June 26, 2014 | 10

The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of
The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of
Contact George Musser via email.

Follow George Musser on Twitter as @gmusser. Or visit their website.

Follow George Musser on Twitter as @gmusser. Or visit their website.

*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*

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Unfortunately, the link to the subject paper seems to be real broke (as of this writing). I think the article is discussing http://arxiv.org/abs/1309.0792

Link to thisvery much enjoyed the article.

Link to thisJames, I think it might be this one…

http://arxiv.org/abs/1406.4898

It is more likely that QM holds and GR diverges near a black hole. Pauli Exclusion should prevent spacetime curvature from becoming infinite, IF spacetime is quantized and fermionic. Extreme gravity is no match for Pauli Exclusion, and the fermions will form a degenerate state and the slope of the spacetime curvature will level off. There will be no infinitely dense singularity and no information loss.

Link to thisWayne, may be, but my guess is authored by both Elias Okon and Daniel Sudarsky, mentioned as the authors in the article…

Link to thislavaroy,

Link to thisI agree that there can be no physical singularity.

However, in the case of a nuclear firewall process occurring at the event horizon, nuclear binding mass-energy might be retained as gravitationally curved spacetime – whose focus is a singular point – while residual fundamental particles are redirected by EM field momentum and ejected via relativistic polar jets…

Sorry for the confusion. I’ve updated the post. (I’d typed “hef” rather than “href” in the HTML tag.)

Link to thisThanks, George.

Link to thisWayne – we had both guessed wrong!

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Because we do not understand causality does not mean we should replace it with non-causality.

Link to thisThe universe doesn’t need to ‘collapse’ a wavefront until it has to. Huh? Why would it postpone this? Well, imagine this is a computer simulation. The advantage of not computing the result until you need to is that you can save computing cycles. Now, many folks doubt this universe is a computer simulation. Sure, but there is a twist to this circumstance that should be understood. At present, we use computer simulations of almost everything in the physical sciences to confirm our theories and make predictions. The hidden assumption is that the universe is -equivalent- to a computer simulation of it. If this is not true, we can’t do science this way. Two exceptions appear: (1) The universe can only be approximated by a computer simulation – sure – by how closely? more computer power closer approximation? (sounds like epsilon-delta); (2) There must be another way (besides computer simulation) to test our theories. Problem is, computer simulations are no more than large batches of numerical computations and we have not other method for doing physical science. Conclusions: [1] The universe is actually a computer simulation (or equivalent to one); or we are in deep trouble. [2] If the universe is at least equivalent to a computer simulation than many other aspects true of computer simulations are true of our universe. (Bruce Maier/www.realityisvirutal.com)

Link to thisAny model that purports to describe reality in between observations is, by definition, untestable. Times between observations are necessarily finite (as is any measurable quantity). Thus claims that continuously time-dependent wave-functions (as in Schrodinger/Dirac equation) describe reality are untestable interpolations relying on counter-factual definiteness. Any realistic interpretation of QM must account for discrete time observation. As regards the supposed self-reference of an observer of the entire universe, I can only say that there is no such thing except at any moment in which the entire universe is coincident (e.g. at the big bang). Only real objects (and ANY real object) can be an “observer” and “observation” is the reception of information (properties) via another object.

Link to this