richard feynman-quantum reality

8/14/2019 Richard Feynman-Quantum Reality

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Quantum Reality John Lindner, Fall 2007

FYS-18: Belief in God in an Age of Science The College of Wooster

Quantum RealityThe macroscopic worldof our classical intuition

emerges fromthe microscopic worldof quantum mechanics

Things on a very small scalebehave like nothing that

you have any direct experience about.

They do not behave like clouds,or billiard balls, or weights on springs,

or like anything that you have ever seen.

Richard Feynman

A series of thought experiments

introduce many of the major ideasof quantum physics


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Beam Splitter (BS)

Light falls on a half-silvered mirrorHalf is transmitted, half is reflected

At high intensity,light behaves like an electromagnetic wave

Intensity is proportional to amplitude squared

What happens if weturn down the light?

BS

I ! A 2light

source


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PMT

P ! A 2smoked

glass

yesnoyesyesnonoyes

Wave-Particle Duality

At low intensity,the graininess of light becomes apparent:photons, individual particles of light, are

detected by photo multipliers (PMTs)

Probability

Photons are transmitted or reflectedwith probability 1/2

More generally,the probability of detecting a photon is proportional to

the square of the electric field amplitude

These probabilities are thought to beontological rather than epistemological

Apparently, God does play dice with the universe


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Interferometer

Recombine the lightusing two mirrors and another beam splitter

At high intensity,waves interfere

constructively rightwardand destructively upward

What happens if weturn down the light?

mirror

I ! A 2

mirror

bright

dark


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Superposition

At low intensity, light is so faint thatthere is only one photon in the interferometer

at any one time

At first BS, photon neither reflected or transmitted!

It somehow interacts with the entire system,

evolving into a superposition of both possibilitiesuntil it is observed/measured by PMTs

It somehow interferes with itself

PMTyesyesyes

yesyesyesyes


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A superposition is a strange kind ofcomplex-number weightedcoexistence of alternatives

(possibilities, potentialities)

photon = a up + b right

Just asthe square of the electric field amplitude

corresponds to the probability

of detecting a photon,in the superposition,

the absolute square of the weightscorrespond to the probabilitiesof measuring the alternatives:

P up( ) = a 2

P right( ) = b 2

Complex numbers havereal and imaginary parts:

z =3

+

2i,

i = "1 z 2 = 32 + 2 2 = 13

So, imaginary numbers are fundamentalto our description of reality!


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Schrdinger

Deterministic and continuousevolution of quantum-level

complex-weighted superpositionswhen system not observed/measured:

photon = a up + b rightphoton S #

Measurement

Probabilistic and discontinuouscollapse of a superposition to classical

probability-weighted alternativeswhen system is observed/measured:

up , P = a 2

right , P = b 2

$%&

'&photon #M

Potentialities/possibilities evolve causallyuntil an observation realizes/actualizes one

Two Types ofQuantum Evolution


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PMT

measure

recoil

yesnoyesyesnonoyes

Which Path?

Can observe the path taken by the photonby measuring the recoil of a floating mirror

But this measurementcollapses the superposition,destroying the interference,allowing the photon to go

rightward or upward

This experimentelicits the particle nature of light,

while the previous experiment with the fixed mirrorselicits the wave nature of light


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The Elitzur-Vaidman

Bomb Testing ProblemConsider a design of bombs

whose detonators are so sensitiveeven slightest touch will explode them

Unfortunately,for some fraction of the manufactured bombs

the detonators are jammedand the bombs are duds

Classically,there is no way to identify good bombs

without exploding them

This one is a dud This one is (uh, was ) good

BANG!


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What is impossible in classical physicsis possible in quantum physics!

Use the silvered bomb detonatoras one of the mirrors in our interferometer

PMTyesyesyesyesyesyesyes

The superposition doesnt collapse untilit is observed/measured by the PMTs

In particular,it doesnt collapse due to the recoil of the detonator

the detonator must be jammed!

This bomb is a dud


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PMTyesyesno

The detonator would have recoiled

(and the bomb would have exploded)if the photon had reflected off it

The mere possibilityof the detonator recoiling

has collapsed the superpositionand destroyed the interference

thereby allowing the photon to gorightward or upward

This bomb is good and undetonated!

(The importance of such null measurementshas been verified experimentally


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PMTno

BANG!

This bomb was good and is detonated

So, we do destroy some of the good bombs,but we positively identify the others

without detonating them

(A different procedure can be employedto reduce the waste to effectively zero)


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What is a Measurement?

Consider a variation on the (in)famousSchrdinger Cat Experiment

wherein a (good) bomb amplifies amicroscopic event to macroscopic proportions

Without measurement,system evolves into superpositions of

transmitted and reflected photon+ exploded and unexploded bomb

+ dead and live observer

Bu we dont observe superpositionsof life and dead people!

So, to collapse the superposition,a measurement must occur atthe beam splitter or the bomb or the observer

But ultimately arent all of these quantum systems?Where in the chain do we cross the micro-macro threshold?

?


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Does ConsciousnessCollapse Superpositions?

Physicists likeEugene Wigner and Henry Stapp

have suggested that the

Mystery of Consciousness

is connected to the

Mystery of Measurementin Quantum Physics*

Physics seemscausally closed

except during the non-deterministiccollapse of quantum superpositions

Perhaps this is an opening for free willto the enter the universe

*Perhaps due to a Law of Minimization of Mysteries?


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Alternative InterpretationsThere exist alternative interpretations

of quantum physicsthat attempt to eliminate

some of the more bizarre aspectsof the standard Copenhagen interpretation,

especially the measurement problem:

Everetts Many Worlds/Histories Interpretation

the de Broglie-BohmPilot Wave Interpretation

CramersTransactional Interpretation

Evolved Copenhagen Interpretationsinvolving Decoherence

However,none of these interpretations

restore classical physicsor classical intuition


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ReferencesRoger Penrose

Shadows of the Mind:A Search for the Missing Science of Consciousness

(Oxford University Press, 1994)

Johnjoe McFaddenQuantum Evolution(W. W. Norton & Company, 2001)

richard feynman-quantum reality

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