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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”