Is it time to end the “no quantum world”?
This is a special issue of the newsletter. It is about the meaning of quantum mechanics, my favorite subject, which touches deeply on Particle Physics. It seems to me that a broader audience is interested in these foundational issues of our world and its reality. Also, New Year festivities are coming and it seems appropriate to celebrate the beginning of our new ride around the Sun with something special. So without further ado, let's begin.
When we reflect today, it is tempting to look at the question of the interpretation of quantum mechanics as a prime example of what happens when you don't speak truth to power. When the majority of physicists reject critical thinking and bend to power. Thanks to a few brave souls who kept asking the question despite risking their academic careers and keeping the question very much alive, we have today a rich set of many interpretations of quantum mechanics. There is still no consensus among physicists on which interpretation of quantum mechanics best corresponds to physical reality. With rapidly emerging quantum computing technology that harnesses the laws of quantum mechanics to solve problems too complex for classical computers, is it time to end the “no quantum world” and demand the true meaning of quantum mechanics? We now have quantum computers, admittedly in their infancy, and we still do not have a mental picture of their inner workings. This is not a question of beautiful math behind quantum mechanics. The math of quantum mechanics is not questionable. This is the question of how that math might correspond to the physical reality we experience since we want to believe that physical reality is capable of being known even at the quantum level.
The following words [1] are attributed to Niels Bohr, one of the greatest thinkers in quantum physics who made foundational contributions to understanding atomic structure and quantum theory: “There is no quantum world, there is only an abstract quantum physical description. It is wrong to think the task of physics is to find out how nature is. Physics concerns what we can say about nature.”
By these words, Bohr ripped off the heart of physics. The task of physics is exactly to find out how nature is in addition to what we can say about nature.
There is no quantum world
To defend this extreme position, Bohr introduced the principle of complementarity into quantum physics to 'prevent' any attempt to interpret quantum physics as a description of anything. The principle holds that objects have certain pairs of complementary properties which cannot all be observed or measured simultaneously [2]. In his essay, “Three views concerning human knowledge” [3], Popper said that the principle of complementarity remained completely sterile within physics. Although complementarity is mathematically expressed by non-commuting operators that represent the observable quantities being measured, even today, almost 70 years since Popper’s essay, the principle seems an unnecessary addition to quantum mechanics.
This extreme line of thought eventually evolved into a full-blown Copenhagen interpretation of quantum mechanics, in which, as Heisenberg said, “The idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them is impossible. The transition from the ‘possible’ to ‘actual’ takes place during the act of observation.”
The act of observation is essentially the interaction of the measuring apparatus with “the quantum stuff”. So if there is no quantum world, what is interacting with the measuring apparatus when we probe “the quantum stuff”? As John Bell said [6], his instructors were never clear “whether it [the wave function] was something real or some kind of bookkeeping operation.” And if the wave function was just a bookkeeping device, just information, then whose information was it? If there is no quantum world, then what was that information about?
“Something” definitely interacting, so it is quite plausible to assume that we have a physical reality even at the quantum level.
There is no need for these hypotheses to be true
We can argue that difficulties in the interpretation of quantum physics were a reason for such a radical dismissal of reality at the quantum level, and overwhelmingly, the physics community, thanks to Bohr and other founding fathers of quantum physics, moved toward the mastery of the mathematical formalism for practical applications. As it turned out, to be able to calculate probabilities of experimental outcomes successfully, you really didn't need to understand that reality, essentially, just playing with math symbols was enough. So the majority of the physics community easily rejected the reality at the quantum level, despite the fact that “something” definitely interacts with the measuring apparatus when we probe “the quantum stuff”. There was no need to establish a correspondence between the mathematical apparatus of quantum mechanics and physical reality. In other words, no need to interpret quantum mechanics. We can draw here an unfortunate similarity with [3] Andreas Osiander who had said in his preface to Copernicus’s De revolutionibus: “There is no need for these hypotheses to be true, or even to be at all like the truth; rather, one thing is sufficient for them – that they should yield calculations which agree with the observations.”
Today we have many interpretations of quantum mechanics, but the question of the interpretation of quantum mechanics is still ignored in the physics community since the mainstream belief is, that Bohr with his Copenhagen interpretation of quantum mechanics, settled that question a long time ago.
This question is far from being settled.
Einstein’s point of view
There is a popular view that Einstein has been ‘beaten’ by Bohr in their dialogue about quantum theory. Contrary to this view, Einstein offered [1] a carefully crafted critique of quantum theory “that has never been satisfactorily answered, or in some ways even addressed, by the quantum establishment, and which, moreover, endures today.” A consistent theme in Einstein’s point of view is the incompleteness of the theoretical description provided by the wave function. For example, Einstein considers the case of radioactive decay in which an alpha particle is emitted from an atom localized for all practical purposes at a point. Mathematically, this system may be modeled by a closed potential barrier which at t=0 encloses the alpha particle. As time passes, the wave function, initially finite only inside the potential barrier, leaks into the surrounding space. The wave function gives the probability that at some instant in time, the alpha particle is found in a certain part of the external space. The problem is that the wave may take many centuries to expand into outer space while the alpha particle may be found there after only a relatively short time. Clearly, the wave function does not say anything concerning the time instant of disintegration of the radioactive atom. In other words, the wave function does not describe the actual individual event revealed by the detector, including also the cause of the event. If it is reasonable to assume that the individual atom really does have a definite moment of decay, then, as we can see, the wave function does not provide a complete description of the individual event. It must be considered incomplete.
Copenhagen
It appears that Einstein understood the wave function as an objective description of a physical system. In the Copenhagen interpretation of quantum mechanics, the wave function is not regarded as an objective description of a physical system. The wave (state) function is understood as a mathematical entity that enables one to make the best statistical predictions that it is possible to make. As per Copenhagen's interpretation, all statements about quantum phenomena are meaningless unless accompanied by a complete description of a classical experimental arrangement which serves as a reference frame to derive respective statements. In other words, we can make statements about the quantum phenomena only “through the classical world” which amounts to a complete description of the respective classical experimental apparatus. Therefore, as Everett noticed [4], the deduction of classical phenomena from quantum theory is impossible in this interpretation because we can’t make meaningful statements without preexisting classical apparatus. The interpretation is built around the principle of complementarity so the totality of quantum phenomena can only be understood by the use of mutually exclusive, complementary, experimental arrangements. An objective description is granted at the classical level, but it is completely forbidden at the quantum level [4]. Thus it’s not possible to know what is going on at the quantum level without a reference to the respective classical experimental apparatus. Essentially, reality at the quantum level does not exist until it is measured. Therefore, this interpretation denies any possibility to interpret the quantum world as a description of anything and favors a “safe” theory that will never be open to contradiction. It is not clear how such a safe theory can prompt the discovery of totally new phenomena.
In addition, there is the question of the consistency of the scheme on which probability calculations are based by which the state function can change as pointed out by Everett in his dissertation [4].
An inconsistency of the ways the state function can change
?In the opening lines of his dissertation [4], Everett states that the state function is thought of as objectively characterizing the physical system. At all times an isolated system is thought of as possessing a state function, independently of our state of knowledge of it. Thus at all times, a physical system is described completely by a state function, which gives information only concerning the probabilities of the results of various observations which can be made on the system. There are two opposing ways in which the state function can change. The discontinuous change is brought about by the observation of an observable with certain eigenstates in which the state function will change to one of these eigenstates with a definite probability (“wave function collapse”). Continuous change in the sense that the state function changes in a casual, deterministic, continuous manner with time as long as the system remains isolated, obeying a differential (Schrodinger) equation. As Everett pointed out the question of the consistency of the above scheme arises if we allow for the existence of more than one observer. We consider the case of one observer A, who is performing measurements upon an object system S, the totality A+S indeed forming the object system for another observer B. Denying the possibility of B’s use of a quantum mechanical description for A+S does not seem right. First, in that case, we would need some alternative description for systems that contain observers or measuring apparatuses. Second, we would have to have a criterion for telling precisely what systems would be considered observers or measuring apparatuses. On the other hand, if we allow B to use a quantum mechanical description for A+S, by assigning a state function to A+S, then, so long as B does not interact with A+S, this state function changes in a casual, deterministic, continuous manner with time, obeying Schrodinger equation, even though A may be performing measurements upon S. So, if A performs measurements upon S, nothing of this discontinuous change can B see, so long as B does not interact with A+S. B only sees a continuous change of the state function that is assigned to A+S and obviously can question the “discontinuous” change that A sees when performing measurements upon S. A conflicting situation arises, either A is incorrect in assuming discontinuous change with its probabilistic implications to apply to his measurements, or B’s state function with it’s purely casual, a deterministic character is an inadequate description of what is happening to A+S. Obviously, this conflicting situation is just a reflection of two opposing ways in which the state function can change.
Is it time to end the “no quantum world”?
Bohr and his associates argued that there is no quantum world, therefore, essentially nothing to interpret, nothing in the quantum mechanical mathematical apparatus to correspond to the physical reality at the quantum level, reducing quantum physics to a tool for calculating probabilities of experimental outcomes. Philosopher Imre Lakatos said [5]: “...Bohr and his associates introduced a new and unprecedented lowering of critical standards for scientific theories. This led to a defeat of reason within modern physics and to an anarchist cult of incomprehensible chaos.” Only a few, like John Bell, David Bohm, and Hugh Everett persisted in seeking the true meaning of quantum mechanics.
Ever since reading David Bohm’s papers in 1952, John Bell [6] knew there was something wrong with von Neumann’s famous proof that purportedly ruled out any interpretation of quantum physics that used so-called hidden variables. Any interpretation of quantum physics that uses so-called hidden variables agrees with Einstein’s view that the wave function does not provide a complete description of the individual event. We arrive at a complete view of the individual event by supplementing the wave function with the hidden variables. Definite locations or other properties are assigned to quantum objects before they are observed, even if those properties can’t be calculated using the wave function. These properties go unseen in the mathematics of quantum physics, hence “hidden” variables.
David Bohm’s papers in 1952 showed [6] that he discovered a totally new way to interpret quantum physics. Contrary to the Copenhagen interpretation and von Neumann’s famous proof, Bohm’s interpretation depicted a world of subatomic particles with definite positions at all times that existed whether or not anyone was looking at them. Each of these particles had “pilot-waves” that determined their motion and the waves behaved in an orderly and predictable fashion. Bohm’s theory was mathematically equivalent to ordinary quantum physics.
Bell disassembled [6] von Neumann’s proof and found that a hidden-variable theory could avoid all obstacles from the proof if it had a property called contextuality. Contextuality means that the outcome of a measurement on a quantum system depends on other measurements on that system at the same time. In other words, the outcome of a measurement depends on the context in which you made the measurement. In demolishing von Neumann’s and other no-hidden variables proofs, Bell discovered his famous theorem [7] and demonstrated that quantum physics describes a contextual world. Bell cited Bohr himself. Bohr said it’s impossible to draw “any sharp distinction between the behavior of atomic objects and [their] interaction with the measuring instruments.” In other words, you can’t look at the quantum world without altering it. This does not mean that the quantum world is not there before you look. If the quantum world weren’t there, you would not be able to alter it by looking! You can’t alter “nothing”!
Bell definitely demonstrated that Bohm’s theory was not impossible. But, there was a price to pay: Bohm’s theory was a nonlocal theory. Was the nonlocality in Bohm’s theory an essential feature of quantum physics? Bell asked this question at the conclusion of his paper. Nonlocality means you can produce instantaneous effects over distant systems. This is what Einstein called “a spooky action at a distance”- quantum entanglement, the ability of separated objects to share a condition or state. This implies that the whole cannot be analyzed in terms of its constituent parts. This is what Bohm [8] called objective wholeness. He introduced the idea of ‘Quantum Potential’, a field that pervades all of space and it does not weaken with distance. Since the quantum potential does not weaken with distance, location does not play any role anymore and this explains the connectedness of the objects when they are entangled without violating the speed of light limit. These correlations crop up all the time when things interact, so essentially, the Universe is entangled all the time and we can talk about the wave function of the Universe which represents the respective entangled state. The wave function is the fundamental entity, objectively real, obeying at all times a deterministic wave equation.
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Everett in his dissertation [4] also asserts that the universal wave function is objectively real and that there is no wave function collapse. This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe. The “wave function collapses” are relative phenomena, meaning that the states of an object system relative to chosen observer states show this effect. In modern parlance, the observer and the observed have become entangled: we can only specify the state of one relative to the other, i.e., the state of the observer and the observed are correlated after the observation is made. Thus probabilistic aspects of a discontinuous change brought about by the observation reappear at the subjective level as relative phenomena to observers. Observers can’t make any predictions better than the limitations imposed by the uncertainty principle. Everett’s formalism alone [11] is sufficient to generate interpretation implying a direct correspondence between formalism and reality, admittedly bizarre reality (Many Worlds interpretation of quantum mechanics).
Everett’s Many Worlds interpretation of quantum mechanics may elegantly explain the inner workings of quantum computers. In David Deutsch’s view [10], ‘Everett’s approach was to look at the quantum theory and see what it actually said, rather than hope it said certain things. What we want is for a theory to conform to reality, and, in order to find out whether it does, you need to see what the theory actually says. Which with the deepest theories is actually quite difficult, because they violate our intuitions.’ Deutsch said that a quantum computer would be “the first technology that allows useful tasks to be performed in collaboration between parallel universes.” In his book [12] Deutsch said: “…we don’t need deep theories to tell us that parallel universes exist – single particle interference phenomena tell us that… The quantum theory of parallel universes is not the problem, it is the solution. It is not some troublesome, optional interpretation emerging from arcane theoretical considerations. It is the explanation – the only one that is tenable – of a remarkable and counter-intuitive reality.”
Coming back to Bell [6], Bell’s theorem is just a statement about the world, independent of quantum physics. If nonlocality is an essential feature of the world, then locality is merely an illusion. But, Einstein’s relativity made it clear that locality (an object is influenced directly only by its immediate surroundings) was a key feature of the world. Thus, it seems a radical revision of our conception of space and time is needed, far beyond Einstein’s relativity. The nonlocal world must be a really strange place. Bell’s theorem does suggest that Everett’s many worlds scheme could be a necessary feature of the world if we don’t want to abandon locality. So we have two assumptions: locality and living in a single universe. One of them must be wrong since Bell’s inequality is violated in real experiments. It boils down to either we live in a single universe and nature is nonlocal, or we live in many worlds and in each of them locality is preserved, or quantum physics gives wrong predictions about experimental setups used in Bell’s paper (highly unlikely I would say).
After Bell’s paper [6] was published in 1964 in some obscure journal, Bell received no correspondence about it for almost five years after it first appeared. But those few people who did read it set things in motion and by the middle of the 1970s, Bell’s work inspired a full-blown quantum rebellion against Copenhagen’s interpretation of quantum mechanics.
Before Bohm’s papers were published [6], he had sent drafts to several of the founding fathers of quantum physics. De Broglie wrote back, pointing out that he had similar ideas twenty-five years earlier, but Pauli and others had set him straight by raising problems with the pilot-wave theory. Pauli threw the same problems at Bohm, but Bohm managed to respond to Pauli in the sense that measurement devices themselves must be incorporated in his quantum descriptions. Pauli conceded that Bohm’s theory was consistent, but that there is no way to test it against “normal” quantum physics. In the end, Pauli thought that Bohm’s ideas were simply “artificial metaphysics”. Bohr never wrote back to Bohm. He thought that Bohm’s theory was “very foolish” and did not say much else.
During March and April 1959, Everett visited Copenhagen to meet with Niels Bohr. Everett was unable to communicate his ideas to Bohr and others at Copenhagen. To them, Everett’s ideas were simply heresy. Rosenfeld, one of Bohr’s devotees, described Everett as “being undescribably stupid and could not understand simplest things in quantum mechanics” [11].
What the founding fathers of quantum mechanics proved with the above responses or no responses is only “lack of imagination”, to use Bell’s comment on impossibility proofs. Quantum computers are a reality now and for them, the “quantum world” is very much alive.
It is time to end the “no quantum world”.
References
[1] Holland, Peter, R. (1993). The Quantum Theory of Motion. Cambridge: University Press.
An account of the de Broglie-Bohm Causal Interpretation of Quantum Mechanics.
[2] Complementarity
[3] Popper, Karl, (1963). Conjectures and Refutations. London: Routledge.
Everett’s dissertation ( the Many-Worlds interpretation of quantum mechanics).
[6] Becker, Adam (2018). What is Real. New York: Basic Books.
The unfinished quest for the meaning of quantum physics.
[7] Bell's theorem
[8] Bohm, D. & Hiley, B. J, (1993). The Undivided Universe. London: Routledge.
A radically different approach to quantum theory.
[9] Physics Today, Vol. 23, No. 9 (September 1970).
[12] Deutsch, D, (1997). The Fabric of Reality. New York: Penguin Books.
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2 年This article echoes back ideas I read in https://www.amazon.com/Einsteins-Unfinished-Revolution-Search-Quantum/dp/0143111167/ Regarding reality vs quantum mechanics: would assuming that reality exists offer any more insight or accurate predictions vs Copenhagen interpretation? e.g. pilot wave theory seems to introduce an unnecessary construct just to make us feel better about reality existing.