% MB DB DB EXPERIMENT ONE SINGLE PHOTON SOURCE BS DA MB •  DA & DB are detectors •  MA & MB are mirrors •  BS is a 50/50 beamspliber Thought Experiment with Single Photons BS MA DA MB DB MA SINGLE PHOTON SOURCE EXPERIMENT ONE DB EXPERIMENT ONE Thought Experiment with Single Photons SINGLE PHOTON SOURCE BS detecPon in DA reflecPon by BS detecPon in DB Reflec5on: Ψ = R
PA = 0.50
Transmission: Ψ = T
DA MB Transmission at BS PB = 0.50
1 21/01/2014 Thought Experiment with Single Photons DA MB BS Ψ =
1
(R + T
2
DB MA SINGLE PHOTON SOURCE EXPERIMENT ONE DB EXPERIMENT ONE Thought Experiment with Single Photons )
SINGLE PHOTON SOURCE MA DA MB BS Single photons trigger either DA or DB, but not both!! What happens when a single photon encounters the beam spliber? (A) Each photon is either reflected or transmibed. (B) Each photon is both reflected and transmibed. (C) There’s no way of knowing. (D) None of the above. Thought Experiment with Single Photons Thought Experiment with Single Photons 1
Ψ =
(R + T
2
MA DB EXPERIMENT TWO DB EXPERIMENT TWO )
MA DA BS2 SINGLE PHOTON SOURCE MB BS1 DA BS2 SINGLE PHOTON SOURCE MB BS1 If detected in DA, the photon could have been reflected at BS1 and then transmibed by BS2 Thought Experiment with Single Photons MA DB EXPERIMENT TWO DB EXPERIMENT TWO Thought Experiment with Single Photons MA DA BS2 SINGLE PHOTON SOURCE BS1 MB If detected in DA, the photon could have been reflected by BS1 and then transmibed at BS2 OR DA BS2 transmibed at BS1 and then reflected by BS2 SINGLE PHOTON SOURCE BS1 MB Whether detected in DA or DB, we have no informaPon about which path the photon might have taken. When the path length difference δ =
B
− A
= 0
, then PA = PB = 0.5
2 21/01/2014 EXPERIMENT TWO NA
NB
δ ( λ / 50 )
δ ( λ / 50 )
EXPERIMENT ONE Slowly change δ = B
− A
by moving just MB For some δ : PA = 1 & PB = 0
For other δ : PA = 0 & PB = 1
The behavior of every photon is influenced by the relaPve length of both paths!! EXPERIMENT TWO What happens when a single photon encounters the beam spliber? (A) Each photon is either reflected or transmibed. (B) Each photon is both reflected and transmibed. (C) It depends on whether we’re conducPng Experiment One or Experiment Two. (D) None of the above. P. Grangier, G. Roger and A. Aspect, Europhys. Le9. 1 (4), 173 (1986) •  [Realist/Sta5s5cal] Each electron is a parPcle that went through one slit or the other. •  [MaHer-­‐Wave] Each electron went through both slits and interfered with itself. hbp://phet.colorado.edu 0.7 Student InterpretaPons of the Double-­‐Slit Experiment Student InterpretaPons of the Double-­‐Slit Experiment Realist REALIST AgnosPc AGNOSTIC •  [Copenhagen/Agnos5c] We can’t say what an electron does between emission and detecPon. Maber-­‐Wave QUANTUM 0.6 0.5 0.4 0.3 0.2 0.1 0 ENG-­‐R/S ENG-­‐MW PHYS-­‐C/A ENG MW/INT Realist/
StaPsPcal Maber-­‐Wave Copenhagen/AgnosPc 3 21/01/2014 0.9 “When not being observed, an electron in an atom exists at a definite but unknown posiPon at each moment in Pme." AGREE 0.8 NEUTRAL "The probabilisPc nature of quantum mechanics is mostly due to the limitaPons of our measurement instruments." 0.9 DISAGREE 0.8 0.7 DISAGREE 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0 0 ENG-­‐R/S ENG-­‐MW PHYS -­‐ C/A ENG-­‐INT Modern Physics for Engineers “I think quantum mechanics is an interesPng subject." 1 NEUTRAL 0.7 0.6 0.9 AGREE AGREE NEUTRAL ENG-­‐R/S ENG-­‐MW PHYS -­‐ C/A ENG-­‐INT IncorporaPng InterpretaPon into QM •  “Spins first” approach (Stern-­‐Gerlach Experiments) DISAGREE •  Give students the tools to arPculate their beliefs §  Local realism §  Complementarity/wave-­‐parPcle duality §  Entanglement/non-­‐locality 0.8 0.7 0.6 0.5 •  Make connecPons with experiment §  Single-­‐quanta experiments §  Distant correlated measurements §  Quantum cryptography/compuPng 0.4 0.3 0.2 0.1 0 Pre-­‐InstrucPon Average Post-­‐InstrucPon Average Post-­‐InstrucPon ENG-­‐MW/INT Modern Physics for Engineers “I think quantum mechanics is an interesPng subject." 1 0.9 AGREE NEUTRAL DISAGREE 0.8 0.7 0.6 0.5 0.4 0.3 0.2 hbp://per.colorado.edu/ModernPhysics 0.1 0 Pre-­‐InstrucPon Average Post-­‐InstrucPon Average Post-­‐InstrucPon ENG-­‐MW/INT 4 21/01/2014 0.7 Student InterpretaPons of the Double-­‐Slit Experiment Realist REALIST AgnosPc AGNOSTIC 0.9 Maber-­‐Wave QUANTUM 0.8 0.6 ”When not being observed, an electron in an atom exists at a definite but unknown posiPon at each moment in Pme." AGREE NEUTRAL DISAGREE 0.7 0.5 0.6 0.4 0.5 0.3 0.4 0.3 0.2 0.2 0.1 0.1 0 ENG-­‐R/S ENG-­‐MW PHYS-­‐C/A ENG-­‐ MW/INT 0 ENG-­‐R/S ENG-­‐MW PHYS -­‐ C/A ENG-­‐ MW/INT "The probabilisPc nature of quantum mechanics is mostly due to the limitaPons of our measurement instruments." 0.9 0.8 AGREE NEUTRAL InterpretaPon is about sense-­‐making DISAGREE We can strongly impact student thinking 0.7 0.6 0.5 Develop quantum intuiPon first, mathemaPcal tools later 0.4 0.3 0.2 0.1 0 ENG-­‐R/S ENG-­‐MW PHYS -­‐ C/A ENG-­‐ MW/INT 5