Bohmian Realism: Non-Local Hidden Variables and Holism Series Outline

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Foundations of Quantum Mechanics

Bohmian realism: non-local hidden variables and holism Series Outline

1. How to be a Quantum Mechanic

2. Entanglement, measurement and decoherence

3. A Gordian knot and Heisenberg’s cut

4. Local hidden realism: Einstein’s “reasonable” solution

5. QM’s classical inheritance

6. Bohmian realism: non-local hidden variables and holism

7. How many cats does it take to solve a paradox? Hidden Variables

Hidden variable theories are attractive:

• allow for unique objective realist view of the world

• no measurement or preferred basis problems

But they are severely constrained:

• Gleason/Kochen-Specker theorems rule out non-contextual HV

• Bell’s theorem and experiments rule out local realism

HV theories are still allowed, but they must be non-local, contextual and reproduce the results of QM:

de Broglie-Bohm (dB-B) Pilot Wave theory is the classic example An Unlikely Partnership

Louis Pierre Victor Raymond 7th Duc de Broglie:

• (1892-1987) born into an aristocratic family

• did a history degree, then a physics degree, completed his PhD in 1924, Nobel Prize in 1929

• invented the wavefunction and predicted electron diffraction

David Joseph Bohm:

• (1917-1992) born in Pennsylvania to Hungarian-Lithuanian Jewish immigrants

• targeted by Sen. McCarthy for alleged communist sympathies

• developed a holistic view of reality (and metaphysics) as “undivided wholeness in ﬂowing movement”

Both ignored and ostracized by their (Copenhagenist) peers “While the founding fathers agonized over the question 'particle' or 'wave', de Broglie in 1925 proposed the obvious answer 'particle' and 'wave'”

–John Bell Pilot Wave Basics

Pilot wave theory is a hidden variable theory:

• wavefunction is completed by an additional variable, Q(t)

! ( (x, t),Q(t))

• Q(t) deﬁnes a ‘trajectory’ in conﬁguration space

The system has two physically real components, wavefunction and particle:

• the wavefunction evolves according to the Schrödinger equation

• the particle moves along a trajectory deﬁned by the “guiding equation”

Indeterminism comes from ignorance of the particle’s initial position:

• the hidden variable (trajectory) is revealed when we make an observation Hamilton-Jacobi Mechanics

Based on the Hamilton-Jacobi (H-J) equation, @S ! + H(q, p)=0 @t Where S is Hamilton’s Principle Function and H the Hamiltonian of the system. @S Conjugate momentum defined by p = @q So we have the H-J equation, @S 1 ! = ( S)2 + V @t 2m r • H-J equation is satisfied by S (a wave on configuration space)

• S deﬁnes a vector ﬁeld whose integral curves particles move along 1 • for a single particle trajectories, Q˙ (t)= S mr Pilot Wave

Write the wavefunction in polar form,

! (x,t)=R(x,t)eiS(x,t)/~

Plug into the Schrödinger equation and separate real and imaginary pieces; leads to two equations:

@ R 2 R 2 S @S ( S)2 ~2 2R + r =0 = + V @t r · m r r ! ✓ ◆ @t m 2m R 1 If we make the connection, ⇢ = 2 = R 2 and v = S we get, | | mr ! @⇢ + (⇢v)=0 ! @t r·

This continuity equation governs a conserved ﬂow of particles Quantum Potential

What about the second equation? @S ( S)2 ! = r + V + Q @t 2m which is just the classical Hamilton-Jacobi equation with the potential modiﬁed by, ~2 2R ! Q = r 2m R known as the “quantum potential”

• depends on the state (wavefunction)… highly contextual

• non-zero even for free particles… non-trivial motion even when V=0

• particles can’t cross nodes of the wavefunction

Can interpret the QM system as a ﬂow of classical particles according to the Hamilton-Jacobi equation including a state-dependent quantum potential Double Slit Experiment

What does de Broglie-Bohm theory say about the famous experiment?

• pilot wave propagates through both slits

• guiding equation deﬁnes trajectories

• particle evolves along a trajectory and deﬁnitely goes through one of the slits; which slit determined by initial position

• wave behaviour (interference) encoded in possible trajectories, determined by the wavefunction

• can also think of particles following minimum of the quantum potential

• particle hits the screen and we see a localized ﬂash

Observables

In QM observables are represented by Hermitian operators acting on a Hilbert space

In dB-B an “observable” is more like a classical “be-able”, an ordinary function of the particle’s wavefunction along a trajectory

Deﬁne a “local expectation value” (LEV)

! (x, t)Aˆ( (x, t)) A(x, t, )=Re ⇤ ! (x, t) 2 ✓ | | ◆ Expectation values average an LEV over the ensemble’s probability distribution

! A = R(x, t)2 A(x, t, ) dx h i ! Z Unlike classical statistical mechanics, LEV and probability density are state dependent Measurements

• begin with a factorized system-apparatus (ready) wavefunction

• premeasurement leads to an entangled correlated wavefunction

c (x) (y) c (x) (y) ! n n 0 ! n n n n n X X The actual value of the observable along a trajectory given by the LEV:

Once measurement is completed, trajectories correspond to non- overlapping pointer states:

• LEV evolves towards an eigenvalue of the operator as pointer states move apart in phase space

• actual result determined by which “branch” the particle ends up in premeasurement

c1 1(x)1(y) (x, t)Aˆ( (x, t)) A(x, t, )=Re ⇤ (x, t) 2 ✓ | | ◆ c2 2(x)2(y)

A(x, t, ) ↵ ! 2

cn n(x)0(y) c3 3(x)3(y) n X

c4 4(x)4(y)

H =0 SA 6 In dB-B particles are distributed according to ! P = R(x, y, t)2 = (x, y, t) 2 | | during premeasurement system-apparatus becomes entangled

! (x0,y0,t0)= cn n(x0,t0) 0(y0,t0) (x, y, t)= cn n(x, t)n(y, t) ! n n ✓ X ◆ X entanglement with environment ⌘ ( z 1 ,z 2 , ,z N ) ensures packets cease to overlap in conﬁguration space ··· 2 2 2 2 ! (x, y, t) cn n(x, t) n(y, t) | | ⇡ | | | | | | n th probability system found in inﬁnitesimalX region of (x,y) space where n summand non-zero P dx dy = c 2 (x, t) 2 (y, t) 2 dx dy ! n | n| | n | | n |

th total probability system found in n state

P dx dy = c 2 n | n| Z Born: a quantum system whose state is given by the superposition,

! (x)= cn n(x) n X may be found in any eigenstate upon measurement. The result is fundamentally th indeterminate and we postulate the probability to be found in the n state is,

2 ! Pn = cn | | Bohm: a quantum system is speciﬁed by the state and initial position,

! (x)= cn n(x) ,Q(t0) n X in a measurement process the state evolves into non-overlapping branches. The particle evolves deterministically along a trajectory into one of those branches, determined by the (unknown) initial position. If particles are initially distributed according to ( x ) 2 then it follows that, | | P = c 2 n | n| Uncertainty Principle

Trajectories deﬁne a deﬁnite position and momentum to the particle, is this allowed?

• QM says system can’t have simultaneous values for non-commuting observables e.g. (x-p)

• dB-B says they merely can’t be simultaneously measured

For a classical ensemble of particles with position x , momentum p = S (x) r and distribution ⇢ ( x )= R ( x ) 2, momentum uncertainty given by, 2 (p)2 = R2 ( S)2 dx R2 Sdx ! classical r r Z ✓ Z ◆ in dB-B, LEV for observables depends on the state and leads to,

2 2 2 2 2 2 R 2 (p) = R ( S) ~ r dx R Sdx dB-B r R r Z  ✓ ◆ ✓ Z ◆ Quantum Equilibrium

So far we have assumed that particles are Born distributed

! P (x, t)= (x, t) 2 = R(x, t)2 | | What’s so special about this distribution?

• if Born distributed at any time t, then Schrödinger equation ensures it is Born distributed for all subsequent times, “equivariance”

• can prove that Born distribution is the only equivariant distribution

What happens if we imagine an initial distribution P ( x, t ) = ( x, t ) 2 ? 6 | | • simulations show that system rapidly relaxes into a Born distribution

• Born rule dynamically derived in dB-B, not a postulate like in QM dB-B theory allows for non-equilibrium dynamics, genuinely new (testable) theory! Dynamical origin of quantum probabilities: numerical simulations Results of evolution

⇢ 2 | |

Results for t =0(a,b), for t =2⇡ (c,d) and for t =4⇡ (e,f). While 2 recurs to its initial value, the smoothed particle distribution ⇢ shows a remarkable evolution towards quantum equilibrium! | | . – Typeset by FoilTEX – 28 Bohmian Path Integral

Can move to a Lagrangian frame moving with the ﬂow

d @ dS @S ( S)2 ( S)2 dR @R R = + v , = + r = r V Q = L Q , = + v R = v dt @t · r dt @t m 2m dt @t · r 2 r · H-J and continuity equations decouple and can solve the system

1 2 v dt ! S(x, t)=S(x0,t0)+ LQ dt R(x, t)=R(x0,t0) e r· Z R Putting this together we get the evolved wavefunction 1 2 v dt iSQ/~ (x, t)=e r· e (x0,t0) ! R KQ(x,t;x0,t0) i.e. the wavefunction is evolved| along{z a single} Bohmian path, weighted by the quantum action! (c.f. Feynman’s path integral) evaluate classical action over evaluate non-local quantum all possible paths , action over one path Relativity dB-B says that nature is fundamentally non-local, so why do we learn relativity?

• EPR correlations are non-local, but outcomes are still random, so cant use the non-local correlations for communication, “no-signalling theorem”

In dB-B theory space-time has preferred frames:

• but still no superluminal communication

• results are relativistic even though the theory is not fundamentally so

• results are not relativistic if we move away from quantum equilibrium!

In dB-B quantum equilibrium allows QM and relativity to emerge dynamically; both phenomena emerge from a more fundamental pilot wave picture in equilibrium Holism

N particle wavefunction deﬁned on 3N dimensional conﬁguration space

! (x1, ,xN ; t) ··· each particle’s motion governed by a guidance equation depending on all other particles, dxi 1 ! = vi(x1, ,xN )= iS(x1, ,xN ) dt ··· mi r ···

• local in 3N-d conﬁguration space, non-local in 3-d real space

• interfering with any particle affects all other particles in the universe

If we can factorize the wavefunction (no entanglement), systems evolve effectively independent of one another dx 1 i(S(x1)+ +S(xN )) k (x1, ,xN )=R(x1) R(xN )e ··· = kS(xk) ··· ··· dt mk r Bohmian Realism

Bohmian mechanics provides a particular view of reality:

• state is uniquely speciﬁed by wavefunction and trajectory

• objectively real (exists independent of observers) and can be applied to closed systems like the universe

• fundamentally deterministic; probability arises only from ignorance of initial position (epistemic)

• observable properties are just values, evolve towards eigenvalues of Hermitian operators in premeasurement process

• highly contextual and non-local, signal causality preserved in quantum equilibrium

Bohmian mechanics provides an ontological but highly non-classical picture of QM Summary dB-B theory has many attractive features of classical physics:

• unique, deterministic, objective, formulated in conﬁguration space but is (necessarily) non-classical:

• dynamics determined by the state as well as the Hamiltonian

• non-local and contextual so evades all no-go theorems for hidden variables

• even deﬁnes a generalization of QM away from quantum equilibrium

Consistency does not make it correct; some things to think about:

• why is position special? why should nature be objective? why should it be unique? what happens to all the empty branches? what does it mean to be able to deﬁne the energy (for example) of the universe objectively? Series Outline