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Light in Lumps or Reach for the Stars Dr Martin Hendry University of Glasgow ? Isaac Newton 1686 White light Prism Particle theory of light Refraction of light Refraction of light Particles move faster in more “optically dense” medium Reflection of light i r Incident angle (i) = Reflected angle (r) Rival theory due to Christian Huygens Light waves propagate through the luminiferous ether Wave theory could explain equally well reflection and refraction Diffraction could, in principle, distinguish the models Barrier Light Wave Intensity Particle theory dominated until early 1800s: Experiments by Thomas Young and Augustin Fresnel changed all that! Diffraction of light Outgoing Circular Waves Barrier Direction of waves Interference of light Direction of waves Interference of light Direction of waves Maxwell’s theory of light Early 1900s: accelerated electron radiates How do atoms persist? Black-body radiation Wilhelm Wien Intensity Ultraviolet Catastrophe Wavelength The UV Catastrophe could be avoided if light energy was quantised in packets, or photons of energy E = h f Max Planck Black-body radiation Quantised assumption keeps the black-body brightness finite Albert Einstein, 1905 The Photoelectric Effect Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect …. Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect No effect for blue light Incoming light, produces electric current Meter A: measures current of ejected electrons …. Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect …. Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons The Photoelectric Effect Effect seen for UV light Incoming light, produces electric current Meter A: measures current of ejected electrons Metal plate Meter B: measures speed of the ejected electrons 1909 It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and the emission theory 1909 It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and the emission theory 1911 I insist on the provisional character of this concept, which does not seem reconcilable with the experimentally verified consequences of the wave theory 1909 It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and the emission theory 1911 I insist on the provisional character of this concept, which does not seem reconcilable with the experimentally verified consequences of the wave theory 1924 There are therefore now two theories of light, both indispensable…without any logical connection The Bohr atom, 1913 Absorption ee- Emission ee- If light waves also behave like particles, why shouldn’t electrons also behave like waves? Pilot Waves Interference of Electrons Direction of waves Louis de Broglie, 1923 Davisson & Germer; Thomson & Reid, 1937 Making Quantum Mechanics Work Erwin Schrodinger Paul Dirac : Werner Heisenberg Max Born Wolfgang Pauli Neils Bohr John von Neumann All physical systems and events are inherently probabilistic, expressed by the Wave Function Only when the quantum system is observed, the wave function collapses Copenhagen Interpretation Heisenberg Uncertainty Principle The precision of measurements in a quantum system is limited in principle Heisenberg Uncertainty Principle The precision of measurements in a quantum system is limited in principle DpDx ~ h Heisenberg Uncertainty Principle The precision of measurements in a quantum system is limited in principle DpDx ~ h Position and momentum are complementary properties: the action of measurement determines which of the two properties the quantum system possesses : Schrodinger’s Cat Radioactive source Poison Gas : Schrodinger’s Cat Radioactive source Poison Gas : Schrodinger’s Cat Radioactive source Poison Gas R.I.P. : Schrodinger’s Cat Radioactive source + Poison Gas R.I.P. versus Complementarity asserts that it is not just meaningless to talk about knowing simultaneously exact values of position and momentum; these quantities simply do not exist simultaneously. versus Complementarity asserts that it is not just meaningless to talk about knowing simultaneously exact values of position and momentum; these quantities simply do not exist simultaneously. You believe in the God who plays dice, and I in complete law and order in a world which objectively exists How are the outcomes chosen? “God does not play dice” Thought experiment, proposed by Einstein, Podolsky & Rosen (1935) “Can quantum-mechanical description of physical reality be considered complete?” The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ A B The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ Can, in principle, measure precisely separation and total momentum before they fly apart The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ The Einstein Podolsky Rosen ‘Paradox’ Decide to measure precisely the momentum of A The Einstein Podolsky Rosen ‘Paradox’ Decide to measure precisely the momentum of A A assumes wave properties The Einstein Podolsky Rosen ‘Paradox’ Decide to measure precisely the momentum of A A assumes wave properties According to the Copenhagen Interpretation, B instantaneously assumes wave properties The Einstein Podolsky Rosen ‘Paradox’ EPR regarded this prediction as unreasonable, as it violated causality. The Einstein Podolsky Rosen ‘Paradox’ EPR regarded this prediction as unreasonable, as it violated causality. “ [It] makes the reality of position and momentum in the second system depend upon the measurement carried out in the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.” The Einstein Podolsky Rosen ‘Paradox’ EPR regarded this prediction as unreasonable, as it violated causality. “ [It] makes the reality of position and momentum in the second system depend upon the measurement carried out in the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.” But this is exactly what does happen, in experiments carried out since the 1970s The Einstein Podolsky Rosen ‘Paradox’ EPR regarded this prediction as unreasonable, as it violated causality. “ [It] makes the reality of position and momentum in the second system depend upon the measurement carried out in the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.” But this is exactly what does happen, in experiments carried out since the 1970s Alain Aspect (1982) provided the final proof The Einstein Podolsky Rosen ‘Paradox’ Decide to measure precisely the momentum of A A assumes wave properties According to the Copenhagen Interpretation, B instantaneously assumes wave properties The Einstein Podolsky Rosen ‘Paradox’ Could the existence of the wave-measuring apparatus at A influence the wave function of the whole system, so that B somehow ‘knows’ before they separate that it is going to ‘be’ a wave?….. Decide to measure precisely the momentum of A A assumes wave properties According to the Copenhagen Interpretation, B instantaneously assumes wave properties The Einstein Podolsky Rosen ‘Paradox’ In Aspect’s experiment, the decision to measure either the wave or particle properties of A is taken only after they have separated (and so are causally disconnected in classical theories). Decide to measure precisely the momentum of A A assumes wave properties According to the Copenhagen Interpretation, B instantaneously assumes wave properties How are the outcomes chosen? “God does not play dice” EPR experiment proves conclusively that he does! Light is both lumps and ripples – but not at the same time! Which aspect is ‘real’ is determined (only) when light interacts with matter (Quantum reality may depend on the intervention of a conscious observer) Quantum states are ‘entangled’: they can influence each other instantaneously, even when separated by great distances “ Those who are not shocked when they first come across quantum theory cannot possibly have understood it”