Aspect's experiment was the first
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
experiment to demonstrate the violation of
Bell's inequalities. Its 1982 result allowed for further validation of the
quantum entanglement
Quantum entanglement is the phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of ...
and
locality
Locality may refer to:
* Locality (association), an association of community regeneration organizations in England
* Locality (linguistics)
* Locality (settlement)
* Suburbs and localities (Australia), in which a locality is a geographic subdivis ...
principles. It also offered an experimental answer to
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
,
Boris Podolsky
Boris Yakovlevich Podolsky (russian: link=no, Бори́с Я́ковлевич Подо́льский; June 29, 1896 – November 28, 1966) was a Russian-American physicist of Jewish descent, noted for his work with Albert Einstein and Nathan ...
, and
Nathan Rosen
Nathan Rosen (Hebrew: נתן רוזן; March 22, 1909 – December 18, 1995) was an American-Israeli physicist noted for his study on the structure of the hydrogen atom and his work with Albert Einstein and Boris Podolsky on entangled wave functio ...
's
paradox
A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...
which had been proposed about fifty years earlier.
The experiment was led by French physicist
Alain Aspect
Alain Aspect (; born 15 June 1947) is a French physicist noted for his experimental work on quantum entanglement.
Aspect was awarded the 2022 Nobel Prize in Physics, jointly with John Clauser and Anton Zeilinger, "for experiments with entangle ...
at the
École supérieure d'optique
The Institut d'optique Graduate School ("Institute of optics"), nicknamed SupOptique or IOGS, is one of the most prestigious French Grandes Ecoles and the leading French ''grande école'' in the field of Optics and its industrial and scientific ap ...
in
Orsay
Orsay () is a commune in the Essonne department in Île-de-France in northern France. It is located in the southwestern suburbs of Paris, France, from the centre of Paris.
A fortified location of the Chevreuse valley since the 8th centur ...
between 1980 and 1982. Its importance was immediately recognized by the scientific community. Although the methodology carried out by Aspect presents a potential flaw, the
detection loophole, his result is considered decisive and led to numerous other experiments (the so-called
Bell test experiments
A Bell test, also known as Bell inequality test or Bell experiment, is a real-world physics experiment designed to test the theory of quantum mechanics in relation to Albert Einstein's concept of local realism. Named for John Stewart Bell, the e ...
) which confirmed Aspect's original experiment.
For his work on this topic, Aspect was awarded part of the 2022
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
.
Scientific and historical context
The experiment must be placed in its historical and scientific context in order to be fully comprehended.
Entanglement, the EPR paradox and Bell inequalities
Quantum entanglement
Quantum entanglement is a phenomenon first theorized by Erwin Schrödinger in 1935.
Quantum mechanics dictates that once two separate quantum systems (two particles for example) have interacted or if they have a common origin, they cannot be considered as two independent systems. The quantum mechanical formalism postulates that if a first system possesses a
state, and the second a
state, then the initial composite system can be represented by the
tensor product
In mathematics, the tensor product V \otimes W of two vector spaces and (over the same field) is a vector space to which is associated a bilinear map V\times W \to V\otimes W that maps a pair (v,w),\ v\in V, w\in W to an element of V \otimes W ...
of both states:
. Once the interaction takes place and the two systems become entangled, the new composite state of the system, say
, can no longer be written as a tensor product of two separate, individual states. It is a characteristic mark of entanglement that we have a global understanding of the properties of the composite system, and an intrinsic lack of knowledge about the two original separate states. The physical distance between the two systems plays no role in the entangled state (because no position variable is present). The entangled quantum state remains identical — all else being equal — whatever the distances between both systems.
Consequently, any measurement operated on the entangled system is applicable to both elements constituting it: ''the measure results of both systems are correlated''.
The EPR paradox
The result of the experiment might have shocked
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
(he died in 1955 long before the experiment was performed) who had a local, realistic outlook on physics. His outlook led him to the conclusion that if the act of measurement influences both systems, then there would exist an influence capable of propagating from one system to the other, at a speed not limited by the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
. Quantum mechanical formalism anticipates that the influence of measuring the components of an entangled system has an immediate effect on both components, no matter the distance.
Later in 1935,
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
,
Boris Podolsky
Boris Yakovlevich Podolsky (russian: link=no, Бори́с Я́ковлевич Подо́льский; June 29, 1896 – November 28, 1966) was a Russian-American physicist of Jewish descent, noted for his work with Albert Einstein and Nathan ...
, and
Nathan Rosen
Nathan Rosen (Hebrew: נתן רוזן; March 22, 1909 – December 18, 1995) was an American-Israeli physicist noted for his study on the structure of the hydrogen atom and his work with Albert Einstein and Boris Podolsky on entangled wave functio ...
(E.P.R.) imagined a
thought experiment
A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences.
History
The ancient Greek ''deiknymi'' (), or thought experiment, "was the most anci ...
which, if one allowed for entangled states to exist, led to a paradox: either some influence travels faster than light (non-causality), or quantum physics is incomplete. None of the two terms of the alternative were admissible at the time, hence the paradox.
This paradox was of great historical importance, but it failed to have an immediate impact. Only
Niels Bohr
Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. B ...
seriously considered the objection it raised, and tried to answer it. But his answer was qualitative, and the paradox remained unsolved. The reality of entanglement remained a matter of opinion, devoid of any direct experimental support. As a matter of fact, the EPR experiment was not practically feasible at the time.
Two major obstacles opposed its undertaking. On one hand, the technical means were insufficient; on the other (and mainly), there seemed to be no effective way of directly measuring the obtained data with quantitative criteria.
The simultaneity of both systems, whatever meaning is attributed to it, could only be observed by comparing two distant measurements, within the constraints of the speed of light. The simultaneity's influence cannot be
causal
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
, nor can it transmit information (which amounts to the same thing). This property is therefore compatible with the
theory of relativity
The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in ...
, according to which no information can travel faster than the speed of light.
Bell inequalities
In 1964, Irish physicist
John Stewart Bell
John Stewart Bell FRS (28 July 1928 – 1 October 1990) was a physicist from Northern Ireland and the originator of Bell's theorem, an important theorem in quantum physics regarding hidden-variable theories.
In 2022, the Nobel Prize in Phy ...
published an article showcasing ''quantitative and measurable effects'' of the EPR experiments. Those are the famous
Bell inequalities. These inequalities are quantitative relations which must be verified by measuring correlations between systems which fully obey relativist
causality
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
. Any violation of these inequalities would allow for instantaneous remote influence.
These inequalities allowed physicists to remove one of the two obstacles preventing the EPR experiments. But in 1964, the available technical means were still insufficient for the experiment's actual undertaking.
First Bell inequalities tests
The EPR experiments became conceivable in 1969, when an article demonstrated their technical feasibility.
Following this article, the universities of Harvard and Berkeley set up an experimental protocol and led experiments in 1972. The results were contradictory: Harvard observed concordance with the Bell inequalities (and therefore a contradiction with the predictions of quantum physics), whereas Berkeley's results violated Bell's inequalities (and therefore verified quantum physics).
These experiments notably suffered from an unreliable and unproductive source of entangled particles, which required days of continuous experimentation. But constant controlled experimental conditions are extremely difficult to maintain for such long periods of time, especially for experiments as sensitive as this one. Both results were thus questionable.
In 1976, the same experiment was repeated in Houston using a better and more prolific source of entangled photons which allowed the duration of the experiment to be reduced to 80 minutes. In return, the photons were not optimally polarized, thus preventing the Bell inequalities from appearing clearly. The experiment nevertheless revealed a violation of Bell's inequalities, although it was too weak to constitute a definitive answer.
Additionally, and mainly, these experiments were not sufficiently elaborate to overrule the possibility of correlations (which entail the violation of Bell's inequalities) owing to some classical, slower-than-light influence or signal propagating between the two particles.
In the end, the experimental scheme used in these experiments was far removed from the "ideal" scheme which John Bell used to demonstrate his inequalities: there was therefore no certainty that the Bell inequalities could be applied as they were in such experiments.
Aspect's experiments (1980-1982)
In 1975, since a decisive experiment based on the violation of Bell's inequalities and verifying the veracity of quantum entanglement was still missing, Alain Aspect proposed in an article an experiment meticulous enough to be irrefutable: ''Proposed experiment to test the non-separability of quantum mechanics''.
Alain Aspect
Alain Aspect (; born 15 June 1947) is a French physicist noted for his experimental work on quantum entanglement.
Aspect was awarded the 2022 Nobel Prize in Physics, jointly with John Clauser and Anton Zeilinger, "for experiments with entangle ...
specified his experiment so that it would be as decisive as possible. Namely:
* Its source of entangled particles must be excellent to shorten the duration of the experiment and to provide as clear a violation of Bell's inequalities as possible.
* It must show correlations in measurements, but also demonstrate that these correlations are indeed the result of a quantum effect (and consequently of an instantaneous influence) and not of a classical slower-than-light effect between the two particles.
* The experimental scheme must match John Bell's as closely as possible to demonstrate his inequalities so that the agreement between the measured and predicted results be as significant as possible.
Reminder of John Bell's "ideal" scheme
![Aspect_epr](https://upload.wikimedia.org/wikipedia/commons/f/f6/Aspect_epr.png)
The illustration above represents the principle scheme from which John Bell demonstrated his inequalities: a source of entangled photons S simultaneously emits two
and
photons whose
polarization is prepared so that both photons' state vector is:
This formula simply means that the photons are in a
superposed state: they are both on a vertical, horizontal or linear polarity with an equal probability.
These two photons are then measured using two polarizers P1 and P2, each with a configurable measuring angle: α and β. the result of each polarizer's measurement can be (+) or (−) according to whether the measured polarization is parallel or perpendicular to the polarizer's angle of measurement.
One noteworthy aspect is that the polarizers imagined for this ideal experiment give a measurable result both in the (−) and (+) situations. Not all real polarizers are able to do this: some detect the (+) situation for example, but are unable to detect anything in the (−) situation (the photon never leaves the polarizer). The first experiments (described above) used the latter sort of polarizer. Alain Aspect's polarizers are much better able to detect both scenarios and therefore much closer to the ideal experiment.
Given the apparatus and the initial state of polarization given to the photons, quantum mechanics is able to predict the probabilities of measuring (+,+), (−,−), (+,−) and (−,+) on the polarizers (P1,P2), oriented on the (α,β) angles. As a reminder:
A maximal violation of Bell's inequalities is predicted for , α−β, = 22.5°
Description of the experimental set-up
Alain Aspect
Alain Aspect (; born 15 June 1947) is a French physicist noted for his experimental work on quantum entanglement.
Aspect was awarded the 2022 Nobel Prize in Physics, jointly with John Clauser and Anton Zeilinger, "for experiments with entangle ...
(with the notable collaboration of physicists Philippe Grangier, Gérard Roger and
Jean Dalibard
Jean Dalibard (born 8 December 1958) is a French physicist, Professor at the École Polytechnique, member of the French Academy of Sciences and a researcher at the École Normale Supérieure. In 2009, Dalibard received the Blaise Pascal medal of ...
) set up several increasingly complex experiments between 1980 and 1982.
Only his most complex experiment, which took place in 1982 and is the closest to the initial specifications, will be described here.
Photon source
The first experiments testing Bell's inequalities possessed low-intensity photon sources and necessitated a continuous week to complete. One of Alain Aspect's first improvements consisted in using a photon source several orders of magnitude more efficient. This source allowed a detection rate of 100 photons per second, thus shortening the length of the experiment to ''100 seconds''.
The source used is a
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
radiative cascade, excited with a krypton laser.
Polarizers with an adjustable orientation variable and on a remote position
One of the main points of this experiment was to make sure that the correlation between the measurements P1 and P2 had not been the result of "classical" effects, especially experimental artefacts.
As an example, when P1 and P2 are prepared with fixed angles α and β, it can be surmised that this state generates parasitic correlations through current or mass loops, or some other effects. As a matter of fact, both polarizers belong to the same setup and could influence one another through the various circuits of the experimental device, and generate correlations upon measurement.
One can then imagine that the fixed orientation of the polarizers impacts, one way or the other, the state the photon couple is emitted with. In such a case, the correlations between the measurement results could be explained by
hidden variables
Hidden variables may refer to:
* Confounding, in statistics, an extraneous variable in a statistical model that correlates (directly or inversely) with both the dependent variable and the independent variable
* Hidden transformation, in computer ...
within the photons, upon their emission. Alain Aspects had mentioned these observations to John Bell himself.
One way of ruling out these kinds of effects is to determine the (α,β) orientation of the polarizers at the last moment — after the photons have been emitted, and before their detection — and to keep them far enough from each other to prevent any signal from reaching any one of them.
This method assures that the orientation of the polarizers during the emission has no bearing on the result (since the orientation is yet undetermined during emission). It also assures that the polarizers do not influence each other, being too distant from one another.
As a consequence, Aspect's experimental set-up has polarizers P1 and P2 set 6 metres apart from the source, and 12 metres apart from one another. With this setup, only 20 nanoseconds elapse between the emission of the photons and their detection. During this extremely short period of time, the experimenter has to decide on the polarizers' orientation and to then orient them.
Since it is physically impossible to modify a polarizer's orientation within such a time span, two polarizers — one for each side — were used and pre-oriented in different directions. A high-frequency shunting randomly oriented towards one polarizer or the other. The setup corresponded to one polarizer with a randomly tilting polarization angle.
Since it was not possible either to have the emitted photons provoke the tilting, the polarizers shunted periodically every 10 nanoseconds (asynchronously with the photon's emission) thus assuring the referral device would tilt at least once between the emission of the photon and its detection.
Two-channel polarizers
Another important characteristic of the 1982 experiment was the use of two-channel polarizers which allowed a measurable result in situations (+) and (−). The polarizers used until Aspect's experiment could detect situation (+), but not situation (−). These single-channel polarizers had two major inconveniences:
* Situation (−) was difficult to discriminate from an experimentation mistake.
* They had to be scrupulously calibrated.
The two-channel polarizers Aspect used in his experiment avoided these two inconveniences and allowed him to use Bell's formulas directly to calculate the inequalities.
Technically, the polarizers he used were polarizing cubes which transmitted one polarity and reflected the other one, emulating a
Stern-Gerlach device.
Experiment results
Bell's inequalities establish a theoretical curve of the number of correlations (++ or −−) between the two detectors in relation to the relative angle of the detectors
. The shape of the curve is characteristic of the violation of Bell's inequalities. The measures' matching the shape of the curve establishes, quantitatively and qualitatively, that Bell's inequalities have been violated.
Aspect's experiments unambiguously confirmed the violation, as predicted the Copenhagen interpretation of quantum physics, thus undermining Einstein's local realistic outlook on quantum mechanics and
local hidden variable scenarios. In addition to being confirmed, the violation was confirmed ''in the exact way predicted by quantum mechanics'', with a statistical agreement of up to 242
standard deviation
In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
.
Given the technical quality of the experiment, the scrupulous avoidance of experimental artefacts, and the quasi-perfect statistical agreement, this experiment convinced the scientific community at large that quantum physics violated Bell's inequalities and, consequently, that quantum physics was
non-local.
Limits of the experiment
After the results, some physicists legitimately tried to look for flaws in Aspect's experiment and to find out how to improve it to resist criticism.
Some theoretical objections can be raised against the setup:
* the quasi-periodical aspect of the shunting oscillations hinders the validity of the experiment because it can induce correlations through quasi-synchronization resulting from two referrals;
* the correlations (+,+), (−,−) etc. were counted in real time, at the moment of detection. Each polarizer's two (+) and (−) channels were therefore linked by physical circuits. Once more, correlations may be induced.
The ideal experiment, which would negate any imaginable possibility of induced correlations should:
* use purely random shunting;
* record the (+) or (−) results on each side of the device, without any physical link between the two sides. The correlations would be calculated after the experiment, by comparing the recorded results of both sides.
The conditions of the experiment also suffered from a
detection loophole.
Recent experiments
The loopholes mentioned could only be solved from 1998. In the meantime, Aspect's experiment was reproduced, and the violation of Bell's inequalities was systematically confirmed, with a statistical certainty of up to 100
standard deviation
In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
.
Other experiments were conducted to test the violations of Bell's inequalities with other
observable
In physics, an observable is a physical quantity that can be measured. Examples include position and momentum. In systems governed by classical mechanics, it is a real-valued "function" on the set of all possible system states. In quantum ph ...
s than polarization, in order to approach the original spirit of the
EPR paradox
EPR may refer to:
Science and technology
* EPR (nuclear reactor), European Pressurised-Water Reactor
* EPR paradox (Einstein–Podolsky–Rosen paradox), in physics
* Earth potential rise, in electrical engineering
* East Pacific Rise, a mid-ocea ...
, in which Einstein imagined measuring two combined variables (such as position and movement quantity) on an EPR pair. An experiment introduced the combined variables (time and energy) which, once again, confirmed quantum mechanics.
In 1998, the Geneva experiment tested the correlation between two detectors set 30 kilometres apart using the Swiss optical fibre telecommunication network. The distance gave more time to commute the angles of the polarizers. It was therefore possible to have a completely random shunting. Additionally, the two distant polarizers were entirely independent. The measurements were recorded on each side, and compared after the experiment by dating each measurement using an atomic clock. The violation of Bell's inequalities was once again verified under strict and practically ideal conditions. If Aspect's experiment implied that a hypothetical coordination signal travel twice as fast as ''c'', Geneva's reached 10 million times ''c''.
An experiment took place in
Boulder
In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive.
In c ...
in 2000 on trapped-ion entanglement using a very efficient correlation-based detection method. The reliability of detection proved to be sufficient for the experiment to violate Bell's inequalities on the whole, even though all detected correlations did not violate them.
In 2001, Antoine Suarez's team, which included Nicolas Gisin who had participated in the Geneva experiment, reproduced the experiment using mirrors or detectors in motion, allowing them to reverse the order of events across the frames of reference, in accordance with
special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
# The laws o ...
(this inversion is only possible for events without any causal relationship). The speeds are chosen so that when a photon is reflected or crosses the semi-transparent mirror, the other photon has already crossed or been reflected from the point of view of the frame of reference attached to the mirror. This an "after-after" configuration, in which sound waves play the role of semi-transparent mirrors.
Another tested configuration allows for each photon to be received by a moving detector so that in that detector's frame of reference, the other photon has not been detected yet, whether it has crossed or been reflected or not ("before-before" configuration). Bell's inequalities are violated in this experiment notably.
Since 2015,
quantum physics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qua ...
' violation of
Bell's inequalities has been clearly established.
Quantum nonlocality
In theoretical physics, quantum nonlocality refers to the phenomenon by which the measurement statistics of a multipartite quantum system do not admit an interpretation in terms of a local realistic theory. Quantum nonlocality has been experimen ...
and
entanglement must therefore be recognized.
Relativistic causality
The question of whether Aspect's experiment challenges relativistic causality is raised by the widespread conception that "a quantum object presents a state which instantaneously depends on the state of another object with which it has been entangled." This "non-local influence" introduction is often used in popular science journals but also (deliberately) by some scientists who adhere to
realism
Realism, Realistic, or Realists may refer to:
In the arts
*Realism (arts), the general attempt to depict subjects truthfully in different forms of the arts
Arts movements related to realism include:
*Classical Realism
*Literary realism, a move ...
, such as
Alain Aspect
Alain Aspect (; born 15 June 1947) is a French physicist noted for his experimental work on quantum entanglement.
Aspect was awarded the 2022 Nobel Prize in Physics, jointly with John Clauser and Anton Zeilinger, "for experiments with entangle ...
himself, or
Bernard d'Espagnat
Bernard d'Espagnat (22 August 1921 – 1 August 2015) was a French theoretical physicist, philosopher of science, and author, best known for his work on the nature of reality. Wigner-d'Espagnat inequality is partially named after him.
''Quote'': ...
.
Three possibilities then subsist:
* The first is that experimenters should only use calculations with results in accordance with the experiment, without referring to an explanation derived from our "macroscopic" logic. This approach, borrowed from the
Copenhagen interpretation
The Copenhagen interpretation is a collection of views about the meaning of quantum mechanics, principally attributed to Niels Bohr and Werner Heisenberg. It is one of the oldest of numerous proposed interpretations of quantum mechanics, as featu ...
, is the most widely recognized among physicists. It is based on the fact that no explanation of the EPR phenomena leads to verifications or measurable predictions. As a consequence, most physicists consider this experiment's explanations to fall outside the field of science (see
Karl Popper
Sir Karl Raimund Popper (28 July 1902 – 17 September 1994) was an Austrian-British philosopher, academic and social commentator. One of the 20th century's most influential philosophers of science, Popper is known for his rejection of the cl ...
's criterion of
falsification). Most explanations indeed lack theoretical formalization, and those which do not, fail to suggest measurable verifications. An
empirical
Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...
approach is therefore at play here, and it aims at avoiding any slide outside the scientific field. In their work ''The Undivided Universe: An Ontological Interpretation of Quantum Theory'', physicists
David Bohm
David Joseph Bohm (; 20 December 1917 – 27 October 1992) was an American-Brazilian-British scientist who has been described as one of the most significant theoretical physicists of the 20th centuryPeat 1997, pp. 316-317 and who contributed u ...
and
Basil Hiley
Basil J. Hiley (born 1935), is a British people, British Quantum mechanics, quantum physicist and professor emeritus of the University of London.
Long-time colleague of David Bohm, Hiley is known for his work with Bohm on implicate orders and for ...
consider objections to the non-locality principle to be baseless.
In answer to those who regard the acceptation of non-locality as an obstacle to the scientific isolation and observation of any given object, Bohm and Hiley argue that, in the macroscopic world, this science is possible since the effects of non-locality are not ''significant'': interpretation allows for exactly the same degree of system separability as what is required of "actual scientific work." To match the theory of
special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
# The laws o ...
with non-locality (see
EPR paradox
EPR may refer to:
Science and technology
* EPR (nuclear reactor), European Pressurised-Water Reactor
* EPR paradox (Einstein–Podolsky–Rosen paradox), in physics
* Earth potential rise, in electrical engineering
* East Pacific Rise, a mid-ocea ...
) is a more complex matter, but Bohm, like
John Stewart Bell
John Stewart Bell FRS (28 July 1928 – 1 October 1990) was a physicist from Northern Ireland and the originator of Bell's theorem, an important theorem in quantum physics regarding hidden-variable theories.
In 2022, the Nobel Prize in Phy ...
,
[John Bell Inequality Video](_blank)
22 January 1990. points out that the transmission of signals is not what is at play in the notion of non-locality.
Bohm and Hiley, like Bell, see extra-scientific factors in the rejection of non-locality:
* The second possibility is that entanglement "unified" the two objects submitted to an interaction: the two objects remain "one" despite their spatial distance ("Bernard d'Espagnat's
non-locality"). This distancing can, in actuality, even be temporal: it is fundamentally spatio-temporal. As of yet, no explanation exists for what is considered as an experiment result rather than an explanation or an interpretation of this result. This approach which aims at explaining experiment facts is that of the
rationalists
In philosophy, rationalism is the epistemological view that "regards reason as the chief source and test of knowledge" or "any view appealing to reason as a source of knowledge or justification".Lacey, A.R. (1996), ''A Dictionary of Philosophy ...
.
* The third one consists in changing our conception of causality and in accepting the principle of a retrograde
causality
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
(a causal flow from the future to the past), which cannot however be assimilated to the classical philosophers' "teleological" "
final cause
The four causes or four explanations are, in Aristotelian thought, four fundamental types of answer to the question "why?", in analysis of change or movement in nature: the material, the formal, the efficient, and the final. Aristotle wrote th ...
." No one is there to orient events according to an objective: the nature of backward causation is identical to causality as we conceive it ("efficient causality" of the classical philosophers), except it flows backwards in relation to time and may "add" itself to "classical" causality. This interpretation requires for the irreversible nature of time to be only true on a macroscopic scale (
second law of thermodynamics
The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
). Numerous physicists are opposed to the idea, such as physicist and philosopher
Étienne Klein who points out that the
arrow of time
The arrow of time, also called time's arrow, is the concept positing the "one-way direction" or " asymmetry" of time. It was developed in 1927 by the British astrophysicist Arthur Eddington, and is an unsolved general physics question. This ...
is, according to him, inscribed in the symmetries of particle physics. This interpretation has some success among those who develop esoteric interpretations of the experiment, and use it to make
parapsychological phenomena (controversial in the scientific community, notably
precognition
Precognition (from the Latin 'before', and 'acquiring knowledge') is the purported psychic phenomenon of seeing, or otherwise becoming directly aware of, events in the future.
There is no accepted scientific evidence that precognition is a ...
.
Olivier Costa de Beauregard Olivier Costa de Beauregard (6 November 1911 - 5 February 2007) was a French engineering, engineer, Physics, physicist and Philosophy, philosopher. He spent much of his career studying quantum physics and General relativity, relativity. From the ear ...
is famous for his defence of such theses.
) But this interpretation blatantly contradicts the experiment's results as they were most often conducted: the
world line
The world line (or worldline) of an object is the path that an object traces in 4-dimensional spacetime. It is an important concept in modern physics, and particularly theoretical physics.
The concept of a "world line" is distinguished from con ...
linking the "P1 measurement" and "P2 measurement"
events
Event may refer to:
Gatherings of people
* Ceremony, an event of ritual significance, performed on a special occasion
* Convention (meeting), a gathering of individuals engaged in some common interest
* Event management, the organization of ev ...
of
spacetime
In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
is a curvature of
space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
. As a matter of fact, in order to disprove a possible alternative interpretation of the correlations observed in these experiments, the experimenters had to show that relativist "causality" was at least partly unable to explain these results, included with scenarios such as: "the photon
informs, by whatever relativistic process, the photon
of its quantum state after the first measurement... ." It is however perfectly clear that the experimenters' precautions to remove all relativistic "causal" explanations remove at the same time, according to the prevailing view, any "retro-causal" explanation. Finally, for the followers of the leading conception, this type of conception is a
conjectural
In mathematics, a conjecture is a conclusion or a proposition that is proffered on a tentative basis without proof. Some conjectures, such as the Riemann hypothesis (still a conjecture) or Fermat's Last Theorem (a conjecture until proven in ...
interpretation and does not really refer to the existing experiments. In their opinion, it leads to interpretations at the frontier of science, or even
pseudoscience
Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or falsifiability, unfa ...
, and involves quantum physics in a debate where it does not belong.
No physicist believes the results of the EPR experiment in general and of Aspect's experiment in particular—in perfect agreement with the Copenhagen interpretation of quantum mechanics—challenge, in any way whatsoever, the
relativity principle according to which no form of energy (matter or force), and therefore no usable information, can travel faster than the speed of light nor does it, as a consequence, challenge the derived relativist causality principle. It can easily be proven that quantum entanglement cannot be used to instantaneously transmit information from one space-time point to the other. The results measured on the first particle are random; the state alterations on the other particle induced by these measurements—as instantaneous as they may be according to the Copenhagen interpretation of quantum mechanics and Aspect's experiment's results—lead to measurement results relative to the second particle which are seemingly just as random: no usable information can be separately obtained upon measuring, and the correlations remain undetectable as long as the two series' results are not compared. This kind of experiment demonstrates the unavoidable need for a "classical" signal in the relativistic sense in order to transmit the information necessary for the detection of these correlations. Without this signal, nothing can be transmitted. It determines the speed of the transmission of information which reaffirms the fundamental principle of relativity. As a result, the relativist causality principle is perfectly compatible with the EPR experiments results.
See also
*
Superdeterminism
In quantum mechanics, superdeterminism is a loophole in Bell's theorem. By postulating that all systems being measured are correlated with the choices of which measurements to make on them, the assumptions of the theorem are no longer fulfilled. ...
Footnotes
References
Bibliography
* Bernard d'Espagnat, ''Traité de physique et de philosophie'', Fayard (in French). See chapter 3. Non-separability and Bell theorem.
* Bernard d'Espagnat, ''À la recherche du réel'', Bordas (in French).
* Bernard d'Espagnat, Étienne Klein, ''Regards sur la matière'' (in French). See chapter VIII. Non-separability of correlating couples.
External links
{{Wikiquote
Video conference on quantum optics(17 min), by
Alain Aspect
Alain Aspect (; born 15 June 1947) is a French physicist noted for his experimental work on quantum entanglement.
Aspect was awarded the 2022 Nobel Prize in Physics, jointly with John Clauser and Anton Zeilinger, "for experiments with entangle ...
, head of research at the
Institut d'Optique
The Institut d'optique Graduate School ("Institute of optics"), nicknamed SupOptique or IOGS, is one of the most prestigious French Grandes Ecoles and the leading French ''grande école'' in the field of Optics and its industrial and scientific ap ...
in Orsay (in French).
Centre for Quantum Philosophy
Thought experiments in quantum mechanics
Quantum measurement