Richard Feynman-Quantum Reality
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Transcript of 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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
FYS-18: Belief in God in an Age of Science The College of Wooster
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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Quantum Reality John Lindner, Fall 2007
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)