In
quantum mechanicsQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
, the
double-slit experiment (often referred to as
Young'sThomas Young was an English polymath who made notable contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony and Egyptology.-Biography:...
experiment) demonstrates the
inseparability of the
waveA wave is a disturbance that propagates through space and time, usually with transference of energy. A mechanical wave is a wave that propagates or travels through a medium due to the restoring forces it produces upon deformation. There also exist waves capable of traveling through a vacuum,...
and
particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
natures of light and other quantum particles. A
coherentIn physics, coherence is a property of waves, that enables stationary interference. More generally, coherence describes all properties of the electronic correlation between physical quantities of a wave....
light source illuminates a thin plate with two parallel slits cut in it, and the light passing through the slits strikes a screen behind them. The wave nature of light causes the light waves passing through both slits to interfere, creating an interference pattern of bright and dark bands on the screen. However, at the screen, the light is always found to be absorbed as discrete particles, called
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s.
If the light travels from the source to the screen as particles, then on the basis of a
classical reasoning the number that strike any particular point on the screen is expected to be equal to the sum of those that go through the left slit and those that go through the right slit. In other words, according to classical particle physics the brightness at any point should be the sum of the brightness when the right slit is blocked and the brightness when the left slit is blocked. However, it is found that unblocking both slits makes some points on the screen brighter, and other points darker. This can only be explained by the alternately additive and subtractive interference of waves, not the exclusively additive nature of particles, so we know that light must have some
particle-wave duality.
Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern, illustrating the
complementarityIn physics, complementarity is a basic principle of quantum theory closely identified with the Copenhagen interpretation, and refers to effects such as the wave–particle duality, in which different measurements made on a system reveal it to have either particle-like or wave-like properties...
principle; that the light can demonstrate both particle and wave characteristics, but not both at the same time.. However, an experiment performed in 1987 produced results that demonstrated that which-path information could be obtained without destroying the possibility of interference. This showed the effect of measurements that disturbed the particles in transit to a lesser degree and thereby influenced the interference pattern only to a comparable extent.
The double slit experiment can also be performed (using different apparatus) with particles of matter such as
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s with the same results, demonstrating that they also show particle-wave duality.
Overview
Normally, when only one slit is open, the pattern on the screen is a diffraction pattern, a fairly narrow central band with dimmer bands parallel to it on each side. (See the top photograph to the right.) When both slits are open, the pattern displayed becomes very much more detailed and at least four times as wide. (See the bottom photograph to the right.) These facts were elucidated by
Thomas YoungThomas Young was an English polymath who made notable contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony and Egyptology.-Biography:...
in a paper entitled "Experiments and Calculations Relative to Physical Optics," published in 1803. To a very high degree of success, these results could be explained by the method of Huygens–Fresnel principle that based its calculations on the hypothesis that light consists of waves propagated through some medium. However, discovery of the
photoelectric effectThe photoelectric effect is a phenomenon in which electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons"...
made it necessary to go beyond classical physics and take the
quantumQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
nature of light into account.
It is a widespread misunderstanding that, when two slits are open but a detector is added to the experiment to determine which slit a photon has passed through, then the interference pattern no longer forms and the experimental apparatus yields two simple patterns, one from each slit, superposed without interference. Such a result would be obtained only if the results of two experiments were superposed in which either one or the other slit is closed. However, there are many other methods to determine whether a photon passed through a slit, for instance by placing an atom at the position of each slit and monitoring whether one of these atoms is influenced by a photon passing it. In general in such experiments the interference pattern will be changed but not be completely wiped out. Interesting experiments of this latter kind have been performed with photons and with neutrons.
Restriction to the two experiments in which either both slits are open or one slit is closed has given rise to the idea of
wave-particle complementarityIn physics and chemistry, wave–particle duality is the concept that all energy exhibits both wave-like and particle-like properties. A central concept of quantum mechanics, duality addresses the inadequacy of classical concepts like "particle" and "wave" in fully describing the behavior of...
(to be distinguished from wave-particle duality)
according to which a microscopic object (photon, electron, etc.) would manifest itself as a
particle in the which-way experiment but as a
wave in the interference experiment. This idea has been felt to be counterintuitive by those not being content with an
instrumentalist interpretationAn Instrumentalist interpretation is a description of quantum experiments.This article aims at proposing a non-interpretative description of some key quantum experiments, based as far as possible on genuine facts, although there is no criterion for deciding whether this goal has been fully achieved...
of quantum mechanics in which that theory is accepted as just describing
phenomena without providing explanations.
The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present. (The experiment works with electrons, atoms, and even some molecules too.)
Richard FeynmanRichard Phillips Feynman was an American physicist known for the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as work in particle physics...
was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.
(The following depictions are relatively slow to load.)
- Animation 1
- Animation 2 (zoom in)
The underpinning of this experiment
Christiaan HuygensChristiaan Huygens, FRS was a prominent Dutch mathematician, astronomer, physicist, horologist, and writer of early science fiction...
understood the basic idea of how light propagates and how to predict its path through a physical apparatus. He understood that a light source emits a series of waves comparable to the way that water waves spread out from something like a fishing float that is jiggled up and down and bobs on the water surface. He said that the way to predict where the next wave front will be found is to generate a series of concentric circles on a sufficiently large number of points on a known wave front and then draw a curve that will pass tangent to all the resulting circles out in front of the known wave front. The diagram given here shows what happens when a flat wave front is extended in this manner, and what happens when a curved wave front is extended in the same way. Augustin Fresnel (1788-1827) based his proof that the wave nature of light does not contradict the observed fact that light propagates in a straight line in homogeneous media on Huygens' work, and also based himself on Huygens'
ideas to give a complete account of diffraction and interference phenomena known at his time. See the article Huygens–Fresnel principle for more information.
The second drawing shows what happens when a flat wave front encounters a slit in a wall. Following the same principle elucidated above, it is clear that the new wave front will "bulge out" from the slit and light will be experienced as having diverged around the edges of the slit. The result is called a diffraction pattern.
The third drawing shows the explanation for interference based on the classical idea of a single wave front that represents all the light energy emitted by a source at one moment. Since photons diverge beyond the barrier wall, the distance between parts of any pattern they form on the target wall increases as the distance they have to travel increases, a fact that is well known from everyday experience with things like automobile headlights whose beams are not parallel. But decreasing the distance between slits will also
increase the distance between fringes (colored bands such as the sixteen shown in the second photograph above). Increasing the wavelength will also increase the distance between fringes as long as the slits are wide enough to permit the passage of light of that wavelength. Slits that are very wide in comparison to the frequency of the photons involved (e.g., two ordinary windows in a single wall) will permit light to appear to go "straight through."
When light came to be understood as the result of electrons falling from higher energy orbits to lower energy orbits, the light that is delivered to some surface in any short interval of time came to be understood as ordinarily representing the arrival of very many photons, each with its own wave front. In understanding what actually happens in the two-slit experiment it became important to find out what happens when photons are emitted one by one.
When it became possible to perform that experiment, it became apparent that a single photon has its own wave front that passes through both slits, and that the single photon will show up on the detector screen according to the net probability values resulting from the co-incidence of the two probability waves coming by way of the two slits. When a great number of photons are sent through the apparatus one by one and recorded on photographic film, the same interference pattern emerges that had been seen before when many photons were being emitted at the same time. The low intensity double-slit experiment was first performed by Taylor in 1909, by reducing the level of incident light until on average only one photon was being transmitted at a time. Note that it is the probabilities of photons appearing at various points along the detection screen that add
or cancel. So if there is a cancellation of waves at some point that does not mean that a photon disappears; it means that the probability of a photon's appearing at that point will disappear, and the probability that it will appear somewhere else increases.
Importance to physics
Although the double-slit experiment is now often referred to in the context of quantum mechanics, it is generally thought to have been first performed by the English scientist
Thomas YoungThomas Young was an English polymath who made notable contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony and Egyptology.-Biography:...
in the year 1801 in an attempt to resolve the question of whether light was composed of particles (Newton's
"corpuscular" theoryIn optics, the corpuscular theory of light, set forward by Sir Isaac Newton, says that light is made up of small discrete particles called "corpuscles" which travel in straight line with a finite velocity and possess kinetic energy.This theory also rules out the presence of any medium for...
), or rather consisted of waves traveling through some
etherIn the late 19th century, "luminiferous aether" , meaning light-bearing aether, was the term used to describe a medium for the propagation of light. The word aether stems via Latin from the Greek αιθήρ, from a root meaning to kindle, burn, or shine...
, just as sound waves travel in air. The interference patterns observed in the experiment seemed to discredit the corpuscular theory, and the wave theory of light remained well accepted until the early 20th century, when evidence began to accumulate which seemed instead to confirm the particle theory of light.
The double-slit experiment, and its variations, then became a classic
thought experimentA thought experiment, sometimes called a gedankenexperiment in German, is a proposal for an experiment that would test or illuminate a hypothesis or theory....
for its clarity in expressing the central puzzles of quantum mechanics.
It was shown experimentally in 1972 that in a Young slit system where only one slit was open at any time, interference was nonetheless observed provided the path difference was such that the detected photon could have come from either slit. The experimental conditions were such that the photon density in the system was much less than unity.
A Young double slit experiment was not performed with anything other than light until 1961, when Clauss Jönsson of the University of Tübingen performed it with electrons, and not until 1974 in the form of "one electron at a time", in a laboratory at the
University of MilanThe University of Milan is one of the largest universities in Italy, with about 62,801 students, a teaching and research staff of 2,455 and a non-teaching staff of 2,200....
, by researchers led by Pier Giorgio Merli, of LAMEL-CNR Bologna.
The results of the 1974 experiment were published and even made into a short film, but did not receive wide attention. The experiment was repeated in 1989 by Tonomura et al. at
Hitachiis a Japanese multinational corporation specializing in high-technology and services headquartered in Marunouchi Itchome, Chiyoda, Tokyo, Japan...
in
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. Their equipment was better, reflecting 15 years of advances in electronics and a dedicated development effort by the Hitachi team. Their methodology was more precise and elegant, and their results agreed with the results of Merli's team. Although Tonomura asserted that the Italian experiment had not detected electrons one at a time—a key to demonstrating the wave-particle paradox—single electron detection is clearly visible in the photos and film taken by Merli and his group.
In September 2002, the double-slit experiment of Claus Jönsson was voted "the most beautiful experiment" by readers of
Physics WorldPhysics World is the membership magazine of the Institute of Physics, one of the largest physical societies in the world. It is an international monthly magazine covering all areas of physics, both pure and applied, and is aimed at physicists in research, industry and education worldwide...
.
Importance to philosophy
The double-slit experiment has been of great interest to philosophers, because the quantum mechanical behavior it reveals has forced them to reevaluate their ideas about classical concepts such as "particles", "waves", "location", and "movement from one place to another".
In contrast to the way of conceptualizing the macroscopic world of everyday experience, attempting to describe the motion of a single photon is problematic. As
Philipp FrankPhilipp Frank was a physicist, mathematician and also an influential philosopher during the first half of the 20th century. He was a logical-positivist, and a member of the Vienna Circle....
observes, investigating the motion of single particles through a single slit can obtain a description of the pattern of photon strikes on a target screen. However, "the pattern of fringes for two slits is not the superposition of the two patterns for single slits. Hence, there is no law of motion that would determine the trajectory of a single photon and allow us to derive the observed facts that occur when photons pass two slits." Experience in the micro world of sub-atomic particles forces us to reconceptualize some of our most commonplace ideas.
One of the most striking consequences of the new science is that it is not in agreement with the belief of Laplace that an omniscient entity, knowing the initial positions and velocities of all particles in the universe at one time, could predict their positions at any future time. (To paraphrase Laplace's idea, the positions and velocities of all things at any given time depend absolutely on their previous positions and velocities and the absolute laws that govern physical interactions.) Laplace believed that such particles would follow the laws of motion discovered by Newton, but twentieth century physics made it clear that the motions of sub-atomic particles and even some small atoms cannot be predicted by using the laws of Newtonian physics. For instance, most of the orbits for electrons moving around atomic nuclei that are permitted by Newtonian physics are excluded by the new physics. And it is not even clear what the "movement" of a particle such as a photon may be when
it is not clear that it "goes through" either one slit or the other, but it is clear that the probability of its arrival at various points on the target screen is a function of its wavelength and of the distance between the slits. Whereas Laplace would expect an omniscient mind to be able to predict with absolute confidence the arrival of a photon at some specific point on the target screen, it turns out that the particle may arrive at one of a great number of points, but that the percentage of particles that arrive at each of such points is determined by the laws of the new physics.
Results observed
The bright bands observed on the screen happen when the light has interfered constructively—where a crest of a wave meets a crest from another wave. The dark regions show destructive interference—a crest meets a trough. Constructive interference occurs when
-
- where
- λ is the wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the light,
- d is the separation of the slits, the distance between A and B in the diagram to the right
- n is the order of maximum observed (central maximum is n = 0),
- x is the distance between the bands of light and the central maximum (also called fringe distance), and
- L is the distance from the slits to the screen centerpoint.
This is only an approximation and depends on certain conditions.
It is possible to work out the wavelength of light using this equation and the above apparatus. If
d and
L are known and
x is observed, then
λ can be easily calculated.
A detailed treatment of the mathematics of double-slit interference in the context of quantum mechanics is given in the article on Englert-Greenberger duality.
Shape of interference fringes
The theoretical shapes of the interference fringes observed in a Young double slit experiment are straight lines, which is easily proved.
In case two pinholes are used instead of slits, as in Young's original experiment, hyperbolic fringes are observed.
If the two sources are placed on a line perpendicular to the screen, the shape of the interference fringes is circular as the individual paths travelled by light from the two sources are always equal for a given fringe. This can be done in simpler way by placing a mirror parallel to a screen at a distance and a source of light just above the mirror. (Note the extra phase difference of
π due to reflection at the interface of a denser medium).
Quantum version of experiment
By the 1920s, various other experiments (such as the
photoelectric effectThe photoelectric effect is a phenomenon in which electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons"...
) had demonstrated that light interacts with matter only in discrete, "quantum"-sized packets called photons.
If sunlight is replaced with a light source that is capable of producing just one photon at a time, or if the beam of light is attenuated to the point that only one photon at a time can get through, as G. I. Taylor did as early as 1909, one photon at a time can pass through the apparatus with the identical result of interference fringes.
If either slit is covered, the individual photons hitting the screen, over time, create an ordinary diffraction pattern. But if both slits are left open, the pattern of photons hitting the screen, over time, again becomes a series of light and dark fringes. This result seems to both confirm and contradict the wave theory. If light were not to behave like a wave, there would be no interference pattern. On the other hand, if light were actually a wave then light energy would not arrive in discrete quantities (quanta) and would be spread over more space the farther the detector screen was placed from the screen with the slits in it.
There is a variation of the double-slit experiment in which detectors are placed in either or both of the two slits in an attempt to determine which slit the photon passes through on its way to the screen. Placing a detector even in just one of the slits will result in the disappearance of the interference pattern. The detection of a photon involves a physical interaction between the photon and the detector of the sort that physically changes the detector. (If nothing changed in the detector, it would not detect anything.) If two photons of the same frequency were emitted at the same time they would be
coherentIn physics, coherence is a property of waves, that enables stationary interference. More generally, coherence describes all properties of the electronic correlation between physical quantities of a wave....
. If they went through two unobstructed slits then they would remain coherent and arriving at the screen at the same time but laterally displaced from each other they would exhibit interference. However, if one or both of them were to encounter a detector, time could be required for each to interact with its detector and they would most likely fall out of
step with each other—that is, they would decohere. They would then arrive at the screen at slightly different times and could not interfere because the first to arrive would have already interacted with the screen before the second got there. If only one photon is involved, it must be detected at one or the other detector, and its continued path goes forward only from the slit where it was detected.
Copenhagen interpretation
The
Copenhagen interpretationThe Copenhagen interpretation is an interpretation of quantum mechanics. A key feature of quantum mechanics is that the state of every particle is described by a wavefunction, which is a mathematical representation used to calculate the probability for it to be found in a location or a state of...
is a consensus among some of the pioneers in the field of quantum mechanics that it is undesirable to posit anything that goes beyond the mathematical formulae and the kinds of physical apparatus and reactions that enable us to gain some knowledge of what goes on at the atomic scale. One of the mathematical constructs that enables experimenters to predict very accurately certain experimental results is sometimes called a probability wave. In its mathematical form it is analogous to the description of a physical wave, but its "crests" and "troughs" indicate levels of probability for the occurrence of certain phenomena (e.g., a spark of light at a certain point on a detector screen) that can be observed in the macro world of ordinary human experience.
The probability "wave" can be said to "pass through space" because the probability values that one can compute from its mathematical representation are dependent on time. One cannot speak of the location of any particle such as photon between the time it is emitted and the time it is detected simply because in order to say that something is located somewhere at a certain time one has to detect it. The requirement for the eventual appearance of an interference pattern is that particles be emitted, and that there be a screen with at least two slits between the emitter and the detection screen. Experiments observe nothing whatsoever between the time of emission of the particle and its arrival at the detection screen. However, it is essential that both slits be an equal distance from the center line, and that they be within a certain maximum distance of each other that is related to the
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the particle being emitted. If a ray tracing is then made
as if a light wave as understood in classical
physics is wide enough to encounter both slits and passes through both of them, then that ray tracing will accurately predict the appearance of maxima and minima on the detector screen when many particles pass through the apparatus and gradually "paint" the expected interference pattern.
Note that the existence of any such particle is known only at the point of emission and the point of detection. If by "object A exists" is meant "object A is detected at point x,y,z,t," then this object "exists" only at the point of emission and the point of detection. In between times it is completely out of sensible interaction with the things of our universe, out of sensible interaction with the macro world. What is going on in the apparatus is something that is not known.
It is perhaps not so astounding that one knows nothing about what a light particle is doing between the time it is emitted from the sun and the time it triggers a reaction in one's retina, but the remarkable consequence discovered by this experiment is that anything that one does to try to locate a photon between the emitter and the detection screen will change the results of the experiment in a way that everyday experience would not lead one to expect. If, for instance, any device is used in any way that can determine whether a particle has passed through one slit or the other, the interference pattern formerly produced will then disappear.
Reason, as applied to the events of our ordinary macro experience, tells us that a particle must pass through one slit or the other. The experiment tells us that there must be at least two slits to produce an interference pattern, and that anything that locates the particle before it hits the screen will destroy the interference pattern. Recent experiments have tried to identify which of the two slits a particle is coming out of on its way to the detection screen. Doing so will also prevent interference. Even less in line with the expectations of human scale interactions with nature, if the information about which slit a given particle came through is "erased" before a photon has time to interact with the detector screen, interference will be restored. (See
Quantum eraser experimentIn physics, the quantum eraser experiment is a double-slit experiment that demonstrates several laws of quantum mechanics, including wave-particle duality, which seeks to explain certain wave and particle properties of matter, complementarity, and the Copenhagen interpretation of quantum mechanics,...
.)
Path Integral Formulation
The Copenhagen interpretation is similar to the
path integral formulationThe path integral formulation of quantum mechanics is a description of quantum theory which generalizes the action principle of classical mechanics...
of quantum mechanics provided by Feynman. (Feynman stresses that his formulation is merely a mathematical description, not an attempt to describe some "real" process that we cannot see.) In the path integral formulation, a particle such as a photon takes
every possible classical path through space-time to get from point A to point B. In the double-slit experiment, point A might be the emitter, and point B the screen upon which the interference pattern appears, and a particle takes every possible path, including paths through both slits at once, to get from A to B. When a detector is placed at one of the slits, the situation changes, and we now have a different point B. Point B is now at the detector, and a new path proceeds from the detector to the screen. In this eventuality there is only empty space between (B =) A' and the new terminus B', no double slit in the way, and so an interference pattern no longer appears.
When observed emission by emission
Regardless of whether it is an electron, a proton, or something else existing on what is considered a "quantum" scale,
where it will arrive at the screen is highly determinate (in that quantum mechanics predicts accurately the probability that it will arrive at any point on the screen). However,
in what sequence members of a series of singly emitted things (e.g., electrons) will arrive is completely unpredictable. The experimental facts are so highly reproducible that there is virtually no argument about them, but the appearance of there being an uncaused event (because of the unpredictability of the sequencing) has aroused a great deal of
cognitive dissonanceCognitive dissonance is an uncomfortable feeling caused by holding two contradictory ideas simultaneously. The "ideas" or "cognitions" in question may include attitudes and beliefs, the awareness of one's behavior, and facts...
and attempts to account for the sequencing by reference to supposed "additional variables."
For example, when electrons are fired at the target screen in bursts, it is easy to account for the interference pattern that results by assuming that electrons that travel in pairs are interfering with each other because they arrive at the screen at the same time, but when a laboratory apparatus was developed that could reliably fire single electrons at the screen, the emergence of an interference pattern suggested that each electron was interfering with itself; and, therefore, in some sense the electron had to be going through both slits. For something that most people continue to imagine to be an unimaginably small particle to be able to interfere with itself would suggest that this "sub-atomic particle" was in two places at once, but that idea is strongly at odds with the truism, "You cannot be in two places at
the same time," (see
principle of contradictionIn logic, the Principle of contradiction is the second of the so-called three classic laws of thought. The oldest statement of the law is that contradictory statements cannot both at the same time be true, e.g. the two propositions A is B and A is not B are mutually exclusive...
). It was easier to conceptualize the electron as a wave than to accept another, more disturbing implication (from the point-of-view of our everyday notions of reality): that quantum objects are able to exist and behave in ways that defy classical interpretation.
However, when one electron (proton, photon, or whatever) is fired at a time, it also becomes possible to detect the point on the screen at which it arrives—and another result was demonstrated that could not easily be squared with experience of the macro world, the world of everyday experience.
In everyday experience we are accustomed to a seemingly analogous result. If one tests a firearm by locking it in a gun mount and firing several rounds at a target, a scatter pattern of bullet holes will appear in the target. We know from long experience that a poorly made gun firing poorly made ammunition will scatter shots fairly widely. We can learn and understand how flight path deviations are caused; more exacting construction of both firearms and ammunition leads to tighter and tighter patterns of bullet holes. But that is not what happens in the new double-slit experiment.
Returning again to electrons, when electrons are fired one at a time through a double-slit apparatus they do
not cluster around two single points directly on lines between the emitter and the two slits, but instead one by one they fill in the same old interference pattern with which we have now become quite familiar. However, they do not arrive at the screen in any predictable order. In other words, knowing where all the previous electrons appeared on the screen and in what order tells us nothing about where the next electron will hit.
The electrons (and the same applies to photons and to anything of atomic dimensions used) arrive at the screen in an unpredictable and arguably
causeless random sequence, and the appearance of a causeless selection event in a highly orderly and predictable formulation of the by now familiar interference pattern has caused many people to try to find additional determinants in the system which, were they to become known, would account for why each impact with the target appears.
Recent studies have revealed that interference is not restricted solely to elementary particles such as protons, neutrons, and electrons. Specifically, it has been shown that large molecular structures like
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
(C
60) also produce interference patterns.
See also
- Afshar experiment
The Afshar experiment is an optical experiment which investigates the principle of complementarity in quantum mechanics. The result of the experiment, which was first devised and carried out by Shahriar Afshar in 2001, is in accordance with the standard predictions of quantum mechanics; however,...
- Delayed choice quantum eraser
A delayed choice quantum eraser is a cross between a quantum eraser experiment and Wheeler's delayed choice experiment. This experiment has been performed first and published by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan O. Scully...
- Elitzur-Vaidman bomb-testing problem
In physics, the Elitzur-Vaidman bomb-testing problem is a thought experiment in quantum mechanics, first proposed by Avshalom Elitzur and Lev Vaidman in 1993. An actual bomb-tester was constructed and successfully tested by Anton Zeilinger, Paul Kwiat, Harald Weinfurter, and Thomas Herzog in 1994...
- Photon dynamics in the double-slit experiment
The dynamics of photons in the double-slit experiment describes the relationship between classical electromagnetic waves and photons, the quantum counterpart of classical electromagnetic waves, in the context of the double-slit experiment...
- Photon polarization
Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photons are completely polarized...
- Quantum eraser experiment
In physics, the quantum eraser experiment is a double-slit experiment that demonstrates several laws of quantum mechanics, including wave-particle duality, which seeks to explain certain wave and particle properties of matter, complementarity, and the Copenhagen interpretation of quantum mechanics,...
- Quantum coherence
- Wheeler's delayed choice experiment
Wheeler's delayed choice experiment is a thought experiment proposed by John Archibald Wheeler in 1978. Wheeler proposed a variation of the famous double-slit experiment of quantum physics, one in which the method of detection can be changed after the photon passes the double slit, so as to delay...
- Dual Polarisation Interferometry
Dual polarisation interferometry is an analytical technique that can probe molecular scale layers adsorbed to the surface of a waveguide by using the evanescent wave of a laser beam confined to the waveguide...
External links