4. The Nature and Meaning of Information in Quantum Physics
4.1 Wave Function and Probability Waves
In quantum physics, equations have been developed that describe the outcomes of experiments with great accuracy; however, physicists increasingly admit that they do not understand how to interpret or conceptualize the terms in the equations (Greene, 2004; Greenstein & Zajonc, 2006; Schlosshauer, 2007). For several decades, physicists focused on applying the equations, while generally ignoring questions about interpreting the equations. This resulted in the development of invaluable quantum-based technologies, including transistors and modern electronics. However, in recent years there has been increasing interest in attempting to understand the nature of reality indicated by these equations. This understanding may be important for developing new technologies such as quantum computing.
The primary equation of quantum physics is in the form of waves that include terms for every potential or possible outcome of an experiment or observation. However, there is intrinsic variability and uncertainty on the quantum level and the waves indicate only the probability that a given outcome will occur. The equations do not deterministically specify which outcome will actually be found. The actual outcome that manifests appears to be random. The waves are described as probability waves,and the equation is called the wave function. There is no known medium or substance for the waves.
Taken at face value, the wave function indicates that the most realistic description of the state of a particle prior to observation is a combination of all the potential outcomes for the observation. Numerous experiments support this interpretation (Greenstein & Zajonc, 2006; Schlosshauer, 2007). The most well known is the double slit experiment, which indicates that an unobserved individual particle sent toward two slits in a screen responds to both slits. The particle behaves as if it were a wave that is spread over space and that passes through both slits, rather than as a discrete particle passing through only one of the slits. The experimental results displayinterference patterns that are exactly in accordance with the wave function. The combination of possible or potential outcomes in a wave function is called asuperposition.
4.2 Entanglement
One of the most perplexing features of quantum physics is that particles can become entangled in a way that is nonlocal (Greenstein & Zajonc, 2006; Schlosshauer, 2007). Two particles become entangled when the wave functions have interaction terms that make the state of one particle related to the state of the other particle. The two particles must be considered as a unitary system. A particle that is not entangled can be completely described with a wave function that does not include terms referring to another particle. The entanglement is nonlocal because the two particles may become widely separated in space, but somehow remain connected. The outcome of a measurement for one particle cannot be predicted, but a measurement of the other particle will always find the expected relationship. A measurement appears to apply to the entangled particles as a unit. Nonlocal entanglement has been verified empirically. The randomness of the outcome that is found with a measurement means entanglement cannot be used to directly transmit useful information between different locations. Entanglement can also occur between a particle and a larger system or the environment.
4.3 Potential Outcomes and Imagination
The terms in the quantum wave function symbolize potential outcomes similar to the human imagination of potential future events. Both involve symbols of potential conditions rather than symbols of existing tangible reality. In both cases, the manifestation of one of the potential outcomes can be viewed as information creation. However, the concepts of media and interpretational infrastructure are clearly applicable for human imagination, but are of doubtful applicability for quantum processes.
4.4 Quantum Physics and Measurement
The Measurement Problem
In quantum physics, a measurement not only obtains information about the state of a system, but also has an active role in forming the state that is found. The system can be in a superposition of possible outcomes prior to measurement. The act of measurement or observation transforms the state of the system from the superposition to a single outcome state consistent with classical physics.
The basic wave function of quantum physics offers no insight into how and when the superposition of probability waves get transformed into the one outcome that becomes manifest (Greenstein & Zajonc, 2006). This is known as the measurement problem and is subsumed by the newer term quantum-to-classical transition. The wave function predicts that when a particle interacts with a measurement apparatus the particle and apparatus may become an entangled superposition. The wave function does not predict a transformation into one outcome. At present there is not a scientific consensus for conceptualizing the probability waves or for understanding how observed physical reality emerges from them.
Several ideas have been proposed for addressing this measurement problem, but none have convincing support. The key concepts of the theories that have received the most attention are briefly summarized below. The historical development and numerous refinements and criticisms of these theories are beyond the scope of the present discussion. Similarly, other lesser-known theories are not discussed.
One notable philosophical difference among the proposed interpretations is the role of mathematical equations. Some physicists view the equations of quantum physics, and perhaps physics in general, as abstract models that can be used to make predictions, but that should not be associated with concepts about mechanisms or the nature of reality. On the other hand, others view the concepts about mechanism and the nature of reality as import in working with the equations, and particularly in developing increased scientific understanding.
Orthodox or Standard Interpretation
The orthodox or standard interpretation presented in most past textbooks on quantum physics postulates that the act of measurement causes a discontinuouscollapse or reduction of the wave function from a state of superposition to one observed outcome (Schlosshauer, 2007, pp. 330-334). There is no explanation of the nature of the collapse or the act of measurement. This interpretation generally takes the position that the equations are useful only for making predictions and that it is not appropriate to try to conceptualize the properties of quantum phenomena prior to measurement. It is sometimes referred to as a “shut-up-and-calculate” approach (Schlosshauer, 2007, pp. 329).
Copenhagen Interpretation
The closely related Copenhagen interpretation adds the postulate that the wave function collapse occurs when a quantum system interacts with a macroscopic measurement apparatus (Schlosshauer, 2007, pp. 335-336). In this dualistic worldview, the realm of classical physics does not emerge from the quantum level, rather the macroscopic realm is the primary reality and the quantum level is secondary. This interpretation implicitly focuses on measurements or observations by humans and treats other situations as not knowable.
External Observer Interpretation
The external observer interpretation proposes that the wave function collapse occurs when a measurement result comes into the consciousness or mind of an observer (Schlosshauer, 2007, pp. 359-365). This interpretation derives from the fact that an observer finds a specific outcome, but the wave function does not describe or predict a collapse to a single state. The transition from a quantum superposition to a discrete classical state is placed at the last step in the process of measurement and observation. This dualistic interpretation distinguishes consciousness from physical matter and has a long and varied history. Some authors argue that it is implied in the orthodox and Copenhagen interpretations.
Many Worlds Interpretation
The many-worlds interpretation proposes that with each measurement interaction, the world splits into separate, parallel, non-interacting worlds with each new world having one of the possible measurement outcomes (DeWitt & Graham, 1973; Schlosshauer, 2007, pp. 336-344). Observers happen to find themselves in a particular world, and are not aware that there are other worlds with different outcomes and counterparts of themselves. This interpretation assumes each possible outcome in the wave function fully represents a parallel reality. This interpretation does not require unexplained collapses or observers that are not part of the wave function, but it does require a continuous, infinite splitting of the world. Many-mindsinterpretations apply the splitting to the consciousness of observers rather than to the physical world.
Bohmian Mechanics Model
Bohmian mechanics is a model developed by David Bohm that proposes that quantum effects are produced by a field or wave that guides discrete particles (Bohm & Hiley, 1993; Schlosshauer, 2007, pp. 354-357). The quantum field consists of “information” rather than energy, and manifests through a quantum potential that includes nonlocal connections and depends on the entire system and environment in a unitary manner. The model assumes that “a particle has a rich and complex inner structure which can respond to information and direct its self-motion accordingly” (Bohm & Hiley, 1993, p. 39). The wave function is irreversibly reduced when the location of a particle is registered on a macroscopic or classical level, such as with an experimental apparatus. This model gives results identical to traditional quantum physics in most situations, and the cases with predicted differences cannot yet be empirically tested. Bohmian mechanics has received relatively little attention—perhaps because it cannot be empirically distinguished from other interpretations and the practical value of the additional complexity is questionable.
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