quantum physics not understandable surely you're joking

7/25/2019 Quantum Physics Not Understandable Surely You'Re Joking

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Quantum Physics not understandable? Surely

Youre Joking, Mr. Feynman!

Detlef Drr

Mathematisches Institut

LMU Mnchen

for a good old friend: Gianfausto DellAntonio
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Gianfausto described me once as a member of a sect (andmaybe he still does); thesect of the understanders - theholders of the truth

sect of the understanders?Nobody understands quantummechanics
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Gianfausto described me once as a member of a sect (andmaybe he still does); thesect of the understanders - theholders of the truth

sect of the understanders?Nobody understands quantummechanics
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Richard P. Feynman (1965) TheCharacter of Physical Law

There was a time when the

newspapers said that only twelvemen understood the theory of re-lativity. I do not believe there everwas such a time. There might ha-ve been a time when only one mandid, because he was the only guywho caught on, before he wrote hispaper. But after people read thepaper, a lot of people understoodthe theory of relativity in some wayor other, certainly more than twel-

ve. On the other hand, I think Ican safely say thatnobodyunder-stands quantum mechanics.
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Why wouldhesay that? Because of

the mad hatter?
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The inventor of observables: Matrix Mechanics - physicalquantities become operator observables

Werner Heisenberg (1901-1976)

How?

by putting hats on x X; p P;XP=PX noncommutative algebra
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How?

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How?

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How?

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How can putting hats achieve a description of the real world?

No problem says W. Heisenberg, there is no real world

...the idea of an objective real world whose smallest parts existobjectively in the same sense as stones or trees exist,independently of whether or not we observe them... isimpossible....

in The Physicists Conception of Nature. Harcourt Brace, page 129,(1958), (Trans. ArnoldJ. Pomerans)
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How can putting hats achieve a description of the real world?

No problem says W. Heisenberg, there is no real world

...the idea of an objective real world whose smallest parts exist

objectively in the same sense as stones or trees exist,independently of whether or not we observe them... isimpossible....

in The Physicists Conception of Nature. Harcourt Brace, page 129,(1958), (Trans. ArnoldJ. Pomerans)
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He did not walk alone. There was also the quantumphilosopher

Niels Bohr 1885-1962
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Complementarity, the new philosophy of nature

A complete elucidation of one and the same object mayrequire diverse points of view which defy a unique description.

Indeed, strictly speaking, the conscious analysis of any conceptstands in a relation of exclusion to its immediate application

Bohr quoted in M. Jammer, The Philosophy of Quantum Mechanics.Wiley, New York, page 102, (1974) introducing complementarity
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Applied Quantum Philosophy
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The thesis light consists of particles and the antithesis lightconsists of waves fought with one another until they wereunited in the synthesis of quantum mechanics. ...Only why notapply it to the thesis Liberalism (or Capitalism), the antithesisCommunism, and expect a synthesis, instead of a completeand permanent victory for the antithesis? There seems to besome inconsistency. But the idea of complementarity goes

deeper. In fact, this thesis and antithesis represent twopsychological motives and economic forces, both justified inthemselves, but, in their extremes, mutually exclusive. ...theremust exist a relation between the latitudes of freedom df andof regulation dr, of the type df dr=p. ...But what is the

political constant p? I must leave this to a future quantumtheory of human affairs.

Max Born: Physics and Politics
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The thesis light consists of particles and the antithesis lightconsists of waves fought with one another until they wereunited in the synthesis of quantum mechanics. ...Only why notapply it to the thesis Liberalism (or Capitalism), the antithesisCommunism, and expect a synthesis, instead of a completeand permanent victory for the antithesis? There seems to besome inconsistency. But the idea of complementarity goes

deeper. In fact, this thesis and antithesis represent twopsychological motives and economic forces, both justified inthemselves, but, in their extremes, mutually exclusive. ...theremust exist a relation between the latitudes of freedom df andof regulation dr, of the type df dr=p. ...But what is the

political constant p? I must leave this to a future quantumtheory of human affairs.

Max Born: Physics and Politics


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While one can safely say that nothing of that sort isunderstandable, this is not what Feynman had in mind

Neither did he fall for that

He did worry about real physics
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While one can safely say that nothing of that sort isunderstandable, this is not what Feynman had in mind

Neither did he fall for that

He did worry about real physics

th d bl lit i t
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the double slit experiment

! #$%&'(%&'& )$*+$ *, *(#',,*-.% /' %0#.1*& *&

1&2 +.1,,*+1. )12 1&3 )$*+$ $1, *& */ /$% $%14/ '5

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(2,/%42 '5 671&/7( (%+$1&+*,

:*+$143 ;%2&(1&
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Landau and Lifshitz about the double slit experiment

It is clear that this result can in no way be reconciled with the

idea that electrons move in paths.... In quantum mechanicsthere is no such concept as the path of a particle
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What is mysterious about that experiment? What is the sourceof all evil?

A particle can only go through one of the slits, a wave can gothrough both, but on the screen only localized dots appear, asif particles arrived, but the ensemble of dots make up a

pattern which looks like an interference pattern of a wave

So? Wheres the problem?

No problem as this man suggested in 1927
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h h i
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the harmonizer

de Broglie 1892- 1987: wave and particle

de Broglie proposed that the wave guides the particles: (Q, )

BUT he was ridiculed at the Solvay conference in 1927. WHY?

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the harmonizer




th h i


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the harmonizer




the harmonizer
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the harmonizer



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Why did the physicists not listen? Why did they continue toagonize over

big mystery: Why a dot at the screen? How can the dots builtup a wave-like interference pattern, if one particle per year issent through the double slit? Whats the physics?

little mystery: Why does the interference pattern vanish if onelooks through which slit the particle goes?

huge mystery: What is the role of the wave, what describesthe factual state of affairs? What is really going on?
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The endless agony revisited
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the unphysical wave function

Erwin Schrdinger 1887-1961


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and the high dimensional unphysical configuration space

Wave function ofn particles is afunction ofn position variables(x1, x2, ...,xn) ! It is not a function on physical space!

This is called entanglement and is the source of decoherence

it is governed by a linearequation

It changes continuously, no jumps

What is its relation to reality? Erwin Schrdinger in Naturwissenschaften23, 807 (1935) in: Die gegenwrtige Situation in der Quantenmechanik

writes about that


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writes about that
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writes about that
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writes about that
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writes about that
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writes about that

and measurement problem
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p

One can even set up quite ridiculous cases. A cat is penned up in a steelchamber, along with the following device (which must be secured against

direct interference by the cat): in a Geiger counter there is a tiny bit ofradioactive substance, so small, that perhaps in the course of the hourone of the atoms decays, but also, with equal probability, perhaps none; ifit happens, the counter tube discharges and through a relay releases ahammer which shatters a small flask of hydrocyanic acid. If one has left

this entire system to itself for an hour, one would say that the cat stilllives if meanwhile no atom has decayed. The psi-function of the entiresystem would express this by having in it the living and dead cat (pardonthe expression) mixed or smeared out in equal parts. It is typical of thesecases that an indeterminacy originally restricted to the atomic domain

becomes transformed into macroscopic indeterminacy, which can then beresolved by direct observation. That prevents us from so naivelyaccepting as valid a blurred model for representing reality. In itself itwould not embody anything unclear or contradictory. There is adifference between a shaky or out-of-focus photograph and a

snapshot of clouds and fog banks.

Clearly the cat decoheres the atomic wave function and Alice
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Clearly, the cat decoheres the atomic wave function and Alicedecoheres the cat, but who or what creates the facts?

the cat waits for the particle, she wants to catch it

and then
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! #$

and then Alice looks
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and then? Dont worry says
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the mad hatter

...the idea of an objective real world whose smallest parts exist objectivelyin the same sense as stones or trees exist, independently of whether ornot we observe them... is impossible...
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we observe and creates facts? who are we? is Alice a we?
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Feynman wouldnt buy that. He says:

Does this mean that my observations become real only when I observe anobserver observing something as it happens? This is a horrible viewpoint.Do you seriously entertain the thought that without observer there is no

reality? Which observer? Any observer? Is a fly an observer? Is a star anobserver? Was there no reality before 109 B.C. before life began? Or areyou the observer? Then there is no reality to the world after you aredead? I know a number of otherwise respectable physicists who havebought life insurance.
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Some still think that this man (and perhaps this man thinks sohimself) can help us to exist! He is an observer, presumablywith life insurance
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Mermin, in Physics Today on Quantum Baysianism, says no,we dont need him:

Albert Einstein famously asked whether a wavefunction could becollapsed by the observations of a mouse. Bell expanded on that, askingwhether the wavefunction of the world awaited the appearance of aphysicist with a PhD before collapsing. The QBist answers both

questions with no. A mouse lacks the mental facility to use quantummechanics to update its state assignments on the basis of its subsequentexperience, but these days even an undergraduate can easily learn enoughquantum mechanics to do just that.

enough of silliness, back to serious physics
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Mermin, in Physics Today on Quantum Baysianism, says no,we dont need him:

Albert Einstein famously asked whether a wavefunction could becollapsed by the observations of a mouse. Bell expanded on that, askingwhether the wavefunction of the world awaited the appearance of aphysicist with a PhD before collapsing. The QBist answers both

questions with no. A mouse lacks the mental facility to use quantummechanics to update its state assignments on the basis of its subsequentexperience, but these days even an undergraduate can easily learn enoughquantum mechanics to do just that.

enough of silliness, back to serious physics

Why comes a dot at the screen, that is the question!
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Why comes a dot at the screen, that is the question!

No said this man

To be, or not to be: that is the question: Whether tis nobler in the mindto suffer The slings and arrows of outrageous fortune, Or to take armsagainst a sea of troubles...

de Broglie took arms against a sea of troubles and suggestedth t th fi ti id th ti l i
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that the wave on configuration space guides the particles inphysical space ((Q1,Q2, ...,Qn), )
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but one man understood it all in 1952: (Q, ) is indeed thetrivial solution to all the mysteries of quantum mechanics.

David Bohm (1917-1992)
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Bohmian Mechanics (Q,)

is a solution of the Schrdinger equation

Q= (Q1, ...QN) positions ofN particles. Q(t) solves theguiding equation

ddt

Q ln

Analyze this in Quantum Equilibrium (Boltzmannian statistical

analysis of BM)
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Bohmian Mechanics (Q,)

is a solution of the Schrdinger equation

Q= (Q1, ...QN) positions ofN particles. Q(t) solves theguiding equation

ddt

Q ln

Analyze this in Quantum Equilibrium (Boltzmannian statistical

analysis of BM)

theoremsabout (Q, ) instead of axioms
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a subsystem Xof a big system Q= (X, Y), (x, y) has theconditional wave function (x) = (x, Y) guiding the particles of

the subsystem and which sometimes is the effective wave functionobserving an autonomous Schrdinger equation for the subsystem.

Quantum equilibrium (DGZ): = ||2 is thetypicalempiricaldistribution for X in the sense of Boltzmann in an ensemble ofsubsystems having wave function

Heisenbergs uncertainty principle holds (of course)

POVMs appear naturally as book keeper of statistics inmeasurement situations, sometimes POVMs are PVs and the bookkeepers define (via the spectral measure) observables A

precise description of the effective collapse of the wave function andemergence of classical behavior

Alice stays completely sane, never mind the grinning cat

arrival times of particles are well defined

wave function must be either fermionic or bosonic

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all mysteries (the tiny, the big and the huge) are gone
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big mystery: Why a dot at the screen? How can the dots built up a

wave like interference pattern, if one particle per year is sent throughthe double slit? Whats the physics?

Answer: Because guides Q, the particle arrives at the screen andquantum equilibrium holds

little mystery: Why does the interference pattern vanish if one looks

through which slit the particle goes?

Answer: Because the wave function of system and apparatus lives onthe configuration space of system and apparatus, and the wave partsin configuration space dont meet anymore: Decoherence!

huge mystery: What is the role of the wave, what describes thefactual state of affairs? What is really going on?

Answer: guides Q and Q is what there is. Just like ordinaryphysics ought to be

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It is all so easy when local beables (like particle positions)exist

It i ll h l l b bl (lik ti l iti )
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computer simulation of Bohmian trajectories by Chris Dewdney

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exist

3000 4000 5000 6000 7000 8000

6

4

2

0

2

4

Propagation distance[mm]

Transverse

coordinate[mm]

Exeperiment: S.Kocsis et al: Observing the Average Trajectories of SinglePhotons in a Two-Slit Interferometer. Science 2011

But hold on! Bohms papers were read and readily dismissed!Why? Because all the romantics of mysticism was gone?Because the swamp was laid dry? How did he overcome the
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ecause t e swa p was a d d y ow d d e o e co e t eproblem de Broglie faced in the Solvay conference. Isnt it a

sin to have a physical field, , is on configuration space? Hedid not overcome that! Quite the contrary!

Exactly, says this man, nature is like that, there is no betterway

John Stuart Bell 1928-1990, proved non locality of nature

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p yproblem de Broglie faced in the Solvay conference. Isnt it a




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p yproblem de Broglie faced in the Solvay conference. Isnt it a




Bohmian mechanics is just what the doctor ordered
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!"#$ &'' (&) *# "&++$,
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And Gianfausto should be too! Welcome to the sect! But-and

there is always a but-: Why isnt Bohmian mechanics taught?

Possible answers:

since it isnt taught there must be something wrong with it

since your admired authority does not like it, there must besomething wrong with it

Heisenberg or Bohr(or the pope)forbids it, because it is bad foryour mind(or for your well being after death)

what must not be cannot be
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Possible answers:




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Possible answers:




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Possible answers:




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Possible answers:



Heisenberg or Bohr (or the pope) forbids it, because it is bad foryour mind (or for your well being after death)

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Possible answers:



Heisenberg or Bohr (or the pope) forbids it, because it is bad foryour mind (or for your well being after death)


Facts cant be impossible or can they?

h I bl
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The Impossible Fact

Palmstroem, old, an aimless rover, walking in the wrong directionat a busy intersection is run over.How, he says, his life restoring and with pluck his death ignoring,can an accident like this ever happen? Whats amiss? Did the stateadministration fail in motor transportation?Did police ignore the need for reducing driving speed?

Isnt there a prohibition, barring motorized transmissionof the living to the dead? Was the driver right who sped ... ?Tightly swathed in dampened tissues he explores the legal issues,and it soon is clear as air: Cars were not permitted there!And he comes to the conclusion: His mishap was an illusion,

for, he reasons pointedly, that which must not, can not be.

Ch. Morgenstern, Die unmgliche Tatsache, taken from Palmstrm undanderen Galgenliedern translated by M. Knight

Facts cant be impossible or can they?

Th I ibl F
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The Impossible Fact

Palmstroem, old, an aimless rover, walking in the wrong directionat a busy intersection is run over.How, he says, his life restoring and with pluck his death ignoring,can an accident like this ever happen? Whats amiss? Did the stateadministration fail in motor transportation?Did police ignore the need for reducing driving speed?

Isnt there a prohibition, barring motorized transmissionof the living to the dead? Was the driver right who sped ... ?Tightly swathed in dampened tissues he explores the legal issues,and it soon is clear as air: Cars were not permitted there!And he comes to the conclusion: His mishap was an illusion,

for, he reasons pointedly, that which must not, can not be.

Ch. Morgenstern, Die unmgliche Tatsache, taken from Palmstrm undanderen Galgenliedern translated by M. Knight
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quantum physics not understandable surely you're joking

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