https://www.scientificamerican.com/article/quantum-theorys-measurement-problem-may-be-a-poison-pill-for-objective-reality/
Solving a notorious quantum quandary could require abandoning some of science’s most cherished assumptions about the physical world
A core mystery of quantum physics hints that objective reality is illusory—or that the quantum world is even weirder than expected. Credit:
Imagine a physicist observing a quantum system whose behavior is akin to a coin toss: it could come up heads or tails. They perform the quantum coin toss and see heads. Could they be certain that their result was an objective, absolute and indisputable fact about the world? If the coin was simply the kind we see in our everyday experience, then the outcome of the toss would be the same for everyone: heads all around! But as with most things in quantum physics, the result of a quantum coin toss would be a much more complicated “It depends.” There are theoretically plausible scenarios in which another observer might find that the result of our physicist’s coin toss was tails.
At the heart of this bizarreness is what’s called the measurement problem. Standard quantum mechanics accounts for what happens when you measure a quantum system: essentially, the measurement causes the system’s multiple possible states to randomly “collapse” into one definite state. But this accounting doesn’t define what constitutes a measurement—hence, the measurement problem.
Attempts to avoid the measurement problem—for example, by envisaging a reality in which quantum states don’t collapse at all—have led physicists into strange terrain where measurement outcomes can be subjective. “One major aspect of the measurement problem is this idea ... that observed events are not absolute,” says Nicholas Ormrod of the University of Oxford. This, in short, is why our imagined quantum coin toss could conceivably be heads from one perspective and tails from another.
But is such an apparently problematic scenario physically plausible or merely an artifact of our incomplete understanding of the quantum world? Grappling with such questions requires a better understanding of theories in which the measurement problem can arise—which is exactly what Ormrod, along with Vilasini Venkatesh of the Swiss Federal Institute of Technology in Zurich and Jonathan Barrett of Oxford, have now achieved. In a recent preprint, the trio proved a theorem that shows why certain theories—such as quantum mechanics—have a measurement problem in the first place and how one might develop alternative theories to sidestep it, thus preserving the “absoluteness” of any observed event. Such theories would, for instance, banish the possibility of a coin toss coming up heads to one observer and tails to another.
But their work also shows that preserving such absoluteness comes at a cost many physicists would deem prohibitive. “It’s a demonstration that there is no pain-free solution to this problem,” Ormrod says. “If we ever can recover absoluteness, then we’re going to have to give up on some physical principle that we really care about.”
Ormrod, Venkatesh and Barrett’s paper “addresses the question of which classes of theories are incompatible with absoluteness of observed events—and whether absoluteness can be maintained in some theories, together with other desirable properties,” says Eric Cavalcanti of Griffith University in Australia. (Cavalcanti, along with physicist Howard Wiseman and their colleagues, defined the term “absoluteness of observed events” in prior work that laid some of the foundations for Ormrod, Venkatesh and Barrett’s study.)
Holding on to absoluteness of observed events, it turns out, could mean that the quantum world is even weirder than we know it to be.
Gaining a sense of what exactly Ormrod, Venkatesh and Barrett have achieved requires a crash course in the basic arcana of quantum foundations. Let’s start by considering our hypothetical quantum system that can, when observed, come up either heads or tails.
In textbook quantum theory, before collapse, the system is said to be in a superposition of two states, and this quantum state is described by a mathematical construct called a wave function, which evolves in time and space. This evolution is both deterministic and reversible: given an initial wave function, one can predict what it’ll be at some future time, and one can in principle run the evolution backward to recover the prior state. Measuring the wave function, however, causes it to collapse, mathematically speaking, such that the system in our example shows up as either heads or tails.
This collapse-inducing process is the murky source of the measurement problem: it’s an irreversible, one-time-only affair—and no one even knows what defines the process or boundaries of measurement. What amounts to a “measurement” or, for that matter, an “observer”? Do either of these things have physical constraints, such as minimal or maximal sizes? And must they, too, be subject to various slippery quantum effects, or can they be somehow considered immune from such complications? None of these questions have easy, agreed-upon answers—but theorists have no shortage of proffered solutions.
Given the example system, one model that preserves the absoluteness of the observed event—meaning that it’s either heads or tails for all observers—is the Ghirardi-Rimini-Weber theory (GRW). In GRW, quantum systems can exist in a superposition of states until they reach some as-yet-underdetermined size, at which point the superposition spontaneously and randomly collapses, independent of an observer. Whatever the outcome—heads or tails in our example—it shall hold for all observers.
But GRW, which belongs to a broader class of “spontaneous collapse” theories, seemingly runs afoul of a long-cherished physical principle: the preservation of information. Just as a burned book could, in principle, be read by reassembling its pages from its ashes (ignoring the burning book’s initial emission of thermal radiation, for simplicity’s sake), preservation of information implies that a quantum system’s evolution through time should allow its antecedent states to be known. By postulating a random collapse, GRW theory destroys the possibility of knowing what led up to the collapsed state—which, by most accounts, means information about the system prior to its transformation becomes irrecoverably lost. “[GRW] would be a model that gives up information preservation, thereby preserving absoluteness of events,” Venkatesh says.
A counterexample that allows for nonabsoluteness of observed events is the “many worlds” interpretation of quantum mechanics. In this view, our example wave function will branch into multiple contemporaneous realities, such that in one “world,” the system will come up heads, while in another, it’ll be tails. In this conception, there is no collapse. “So the question of what happens is not absolute; it’s relative to a world,” Ormrod says. Of course, in trying to avoid the collapse-induced measurement problem, the many worlds interpretation introduces the mind-numbing branching of wave functions and runaway proliferation of worlds at each and every fork in the quantum road—an unpalatable scenario for many.
Nevertheless, the many worlds interpretation is an example of what are called perspectival theories, wherein the outcome of a measurement depends on the observer’s perspective.