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Dr Martin Hendry University of Glasgow Lumps Lumps Light in or ? Reach Reach for the for the Stars Stars
Dr Martin HendryUniversity of Glasgow
LumpsLumpsLight in
or ?ReachReachfor thefor theStarsStars
Isaac Newton
1686
Particle theory of light
Prism
White 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 toChristian 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 Particle theory dominated until early 1800s:until early 1800s:
Experiments byThomas Young and Augustin Fresnel changed all that!
Direction of waves
Barrier
OutgoingCircular 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
Inte
nsi
tyUltravioletCatastrophe
WilhelmWien
The UV Catastrophe could be avoided if light energy was quantised in packets, or
photons of energy E = hf
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
1909It 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
1909It 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
1909It 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
1924There are therefore now two theories of light, both indispensable…without any logical connection
The Bohr atom, 1913
Absorption
e -
e -
Emission
e - e -
Louis de Broglie, 1923
If light waves also behave like particles, why shouldn’t electrons also behave like waves?
Pilot Waves
Direction of waves
Interference of Electrons
Davisson & Germer; Thomson & Reid, 1937
Making Quantum Mechanics Work
Werner Heisenberg Erwin Schrodinger Max Born Neils Bohr
Paul Dirac Wolfgang Pauli John 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 PrincipleThe precision of measurements in a quantum system is limited in principle
Heisenberg Uncertainty Principle
px ~ hThe precision of measurements in a quantum system is limited in principle
Heisenberg Uncertainty Principle
px ~ hThe 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 CatSchrodinger’s Cat
PoisonGas
Radioactive source::
Schrodinger’s CatSchrodinger’s Cat
PoisonGas
Radioactive source::
Schrodinger’s CatSchrodinger’s Cat
PoisonGas
Radioactive source
R.I.P.
::
Schrodinger’s CatSchrodinger’s Cat
PoisonGas
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’
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, 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 experimentproves conclusively that he does!
Light is Light is bothboth lumps and ripples – lumps and ripples – but not at the same time!but not at the same time!
Which aspect is ‘real’ is determined Which aspect is ‘real’ is determined (only) when light interacts with matter(only) when light interacts with matter
(Quantum reality may depend on the (Quantum reality may depend on the intervention of a conscious observer)intervention of a conscious observer)
Quantum states are ‘entangled’: they Quantum states are ‘entangled’: they can influence each other instantaneously, can influence each other instantaneously, even when separated by great distanceseven when separated by great distances
Those who are not Those who are not shocked when they shocked when they first come across first come across quantum theory quantum theory cannot possibly cannot possibly have understood it”have understood it”
““
