What does quantum theory actually say about reality? Quantum Theory of Society Other books on similar topics.
Are our efforts to describe reality nothing more than a game of dice trying to predict the desired outcome? James Owen Weatherall, Professor of Logic and Philosophy of Science at the University of Irvine, reflected on the riddles in Nautil.us quantum physics, the problem of the quantum state and how it depends on our actions, knowledge and subjective perception of reality, and why, when predicting different probabilities, we all turn out to be right.
Physicists are well aware of how to apply quantum theory - your phone and computer are proof of that. But knowing how to use something is far from fully understanding the world described by the theory, or even what the various mathematical tools that scientists use mean. One such mathematical tool, the status of which physicists have been arguing for a long time, is the "quantum state" ⓘ A quantum state is any possible state that a quantum system can be in. In this case, the "quantum state" should also be understood as all the potential probabilities of falling out of one or another value when playing "dice". — Approx. ed..
One of the most striking features of quantum theory is that its predictions are probabilistic. If you are doing an experiment in a lab and using quantum theory to predict the outcome of various measurements, at best the theory can only predict the likelihood of the outcome: for example, 50% for predicting the outcome and 50% for it being different. The role of the quantum state is to determine the probability of outcomes. If the quantum state is known, you can calculate the probability of getting any possible outcome for any possible experiment.
Does the quantum state represent an objective aspect of reality, or is it just a way of characterizing us, that is, what a person knows about reality? This question was actively discussed at the very beginning of the study of quantum theory and has recently become topical again, inspiring new theoretical calculations and subsequent experimental verifications.
“If you change only your knowledge, things will no longer seem strange.”
To understand why a quantum state illustrates someone's knowledge, imagine a case in which you are calculating a probability. Before your friend rolls the dice, you guess which side they will land on. If your friend rolls a regular six-sided die, the probability that your guess will be correct will be approximately 17% (one sixth), no matter what you guess. In this case, the probability says something about you, namely, what you know about the die. Let's say you turn your back while throwing, and your friend sees the result - let it be six, but you do not know this result. And until you turn around, the outcome of the roll remains uncertain, even though your friend knows it. Probability representing human uncertainty, even if reality is certain, is called epistemic, from the Greek word for "knowledge".
This means that you and your friend could determine different probabilities, and neither of you would be wrong. You will say that the probability of rolling a six on a die is 17%, and your friend, who already knows the result, will call it 100%. This is because you and your friend know different things, and the probabilities you named represent varying degrees your knowledge. The only incorrect prediction would be one that rules out the possibility of a six coming up at all.
For the past fifteen years, physicists have wondered whether a quantum state could be epistemic in the same way. Suppose some state of matter, such as the distribution of particles in space or the outcome of a game of dice, is certain, but you don't know. The quantum state, according to this approach, is just a way of describing the incompleteness of your knowledge about the structure of the world. In different physical situations, there may be several ways to define a quantum state, depending on the known information.
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It is tempting to think of a quantum state in this way because it becomes different when the parameters of a physical system are measured. Making measurements changes this state from one where each possible outcome has a non-zero probability to one where only one outcome is possible. This is similar to what happens in a game of dice when you know the result. It may seem strange that the world can change just because you take measurements. But if it's just a change in your knowledge, it's no longer surprising.
Another reason to consider a quantum state to be epistemic is that it is impossible to determine what the quantum state was like before it was carried out using a single experiment. It also resembles a game of dice. Let's say your friend offers to play and claims that the probability of rolling a six is only 10%, while you insist on 17%. Can one single experiment show which one of you is right? No. The fact is that the resulting result is comparable to both probability estimates. There is no way to know which of the two of you is right in any particular case. According to the epistemic approach to quantum theory, the reason why most quantum states cannot be determined experimentally is like a game of dice: for every physical situation there are several probabilities consistent with the multiplicity of quantum states.
Rob Speckens, a physicist at the Institute for Theoretical Physics (Waterloo, Ontario), published in 2007 scientific work, where he presented a "toy theory" designed to mimic quantum theory. This theory is not exactly analogous to quantum theory, as it is simplified to an extremely simple system. The system has only two options for each of its parameters: for example, "red" and "blue" for color, and "top" and "bottom" for position in space. But, as with quantum theory, it included states that could be used to calculate probabilities. And the predictions made with its help coincide with the predictions of quantum theory.
Speckens' "toy theory" was exciting because, as in quantum theory, its states were "undefinable" - and this uncertainty was entirely due to the fact that the epistemic theory does indeed relate to real physical situations. In other words, the "toy theory" was similar to the quantum one, and its states were uniquely epistemic. Since in the case of rejecting the epistemic view, the uncertainty of quantum states does not have a clear explanation, Speckens and his colleagues considered this a sufficient reason to consider quantum states also epistemic, but in this case the "toy theory" should be extended to more complex systems ( i.e. physical systems explained by quantum theory). Since then, it has led to a number of studies in which some physicists tried to explain all quantum phenomena with its help, while others tried to show its fallacy.
"These assumptions are consistent, but that doesn't mean they're true."
Thus, opponents of the theory raise their hands higher. For example, one widely discussed 2012 result published in Nature Physics showed that if one physics experiment can be done independently of another, then there can be no uncertainty about the "correct" quantum state describing that experiment. That. all quantum states are "correct" and "correct", except for those that are completely "unreal", namely: "incorrect" are states like those when the probability of rolling a six is zero.
Another study published in Physical Review Letters in 2014 by Joanna Barrett and others showed that the Speckens model cannot be applied to a system in which each parameter has three or more degrees of freedom—for example, red, blue, and green for colors, and not just "red" and "blue" - without violating the predictions of quantum theory. Proponents of the epistemic approach propose experiments that could show the difference between the predictions of quantum theory and the predictions made by any epistemic approach. Thus, all the experiments carried out within the framework of the epistemic approach could be consistent to some extent with the standard quantum theory. In this regard, it is impossible to interpret all quantum states as epistemic, since there are more quantum states, and epistemic theories cover only a part of quantum theory, because they give results different from those of the quantum one.
Do these results rule out the idea that a quantum state indicates characteristics of our mind? Yes and no. The arguments against the epistemic approach are mathematical theorems proven by a special structure applied to physical theories. Developed by Speckens as a way of explaining the epistemic approach, this framework contains several fundamental assumptions. One of them is that the world is always in the objective physical condition, independent of our knowledge of it, which may or may not coincide with the quantum state. Another is that physical theories make predictions that can be represented using standard probability theory. These assumptions are consistent, but this does not mean that they are correct. The results show that in such a system there cannot be results that are epistemic in the same sense as Speckens' "toy theory" as long as it is consistent with quantum theory.
Whether you can put an end to this depends on your view of the system. Here opinions differ.
For example, Owee Maroni, a physicist and philosopher at the University of Oxford and one of the authors of a paper published in 2014 in Physical Review Letters, said in an email that "the most plausible psi-epistemic models" (i.e. those that can be fitted to the system Speckens) are excluded. Also, Matt Leifer, a physicist at the University of Champagne who has written many papers on the epistemic approach to quantum states, said that the issue was closed back in 2012 - if you, of course, agree to accept the independence of the initial states (which Leifer tends to).
Speckens is more vigilant. He agrees that these results severely limit the application of the epistemic approach to quantum states. But he emphasizes that these results are obtained within his system, and as the creator of the system, he points out its limitations, such as assumptions about probability. Thus, the epistemic approach to quantum states remains relevant, but if so, then we need to reconsider the basic assumptions of physical theories, which many physicists accept without question.
Nevertheless, it is clear that significant progress has been made in fundamental questions of quantum theory. Many physicists tend to call the question of the meaning of a quantum state merely interpretive, or worse, philosophical, as long as they don't have to develop a new particle accelerator or improve a laser. Calling the problem "philosophical", we seem to take it out of the redistribution of mathematics and experimental physics.
But work on the epistemic approach shows the illegitimacy of this. Speckens and his colleagues took the interpretation of quantum states and turned it into an exact hypothesis, which was then filled with mathematical and experimental results. This does not mean that the epistemic approach itself (without mathematics and experiments) is dead, it means that its advocates need to put forward new hypotheses. And this is an undeniable progress - for both scientists and philosophers.
James Owen Weatherall is Professor of Logic and Philosophy of Science at the University of Irvine, California. His latest book, Strange Physics of the Void, examines the history of the study of the structure of empty space in physics from the 17th century to the present day.
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The demonstration that turned the great Isaac Newton's ideas about the nature of light upside down was incredibly simple. It "can be repeated with great ease, wherever the sun shines," said English physicist Thomas Young in November 1803 to the Fellows of the Royal Society of London, describing what is now called the double slit experiment. And Yang was not an enthusiastic youth. He came up with an elegant and elaborate, demonstrating the wave nature of light, and thereby disproved Newton's theory that light consists of corpuscles, that is, particles.
Quantum theory much more complicated than this visualization.
But the birth of quantum physics in the early 1900s made it clear that light is made up of tiny, indivisible units—or quanta—of energy that we call photons. Whether done with single photons or even single particles of matter such as electrons and neurons, Young's experiment is a puzzle that raises questions about the very nature of reality. Some have even used it to claim that the quantum world is influenced by human consciousness. But can a simple experiment really demonstrate this?
Can consciousness determine reality?
In modern quantum form, Young's experiment involves firing individual particles of light or matter through two slits or holes cut into an opaque barrier. On one side of the barrier is a screen that records the arrival of the particles (say, a photographic plate in the case of photons). Common sense leads us to expect that photons will pass through either one or the other slit and accumulate behind the corresponding passage.