Dr Martin Hendry University of Glasgow Lumps Light in or ? Reach for the Stars

Isaac Newton 1686

Particle theory of light Prism White light

Refraction of light

Particles move faster in more “optically dense” medium

Reflection of light ir 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 Light Wave Intensity Barrier

Particle theory dominated until early 1800s: Experiments by Thomas Young and Augustin Fresnel changed all that!

Direction of waves Barrier Outgoing Circular Waves Diffraction of light

Direction of waves Interference of light

Direction of waves Interference of light

Maxwell’s theory of light Early 1900s: accelerated electron radiates

How do atoms persist?

Black-body radiation

Wavelength Intensity Ultraviolet Catastrophe Wilhelm Wien

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

Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

Metal plate The Photoelectric Effect Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons Incoming light, produces electric current

Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

…. Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

…. Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons No effect for blue light

…. Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

Metal plate The Photoelectric Effect Incoming light, produces electric current Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons Effect seen for UV light

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 e -

Emission e -

Louis de Broglie, 1923 If light waves also behave like particles, why shouldn’t electrons also behave like waves? Pilot Waves Davisson & Germer; Thomson & Reid, 1937

Making Quantum Mechanics Work Werner HeisenbergErwin Schrodinger Max Born Neils Bohr Paul DiracWolfgang PauliJohn von Neumann :

All physical systems and events are inherently probabilistic, expressed by the Wave Function when the quantum system is observed, the wave function collapses Only Copenhagen Interpretation

Heisenberg Uncertainty Principle The precision of measurements in a quantum system is limited in principle

Heisenberg Uncertainty Principle  p  x ~ h The precision of measurements in a quantum system is limited in principle

Heisenberg Uncertainty Principle  p  x ~ h The precision of measurements in a quantum system is limited in principle Position and momentum are complementary properties: the action of measurement determines which of the two properties the quantum system possesses

Schrodinger’s Cat Poison Gas Radioactive source :

Schrodinger’s Cat Poison Gas Radioactive source :

Schrodinger’s Cat Poison Gas Radioactive source R.I.P. :

Schrodinger’s Cat Poison Gas Radioactive source + 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’

AB

Can, in principle, measure precisely separation and total momentum before they fly apart

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, instantaneously assumes wave properties B

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, instantaneously assumes wave properties B

The Einstein Podolsky Rosen ‘Paradox’ Decide to measure precisely the momentum of A A assumes wave properties According to the Copenhagen Interpretation, instantaneously assumes wave properties B 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?…..

The Einstein Podolsky Rosen ‘Paradox’ Decide to measure precisely the momentum of A A assumes wave properties According to the Copenhagen Interpretation, instantaneously assumes wave properties B 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).

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” “