Quantum nonlocality does not exist Frank J. Tipler1 Department of Mathematics, Tulane University, New Orleans, LA 70118 Edited* by John P. Perdew, Temple University, Philadelphia, PA, and approved June 2, 2014 (received for review December 30, 2013) Quantum nonlocality is shown to be an artifact of the Copenhagen instantaneously the spin of particle 2 would be fixed in the op- interpretation, in which each observed quantity has exactly one posite direction to that of particle 1—if we assume that [2] col- value at any instant. In reality, all physical systems obey quantum lapses at the instant we measure the spin of particle 1. The mechanics, which obeys no such rule. Locality is restored if observed purported mystery of quantum nonlocality lies in trying to un- and observer are both assumed to obey quantum mechanics, as in derstand how particle 2 changes—instantaneously—in re- the many-worlds interpretation (MWI). Using the MWI, I show that sponse to what has happened in the location of particle 1. the quantum side of Bell’s inequality, generally believed nonlocal, There is no mystery. There is no quantum nonlocality. Particle is really due to a series of three measurements (not two as in the 2 does not know what has happened to particle 1 when its spin is standard, oversimplified analysis), all three of which have only measured. State transitions are entirely local in quantum me- local effects. Thus, experiments confirming “nonlocality” are actu- chanics. All these statements are true because quantum me- ally confirming the MWI. The mistaken interpretation of nonlocal- chanics tells us that the wave function does not collapse when ity experiments depends crucially on a question-begging version the state of a system is measured. In particular, nonlocality dis- of the Born interpretation, which makes sense only in “collapse” appears when the many-worlds interpretation (2–5) is adopted. versions of quantum theory, about the meaning of the modulus of The many-worlds interpretation (MWI) dispels the mysteries of the wave function, so I use the interpretation based on the MWI, quantum mechanics. Collapse interpretations are nonlocal. So namely that the wave function is a world density amplitude, not the standard argument that quantum phenomena are nonlocal a probability amplitude. This view allows the Born interpretation goes like this: (i) Let us add an unmotivated, inconsistent, un- to be derived directly from the Schrödinger equation, by applying observable, nonlocal process (collapse) to local quantum me- the Schrödinger equation to both the observed and the observer. chanics; (ii) note that the resulting theory is nonlocal; and (iii) PHYSICS conclude that quantum mechanics is nonlocal. Bell’s theorem | Einstein–Podolsky–Rosen experiment | multiverse | I outlined the arguments in an earlier paper (6). Everett was indistinguishability the first to suggest (ref. 3, p. 149) that nonlocality would disap- pear in the MWI, but this paper is to my knowledge the first to onlocality is a standard example of a quantum mechanical prove what Everett claimed. Here I directly address Bell’sin- Nproperty not present in classical mechanics. A huge number equality, which requires a derivation of the Born interpretation of papers are published each year in the major physics journals of the wave function. My derivation starts from the standard [e.g., 5 in Physical Review Letters (PRL) in 1997 and 23 in PRL in MWI idea that the wave function is not a probability amplitude, 2004], purporting to clarify the meaning of “nonlocality.” The but instead a “world density amplitude,” which is to say jψj2 is phenomenon of nonlocality was first described in 1935 by Einstein, proportional to the density of universes in the multiverse. The et al. (1), in their classic paper, “Can quantum mechanical de- problem is to derive the Born frequencies from this assumption. scription of physical reality be considered complete?” Previous derivations have been unsatisfactory, in my judgment, The basic idea in Einstein, et al.’s paper (1) is best described in because an essential part of the physics has been left out. The the well-known formulation in terms of two electrons and their physics that has been heretofore omitted has been quantum spins. We have two spin 1/2 particles, and the two-particle system mechanical indistinguishability, applied to the experimenters and is in the rotationally invariant singlet state with zero total spin their experimental apparatus. From the MWI viewpoint, humans angular momentum. Thus, if we decide to measure the particle and their equipment are quantum mechanical objects no less than spins in the up–down direction, we would write the wave function atoms and are thus subject to indistinguishability no less than atoms. of such a state as Universes in the same quantum state are indistinguishable and j↑i j↓i − j↓i j↑i jΨi = 1 2pffiffiffi 1 2; [1] Significance 2 I show that quantum nonlocality is an artifact of the as- where the direction of the arrow denotes the direction of spin, sumption that observers obey the laws of classical mechanics, and the subscript identifies the particle. If we decide to measure whereas observed systems obey quantum mechanics. Locality the particle spins in the left–right direction, the wave function is restored if observed and observer both obey quantum me- would be written in a left–right basis as chanics, as in the many-worlds interpretation (MWI). Using the MWI, I show that the quantum side of Bell’s inequality is j ← i j → i − j → i j ← i “ ” jΨi = 1 2pffiffiffi 1 2: [2] entirely local. Thus, experiments confirming nonlocality are 2 actually confirming the MWI. The mistaken interpretation of Bell’s inequality depends on the idea that the wave function Nonlocality arises if and only if we assume that the measure- is a probability amplitude, but the MWI holds that the wave ment of the spin of a particle “collapses the wave function” from function is a world density amplitude. Assuming the wave func- the linear superposition to either j↑〉1j↓〉2 or j↓〉1j↑〉2 in [1]. If tion is a world density amplitude, I derive the Born interpreta- such a collapse occurs, then measuring the spin of particle 1 tion directly from Schrödinger’s equation. would fix the spin of particle 2. The spin of particle 2 would be fixed instantaneously, even if the particles were allowed to sep- Author contributions: F.J.T. performed research and wrote the paper. arate to large distances. If at the location of particle 1, we make The author declares no conflict of interest. a last-minute decision to measure the spin of particle 1 in the *This Direct Submission article had a prearranged editor. left–right direction rather than the up–down direction, then 1Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1324238111 PNAS Early Edition | 1of6 Downloaded by guest on September 29, 2021 hence if interchanged, nothing has happened. I use this fact to are carried out at space–time events that are space-like separated, derive the Born frequencies. In outline, the indistinguishability then there is no Lorentz invariant way of determining which allows probabilities in the Bayesian sense to be assigned to the measurement was carried out first. At space-like separation, the likelihood that we will be in a particular universe observing measuring operators U1 and U2 commute, and so we can equally a particular sequence of paired electron spins, and Bayesian well perform the measurement of the spins of the electrons in probability theory tells us how to calculate the most likely fre- reverse order and obtain the same splits, quencies from these probabilities. I show that these most j↑i j↓i − j↓i j↑i likely frequencies are the Born frequencies. 1 2 1 2 U1U2M1ð ...ÞM2ð ...Þ pffiffiffi 2 The Disappearance of Nonlocality in the MWI M ð↓Þj↑i j↓i M ð↑Þj↓i j↑i 2 1 2 2 1 2 To see how nonlocality disappears in detail, let us analyze the = U1M1ð ...Þ pffiffiffi − pffiffiffi [7] measure of the spins of the two particles from the many-worlds 2 2 perspective. Let Mi(...) denote the initial state of the device that M1ð↑ÞM2ð↓Þj↑i j↓i M1ð↓ÞM2ð↑Þj↓i j↑i measures the spin of the ith particle. The ellipsis denotes a = pffiffiffi 1 2 − pffiffiffi 1 2; measurement not yet having been performed. We can for sim- 2 2 plicity assume that the apparatus is 100% efficient and that the the last line of which is the same as that of [6] (except for the measurement does not change the spin being measured (putting order of states, which is irrelevant). in a more realistic efficiency and taking into account the fact that The effect of measurements in which both observers happen to measurement may affect the spin slightly would complicate the choose to measure with respect to the left/right basis is notation but the conclusions would be unchanged). That is, if each particle happens to be in an eigenstate of spin, a measure- j ← i j → i − j → i j ← i U U M ð ...ÞM ð ...Þ 1 2pffiffiffi 1 2 ment of the ith particle changes the measuring device—but not 2 1 2 1 2 the spin of the particle—as M ð←Þj ← i j → i M ð→Þj → i j ← i = U M 1 pffiffiffi 1 2 − 1 pffiffiffi 1 2 U M ð ...Þj↑i = M ð↑Þj↑i 2 2 1 1 1 1 1 [3] 2 2 U M ð ...Þj↓i = M ð↓Þj↓i [8] 1 1 1 1 1 M ð → ÞM ð←Þj ← i j → i = 2 1 pffiffiffi 1 2 U M ð ...Þj↑i = M ð↑Þj↑i 2 2 2 2 2 2 [4] U M ð ...Þj↓i = M ð↓Þj↓i ; M ð ← ÞM ð → Þj → i j ← i 2 2 2 2 2 − 2 1 pffiffiffi 1 2: 2 where Ui are linear operators that generate the change of state in the measurement apparatus, corresponding to the measurement.