Observer (quantum physics)

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Some interpretations of quantum mechanics posit a central role for an observer of a quantum phenomenon. [1] The quantum mechanical observer is tied to the issue of observer effect, where a measurement necessarily requires interacting with the physical object being measured, affecting its properties through the interaction. The term "observable" has gained a technical meaning, denoting a Hermitian operator that represents a measurement. [2] : 55 

Contents

Foundation

The theoretical foundation of the concept of measurement in quantum mechanics is a contentious issue deeply connected to the many interpretations of quantum mechanics. A key focus point is that of wave function collapse, for which several popular interpretations assert that measurement causes a discontinuous change into an eigenstate of the operator associated with the quantity that was measured, a change which is not time-reversible.

More explicitly, the superposition principle (ψ = Σnanψn) of quantum physics dictates that for a wave function ψ, a measurement will result in a state of the quantum system of one of the m possible eigenvalues fn , n = 1, 2, ..., m, of the operator ∧F which in the space of the eigenfunctions ψn , n = 1, 2, ..., m.

Once one has measured the system, one knows its current state; and this prevents it from being in one of its other states ⁠— it has apparently decohered from them without prospects of future strong quantum interference. [3] [4] [5] This means that the type of measurement one performs on the system affects the end-state of the system.

An experimentally studied situation related to this is the quantum Zeno effect, in which a quantum state would decay if left alone, but does not decay because of its continuous observation. The dynamics of a quantum system under continuous observation are described by a quantum stochastic master equation known as the Belavkin equation. [6] [7] [8] Further studies have shown that even observing the results after the photon is produced leads to collapsing the wave function and loading a back-history as shown by delayed choice quantum eraser. [9]

When discussing the wave function ψ which describes the state of a system in quantum mechanics, one should be cautious of a common misconception that assumes that the wave function ψ amounts to the same thing as the physical object it describes. This flawed concept must then require existence of an external mechanism, such as a measuring instrument, that lies outside the principles governing the time evolution of the wave function ψ, in order to account for the so-called "collapse of the wave function" after a measurement has been performed. But the wave function ψ is not a physical object like, for example, an atom, which has an observable mass, charge and spin, as well as internal degrees of freedom. Instead, ψ is an abstract mathematical function that contains all the statistical information that an observer can obtain from measurements of a given system. In this case, there is no real mystery in that this mathematical form of the wave function ψ must change abruptly after a measurement has been performed.

A consequence of Bell's theorem is that measurement on one of two entangled particles can appear to have a nonlocal effect on the other particle. Additional problems related to decoherence arise when the observer is modeled as a quantum system.

Description

The Copenhagen interpretation, which is the most widely accepted interpretation of quantum mechanics among physicists, [1] [10] : 248  posits that an "observer" or a "measurement" is merely a physical process. One of the founders of the Copenhagen interpretation, Werner Heisenberg, wrote:

Of course the introduction of the observer must not be misunderstood to imply that some kind of subjective features are to be brought into the description of nature. The observer has, rather, only the function of registering decisions, i.e., processes in space and time, and it does not matter whether the observer is an apparatus or a human being; but the registration, i.e., the transition from the "possible" to the "actual," is absolutely necessary here and cannot be omitted from the interpretation of quantum theory. [11]

Niels Bohr, also a founder of the Copenhagen interpretation, wrote:

all unambiguous information concerning atomic objects is derived from the permanent marks such as a spot on a photographic plate, caused by the impact of an electron left on the bodies which define the experimental conditions. Far from involving any special intricacy, the irreversible amplification effects on which the recording of the presence of atomic objects rests rather remind us of the essential irreversibility inherent in the very concept of observation. The description of atomic phenomena has in these respects a perfectly objective character, in the sense that no explicit reference is made to any individual observer and that therefore, with proper regard to relativistic exigencies, no ambiguity is involved in the communication of information. [12]

Likewise, Asher Peres stated that "observers" in quantum physics are

similar to the ubiquitous "observers" who send and receive light signals in special relativity. Obviously, this terminology does not imply the actual presence of human beings. These fictitious physicists may as well be inanimate automata that can perform all the required tasks, if suitably programmed. [13] : 12 

Critics of the special role of the observer also point out that observers can themselves be observed, leading to paradoxes such as that of Wigner's friend; and that it is not clear how much consciousness is required. As John Bell inquired, "Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer—with a PhD?" [14]

Anthropocentric interpretation

The prominence of seemingly subjective or anthropocentric ideas like "observer" in the early development of the theory has been a continuing source of disquiet and philosophical dispute. [15] A number of new-age religious or philosophical views give the observer a more special role, or place constraints on who or what can be an observer. There is no credible peer-reviewed research that backs such claims. As an example of such claims, Fritjof Capra declared, "The crucial feature of atomic physics is that the human observer is not only necessary to observe the properties of an object, but is necessary even to define these properties." [16]

Confusion with uncertainty principle

The uncertainty principle has been frequently confused with the observer effect, evidently even by its originator, Werner Heisenberg. [17] The uncertainty principle in its standard form describes how precisely it is possible to measure the position and momentum of a particle at the same time. If the precision in measuring one quantity is increased, the precision in measuring the other decreases. [18] An alternative version of the uncertainty principle, [19] more in the spirit of an observer effect, [20] fully accounts for the disturbance the observer has on a system and the error incurred, although this is not how the term "uncertainty principle" is most commonly used in practice.

See also

Related Research Articles

The Copenhagen interpretation is a collection of views about the meaning of quantum mechanics, stemming from the work of Niels Bohr, Werner Heisenberg, Max Born, and others. While "Copenhagen" refers to the Danish city, the use as an "interpretation" was apparently coined by Heisenberg during the 1950s to refer to ideas developed in the 1925–1927 period, glossing over his disagreements with Bohr. Consequently, there is no definitive historical statement of what the interpretation entails.

<span class="mw-page-title-main">Many-worlds interpretation</span> Interpretation of quantum mechanics

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe. The evolution of reality as a whole in MWI is rigidly deterministic and local. Many-worlds is also called the relative state formulation or the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957. Bryce DeWitt popularized the formulation and named it many-worlds in the 1970s.

<span class="mw-page-title-main">Quantum mechanics</span> Description of physical properties at the atomic and subatomic scale

Quantum mechanics is a fundamental theory that describes the behavior of nature at and below the scale of atoms. It is the foundation of all quantum physics, which includes quantum chemistry, quantum field theory, quantum technology, and quantum information science.

<span class="mw-page-title-main">Quantum entanglement</span> Correlation between quantum systems

Quantum entanglement is the phenomenon of a group of particles being generated, interacting, or sharing spatial proximity in such a way that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics not present in classical mechanics.

<span class="mw-page-title-main">Schrödinger's cat</span> Thought experiment in quantum mechanics

In quantum mechanics, Schrödinger's cat is a thought experiment concerning quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead, while it is unobserved in a closed box, as a result of its fate being linked to a random subatomic event that may or may not occur. This experiment viewed this way is described as a paradox. This thought experiment was devised by physicist Erwin Schrödinger in 1935 in a discussion with Albert Einstein to illustrate what Schrödinger saw as the problems of the Copenhagen interpretation of quantum mechanics.

<span class="mw-page-title-main">Uncertainty principle</span> Foundational principle in quantum physics

The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position and momentum, can be simultaneously known. In other words, the more accurately one property is measured, the less accurately the other property can be known.

The de Broglie–Bohm theory is an interpretation of quantum mechanics which postulates that, in addition to the wavefunction, an actual configuration of particles exists, even when unobserved. The evolution over time of the configuration of all particles is defined by a guiding equation. The evolution of the wave function over time is given by the Schrödinger equation. The theory is named after Louis de Broglie (1892–1987) and David Bohm (1917–1992).

An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments. However, there exist a number of contending schools of thought over their interpretation. These views on interpretation differ on such fundamental questions as whether quantum mechanics is deterministic or stochastic, local or non-local, which elements of quantum mechanics can be considered real, and what the nature of measurement is, among other matters.

In quantum mechanics, wave function collapse, also called reduction of the state vector, occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an observation, and is the essence of a measurement in quantum mechanics, which connects the wave function with classical observables such as position and momentum. Collapse is one of the two processes by which quantum systems evolve in time; the other is the continuous evolution governed by the Schrödinger equation.

<span class="mw-page-title-main">Quantum decoherence</span> Loss of quantum coherence

Quantum decoherence is the loss of quantum coherence. Quantum decoherence has been studied to understand how quantum systems convert to systems which can be explained by classical mechanics. Beginning out of attempts to extend the understanding of quantum mechanics, the theory has developed in several directions and experimental studies have confirmed some of the key issues. Quantum computing relies on quantum coherence and is one of the primary practical applications of the concept.

In physics, a hidden-variable theory is a deterministic physical model which seeks to explain the probabilistic nature of quantum mechanics by introducing additional variables.

In quantum physics, a measurement is the testing or manipulation of a physical system to yield a numerical result. A fundamental feature of quantum theory is that the predictions it makes are probabilistic. The procedure for finding a probability involves combining a quantum state, which mathematically describes a quantum system, with a mathematical representation of the measurement to be performed on that system. The formula for this calculation is known as the Born rule. For example, a quantum particle like an electron can be described by a quantum state that associates to each point in space a complex number called a probability amplitude. Applying the Born rule to these amplitudes gives the probabilities that the electron will be found in one region or another when an experiment is performed to locate it. This is the best the theory can do; it cannot say for certain where the electron will be found. The same quantum state can also be used to make a prediction of how the electron will be moving, if an experiment is performed to measure its momentum instead of its position. The uncertainty principle implies that, whatever the quantum state, the range of predictions for the electron's position and the range of predictions for its momentum cannot both be narrow. Some quantum states imply a near-certain prediction of the result of a position measurement, but the result of a momentum measurement will be highly unpredictable, and vice versa. Furthermore, the fact that nature violates the statistical conditions known as Bell inequalities indicates that the unpredictability of quantum measurement results cannot be explained away as due to ignorance about "local hidden variables" within quantum systems.

<span class="mw-page-title-main">Quantum Zeno effect</span> Quantum measurement phenomenon

The quantum Zeno effect is a feature of quantum-mechanical systems allowing a particle's time evolution to be slowed down by measuring it frequently enough with respect to some chosen measurement setting.

In quantum mechanics, the measurement problem is the problem of definite outcomes: quantum systems have superpositions but quantum measurements only give one definite result.

In physics, complementarity is a conceptual aspect of quantum mechanics that Niels Bohr regarded as an essential feature of the theory. The complementarity principle holds that certain pairs of complementary properties cannot all be observed or measured simultaneously. For example, position and momentum or wave and particle properties. In contemporary terms, complementarity encompasses both the uncertainty principle and wave-particle duality.

The Born rule is a postulate of quantum mechanics that gives the probability that a measurement of a quantum system will yield a given result. In its simplest form, it states that the probability density of finding a system in a given state, when measured, is proportional to the square of the amplitude of the system's wavefunction at that state. It was formulated and published by German physicist Max Born in July, 1926.

Objective-collapse theories, also known spontaneous collapse models or dynamical reduction models, are proposed solutions to the measurement problem in quantum mechanics. As with other interpretations of quantum mechanics, they are possible explanations of why and how quantum measurements always give definite outcomes, not a superposition of them as predicted by the Schrödinger equation, and more generally how the classical world emerges from quantum theory. The fundamental idea is that the unitary evolution of the wave function describing the state of a quantum system is approximate. It works well for microscopic systems, but progressively loses its validity when the mass / complexity of the system increases.

The Ghirardi–Rimini–Weber theory (GRW) is a spontaneous collapse theory in quantum mechanics, proposed in 1986 by Giancarlo Ghirardi, Alberto Rimini, and Tullio Weber.

The von Neumann–Wigner interpretation, also described as "consciousness causes collapse", is an interpretation of quantum mechanics in which consciousness is postulated to be necessary for the completion of the process of quantum measurement.

In quantum mechanics, weak measurement is a type of quantum measurement that results in an observer obtaining very little information about the system on average, but also disturbs the state very little. From Busch's theorem any quantum system is necessarily disturbed by measurement, but the amount of disturbance is described by a parameter called the measurement strength.

References

  1. 1 2 Schlosshauer, Maximilian; Kofler, Johannes; Zeilinger, Anton (2013-08-01). "A snapshot of foundational attitudes toward quantum mechanics". Studies in History and Philosophy of Science Part B. 44 (3): 222–230. arXiv: 1301.1069 . Bibcode:2013SHPMP..44..222S. doi:10.1016/j.shpsb.2013.04.004. S2CID   55537196.
  2. ↑ Rieffel, Eleanor G.; Polak, Wolfgang H. (2011-03-04). Quantum Computing: A Gentle Introduction. MIT Press. ISBN   978-0-262-01506-6.
  3. ↑ B.D'Espagnat, P.Eberhard, W.Schommers, F.Selleri. Quantum Theory and Pictures of Reality. Springer-Verlag, 1989, ISBN   3-540-50152-5
  4. ↑ Schlosshauer, Maximilian (2005). "Decoherence, the measurement problem, and interpretations of quantum mechanics". Rev. Mod. Phys. 76 (4): 1267–1305. arXiv: quant-ph/0312059 . Bibcode:2004RvMP...76.1267S. doi:10.1103/RevModPhys.76.1267. S2CID   7295619 . Retrieved 28 February 2013.
  5. ↑ Giacosa, Francesco (2014). "On unitary evolution and collapse in quantum mechanics". Quanta. 3 (1): 156–170. arXiv: 1406.2344 . doi:10.12743/quanta.v3i1.26. S2CID   55705326.
  6. ↑ V. P. Belavkin (1989). "A new wave equation for a continuous non-demolition measurement". Physics Letters A. 140 (7–8): 355–358. arXiv: quant-ph/0512136 . Bibcode:1989PhLA..140..355B. doi:10.1016/0375-9601(89)90066-2. S2CID   6083856.
  7. ↑ Howard J. Carmichael (1993). An Open Systems Approach to Quantum Optics. Berlin Heidelberg New-York: Springer-Verlag.
  8. ↑ Michel Bauer; Denis Bernard; Tristan Benoist. Iterated Stochastic Measurements (Technical report). arXiv: 1210.0425 . Bibcode:2012JPhA...45W4020B. doi:10.1088/1751-8113/45/49/494020.
  9. ↑ Kim, Yoon-Ho; R. Yu; S.P. Kulik; Y.H. Shih; Marlan Scully (2000). "A Delayed "Choice" Quantum Eraser". Physical Review Letters . 84 (1): 1–5. arXiv: quant-ph/9903047 . Bibcode:2000PhRvL..84....1K. doi:10.1103/PhysRevLett.84.1. PMID   11015820. S2CID   5099293.
  10. ↑ Jammer, Max (1974). The Philosophy of Quantum Mechanics. John Wiley and Sons. ISBN   0-471-43958-4.
  11. ↑ Werner Heisenberg, Physics and Philosophy, p. 137
  12. ↑ Niels Bohr (1958), "Quantum Physics and Philosophy—Causality and Complementarity", Essays 1958-1962 on Atomic Physics and Human Knowledge, p. 3
  13. ↑ Peres, Asher (1993). Quantum Theory: Concepts and Methods . Kluwer. ISBN   0-7923-2549-4. OCLC   28854083.
  14. ↑ John Stewart Bell, 1981, "Quantum Mechanics for Cosmologists". In C. J. Isham, R. Penrose and D.W. Sciama (eds.), Quantum Gravity 2: A Second Oxford Symposium. Oxford: Clarendon Press, p. 611.
  15. ↑ Mermin, N. David (2019). "Making better sense of quantum mechanics". Reports on Progress in Physics. 82 (1): 012002. arXiv: 1809.01639 . Bibcode:2019RPPh...82a2002M. doi:10.1088/1361-6633/aae2c6. PMID   30232960. S2CID   52299438.
  16. ↑ Fritjof Capra. The Tao of Physics , p. 127
  17. ↑ Furuta, Aya. "One Thing Is Certain: Heisenberg's Uncertainty Principle Is Not Dead". Scientific American. Retrieved 2018-09-23.
  18. ↑ Heisenberg, W. (1930), Physikalische Prinzipien der Quantentheorie, Leipzig: Hirzel English translation The Physical Principles of Quantum Theory. Chicago: University of Chicago Press, 1930. reprinted Dover 1949
  19. ↑ Ozawa, Masanao (2003), "Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement", Physical Review A, 67 (4): 042105, arXiv: quant-ph/0207121 , Bibcode:2003PhRvA..67d2105O, doi:10.1103/PhysRevA.67.042105, S2CID   42012188
  20. ↑ V. P. Belavkin (1992). "Quantum continual measurements and a posteriori collapse on CCR". Communications in Mathematical Physics. 146 (3): 611–635. arXiv: math-ph/0512070 . Bibcode:1992CMaPh.146..611B. doi:10.1007/BF02097018. S2CID   17016809.
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