Is the "many-worlds" interpretation of quantum mechanics viable

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Is the "many-worlds" interpretation of quantum mechanics viable?

Viewpoint: Yes, the "many-worlds" interpretation of quantum mechanics is viable, as it provides the simplest solution to the measurement dilemma inherent in the standard model.

Viewpoint: No, the "many-worlds" interpretation of quantum mechanics is not viable for numerous reasons, including measurement problems and the inability to test it scientifically.

Quantum mechanics is one of the most successful theories of modern physics. Chemists, physicists, materials scientists, molecular biologists, and electrical engineers use the predictions and terminology of the theory to explain and understand numerous phenomena. Looked at closely, however, the theory has certain unsettling features. Particles such as the electron have wavelike properties, while electromagnetic waves behave, at times, like particles. One must abandon the notion that each particle has a definite location at all times. These counter-intuitive aspects of the theory led many physicists, most notably Albert Einstein (1879-1955), to reject it or at least regard it as temporary and inherently incomplete.

Among the strangest aspects of quantum theory is the sudden change in state that is alleged to occur when a measurement is made. If a beam of light is sent through a polarizing filter, it is found that the emerging beam is completely polarized, and thus it will pass through any subsequent polarizing filters with the same orientation as the first without any loss of intensity. If the intensity of the light source is reduced so that one can be sure that only one photon or quantum of light energy is passing through each filter at a time, the result is unchanged. If one of the filters is rotated by 45 degrees, however, only half of the photons pass through. For any particular photon, there is no way of determining whether or not it will pass through the rotated filter. This appears to be a truly random process—an example of God playing dice, as Einstein would have said.

A further problem in quantum measurement theory concerns the existence of so-called entangled states. An electron and an anti-electron can annihilate each other, creating two photons traveling in opposite directions. The polarizations of the photons will be highly correlated. If one of them goes through a polarizing filter, one knows with certainty that the other will not pass through a filter with the same direction of polarization, and this conclusion is unaffected by rotating both filters by an equal amount. It is as if the second photon knew what the first had decided to do, even though it could be light years away by the time the first measurement was made.

According to the standard version of quantum mechanical theory, the state of any system evolves continuously in time according to the so-called time-dependent Schrödinger equation until a measurement is made. The state is described by a wave function—sometimes called a state vector, referring to an abstract mathematical space, called Hilbert space, with an infinite number of dimensions. According to the theory, the wave function contains all the information that one can have about the system and gives the probability of each possible outcome of any measurement. Once the measurement is made, the wave function is claimed to change to a function that predicts with 100% certainty that the outcome of the same measurement performed immediately after the first will be the same. Before the measurement, the wave function is said to represent a superposition of states, each corresponding to a possible outcome of the measurement. When the measurement is made, the wave function is said to collapse, in that only the part of the original wave function corresponding to the observed result remains.

To highlight some of the paradoxical aspects of the standard quantum theory of measurement, Erwin Schrödinger (1887-1961) proposed a thought experiment now generally referred to as Schrödinger's cat. In this case we are asked to consider a cat enclosed in a cage along with a vial of poison gas, a Geiger counter, and a small amount of a radioactive material. At the start of the experiment, the amount of radioactive material is adjusted so that there is a 50-50 chance that the Geiger counter will record at least one decay in the next hour. The Geiger counter is connected to an apparatus that will break open the vial of poison gas if a decay is registered, killing the cat. The whole apparatus—cage, cat, vial, and counter—is covered for an hour. At the end of the hour, according to the quantum theory of measurement, the system will be in a superposition of states, half of which involve a live cat and the other half a dead one. Uncovering the cage constitutes a measurement that would collapse the wave function into one involving a dead or a live cat. The notion of a creature, half-dead, half alive, is inconsistent with human experience and perhaps even less acceptable than the blurring of particle and wave behavior that quantum mechanics describes.

While a graduate student in physics at Princeton University, Hugh Everett III proposed what he termed a "relative state formulation" of quantum mechanics, which might provide an alternative to some of the more paradoxical aspects of quantum measurement theory. Everett advocated thinking about a single wave function for the entire universe, which always evolved in time in a continuous fashion. The wave functions would describe a superposition of states involving observers and experimental apparatus. In the Schrödinger cat case the superposition would involve states in which the cat was dead and known to observers as dead, and states in which the cat was alive and known to observers as alive. Everett's proposal solves the entangled states paradox as well, since the wave function provides a superposition of states with each possible outcome of the two polarization measurements. Everett's original paper said nothing about multiple worlds.

The multiple-worlds idea emerged in subsequent work by Everett, his advisor, John Wheeler, and physicist Neill Graham. In this work it became apparent that the wave function for the entire universe envisioned by Everett was actually a superposition of wave functions for alternative universes. In each, the observers had observed definite outcomes of the Schrödinger cat experiment, but every time a measurement was made, the universe split into multiple universes, each with a separate outcome.

Opinions on the many-worlds interpretation vary greatly among physicists. Since there are no testable predictions, some feel that it falls outside the scope of scientific investigation and is merely an interesting speculation. Scientists generally reject theories that cannot be proven false by an experiment, but some exception is made for cosmological theories, those about how the whole universe behaves, since we cannot set up a test universe. Others feel that since the many worlds theory eliminates some counterintuitive aspects of the standard theory, it has some merit. The notion of alternate realities has long been a favorite of science fiction writers. The idea that all conceivable—that is, not self-contradictory—universes ought to exist also has appeal, in that it eliminates the need to explain why our universe has the particular properties (electron mass, gravitational constant, and so on) that it has—though this goes far beyond the original theory. The pro and con articles illustrate two of the many possible positions.

—DONALD R. FRANCESCHETTI

Viewpoint: Yes, the "many-worlds" interpretation of quantum mechanics is viable, as it provides the simplest solution to the measurement dilemma inherent in the standard model.

The Copenhagen, or standard, interpretation of quantum physics, as defined by Niels Bohr (1885-1962) and others during the 1920s, has proved to be one of the most successful scientific theories ever created. Its predications have been confirmed time and time again, and not a single experiment has contradicted the theory. Yet many scientists are profoundly unhappy with the philosophical consequences of standard quantum theory.

The Copenhagen interpretation of quantum theory contains a number of worrying paradoxes and problems. One of the strangest, and most worrying, is the role that measurement plays in quantum physics. The act of measuring is outside quantum theory, and the standard explanation involves ad hoc assumptions and generates uncomfortable paradoxes. The simplest solution to this dilemma is the many-worlds (or many-universes) interpretation of quantum theory, first proposed by Hugh Everett III in 1957.

The Problem of Measurement

Quantum mechanics is essentially a statistical theory that can be used to calculate the probability of a measurement. It is a epistemological theory, not an ontological theory, in that it focuses on the question of how we obtain knowledge, and does not concern itself with what is. It describes and predicts experimental observations, but as it stands it does not explain any of these processes. Yet despite the focus of the theory on the probabilities of measurement, the act of measurement is beyond the scope of the Copenhagen interpretation.

Imagine a single photon moving toward a strip of photographic film. Before measurement takes place the Copenhagen interpretation describes the photon as a wave function, as defined by Erwin Schrödinger's (1887-1961) wave equation. The wave function describes a broad front of probabilities where the photon could be encountered, and does not give any specific values to the position until measurement. When the wave function hits the film only a single grain of the film is exposed by the single photon. The wave function appears to have collapsed from a broad front of probability to a single point of actual impact. This collapse, or "reduction of the state," takes place instantaneously, even though the front of probability could be very large.

The collapse of the wave function, however, is not described by the Schrödinger wave equation. It is an addition to quantum mechanics to make sense of the fact that observations do appear to occur. Many-worlds theory solves the measurement problem of quantum physics, by allowing for all outcomes of the wave function to be correct, so the wave function does not collapse. Instead all outcomes exist, but in separate realities, unable to interact with each other. Each measurement results in the branching (or grouping depending on which many-worlds theory you use) of universes corresponding to the possible outcomes. No additions to quantum theory are required, and the Schrödinger wave equation as a whole is taken as an accurate description of reality. Many-worlds theory is therefore an ontological theory that explains what the quantum world is, and how the act of measurement takes place.

Measurement is even more of a problem in the standard quantum interpretation when you consider the notion of entanglement. A measuring device interacting with a quantum system to be measured, according to the Schrödinger wave equation, inherits the quantum measurement problem, ad infinitum. In effect the measuring device and the object to be measured just become a larger quantum system. According to the mathematics that quantum physics uses so effectively, there can be no end to this entanglement, even if more and more measuring devices are added to measure the previous entangled systems. Again, the standard interpretation of quantum physics seems to suggest that the act of measurement is impossible.

Cats, Friends, and Minds

Several of the originators of quantum theory expressed their concerns over the direction in which quantum mechanics developed, including Albert Einstein (1879-1955) and Schrödinger. While Schrödinger may have provided quantum theory with some of its most important mathematical tools, namely his wave equation, he could not agree philosophically with Bohr and other proponents of the Copenhagen interpretation. Schrödinger proposed a famous thought experiment involving a cat in a box that will be killed if a radioactive particle decays, in order to show how the assumptions of the quantum world fall down when applied to larger objects.

The experiment focuses on a typical quantum probability outcome, a radioactive particle that has a certain chance of decaying in a certain time. The Copenhagen interpretation suggests that the particle exists in a superposition of states until observed, and then the wave function collapses. However, while it seems reasonable to describe a quantum system as in a superposition of states, the idea that the cat is somehow both dead and not-dead until observed seems a little strange. If we replace the cat with a human volunteer, often referred to as Wigner's friend after Eugene Wigner (1902-1995), who first suggested it, then things become even more unreal. Until observed, quantum theory tells us that Wigner's friend is both alive and dead, in a superposition of states, but surely Wigner's friend himself knows if he is dead or alive? Wigner invoked the idea that the conscious mind is the key to observations. It is the observer's mind that collapses the wave function, not just its interaction. While this interpretation avoids the problem of entanglement, it makes the mind something that exists outside of quantum mechanics, and it further adds to the measurement problems. If we consider the exposed grain of film in the first example, Wigner would claim that until a conscious mind views the developed film the wave function has not collapsed. It remains as a broad wave front until observed, perhaps years later, and then instantaneously collapses. The power of the conscious mind seems to create reality. But what counts as a conscious mind? Can a cat determine reality? An insect or an amoeba? And what happened before the evolution of conscious minds in the universe? Was nothing real in the universes until the first conscious life-form looked up at the sky and observed it?

Many-worlds theory is the simplest and most economical quantum theory that removes such problems. The cat and the human volunteer are both alive and dead, but in different universes. There is no need to invoke a special power of the mind, or to add in the collapse of the wave function. Many-worlds model proposes that the same laws of physics apply to animate observers as they do to inanimate objects. Rather than an ad hoc explanation for measurement, or the elevation of the mind to cosmic status, many-worlds theory restores reality to quantum theory.

Some Criticisms

The many-worlds interpretation had been challenged on a number of grounds, and like any other quantum interpretation it does have its problems. However, most objections are baseless and ill-informed. Some critics have charged that it violates the law of conservation of energy principle, but within each world there is perfect agreement with the law. Another common objection is that it removes free will, as all outcomes of a choice will exist in some universe. Yet, many-worlds theory actually has less problem with free will than some other interpretations, and its expression can be thought of as a weighting of the infinite universes.

Related to this is the criticism that many-worlds theory removes probabilities from quantum theory. When the odds of Schrödinger's imaginary cat dying are 50:50, the splitting (or grouping) of worlds seemed straight forward. But when the odds are tilted to 99:1, how many worlds are created? If just two worlds are formed, each relating to one of the possible outcomes, then there is no difference from the 50:50 situation. However, if you regard the worlds created as a grouping or differentiating of the universes, with 99% having one outcome and 1% the other, then it is obvious that probability is retained in the theory.

It has been argued that many-worlds theory violates Ockham's Razor, the basic principle of the simplest explanation usually being the correct one. Opponents claim that by introducing an infinite number of universes many-worlds model introduces far too much "excess metaphysical baggage" into quantum theory. However, the theory actually requires less additions to quantum mechanics than any other interpretation. Depending on the particular version of many-worlds theory, there are either no extra assumptions (it simply uses Schrödinger's wave equation), or there are a very small number of assumptions relating to how the many worlds are created or grouped.

One valid criticism of the many-worlds interpretation is that there is no agreed upon version of the theory. Indeed, it seems that every major proponent of the theory has formed their own, slightly different, take on the basic concept, such as whether the worlds physically split, split without a physical process, or if there is just a grouping or reordering of existing universes. There are spin-off theories, such as the many-minds interpretation, in which it is the mind of the observer that splits, rather than the universe. There is also the modal logic theory of possible worlds, which is derived from philosophical precepts rather than quantum theory. It should be possible, one day, to differentiate between all these theories experimentally, as they produce slightly different predictions. It is often claimed that many-worlds theory is untestable. However, there have been experiments conceived that would help determine between many-worlds and other interpretations of quantum theory, but currently the means are not available.

Local, Deterministic, and Universal

The many-worlds interpretation has a number of advantages over other quantum theories. It is a local theory, in that it does not rely at any point on faster-than-light signaling or cooperation within quantum systems. It therefore retains the theory of relativity, unlike many other interpretations, which introduce non-local effects to explain measurement and cooperation between particles. Many-worlds theory enables the Schrödinger wave equation to retain its smooth, continuous, and deterministic nature by removing the collapse of the wave function. This is in marked contrast to the standard interpretation, in which the world is a very undetermined entity in which particles have no fixed positions until measured (and we have seen the problems associated with measurement).

While the many-worlds theory is unpopular with many physicists—after all, the standard interpretation works incredibly well as long as you do not consider the philosophical consequences—it does have strong support from cosmologists. Quantum theory implies that the entire universe can be described by a wave equation, and therefore treated as a single quantum system. However, if the observer must be outside the system to be viewed, then it would seem that the universe can never be real. The wave function that describes the universe as a whole could never collapse to an actual value. Many-worlds theory also explains why the universe has developed the way it has to allow life to evolve. The seemingly fortunate distribution of matter in the universe, and the fortuitous values of various constants, and the position of the Earth relative to the Sun all appear to conspire to allow life. However, if there are an infinite number of universes our fortunate circumstances are just a subgroup of the many possible worlds, most of which would not have developed life.

Multiple-worlds theory offers a number of subjective advantages over the standard, Copenhagen interpretation of quantum theory. It is at least as viable as any other interpretation of quantum theory, but has the advantage of requiring few, if any, additional assumptions to the mathematics of quantum physics. It also solves the problems of measurement, entanglement, and the suggestion that the mind is somehow outside physics. It restores the determinism of classical physics, while retaining the utility of the Copenhagen interpretation. Although it does suffer from a current inability to be tested, and a bewildering array of competing versions, the advantages are enough to make it a valuable theory until such time as experiments can be performed to test any differences in its prediction.

—DAVID TULLOCH

Viewpoint: No, the "many-worlds" interpretation of quantum mechanics is not viable for numerous reasons, including measurement problems and the inability to test it scientifically.

A standard plot device in science fiction is the existence of alternate realities in which another "version" of the self exists, usually in counterpoint to the "real" self. For example, Joe A is a nice guy who likes kids and animals and goes to church every Sunday. However, in the "other" world, Joe B may look and talk exactly like Joe A and even have the same genetic makeup; but he hates kids, kicks dogs, and only goes to church when he thinks he can pry open the contribution box and steal money. The "many-worlds" interpretation (MWI) takes this idea a step further by having countless Joes, A through Z and beyond, with a new Joe being created every time a measurement (or "decision") is made. As fodder for science fiction stories, it is a great idea. As an explanation of reality, even in the most theoretical realms of quantum physics, it is problematic at best.

At first look, the many-worlds theory seems to answer one of quantum physics' most nagging paradoxes in that it does not require the collapse of the wave function as set forth by the Copenhagen interpretation. Developed by Niels Bohr (1885-1962) and others, it has long been the generally accepted interpretation of quantum mechanics. The Copenhagen interpretation states that an "unobserved" system evolves in a deterministic way according to a wave equation. However, when "observed" the system's wave function "collapses" to a specific observed state (for example, from a diffuse wave not restricted to one location in space and time to a particle that is restricted in space and time and thus observable). The problem arises because this interpretation gives the observer a special status unlike any other object in quantum theory. Furthermore, it cannot define or explain the "observer" through any type of measurement description. As a result, detractors say that the Copenhagen interpretation describes what goes on in the observer's mind rather than a physical event. Some physicists have espoused that the wave function is not real but merely a representation of our knowledge of an object.

The many-worlds interpretation is an unorthodox alternative that promises to model the complete system (both the macro and the micro world) while relegating the observer to a simple measuring device. By leaving out the wave function collapse, the MWI in quantum physics says that a quantum system that has two or more possible outcomes does not collapse into one outcome when observed but branches out or realizes all the outcomes in other universes or worlds. So, whether an atom might decay or not decay, the world "branches" so that both possibilities come to pass. Furthermore, each world and the observers in it are unaware of the existence of the other worlds. Unlike the Copenhagen interpretation, MWI does not depend on our observation of it to create a specific reality but chooses one of two or more possible worlds, all of which are real. Although enticing and interesting, the viability of this model fades upon closer examination. It does not really solve the problems facing many of the theories in quantum physics, which seems to describe microsystems accurately but encounters numerous problems when applied to larger, classical systems like the material world that we taste, touch, see, and smell everyday.

One of the most fundamental problems with MWI is that it violates the law of conservation of energy. This law states that the universe always has the same amount of energy, which can neither be increased nor decreased. Mainstream physicists generally agree that energy cannot be created from nothing or annihilated into nothing. As a result, conservation of energy places significant constraints on what states the world can pass through. But the propagation of ever-expanding multiple worlds violates this law. For example, when a world diverges, where does the extra energy come from to create this divergent world? As energy (in the form of fundamental particles like electrons, protons, neutrons, and photons) is exchanged between these worlds, the amount of energy in them would fluctuate, resulting in one of the worlds having more or less energy. This result contradicts the conservation of energy. In fact, the ever-expanding universe in the many-worlds interpretation would not conserve but dissipate energy. To represent a valid or unified cosmology, the theory would require an unlimited amount of energy or the creation of energy out of nothing.

The Measurement Problem

One of the goals of the MWI was to create a formal mathematical theory that corresponded to reality in a well defined way and solved the measurement problem of quantum mechanics. To account for the wave-like behavior of matter, quantum mechanics has incorporated two primary laws. The first uses a linear wave equation to account for time trajectory of all unobserved systems and their wave-like properties. The second is the standard collapse theory that accounts for the deterministic qualities we see in systems when we observe them. The two laws, however, are counterintuitive and, to a certain extent, mutually exclusive in that one describes a system when we are not looking and the other when we are. Although dropping the standard collapse theory solves the ambiguity and, some might say, logical inconsistencies in these two laws, it is the collapse theory that ensures we end up with determinate measurements. Without the theory, there is no plausible explanation for these measurements or our perception of reality.

Since the theory claims to represent our reality as well as other classical approaches in physics and mathematics, then it should be rigorously defined in the same manner. In an article in the International Journal of Modern Physics, A. Kent puts it this way, "For any MWI worth the attention of physicists must surely be a physical theory reducible to a few definite laws, not a philosophical position irreducibly described by several pages of prose." Such a theory, says Kent, must include "mathematical axioms defining the formalism and physical axioms explaining what elements of the formalism correspond to aspects of reality." These criteria are concretely defined by other theories, including the theories of general relativity and electrodynamics. According to Kent and many others, the many-worlds model fails to produce such axioms for various reasons, primarily the fact that the axioms have an extremely complicated notion of a measuring device.

To work, the MWI says that we must choose a preferred basis to explain the determinate measurement. Rudimentarily, the preferred basis refers to the fact there are always many ways one might write the quantum-mechanical state of a system as the sum of vectors in the Hilbert space (for purpose of simplicity, defined as a complete metric space). Choosing a preferred basis means choosing a single set of vectors that can be used to represent a state. However, what basis would make the observers' experiences and beliefs determinate in every world? The correct preferred basis in the MWI would depend on numerous factors, including our physiological and psychological makeup. Taking this a step further, a detailed theory of consciousness would have to be developed to validate the MWI. To put it another way, it would be necessary to classify all known measuring devices to make MWI axioms consistent with the known outcomes of experimental results. Since most theories can describe physical phenomena in precise and relatively simple mathematics, the need for such an extensive classification precludes MWI axioms from being accepted as fundamental physical laws.

MWI also raises questions about any meaning for statistical significance or "weight" since statistical predictions are subjective experiences made by an observer. In fact, the theory has no validity in making any type of statistical predictions for the standard collapse theory. For example, say the probability of an electron spinning up or down is one-half after passing through the Stern-Gerlach device. But what is the significance of "one-half" if the world splits into another world, resulting in the electron spinning both up and down? In the MWI the same branching would occur whatever the odds—50:50, 60:40, or 70:30; all would result in the electron spinning both up and down. Furthermore, the standard predictions of quantum mechanics concerning probabilities are made irrelevant because it is unknown which of two future observers is making the measurement. As a result, it means nothing to say that an experience will be "this" rather than "that" when "that" is happening in another world or perhaps in our world. Who can tell? If the MWI is valid, nothing occurs in our world according to probabilistic laws.

To Be or Not to Be?

In many ways, the MWI directly invalidates the foundation of our conscious interpretations and interactions with the world. It does this by insisting on a deterministic reality in a microphysical world that leads to branching of various "macro" realities that all exist but are not accessible. As a result, our consciousness and experiences are meaningless in terms of having a functional role. There is no free will because ultimately there are no important questions to ask since everything that can happen will happen. If the theory itself is designed so that our minds have no effect on the outcome of an event, why then does such a theory grapple with the question of mind? It also does not address the notion that peoples' personal experiences are unified in that we perceive our experiences as a whole following only one path and, in turn, never "experience" the branching worlds. As a result the MWI is more of a metaphysical philosophy than a science. It presents us with a view of nature that is vague, complex, and schizophrenic.

Another fundamental argument against the many-worlds model is that it violates Ockham's Razor, which is science's way of saying "keep it simple stupid." Developed in the fourteenth century by English philosopher William of Ockham (c. 1300-1349), the maxim states that hypotheses or entities should not be multiplied beyond necessity. It proposes that scientists should favor the simplest possible explanation as opposed to more complex ones. It is hard to imagine any theory more complex than one that relies on the existence of numerous worlds that are unseen and unknowable to explain what we do see in this world. In other words, if we want to explain our world and our experiences, then we should do so via the world or worlds we know. So far, there is only one world we know of, and that is the one we live in. If it is to be considered viable, quantum physics must rely on a quantum conception of reality based on the empirical data available to us and that fits a formula of physical theory. This is the only way that we would have a true "place" in a reality that we could both know and act upon in a way that meant anything.

Inherent in the phrase "scientific theory" is the notion that such a theory can ultimately be proven correct or incorrect through experiments. In the case of the many-worlds model, since the inhabitants of these many worlds are not aware of each other, to verify the theory experimentally is impossible. Does that mean we should accept the model on faith? This sounds more like religion than science. Even if it were a logical deduction, which it is not, it would still need to be proven.

Proponents of the many-worlds model say that this is the only solution that "fits" so it must be true because no one has come up with a better alternative. For science, this is as close to blasphemy as you can get. It is not that no alternatives exist—it is just that we have not found them yet in terms of theories, axioms, and mathematics. It is like the story about the nineteenth-century U.S. patent office director who suggested that the patent office be closed because everything that could be discovered had already been discovered. There are other alternatives to the many-worlds model, and time will surely reveal them.

—DAVID PETECHUK

KEY TERMS

AXIOM:

Something that is accepted as true for the basis of an argument or theory.

OCKHAM'S RAZOR:

A principle proposed by William of Ockham (c.1300-1349), who argued that the best answer was almost always the one with the fewest assumptions. It has become a guiding principle of scientific theory.

SCHRÖDINGER EQUATION:

The quantum mechanical equation that describes the evolution of a physical system in time.

STERN-GERLACH DEVICE:

An apparatus used by O. Stern and W. Gerlach in their experiment to measure the intrinsic spin angular momentum of silver atoms. They found that it takes only two discrete values, commonly called "spin up" and "spin down."

THOUGHT EXPERIMENT:

An experiment that cannot, or should not, be performed (at least at the time proposed) in the real world, but can be imagined, and the consequences calculated. This technique was made famous by Albert Einstein, who used a number of thought experiments in his theories of relativity, and in his debates over the nature of reality and physics with other scientists.

WAVE FUNCTION:

In quantum mechanics the mathematical function that describes the state of a physical system, sometimes also called the wave vector, as it can be represented as a vector in an abstract, many dimensional space.

Science in Dispute