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Blaylock - P424 Seminar 10/30/09

! hair balls, litter boxes, decapitated birds

! Schroedingers Cat & Wigners Friend

! altering the past & Wheelers Delayed Choice

! interference and choice! the remaining options

! hair balls, litter boxes, decapitated birds

! Schroedingers Cat & Wigners Friend

! altering the past & Wheelers Delayed Choice

! interference and choice! the remaining options

The problem with catslessons in quantum collapse


2/21


ShroedingersCat

Until the box is

opened and

examined by the

researcher, the

cat is in a super-

position of being

alive and dead.

with apologies to Berk Breathed

The Measurement Problem I


3/21


Wigners Friend

When and how does collapse occur?

WignerWigners

friend

Wigners

press agent

The Measurement Problem II


4/21


A Question of Consciousness?

Would the collapse

take place if the

researcher were in

the box?

Would the collapse

take place if it were

recorded by

computer?

Would the collapse

take place if an

amoeba were in the

box?

(I dont think so)


5/21


Electron doubleslit experiment

Each single electron by itself follows the

rules of the interference pattern.

The interference pattern shows

contributions from both slits.

Each electron must in some sense go

through both slits.

Buildup of electron

interference pattern in

Jnssons experiment.


6/21


Wheelers delayed choice

BS1

BS2

M2

M1

D2

D1

In a Mach-Zender

interferometer, each

photon travels both

paths and interferes,

destructively at D1 andconstructively at D2.


7/21


If the second beam

splitter is removed, the

detection of the photon

at D1 or D2 collapses

the trajectory to one orthe other path.

Wheelers delayed choice

BS1

M2

M1

D2

D1

If the decision to use or remove BS2 is made at the

last instant, then it seems the collapse to one path or

both paths occurs after the photon has passed BS1!!!

Vincent Jacques et al.,

Experimental

Realization of

Wheeler's Delayed-

Choice Gedanken

Experiment, Science315 966 - 968 (2007).


8/21


Gravitational Lenses

Light from a distant source can travel more than one patharound a source of gravity to get to an observer on Earth.

There appears to be more than one source, or the source is distorted.

View without

gravity.View with gravity.

If the source, mass and Earth areall exactly in line the source will

appear to be spread into a ring.

Big Mass

Distant

Source

Big Mass

Distant

Source


9/21


Big Mass

Distant

Source

10 billion light years

The choice about whether to

collapse the photons to a

single path or to both paths

occurs 10 billion years afterthe photons left the source.

Distant

Source

Does our choice of measurement affect the past?

cosmic delayed choice


10/21


Collapse is nonlocal

This is a very special, very restricted form of

nonlocality. We still cant make anything travelfaster than the speed of light, or send a signal

faster than the speed of light. Collapse is no use

for intergalactic telephone calls.

Collapse changes a wave that is spread out over a region to a

wave that is localized at one spot. This is a case of action-at-a-

distance!

Rather like voodoo.


11/21


Collapse is not relativistically invariant

"twin

=

1

2VV + HH( )

Consider two photons

produced in an entangled state:

Copenhagen says when Alice

measures one photon, both

collapse simultaneously.

From Bobs perspective

in his relativistic space

ship, Alices photon

collapses afterthe other.

I what reference

frame is the

collapse

simultaneous, and

why that frame?


12/21


Not Even Wrong

Wolfgang Pauli, when

shown the paper of a

colleague, once said:

Not only is that not right,

its not even wrong!

Copenhagen: An isolated

cat is in a superposition of

two possibilities until he isobserved. Then suddenly,

The idea of collapse was an ill-defined ad hoc addition to the (very

well defined) evolution described by the Schroedinger equation.


13/21


two parts to collapse

Disappearance of interference (a.k.a. decoherence)

recognized as a change in the rules of probability from

those of a superposition to those of a mixture

can be understood in terms of interaction with an

environment

Choice of a single outcome (a.k.a. projection)

who knows how to understand it

is this a deterministic, reversible process or not?


14/21


Combining Probabilities

mixtures add:

superpositions interfere:

+ =

or+ = or or

Superposition is distinguished

from the more common type

of combination, which is

described as a mixture.


15/21


which path collapse

In the electrondouble slit

experiment, the

interference pattern

implies the electron

is in a superposition

of going throughboth slits.

If we shine a lighton the mask, the

superposition

collapses to one

path and the

interference pattern

disappears.

Superposition:add amplitudes then

square to get probability

Mixture:add probabilities

directly


16/21


Double slitIn an undisturbed electron double slit experiment two electron waves

combine constructively in the center of the screen, giving maximumprobability of detecting an electron at that location.

minimum*destructively*

*Electrons (fermions) have an extra minus sign in the combination rules.


17/21


Scattering shift

If the wave has shifted by as much as half a wavelength, there will be

zero probability of detecting an electron in the center of the screen, and

maximum probability somewhere else.

What if a photon scatters off the electron on its way to the screen.

Scattering will change the direction and the momentum(wavelength) of the electron.

original

new

n.b.

If the wavelength/frequency

has changed, the interference

pattern will change as a

function of time.

minimum

high


18/21


Scattering uncertaintyBut the location and the momentum transfer of interaction is uncertain!There are many possibilities for where the scattering takes place, and how muchmomentum is transferred. Many possibilities for interaction form manysuperposing interference patterns.

If the photon is low energy and doesnt disturb the electron too

much, the interference pattern gets smeared out just a bit.

original

new


19/21


DecoherenceWith many small interactions, or several high-momentuminteractions, the disturbance is big enough to completely

smear out the interference.

The pattern smears out to the appearanceof a mixture.

If interactions are strong enough and energetic enough

to resolve the slits, the pattern is completely smeared.


20/21


potential solutions

Specify the collapse

dynamical collapse

Get rid of the collapse

multiple realities: Many Worlds

one reality: (nonlocal) Hidden Variable Theory

Sweep it under the rug

shut up and calculate


21/21


John Wheeler

What part of

quantum theory

do you not

understand?

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