June 26, 2014 | 10
It’s lucky that debates over the meaning of quantum mechanics are so entertaining, because they seem to go on forever. The sundry proposed interpretations make the same experimental predictions, so many people fret that there’ll never be a way to decide among them. Fret no longer. Some “interpretations” aren’t really interpretations so much as separate theories that do make distinctive and testable predictions. In a new paper, physicists Elias Okon and Daniel Sudarsky of the National Autonomous University of Mexico argue that one such interpretation—in which the “collapse” of the quantum wavefunction, often taken to be merely hypothetical, is an actual physical process—not only could be tested by looking at data such as the cosmic microwave background radiation, but could also resolve long-standing mysteries such as the black hole information paradox. I’ve invited them to collapse their ideas into a guest post.—George Musser
The act of measuring any quantity, such as a particle’s position, plays a central role in the standard formulation of quantum mechanics. According to this view, known as the Copenhagen Interpretation, measurement causes the wavefunction of the particle or larger system to collapse from a haze of probability to a single definite outcome. But what is a measurement, really? Who conducts the measurement—does it require a conscious observer? And if an observer must stand outside the system being observed, what happens when the system is not an isolated small object, but the whole universe?
These problems have motivated people to develop modifications and alternative versions of quantum mechanics which are not fundamentally based on the notion of measurement or an external observer: Bohmian mechanics, the transactional interpretation, many-worlds scenarios, and others. In our work, we focus on so-called objective collapse models, which modify the standard equations of quantum mechanics to provide a mechanism for collapsing the wavefunction. Such models come in various forms, notably the Ghirardi-Rimini-Weber theory (GRW) and Philip Pearle’s continuous spontaneous localization (CSL) theory.
A nice feature of these models is that they predict observable departures from ordinary quantum mechanics. For instance, consider experiments that are analogous to Schrödinger’s infamous cat. Physicists place a system in a condition of ambiguity, such as spinning both clockwise and counterclockwise at the same time. The Copenhagen Interpretation says the system will remain in that condition until an observer measures it. Objective collapse models say the system will spontaneously resolve the ambiguity. In that case, the system will also spontaneously lose its characteristic delocalized wavelike properties, and physicists studying the system with an interferometer should notice a change of behavior. The larger the system is, the faster its superposition will collapse.
Some years ago, one of us (Sudarsky) and colleagues Alejandro Perez and Hanno Sahlmann suggested that the cosmos itself could serve as a laboratory. Lately the theory of cosmic inflation has been in the news on account of the possible detection of the gravitational waves it predicts. Inflationary theory says that galaxies and galaxy clusters have their origins in quantum fluctuations that were locked in place and enlarged to astronomical size by the early rapid expansion of the universe. Yet the theory assumes that a symmetric quantum state can turn into an essentially classical asymmetric one. The Copenhagen Interpretation fails to explain how this transition would happen in the absence of observers or measurements. Indeed, standard quantum evolution preserves the symmetries of the initial state, so it is inherently unable to account for the breakdown of the initial symmetry.
Objective collapse models, on the other hand, provide an explicit, observer-independent mechanism for the transition. In these models, a new type of indeterminism enters into play and transforms the quantum fluctuations into inhomogeneities and anisotropies that behave classically. Specific models lead to different signatures in the cosmic microwave background radiation. In the CSL theory, for example, deviations from standard predictions occur on small scales. In addition, unexpected statistical features should appear in the distribution of microwave-background fluctuations.
This year, we have explored how objective collapse models could transform our understanding of black holes. As Stephen Hawking famously showed, quantum theory predicts that material falling into a black hole will reemerge as formless radiation. All the information the material contained is lost, which strikes most physicists as paradoxical, because ordinary quantum mechanics preserves information. In its latest incarnation, the paradox confronts physicists with an unpalatable choice: either blazing firewalls of radiation will form near the black hole horizon, contradicting general relativity, or each infalling particle has to be maximally quantum-entangled with two other particles (one falling into the central singularity and one escaping outward) at once, contradicting quantum mechanics.
This paradox crucially assumes that quantum theory preserves information fully even when including quantum-gravity effects. Hawking, Roger Penrose, and Lajos Diósi have questioned this assupmption, and so do objective collapse models. These models suppose that information destruction and creation is a fundamental feature of nature, so they hold out hope for eliminating the conflict. The critical issue is whether this hope is realized when you get into the details. We suggest that it is. We conjecture that the rate of spontaneous collapse increases as the curvature of spacetime becomes large—in effect, quantum mechanics would be substantially modified in the highly curved region near the singularity. In concrete terms, we think that an astronaut falling into a black hole will observe quantum effects gradually fading away, so that particles behave, in a sense, in a more classical manner. Thus the paradox simply evaporates.
In fact, we think the connection between objective collapse and black holes goes even deeper. Following a suggestion by John Wheeler, we think that spontaneous collapse events could occur because of the creation and destruction of black holes at a microscopic level. In our proposal, this process is amplified in the highly curved regions inside a large black hole. The bottom line is that the incredible accuracy of quantum mechanics in explaining the phenomena we see does not mean that the theory will continue to apply, unmodified, under more extreme circumstances.
We believe that these studies should offer clues that would help in the construction of fully satisfactory theories of quantum dynamical collapse. Those, in turn, should help resolve some of the most pressing conundrums facing our understanding of the fundamental laws of physics. Perhaps the reason that theorists have been having so much trouble coming to terms with some of the puzzles of modern physics is that many of us have been attempting to do so while ignoring the conceptual difficulties that afflict quantum theory itself.
Hubble Space Telescope image courtesy of NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team