In the famous Solvay 1927 conference, Einstein discussed a gedanken experiment involving a single photon diffracted at an aperture and impinging on a screen. He devised the example to support De Broglie's hypothesis of the pilot wave, and his own ideas on the incompleteness of the description of physical reality provided by Quantum Mechanics. Partial realizations of Einstein's example have been performed, but the complete experiment has not been attempted (for good practical reasons) yet. Here I describe how to do it with accessible means. The setup will make possible to test Hellwig and Kraus postulate of covariant reduction of the quantum state.
In the famous Solvay 1927 conference, Einstein discussed a gedanken experiment involving a single photon diffracted at an aperture and impinging on a screen. He devised the example to support De Broglie's hypothesis of the pilot wave, and his own ideas on the incompleteness of the description of physical reality provided by Quantum Mechanics. Partial realizations of Einstein's example have been performed, but the complete experiment has not been attempted (for good practical reasons) yet. Here I describe how to do it with accessible means. The setup will make possible to test Hellwig and Kraus postulate of covariant reduction of the quantum state.
Einstein's 1927 argument for incompleteness A huge literature arose out of the famous ‘EPR’ paper by Einstein, Podolsky and Rosen (1935), entitled ‘Can quantum-mechanical description of physical reality be considered complete?’. The EPR paper argued, on the basis of (among other things) the absence of action at a distance, that quantum theory must be incomplete. It is less well-known that a much simpler argument, leading to the same conclusion, was presented by Einstein eight years earlier in the general discussion at the fifth Solvay conference (pp. 440ff.). Einstein compares and contrasts two views about the nature of the wave function ψ , for the specific case of a single electron. According to view I, ψ represents an ensemble (or ‘cloud’) of electrons, while according to view II, ψ is a complete description of an individual electron. Einstein argues that view II is incompatible with locality, and that to avoid this, in addition to ψ there should exist a localised particle (along the lines of de Broglie's theory). Thus, according to this reasoning, if one assumes locality, then quantum theory (as normally understood today) is incomplete. The conclusion of Einstein's argument in 1927 is the same as that of EPR in 1935, even if the form of the argument is rather different. Einstein considers electrons striking a screen with a small hole that diffracts the electron wave, which on the far side of the screen spreads out uniformly in all directions and strikes a photographic film in the shape of a hemisphere with large radius (see Einstein's figure, p. 440).
Single electron may have wavefunction of a macroscopic lengthscale but cannot be detected in two places. At the Solvay Conference of 1927, Einstein argued that in a combination with Bohr's postulate about wavefunction as an exhaustive characteristics of electron, this fact implies nonlocality related to the collapse of a macroscopic wavefunction of single electron. This was his objection to Bohr's quantum mechanics (QM) in favor of de Broglie's quantum theory. We perform an experimental test in low-temperature state of matter where electrons have macroscopic orbit-like wavefunctions and surprisingly observe nonlocality of that type. This nonlocality differs in principle from the well-known Einstein-Podolsky-Rosen (EPR) nonlocality because the former does not imply quantum entanglement and hence is beyond the no-communication theorem. Our observation is the first empirical justification of the choice in favor of QM in compare with de Broglie's theory. Moreover, now we can expand the current concept of reality and thus open the door to a realistic interpretation of quantum formalism. Ultimately, we thus come to a quantum model of Universe based on the standard QM.
At the 1927 Solvay conference, Albert Einstein presented a thought experiment intended to demonstrate the incompleteness of the quantum mechanical description of reality. In the following years, the experiment was modified by Einstein, de Broglie, and several other commentators into a simple scenario involving the splitting in half of the wave function of a single particle in a box. This paper collects together several formulations of this thought experiment from the literature, analyzes and assesses it from the point of view of the Einstein–Bohr debates, the EPR dilemma, and Bell’s theorem, and argues for “Einstein’s Boxes” taking its rightful place alongside similar but historically better known quantum mechanical thought experiments such as EPR and Schrödinger’s Cat.
Polarization entangled photon pairs Throughout his life, Albert Einstein was never reconciled to quantum theory being an essentially indeterministic description of natural processes, even though he himself contributed fundamental ideas to its development. “God does not play dice” was his inner conviction. In his opinion, quantum theory was only makeshift. His doubts about the completeness of the quantum mechanical description were expressed concisely in a paper published jointly with Podolsky and Rosen (Einstein, Podolsky and Rosen, 1935). This paper analyzes a sophisticated Gedanken experiment, now famous as the Einstein–Podolsky–Rosen paradox, which has excited theoreticians ever since. The Gedanken experiment was recently realized in a laboratory. The analyzed objects are photon pairs – and this is what has motivated us to dedicate a chapter to this problem which has bearing upon the foundations of quantum mechanics. The photon pairs are formed by two photons generated in sequence (in a so-called cascade transition, as shown in Fig. 11.1). Due to the validity of the angular momentum conservation law (discussed in Section 6.9) for the elementary emission process, the two photons exhibit specifically quantum mechanical correlations, which are incompatible with the classical reality concept, as will be discussed in detail below. How do the correlations appear in detail? Let us assume the initial state of the atom to be a state with angular momentum (spin) J = 0, the intermediate state to have angular momentum J = 1, and the final state to have again J = 0.
The wave-particle duality of photons and electrons was thoroughly discussed at a conference held at the Solvay Institute in October 1927, where Albert Einstein and Niels Bohr were the most prominent participants. From the beginning Einstein manifested great hesitation in accepting the renunciation of causal description of physical phenomena introduced by quantum theory. He clearly demonstrated his basic attitude by discussing the example of an atomic object (electron or photon) impinging upon a screen with a single slit (see figure 4). To the left of the screen the object is supposed to have a precisely defined momentum p and hence it is represented as a plane, harmonic wave. When passing through the slit, the plane wave is scattered into a cylindrical wave with an opening angle determined by the wavelength and the slit aperture. Einstein’s problem consisted of the fact that as soon as the object is detected at some point A on a photographic plate placed to the right of the screen, the object-wavecannot be detected in any other part of space whatever, which is directly contradictory to any wave phenomenon in classical physics. But the object-wave cannot be detected in any other part of space whatever, which is directly contradictory to any wave phenomenon is classical physics. But the object-wave just cannot be a classical wave phenomenon because this would imply the possibility of knowing the dynamical state of the object far more precisely than is compatible with the indeterminacy relations.
From a fundamental point of view we must acknowledge that, since about 1935, our conceptual understanding of quantum mechanics has not progressed so much. Really new ideas are few and far between – except of course the major line initiated by the contribution of Bell [6]. This is in big contrast with the rest of physics, where new theoretical and experimental discoveries in many fields have flourished, very often with the help of the tools of quantum mechanics. The fantastic evolution of the experimental techniques has completely changed the situation. At the beginning of quantum mechanics, the observation of the tracks of single particles in Wilson chambers [348] played the essential role in the introduction of the postulate of state vector reduction, but otherwise it was impossible to observe continuously a single electron, atom, or ion; the experiments that theorists were proposing in discussions on the principles of quantum measurement were therefore “thought experiments” (“Gedanken Experiment”), as for instance in the famous Solvay meetings [1, 21]. But nowadays, after almost of century of experimental progress, experiments that were then unthinkable have become a reality. A huge number of contemporary experiments involves the laws of quantum mechanics in general; several books would not be sufficient to describe all of them. Nevertheless, in the majority of experiments, what is really observed is the sum over a very large number of particles of one individual microscopic observable (sum of atomic dipoles for instance), which is accurately described by the average value of this observable.