Physical reality gets personal
In this book, the authors have suggested alternative interpretations of quantum reality contrasting the well-known Copenhagen interpretation for the results of quantum physical measurements in the double-slit experiment. This experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles (wave-particle duality). In addition, it displays the fundamentally probabilistic nature of quantum physical phenomena.
The following brief summary could help us understand how QBism described in this book interprets quantum reality: Physical reality we experience in this universe is due to operation of the laws of physics on matter (and energy) in a four dimensional spacetime. The fabric of reality is made of space and time fused together to form a unified spacetime in four dimensions, and speed of light is a part of this fabric. In the basic energy/force equations of classical physics, relativistic physics and quantum physics, time does not appear. Hence laws of physics allows physical reality to operate from past to future or future to past. In addition, our perception of the reality, of what is “now” (present tense) is not absolute, but only relative. Space and time looks differently for different observers in the universe, since the speed of light is a part of the fabric of reality. This speed is a universal constant for all observers, and matter/energy cannot travel faster than the speed of light (according special relativity.) As a result, for example, for inhabitants of a planet 4.5 billion light years away from us, solar system doesn’t even exist, let alone see human beings. Because light takes 4.5 billion years to reach them from us and hence in their “reality” we don’t exist (Solar system is exists for the last 4.5 billion years.) The speed of light is packaged into the fabric of reality, and this speed limit constrains only the motion of matter/energy, and not to the motion of space. Space can expand much faster than speed of light, and it has done that at about 13.7 billion years ago. Space or spacetime is also not a continuous fabric as we understand physical reality, but it also curves, bends, rips, repairs the rips and connects distant parts of the universe through bridges (cosmic worm holes).
Physics thus far helps in describing the objective external world: fundamental particles, waves, fields, atoms, molecules, biomolecules, and biological systems. How waves/particles change in time; and how they give rise to the different forms of matter, the life-forms, machines, planets, stars and galaxies make the universe. This world makes itself known to each of us through our own perceptions. Yet physics has ignored one other critical factor of reality, the consciousness. Classical and relativistic physics say that reality exists as they appear to us and we are mere observers of its presence. But the laws quantum physics that operate at the microscopic scales on matter and energy, like electrons and photons, deny objective reality, determinism, free will or our understanding that cause must precede an effect. Quantum properties like entanglement, uncertainty principle, quantum gravity (bending of spacetime in presence of matter), vacuum energy, quantum bubbles, black holes, spacetime ripples (also called gravitational waves), etc. deny common sense arguments since spacetime at the most fundamental level is not clearly understood. Hence this has given rise to a wide range of conjectures that suggest that reality could be an illusion or the universe is a two dimensional hologram that contains information for our existence in three dimensional space. Some physicists also suggest that space is real and time is an illusion; others say, time is an illusion and space is real. We have taken for granted for too long that space and time is a non-interactive fabric of reality. But this is turning out be a major error. In fact spacetime may be discontinuous and exists in discrete quanta or units and how this works at subatomic scales is still unclear.
According to wave mechanics, the quantum reality of a physical system is described in terms of a wave function. Matter in most fundamental form, like an electron, exists in both wave and particle forms (wave-particle duality). The shape of the wave function derived from wave mechanics describes wave’s existence in terms of probabilities. It could exist anywhere in the universe with a given (calculated) probability. When physical measurements are made on this system by an observer, this “act” would “collapse the wave function” to a physically identifiable particle form. This explanation is referred to as Copenhagen interpretation of quantum reality. There are other interpretations to account for this result such as many world’s interpretation and Bohmian interpretation, etc. All of them have something in common. They treat the wave function as a description of an objective reality shared by multiple observers. QBism, on the other hand, treats the wave function as a description of a single observer’s subjective knowledge. Q is for quantum and B is for Bayesian; a view of probability that includes an agent who makes bets and updates odds. In this new interpretation of quantum reality, the wave function does not describe the world, but it describes the observer. The wave function’s probabilities are described as Bayesian probabilities, which is nothing but the subjective degrees of “belief” about a system. It is like gambling by placing bets on measurement outcomes, and bets are updated as new data emerges. According to QBism, the wave function’s “collapse” is simply the observer updating the personal beliefs after making a measurement. And quantum entanglement (two particles entangled quantum physically), which is another strange property of quantum physics is when one observer’s measurement of a particle at position 1 collapses the wave function of its entangled particle somewhere else in the universe at position 2. QBism says that the measurement at position 1 simply provides information for the observer 1, who can use to bet on the state of the distant entangled particle at position 2. But how does his/her measurement at one place affect the outcome of a measurement of a second observer at position 2? In fact, it doesn’t. Since QBism wave function doesn’t belong to the physical system being observed, but to the observer himself. Hence, observer 1’s wave function doesn’t have to align with observer 2. This argument circumvents several shortcomings of Copenhagen interpretation of quantum reality. But fails to answer numerous bigger questions that confronts reality, and possible existence of multiverse.
Physicist Niels Bohr once said that the purpose of science was not to reveal “the real essence of the phenomena” but only to find “relations between the manifold aspects of our experience”. His idea still holds pretty well. QBism is a short-cut to understand the quantum phenomenon that is played out on a larger screen, but we are taking a narrow look at one end of the spectrum. I am not too enthusiastic about this interpretation of our physical reality. But nevertheless, it is an interesting alternative idea which I recommend to readers interested in quantum physics and physical reality.
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