dr martin hendry university of glasgow lumps light in or ? reach for the 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

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





….Metal plate





….Metal plate





No effect for blue light

….Metal plate





Metal plate





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


1911

I insist on the provisional character of this concept, which does not seem reconcilable with the experimentally verified consequences of the wave 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::


PoisonGas

Radioactive source

R.I.P.

::


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’


A B


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


Decide to measure precisely the momentum of A



A assumes wave properties




According to the Copenhagen Interpretation, instantaneously assumes wave properties

B


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




“

But this is exactly what does happen, in experiments carried out since the 1970s

Alain Aspect (1982) provided the final proof





B





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?…..





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”

““

dr martin hendry university of glasgow lumps light in or ? reach for the stars

Documents

ejected electronsmeter

measures speed

electric currentmeter

light energy

blue light

luminiferous etherwave

reflected angle r rival

wave theory1909it