By Michael Betthauser

"It is the mark of an educated mind to be able to entertain a thought without accepting it." ~ Aristotle

With the advent and mathematical understanding of quantum mechanics humans have made incredible advancements in technology, medicine, and science. However there are some features of quantum mechanics that, although nobody disagrees with the math, the interpretation of the meanings of these features have been held in great controversy for nearly a century. I hope to show that it is the predominately held interpretation by the scientific community that an observer is a critical component to understanding the behavior of subatomic particles and that although a lot of work has been done to disprove this interpretation, no substantial evidence has come forth to falsify it. In fact, quite the opposite.

The Copenhagen Interpretation posits that the quantum wave function collapses when detected by an observer. The von Neumann-Wigner interpretation goes further to say that it is a conscious observer (and not merely a detecting apparatus) that's responsible for the collapse. When the Copenhagen Interpretation first came out in the 1920s most scientists could not accept the idea that the conscious observer has anything to do with "outside" physical systems. It has been a long held goal of scientists to be objective when doing research. These interpretations go directly against the notion of objectivity since, according to the interpretation, the very act of observing the physical system changes it. The Copenhagen Interpretation also includes the notion that One can never truly know what is happening at the two slits. In other words, One cannot know both the location and the velocity of a subatomic particle simultaneously, and that all One can ever know are the results seen, or observed, on the detector screen. According to the Copenhagen Interpretation this is not a limitation of the detecting equipment, but a limitation of Nature itself. This facet of the interpretation is called the Heisenberg Uncertainty Principle or otherwise referred to as wave/particle duality (see below). The principle states that the "location" of the particle acts like a wave, a spread out and vague thing of "probability" that does not become precisely known until One observes it. The act of observing the particle's location obscures the precise knowledge of the particle's velocity. While One observes the precise location, the velocity acts like a wave of probability, with no specific value, and vice versa. This paradox seemingly goes directly against the reductive materialism and the philosophies of Descartes and other deterministic thinkers that helped create the foundations of Science and the scientific method since the mere idea of probability is in direct opposition to determinism.

Ever since these initial interpretations determinist and materialist scientists around the world have been trying to find proof that would falsify them entirely. Even Wigner retracted his initial interpretation saying he was afraid it might lead to a type of solipsism. Einstein said "God does not play with dice," and much work has been put forth to disprove the Copenhagen Interpretation.

In order to compare the various differences between the Copenhagen Interpretation and other interpretations it's important to do so by outlining the major features of quantum mechanics.

Besides wave/particle duality, another facet of quantum mechanics is called entanglement. It's when two photons (or two electrons, or two protons, etc.) share a single state with each other, despite the amount of distance between them. It can be the state of any property, such as spin, excitement, velocity, or even location. When two photons are entangled, if the state of one photon is changed, the other photon will change it's state to correspond instantly. It's as if one photon is "communicating" to it's paired or entangled photon instantly, letting the other "know" it's state has been changed. Theoretically the two photons, once entangled, will share this ability to update state changes instantly even if separated by light years. It is unknown how information is being shared between the two particles, however, because the particles "update" instantly, and because it's a fundamental law of thermodynamics that information cannot propagate faster than light, it was believed that no actual "space" must exist between the particles. In other words, the photons are invisibly connected by "non-local" space. Even another way of saying it is that the two photons are the SAME photon and that the separation of "space" is an illusion. This notion of "no space" or "non-local space" is called non-locality.

Entanglement and non-locality in and of themselves have been largely ignored and left uninterpreted, although they play a major role in almost every interpretation and are used by quantum physicists regularly. I do know of one interpretation from the book, "The Tao of Physics" which suggests the Vedic scriptures were right when they said everything in the Universe is all just one interconnected thing. After all, even the Big Bang Theory suggests everything came from a "singularity." In Bohmian Mechanics (more on that below) it is said that all particles are entangled with each other, and in a sense, the entire Universe. This is called the Holographic Principle. More importantly to this topic though, entanglement and non-locality, despite their own implications, have been used by detractors of the Copenhagen Interpretation in various thought and real experiments in an effort to prove that it is indeed possible to know both the location and velocity of a particle simultaneously. In other words it was hoped that entanglement could disprove wave/particle duality and hence the notion of a probabilistic Universe. The idea is that since the entangled particles share a state, if the velocity of one entangled particle could be measured precisely and then the location of the other entangled particle could be measured precisely, One could then know the precise values of both properties. It was (and in large part still is) hoped that by finding a way to resolve the probabilistic and paradoxical nature of wave/particle duality into precisely determined values it could be dismissed. Unfortunately for Einstein and all other deterministic thinkers, quantum entanglement further proved the observer effect for it was found that if one observed the velocity of one particle the other particle would obscure it's location instantly and vice versa. Once again, it's as if the particle "knows" what experimental question is being asked and is transmitting that to the other particle. That entanglement exists is now totally proven. (See Entanglement below).

One way to resolve the paradoxes and return to a deterministic view of reality is to take a completely different interpretation of the results. The Everett Interpretation, or Many Worlds Interpretation, postulates that instead of the particle acting like a wave, it is always acting like a particle with a deterministic path. It accounts for the particles seemingly statistical behavior by stating that all the possible statistical outcomes actually happen, and that events we don't perceive are actually taking place in another Universe. In other words, when the particle encounters the two slits, instead of behaving like a wave in order to pass through both slits simultaneously the entire Universe splits into two Universes, each an exact copy of the other except that in one Universe the particle went through one slit, and in the other Universe it went through the other slit. In this way, every single quantum possibility in the Universe, every electron that jumps orbital shells, every single quantum action everywhere throughout the Universe, causes the Universe to split into two Universes where each of the "possible" outcomes "actually" take place. This brings One to the notion that there must be an infinite number of Universes being infinitely created at near infinitely fast speeds in which to account for all possible quantum actions that are taking place. This interpretation is distasteful to a number of physicists (discussed in the video below) because the existence of other Universes in which the unobserved phenomenon is occurring cannot be experimentally confirmed and the interpretation does not account for why the observer seems to only be in one Universe but not the other.

The Many Worlds Interpretation, like other similar interpretations, relies largely on the concept of decoherence for it's experimental data. Decoherence has to do with superposition, the idea that two quantum states can exist simultaneously, similar to entanglement. In other words, a single particle can be spinning in two opposite directions, or be both excited or not excited simultaneously. The two opposing states of the particle are said to be "superimposed" on each other, or to have superposition. When the particle is observed by measurement the superposition vanishes and only one of the two states is seen. Which state that is seen is random due to INcoherence. In the process of creating superposition a beam of electromagnetic energy is "sent across" the particle. If this beam is set to a specific frequency it will cause the the superposition of the states to become COherent. Another way of looking at it is if an object's wave-like nature is split in two, then the two waves may coherently interfere with each other in such a way as to form a single state that is a superposition of the two states. This concept of superposition is famously represented by Schrödinger's cat, which is both dead and alive at the same time when it's in a coherent state inside a closed box. DEcoherence is the idea that once a particle in superposition interacts with the outside environment it loses superposition and coherence, which is said to "leak" into the environment. Materialists advocate this interpretation because the concept of the decoherence model is said to replace the collapse of the wave function and thus the observer. In this case its the environment that causes the "collapse." But this presents a problem: if a particle from the environment collapsed the particle in question, what collapsed that particle? And what collapsed the particle before it? This logically creates a chain of "collapsing measurements" that require a previous measurement to "collapse" the measuring device ad infinitum. This is referred to as the von Neumann Chain (see Measurement Problem below) . And although this may seem to remove the observer from the wave function collapse, it still does not explain why observation and measurement affect superposition, or entanglement, and thus does not actually disprove the Copenhagen Interpretation, but simply provides another explanation for some, but not all, of the quantum phenomenon. In fact, a recent research team found that for all practical purposes coherence and entanglement are two sides of the same coin. (See Quantum Decoherence below) The inability of the Many Worlds Interpretation or any other interpretations to account for the observer or measurement effect is referred to by physicists as the Measurement Problem (see below).

Now we examine Pilot Wave Theory which was used to develop The De Broglie-Bohm Theory otherwise known as Bohmian Mechanics. This theory also hopes to be deterministic and hopes to prove that a particle is always a particle, and that the wave is a real wave, with the two interacting together to create the observed quantum effects. In this case, the wave is a "pilot-wave" or "guiding wave" that carries the particle around. New evidence has been presented recently that shows a classical demonstration of several parts of this theory. In the demonstration a droplet of silicon oil is bounced on a vibrating silicon oil bath which then performs many behaviors previously only seen at the quantum level. The droplet is carried around by a pilot wave that is created by it's own bouncing and has replicated the double slit experiment in a way that is deterministic and classical (see video below). Although this is an impressive demonstration of the Pilot Wave Theory, there are a number of reasons that both Pilot Wave Theory and this experiment do not disprove the Copenhagen Interpretation.

Pilot Wave Theory assumes a new medium in which the pilot wave is waving. This medium is referred to as the sub-quantum medium and has yet to be experimentally detected. Although the MIT experiment used an oil bath as the medium for the droplet experiments this in no way gives any evidence for the medium that would need to exist at the quantum level. Many physicists compare this medium to the outdated idea of the "ether" that was thought to be the transmission medium for the propagation of electromagnetic or gravitational forces. Another notion was put forth that it is space-time itself that is waving, but that also has yet to be experimentally proven. Bohm himself said it is a new force called Quantum Force which is, incidentally, calculated using equations that were derived from Schrodinger’s equations that provide the wave function for quantum mechanics. In fact, the Quantum Force was purposefully designed by Bohm to give the same answers as quantum mechanics. In this sense his colleagues criticized him by saying his theory looked "rigged." (see Problem With Bohmian Mechanics below).

Pilot Wave Theory is also referred to as a "Hidden Variables" theory. This is because although it is a deterministic theory it must account for the apparent randomness (probabilistic nature) in quantum phenomena. It is said this randomness is due to the observers lack of knowledge about the starting variables in the particle's history. The theory states that if all the starting variables, such as location, velocity, etc., are known, then the precise outcome of the experiment can be determined (and not random). However, the theory also states that it is impossible to know the starting variables since that would require the knowledge of all the variables of the "system" containing the particle in question. To know this would require the knowledge of the system containing that system, and so on, until one reaches the ultimate system known as the entirety of the Universe. The theory goes on to say that since it is impossible to know all the starting variables of the Universe, it is impossible to know the starting variables for the particles in any given experiment. Whereas in quantum mechanics the randomness is "true randomness" calculated as waves of probability, Pilot Wave Theory calls this "randomness due to ignorance." The theory states that since the observer does not know the starting conditions of the particle before measurement the results only seem random. However this is unsatisfactory to some physicists because it seems contrived that one thing should be made "real" at the expense of making something else "unknown." The Copenhagen Interpretation states that it's meaningless to ask "where is the electron" when it is not being measured since it is in a state of superposition between both a wave and a particle. Bohmian Mechanics on the other hand try to show a clear and determined path of the particle at the expense of creating hidden variables. The ignorance of the starting properties creates a certain kind of problem for Bohmian Mechanics called contextuality. Non-contextuality simply means that if One measures a property in two different ways, both measurements should give the same results. In other words, it shouldn't matter on the context of the measurement. However, Bohmian Mechanics suffers the problem that in certain circumstances (see Spin) if you measure a particle one way it gives one answer, and if you measure it another way it gives a different answer. (See Problem With Bohmian Mechanics below). Both answers are completely deterministic, however they don't match, so it is said that Bohmian Mechanics has a problem with contextuality. It is argued that since the observer is the one asking the experimental question and hence controlling the experimental context, Bohmian mechanics hasn't really removed the observer effect from quantum phenomenon at all, but rather "moved" the observer effect to a different part of the equations.

It's been demonstrated experimentally that you can't have a local, deterministic, real, theory. People intuitively expect all of those things to be true, but Bell's Inequality proves this isn't the case (see video under Pilot Wave Videos). The various interpretations all try to give up one of their "local", "real", or "definite" features in order to preserve the others, thus preserving at least a modicum of One's intuition. In the case of The De Broglie-Bohm Theory, deterministic values for the particle's location and velocity are gained at the cost of locality. In this theory, because every measured particle is intrinsically entangled with the surrounding environmental/experimental system, it is said to be non-locally connected. In other words, because the "hidden" starting variables are determined by the surrounding experimental setup and measuring system the particle "in question" and the "measuring" particle are said to be entangled, or non-locally connected. Since every system is a subsystem of a subsystem (etc.) of the whole Universe, this theory states that every single particle in the Universe is non-locally connected to every other particle. Yet another way of thinking about this is that every other particle IS every other particle. This non-local, or sometimes referred to as holographic property of the Universe has caused Bohm and other proponents of Bohmian Mechanics and Pilot Wave Theory to suggest there is a correlation between consciousness and matter since the brain is made of the very same entangled particles as the rest of the Universe (see Wikipedia article on Implicate/Explicate Order below).

It actually shouldn't be a surprise that a classical example of pilot waves replicating the double slit experiment has been found since the idea of Faraday Waves has been known since the 1800s. Bohmian Mechanics, for the most part, uses classical physics to describe the wave and particle so it stands to reason that a macroscopic version of that part of the theory should be found to be demonstrated in a classical way. The problem isn't whether pilot waves exist or even whether they can cause an oil droplet to exhibit certain quantum-like behaviors. Nobody is disagreeing with the math of Pilot Wave Theory or it's classical demonstrations. The problem is that it doesn't provide any new insights to what is actually happening at the quantum level. If the macroscopic oil droplet experiments were to prove every feature of Pilot Wave Theory (and quantum mechanics) they would have to demonstrate non-local entanglement, spin, and quite a few other features, which at this point doesn't seem experimentally possible.

"In its current, immature state, the pilot-wave formulation of quantum mechanics only describes simple interactions between matter and electromagnetic fields, according to David Wallace, a philosopher of physics at the University of Oxford in England, and cannot even capture the physics of an ordinary light bulb. 'It is not by itself capable of representing very much physics,' Wallace said. 'In my own view, this is the most severe problem for the [pilot wave] theory, though, to be fair, it remains an active research area.'" ~

Wheeler's Delayed Choice, Quantum Eraser, and the more recent Delayed Choice Quantum Eraser and other No-Loophole experiments have now completely confirmed beyond anyone's doubt that entanglement is a "real" phenomenon. At first this might seem like excellent news for The De Broglie-Bohm Theory since it so heavily relies upon the concept on non-locality. However, these experiments proved beyond any doubt the existence of wave/particle duality and the Heisenberg Uncertainty Principle. This is actually bad news for hidden variable theories since it proved that the wave function cannot be a physical thing (see It's A Bad Week For Hidden Variable Theories below). Also, it doesn't disprove the Copenhagen Interpretation since this interpretation also includes non-locality. In fact, for a lot of physicists, it reaffirms the role of the observer effect (see Quantum Eraser videos below).

A very common misconception about these experiments is that they prove the possibility of a machine/detector-caused collapse, which simply isn't true. The experiments were designed to investigate wave particle duality and entanglement and have actually stirred the debate about the Measurement Problem more than anything else. However, there is a really good logically equivalent thought experiment (that I borrowed from the Delayed Choice Quantum Eraser video below) that will help to bring much understanding to these very misunderstood experiments and their "weird" results.

The Quantum Eraser experiment is really just the Double Slit experiment with a twist. According to the Copenhagen Interpretation, when an observer looks to see "which way" the particle went, (which slit it went through) the wave function collapses, and we see no interference pattern, just the behavior of a particle. The Quantum Eraser experiments asked the question, "what will happen if we erase the 'which-way' information?" In other words, if we have a device that records which slit the particle went through (the "which-way information") but no one consciously observes it, and then we erase the which-way information before the particle reaches the detector plate, will we see an interference pattern? The quick answer by determinists and materialists is a resounding "no!" because it would seem that it was the device that collapsed the wave function when the which-path information was recorded. But this is a fallacy and goes against experimental evidence and I'll explain why.

To better explain the Quantum Eraser experiment let's continue to imagine that at the two slits we have a special machine that can record the which-way information. This is not exactly how they did the experiment but it's the logical equivalent and will help me get across some important ideas. This information is binary, and can be simply recorded as either a 1 if the particle went through one slit, and 0 if it went through the other. The particle is said to be "marked" or "tagged" with the which-way information because the 1 or 0 is only associated with this particle and no other. Now, the question is: if we erase the which-way information before the particle hits the detector plate which result will we see: The interference pattern or no interference pattern? If you believe that the machine (or the environment) collapsed the wave function when it recorded the which-way information, then it shouldn't matter what happens to the information after the particle passes the slits. If the wave function has collapsed then we should expect to see no interference pattern. However, that's not the case. Once the which-way information is erased, the particle returns to a wave function, as if it had never been a particle, and creates an interference pattern. This is impossible to account for with Pilot -Wave Theory since if the pilot wave were disturbed by the detector when it traversed a slit, and did not have a chance to interfere with itself on the other side, it should remain disturbed all the way to the detector plate and result in a non-interference pattern. But it doesn't.

Wheeler's Delayed Choice Experiments were a series of thought experiments that finally led to real experiments carried out by Alaine Aspect et al. from 1987 to 2006. The Delayed Choice experiments also investigate wave/particle duality but ask a different question. According to the Heisenberg Uncertainty Principle, a particle is both a particle and a wave at the same time, until it is observed by measurement. John Wheeler asked, what will happen if we investigate the wave/particle duality of a single photon? A "single photon" is just another way of saying "a particle", right? How do you obtain and detect a single photon? To answer that question, you use a beam splitter, which is just a piece of glass that allows almost exactly 50% of the straight light through and reflects the other 50%. We are all familiar with being able to see our reflection in glass while also being able to see through it. Detectors are placed at equal distances on either side of the beam splitter so that each detector is in line with each of the two beams. The classical or deterministic view is that, if the photon is a particle, when it enters the beam splitter it will go only one path or the other, but never both paths simultaneously. If it is a single photon then only one detector will give a positive result. However, If it is a wave then both detectors will give a positive result simultaneously, since the wave would be split into two "wavelets" that reach the detectors at the same time. This simultaneous result is called a "coincidence." How to obtain and emit a single photon is a bit more complex and I refer you to Alaine Aspect's lecture which I include below under "Delayed Choice". But as Aspect explains, if you use attenuated light you will only ever get an interference pattern, which is a wave with a probability of being multiple photons at any given moment. However, If you excite an atom with light and then wait for it to drop to the ground state, it will release a single photon (just go watch the video). When a single photon goes through the beam splitter it does indeed exhibit particle behavior and no coincidences are detected. However, if you add a second beam splitter, the interference pattern is restored, and there is coincidence.

In both of these setups a photon is emitted from the bottom left and goes through the first beam splitter (marked BS). In the first setup the two beams take their own path past the two detectors. However, in the second setup the paths are redirected by mirrors to a second beam splitter which recombines the two paths causing them to interfere with each other. This second setup is called a Mach-Zender interferometer. The interesting part about the the second setup is that a single photon will consistently show an interference pattern, exhibiting wave-like behavior. But if we take a classical, deterministic viewpoint, how can this be? Clearly the first experiment shows that the single photon travels either one path or the other, never both at the same time. However, If a second beam splitter is introduced to the setup then the particle behaves as though it took both paths, like a wave. Once again, it's as if the particle is "aware" of the experimental setup, and therefore the experimental question to which it's being asked. These experiments show that If we (experimentally) ask the photon if it's a particle it will be a particle, and if we ask if it is a wave it will be a wave. But John Wheeler was smart and asked "what if we delay the choice to introduce the second beam splitter until after the single photon has passed the first beam splitter?" In other words, if the particle is so seemingly "aware" of the experimental setup, what happens if we delay the moment we choose to ask the experimental question until after the particle has had to "decide" how to behave at the first beam splitter? When this experiment was performed by Alaine Aspect the photon did indeed switch it's behavior according to the experimental question. Or, as John Wheeler said, "Thus One decides the photon shall have come by one route or by both routes after it has already done it's travel."

A common misconception about the Delayed Choice experiment comes from how the second beam splitter was introduced. Because a computer was used (human hands aren't fast enough) materialists argue that it was the computer that made the "decision" about the experimental setup, and not human consciousness. However this is fallacy since it makes no difference to the results if a human or a machine flips the switch. However, the computer did not design the experiment and therefore is not the one asking the experimental question; it is merely flipping a switch randomly, as it was programmed to do. It is the experimenter that has "chosen" or "decided" the setup of the two experiments. But regardless of your stance on on how this experiment has impacted the Measurement Problem, all physicists agree that classical physics and determinism cannot account for these results. The confirmation of non-locality denies any sense of "naive realism."

Now we finally come to the much hyped Delayed Choice Quantum Eraser experiment. Just as the name implies this experiment combines Wheeler's Delayed Choice experiment with the Quantum Eraser experiment to settle once and for all the debate on complimentarity. This experiment has the most misconceptions surrounding it, mostly due to it's complexity, so I will once again use a more simple, logically equivalent thought experiment to explain the results. To go back to the original Quantum Eraser experiment, imagine a machine that records the which-way information as a 1 or 0. Now imagine we separate the first beam splitter from the detectors by one light minute. This means it will take one minute before the result of interference or no interference is observed for each photon. After the photon has traveled through the first beam splitter the computer/machine records the which-way information then prints out either a solid white (1) or solid black (0) piece of paper which is perfectly concealed in a manilla envelope. The which-way information is erased from the computer memory so it is only contained in the manilla envelope and no conscious observer knows the which-way information. We only needed the light-minute to perform this operation and once the photon hits the detector a result is recorded as either a coincidence or no coincidence. If there is a coincidence it's a white piece of paper and vice versa. The result is also stored in a manilla envelope which is not consciously observed. The two envelopes are labeled "which-way" and "result," respectively, and are set side by side, thus "marking" or "tagging" them as being paired. Let's say we send several photons through the experiment in this way and two stacks of envelopes are collected. Now the experiment has ended and we have our information but none of it has been observed yet. This is the delayed choice part of the experiment because the choice to observe the which-way information has been delayed until after the entire experiment has been run! Now here's the interesting part. If we take the first pair of envelopes from the two stacks and look at the which-way information, the result envelope will contain no coincidence and thus no interference pattern (a particle). However, if we take the which-way envelope and burn it so that no observation can be performed, it's paired result envelope will show an interference pattern (a wave). But let's suppose you look at the result envelope first, before anything is done to it's paired which-way envelope. If you do this you will find a wave-like interference pattern every time. And when you open the paired "which-way" envelope the piece of paper will be gray, concealing the which-way information! This is precisely the part that spooks everyone out and makes people's head spin. It's the part that cause some to interpret the results as a form of retro-causality. It's as if the photon went back in time to ensure that when the machine printed the result it would be "fuzzy."

The original Delayed Choice Quantum Eraser experiment performed by Yoon-Ho Kim and Marlan O. Scully et al. in 1999 was not carried out with single photons, but rather entangled pairs of photons, and the results as I just described were carried out by correlating complete sets of data. Kim says in their released research paper, "The triggering of detectors D1 or D2 erases the which-path information so that either the absence of the interference or the restoration of the interference can be arranged via an appropriately contrived photon correlation study." In other words, the pattern of photons registered at D0 (Detector Zero) is meaningless until it is correlated with the entangled partner photons at the other detectors. When this is done, detectors used to detect which-path information, when correlated with D0, show no interference, and detectors used to erase which-way information, once again, when correlated with D0, do show interference. It should be easy to see why the data from D0 is meaningless when it's uncorrelated since it contains data from both lasers and not a single light source as in Young's original Double Slit experiment. I provide a very good explanation of how the data was correlated below under "Delayed Choice Quantum Eraser." Also included at the end of that section is a research interpretation article on all of the delayed choice experiments up to 2015 which include single photon experiments that further back up previous results.

In the end, to summarize, the question of which interpretation is "correct" is that which is one of "completeness." Einstein and other determinist detractors of the Copenhagen Interpretation argue that since quantum mechanics cannot know precisely where the particle is at all times it must be incomplete. However, proponents of current quantum mechanics and the Copenhagen Interpretation state that quantum mechanics is complete so long as you're willing to accept this "barrier" of knowledge (to the unobserved, precise location of the electron, for example) as being an actual feature of Nature. As it stands, quantum mechanics is very precise in it's predictions. So precise that it has been said to be the most successful theory every put forward. One of the biggest reasons physicists continue to use quantum mechanics (and accept all the funky Copenhagen weirdness that comes with it) is exactly because it is so precise, and because the equations seem to describe all the verifiable, observable phenomena that have yet been discovered. Perhaps when a new phenomenon is encountered a new theory (and interpretation) will be needed to account for it. But right now, quantum mechanics works, and it works very well. The biggest argument made by quantum physicists against the other theories, models and interpretations has mostly to do with the additional, cumbersome and often very overly complicated math formulas that come with them. They just aren't as "nice" to work with as quantum mechanics and in some cases not as precise. And they provide no new insights to quantum phenomenon. Perhaps more importantly is the fact that current quantum theory is, so far, able to handle current advances in quantum relativity, black hole theory, and string theory, something the other theories either haven't attempted or are disturbingly overly complicated in their attempt. That isn't to say the other theories aren't useful and that knowledge cannot be gleaned from them. Each model is a useful tool for the experimenter to play out their theories and to conduct experiments. Each interpretation provides it's own mental framework by which to consider new theoretical avenues of thought. But so far, quantum mechanics and the Copenhagen Interpretation are the best models developed for describing reality.

I hope I have shown that regardless of the interpretation One decides to adopt, each interpretation must still handle the role of the observer effect in one way or another. The inability to completely disprove the observer effect is called the Measurement Problem, and as it stands to date, no theory or interpretation has solved the Measurement Problem. This can only make sense, since how does One completely remove the observer from the experiment? It is the observer that asks the experimental question, the observer that sets up the experiment, and it is the observer that analyzes the results. And, according to quantum mechanics, when the observer asks Nature "What are you made of," Nature responds, "That depends on what you're asking," thus intrinsically linking consciousness to the behavior of quantum phenomena, as thus to the nature of matter itself.

"Quantum mechanics must be one of the most successful theories in science. Developed at the start of the twentieth century, it has been used to calculate with incredible precision how light and matter behave – how electrical currents pass through silicon transistors in computer circuits, say, or the shapes of molecules and how they absorb light. Much of today’s information technology relies on quantum theory, as do some aspects of chemical processing, molecular biology, the discovery of new materials, and much more." ~

Below you will find all the references you will ever need. After the videos are statements by Universities that show a bias towards the Copenhagen Interpretation. This includes a PDF from the Physics Department at UC Boulder that although it's goal is to encourage the teaching of more interpretations, it also shows that currently the Copenhagen Interpretation is the preferred interpretation.


MIT Graduate Student demonstrates actual double slit experiment using a laser.

Discovery Channel -Through The Wormhole - How Does the Universe Work.
Anton Zeilinger explains the observer effect with actual laser experiment.

The Fabric of the Cosmos: Quantum Leap - NOVA
Theoretical Physicist Brian Greene explains quantum mechanics
Explains observer effect at about 23:20
Explains quantum entanglement starting at 25:00

The Wonderful Weirdness of the Quantum World
David Z. Albert, Alan Alda, Brian Greene, Max Tegmark, William Phillips discuss the various interpretations, specifically the Copenhagen Interpretation vs. the Many Worlds Interpretation.
Discuss entanglement and observer effect at about 51:00

BBC: The Secrets Of Quantum Physics
Explains observer effect at about 25:00
Explains entanglement at about 32:00


Quantum Entanglement Lab - by Scientific American
SA editors George Musser and John Matson pay a visit to Professor Enrique Galvez of Colgate University, who has built a machine to observe quantum entanglement, the strange phenomenon that Einstein called "spooky action at a distance."
Explains act of measurement at 2:20
Non-locality explained at about 6:00

New York Times - Sorry, Einstein. Quantum Study Suggests ‘Spooky Action’ Is Real.

Nobel Peace Prize Awarded For Proof Of Entanglement


Coherent Light Explained

Quantum superposition of states and decoherence
A great explanation of decoherence.
Mentions measurement or observer effect at 0:48

Decoherence and consciousness - Max Tegmark

Mentions "something in the environment is learning" around 3:20

Physicists find quantum coherence and quantum entanglement are two sides of the same coin


The Measurement Problem
What constitutes as a measurement in quantum mechanics? Can it be completed with a measuring apparatus or does it extend further than that?

What Is A Measurement?
Although this video makes this assumption, One does not need to believe humans are the center of the Universe in order to explain Universal wavefunction collapse.

The Hard Problem discussed by physicist Tom Campbell

Adam Caulton, Assistant Professor at the Munich Center for Mathematical Philosophy
The Measurement Problem 1: Copenhagen and dynamical collapse theories
The Measurement Problem 2: Pilot Wave and Many Worlds theories


The pilot-wave dynamics of walking droplets

Yves Couder . Explains Wave/Particle Duality via Silicon Droplets [Through the Wormhole]

Bohmian Mechanics- An Alternative to Quantum
Explains why Bohmian Mechanics and Quantum Mechanics are indistinguishable at 3:50
Explains "weird results" at about 6:25

A problem with Bohmian Mechanics? Contextuality
'Contextuality' might mean that there are no alternatives to Quantum mechanics that are sensible. Given Quantum isn't sensible either, there may just not be any sensible theories at all.

Bell's theorem explained

Quantum Spin Explained


Alaine Aspect Public Lecture On His Delayed Choice Experiment
Professor Alain Aspect gave a public lecture at the Frederiksberg Gymnasium on the wave-particle duality for a single photon.


Delayed Choice Quantum Eraser Explained
Starts with a recap of the double slit experiment at 0:00
DCQE explained starting at about 5:00

Quantum Eraser (This is different than the above experiment!)

Quantum Eraser Explained | Quantum Mechanics ep 5
This is an important example of the type of misconception about the eraser experiments. The narrator in this video falsely states that this experiment proves a machine collapses the wave function. This isn't true and is explained further in the following videos. It's unfortunate because so far I found all the rest of her videos very accurate.

Delayed Choice Quantum Eraser | Quantum Mechanics ep 6
Although the narrator is "disturbed" and "doesn't like the results" this experiment once again demonstrates the observer effect.

Physicist Thomas Campbell explains the Delayed Choice Quantum Eraser experiment with a logical equivalent.

Great explanation showing the way the data was correlated

Another excellent explanation of the Delayed Choice and Eraser experiments.

Delayed Choice Quantum Eraser Research Paper
Yoon-Ho Kim, Rong Yu, Sergei P. Kulik, Yanhua Shih, and Marlan O. Scully Phys. Rev. Lett. 84, 1 – Published 3 January 2000

Delayed-choice gedanken experiments and their realizations


University of Oregon
"The formation of the interference pattern requires the existence of two slits, but how can a single photon passing through one slit `know' about the existence of the other slit? We are stuck going back to thinking of each photon as a wave that hits both slits. Or we have to think of the photon as splitting and going through each slit separately (but how does the photon know a pair of slits is coming?). The only solution is to give up the idea of a photon or an electron having location. The location of a subatomic particle is not defined until it is observed (such as striking a screen)."

Department of Physics University of Toronto
"Evidently, when we look at what is going on at the slits we cause a qualitative and irreversible change in the behavior of the electrons. This is usually called the 'Heisenberg Uncertainty Principle.'"

University of Southern California
"These characteristics lead to the belief that an observer interacts with a system under study to such an extent that the system cannot be thought of as having an independent existance. This goes contrary to one of classical physics key postulates (unproven assumptions) that the universe exists independent of an observer."

Northern State University
"Heisenberg's principle stressed again the recurrent motif of the scientist's active role in creating the observed reality. It implied that we cannot observe the world without influencing its course."

University of California at Davis
Introduction to Quantum Mechanics and Consciousness

The Physics Department for the University of Colorado at Boulder
In an effort to promote more awareness for other interpretations, UC Boulder conducted this survery and found that the Copenhagen Interpretation or "Quantum" interpretation was the one favored by students.



During most of the 20th century, collapse theories were clearly the mainstream view, and the question of "interpretation" of quantum mechanics mostly revolved around how to interpret "collapse". Proponents of either "pilot-wave" (de Broglie-Bohm-like) or "many-worlds" (Everettian) interpretations tend to emphasize how their respective camps were intellectually marginalized throughout 1950s to 1980s. In this sense, all non-collapse theories are (historically) "minority" interpretations.

However, since the 1990s, there has been a resurgence of interest in non-collapse theories. The Stanford Encyclopedia as of 2015 groups interpretations of quantum mechanics into "Bohmian mechanics" (pilot-wave theories),[11] "collapse theories",[12] "many-worlds interpretations",[13] "modal interpretation"[14] and "relational interpretations"[15] as classes of into which most suggestions may be grouped.

As a rough guide development of the mainstream view during the 1990s to 2000s, consider the "snapshot" of opinions collected in a poll by Schlosshauer et al. at the 2011 "Quantum Physics and the Nature of Reality" conference of July 2011.[16] The authors reference a similarly informal poll carried out by Max Tegmark at the "Fundamental Problems in Quantum Theory" conference in August 1997. The main conclusion of the authors is that "the Copenhagen interpretation still reigns supreme", receiving the most votes in their poll (42%), besides the rise to mainstream notability of the many-worlds interpretations.


The Copenhagen interpretation is an expression of the meaning of quantum mechanics that was largely devised in the years 1925 to 1927 by Niels Bohr and Werner Heisenberg. It remains one of the most commonly taught interpretations of quantum mechanics.

Those who hold to the Copenhagen interpretation are willing to say that a wave function involves the various probabilities that a given event will proceed to certain different outcomes. But when the apparatus registers one of those outcomes, no probabilities or superposition of the others linger.[20]

According to Howard, wave function collapse is not mentioned in the writings of Bohr.[3]

Some argue that the concept of the collapse of a "real" wave function was introduced by Heisenberg and later developed by John von Neumann in 1932.[21] However, Heisenberg spoke of the wavefunction as representing available knowledge of a system, and did not use the term "collapse" per se, but instead termed it "reduction" of the wavefunction to a new state representing the change in available knowledge which occurs once a particular phenomenon is registered by the apparatus (often called "measurement").[22]

In 1952 David Bohm developed decoherence, an explanatory mechanism for the appearance of wave function collapse. Bohm applied decoherence to Louis DeBroglie's pilot wave theory, producing Bohmian mechanics,[23][24] the first successful hidden variables interpretation of quantum mechanics. Decoherence was then used by Hugh Everett in 1957 to form the core of his many-worlds interpretation.[25] However decoherence was largely[26] ignored until the 1980s.[27][28]

Throughout much of the twentieth century the Copenhagen interpretation had overwhelming acceptance among physicists. Although astrophysicist and science writer John Gribbin described it as having fallen from primacy after the 1980s,[33] according to a poll conducted at a quantum mechanics conference in 1997,[34] the Copenhagen interpretation remained the most widely accepted specific interpretation of quantum mechanics among physicists. In more recent polls conducted at various quantum mechanics conferences, varying results have been found.[35][36][37] Often, as is the case with the 4 referenced sources, the acceptance of the Copenhagen interpretation as the preferred view of the underlying nature was below 50% amongst the surveyed.


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

Objections to the interpretation

There are other possible solutions to the "Wigner's friend" thought experiment, which do not require consciousness to be different from other physical processes. Moreover, Wigner actually shifted to those interpretations (and away from "consciousness causes collapse") in his later years. This was partly because he was embarrassed that "consciousness causes collapse" can lead to a kind of solipsism, but also because he decided that he had been wrong to try to apply quantum physics at the scale of everyday life (specifically, he rejected his initial idea of treating macroscopic objects as isolated systems).[6]

To many scientists this interpretation fails to compete with other interpretations of quantum mechanics because "consciousness causes collapse" relies upon an interactionist form of Dualism (philosophy of mind) that is inconsistent with the materialism presupposed by many physicists.[3] The measurement problem not withstanding, they point to a causal closure of physics, suggesting a problem with how consciousness and matter might interact, reminiscent of objections to Descartes' substance dualism.


Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe"). In lay terms, the hypothesis states there is a very large-perhaps infinite[2]-number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes.

In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence, and this is supposed to resolve all of the correlation paradoxes of quantum theory, such as the EPR paradox[13][14] and Schrödinger's cat,[1] since every possible outcome of every event defines or exists in its own "history" or "world".

A consequence of removing wavefunction collapse from the quantum formalism is that the Born rule requires derivation, since many-worlds derives its interpretation from the formalism. Attempts have been made, by many-world advocates and others, over the years to derive the Born rule, rather than just conventionally assume it, so as to reproduce all the required statistical behaviour associated with quantum mechanics. There is no consensus on whether this has been successful.[24][25][26]

One of the salient properties of the many-worlds interpretation is that it does not require an exceptional method of wave function collapse to explain it. "It seems that there is no experiment distinguishing the MWI from other no-collapse theories such as Bohmian mechanics or other variants of MWI... In most no-collapse interpretations, the evolution of the quantum state of the Universe is the same. Still, one might imagine that there is an experiment distinguishing the MWI from another no-collapse interpretation based on the difference in the correspondence between the formalism and the experience (the results of experiments)."[53]

However, in 1985, David Deutsch published three related thought experiments which could test the theory vs the Copenhagen interpretation.[54] The experiments require macroscopic quantum state preparation and quantum erasure by a hypothetical quantum computer which is currently outside experimental possibility. Since then Lockwood (1989), Vaidman and others have made similar proposals.[53] These proposals also require an advanced technology which is able to place a macroscopic object in a coherent superposition, another task for which it is uncertain whether it will ever be possible. Many other controversial ideas have been put forward though, such as a recent claim that cosmological observations could test the theory,[55] and another claim by Rainer Plaga (1997), published in Foundations of Physics, that communication might be possible between worlds.[56]


Objective collapse theories differ from the Copenhagen interpretation in regarding both the wavefunction and the process of collapse as ontologically objective. In objective theories, collapse occurs randomly ("spontaneous localization"), or when some physical threshold is reached, with observers having no special role. Thus, they are realistic, indeterministic, no-hidden-variables theories. The mechanism of collapse is not specified by standard quantum mechanics, which needs to be extended if this approach is correct, meaning that Objective Collapse is more of a theory than an interpretation. Examples include the Ghirardi-Rimini-Weber theory[39] and the Penrose interpretation.[40]

GRW collapse theories have unique problems.
In order to keep these theories from violating the principle of the conservation of energy, the mathematics requires that any collapse be incomplete. Almost all of the wave function is contained at the one measurable (and measured) value, but there are one or more small "tails" where the function should intuitively equal zero but mathematically does not. It is not clear how to interpret these "tails". They might mean that a small bit of matter has collapsed elsewhere than the measurement indicates, that with very low probability an object might "jump" from one collapsed state to another, or something else entirely. All of these options are counterintuitive.

The original QMSL models had the drawback that they did not allow dealing with systems with several identical particles, as they did not respect the symmetries or antisymmetries involved. This problem was addressed by a revision of the original GRW proposal known as CSL (continuous spontaneous localization) developed by Ghirardi, Pearle, and Rimini in 1990.[2]


Decoherence does not generate actual wave function collapse. It only provides an explanation for the observation of wave function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. Specifically, decoherence does not attempt to explain the measurement problem. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".

Decoherence represents a challenge for the practical realization of quantum computers, since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences. Simply put, they require that coherent states be preserved and that decoherence is managed, in order to actually perform quantum computation.


The quantum eraser experiment described in this article is a variation of Thomas Young's classic double-slit experiment. It establishes that when action is taken to determine which slit a photon has passed through, the photon cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. The experiment described in this article is capable of creating situations in which a photon that has been "marked" to reveal through which slit it has passed can later be "unmarked." A photon that has been "marked" cannot interfere with itself and will not produce fringe patterns, but a photon that has been "marked" and then "unmarked" can thereafter interfere with itself and will cooperate in producing the fringes characteristic of Young's experiment.[1]


The experiment was designed to investigate peculiar consequences of the well-known double slit experiment in quantum mechanics as well as the consequences of quantum entanglement [not wave function collapse].

The delayed choice quantum eraser experiment investigates a paradox. a photon manifests itself as though it had come by a single path to the detector, then "common sense" (which Wheeler and others challenge) says it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid-flight, then the photon shouldreverse its original "decision" as to whether to be a wave or a particle. Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on earth on how to observe a photon could alter a decision made millions or even billions of years ago.

This result is similar to that of the double-slit experiment since interference is observed when it is not known which slit the photon went through, while no interference is observed when the path is known.

However, what makes this experiment possibly astonishing is that, unlike in the classic double-slit experiment, the choice of whether to preserve or erase the which-path information of the idler was not made until 8 ns after the position of the signal photon had already been measured by D0.

In addition to challenging our common sense ideas of temporal sequence in cause and effect relationships, this experiment is among those that strongly attack our ideas about locality, the idea that things cannot interact unless they are in contact, if not by being in direct physical contact then at least by interaction through magnetic or other such field phenomena.[21]:199


De Broglie–Bohm theory gives the same results as quantum mechanics. It treats the wavefunction as a fundamental object in the theory as the wavefunction describes how the particles move. This means that no experiment can distinguish between the two theories.

It requires a special setup for the conditional wave function of a system to obey a quantum evolution. When a system interacts with its environment, such as through a measurement, the conditional wave function of the system evolves in a different way. The evolution of the universal wave function can become such that the wave function of the system appears to be in a superposition of distinct states. But if the environment has recorded the results of the experiment, then using the actual Bohmian configuration of the environment to condition on, the conditional wave function collapses to just one alternative, the one corresponding with the measurement results.

De Broglie–Bohm theory is often referred to as a "hidden variable" theory
. Bohm used this description in his original papers on the subject, writing, "From the point of view of the usual interpretation, these additional elements or parameters [permitting a detailed causal and continuous description of all processes] could be called 'hidden' variables." Bohm and Hiley later stated that they found Bohm's choice of the term "hidden variables" to be too restrictive. In particular, they argued that a particle is not actually hidden but rather "is what is most directly manifested in an observation [though] its properties cannot be observed with arbitrary precision". However, others nevertheless treat the term "hidden variable" as a suitable description.

The De Broglie–Bohm theory makes the same (empirically correct) predictions for the Bell test experiments as ordinary quantum mechanics. It is able to do this because it is manifestly non-local. It is often criticized or rejected based on this; Bell's attitude was: "It is a merit of the de Broglie–Bohm version to bring this [non-locality] out so explicitly that it cannot be ignored."

The Bohm interpretation preserves realism, hence it needs to violate the principle of locality in order to achieve the required correlations . It does so by maintaining that both the position and momentum of a particle are determinate in that they correspond to the definite trajectory of the particle; however, that trajectory cannot be known without knowing the physical state of the entire universe.

The recurrence and stability of our own memory as a relatively independent sub-totality is thus brought about as part of the very same process that sustains the recurrence and stability in the manifest order of matter in general. It follows, then, that the explicate and manifest order of consciousness is not ultimately distinct from that of matter in general.[9]

are several equivalent mathematical formulations of the theory and it is known by a number of different names. The de Broglie wave has a macroscopic analogy termed Faraday wave.[4]

Faraday waves, also known as Faraday ripples, named after Michael Faraday, are nonlinear standing waves that appear on liquids enclosed by a vibrating receptacle. When the vibration frequency exceeds a critical value, the flat hydrostatic surface becomes unstable. This is known as the Faraday instability. Faraday first described them in an appendix to an article in the Philosophical Transactions of the Royal Society of London in 1831.[1][2]


Bohm's original aim was not to make a serious counterproposal but simply to demonstrate that hidden-variable theories are indeed possible.[24] (It thus provided a supposed counterexample to the famous proof by John von Neumann that was generally believed to demonstrate that no deterministic theory reproducing the statistical predictions of quantum mechanics is possible.) Bohm said he considered his theory to be unacceptable as a physical theory due to the guiding wave's existence in an abstract multi-dimensional configuration space, rather than three-dimensional space.[24] His hope was that the theory would lead to new insights and experiments that would lead ultimately to an acceptable one;[24] his aim was not to set out a deterministic, mechanical viewpoint, but rather to show that it was possible to attribute properties to an underlying reality, in contrast to the conventional approach to quantum mechanics.[25]