Weekly Papers on Quantum Foundations (42)

Authors: G.E. Volovik

Khalatnikov created the unique Institute, where practically all the important areas of theoretical physics have been represented, opening the broad way for collaboration. Here I discuss the influence of the multilayer environment of Landau Institute on my work during 50 years (1968-2018).

Authors: Jochen Szangolies

In-principle restrictions on the amount of information that can be gathered about a system have been proposed as a foundational principle in several recent reconstructions of the formalism of quantum mechanics. However, it seems unclear precisely why one should be thus restricted. We investigate the notion of paradoxical self-reference as a possible origin of such epistemic horizons by means of a fixed-point theorem in Cartesian closed categories due to F. W. Lawvere that illuminates and unifies the different perspectives on self-reference.

Authors: Andrea AddaziGianluca CalcagniAntonino Marciano

Using known estimates for the kaon–antikaon transitions, the mean lifetime of the muon and the mean lifetime of the tau, we place new and stronger constraints on the scales of the multi-fractional theories with weighted and $q$-derivatives. These scenarios reproduce a quantum-gravity regime where fields live on a continuous spacetime with a scale-dependent Hausdorff dimension. In the case with weighted derivatives, constraints from the muon lifetime are various orders of magnitude stronger than those from the tau lifetime and the kaon–antikaon transitions. The characteristic energy scale of the theory cannot be greater than $E_*>3\times 10^2\,{\rm TeV}$, and is tightened to $E_*>9\times 10^{8}\,{\rm TeV}$ for the typical value $\alpha=1/2$ of the fractional exponents in the spacetime measure. We also find an upper bound $d_{\rm H}<2.9$ on the spacetime Hausdorff dimension in the ultraviolet. In the case with $q$-derivatives, the strongest bound comes from the tau lifetime, but it is about 10 orders of magnitude weaker than for the theory with weighted derivatives.

Authors: Flavio MercatiMatteo Sergola

Noncommutative spacetimes are a proposed effective description of the low-energy regime of Quantum Gravity. Defining the microcausality relations of a scalar quantum field theory on the $\kappa$- Minkowski noncommutative spacetime allows us to define for the first time a notion of light-cone in a quantum spacetime. This allows us to reach two conclusions. First, the majority of the literature on $\kappa$-Minkowski suggests that this spacetime allows superluminal propagation of particles. The structure of the light-cone we introduced allows to rule this out, thereby excluding the possibility of constraining the relevant models with observations of in-vacuo dispersion of Gamma Ray Burst photons. Second, we are able to reject a claim made in [Phys. Rev. Lett. 105, 211601 (2010)], according to which the light-cone of the $\kappa$-Minkowski spacetime has a “blurry” region of Planck- length thickness, independently of the distance of the two events considered. Such an effect would be hopeless to measure. Our analysis reveals that the thickness of the region where the notion of timelike and spacelike separations blurs grows like the square root of the distance. This magnifies the effect, e.g. in the case of cosmological distances, by 30 orders of magnitude.

Authors: Juven WangXiao-Gang Wen

The standard models contain chiral fermions coupled to gauge theory. It has been a long-standing problem to give such gauged chiral fermion theories a non-perturbative definition. Based on the classification of quantum anomalies and symmetric invertible topological orders via a mathematical cobordism theorem, and the existence of non-perturbative interactions gapping the mirror world’s chiral fermions for any all-anomaly-free theory, here we show rigorously that the standard models from the SO(10) and SO(18) grand unifications (more precisely, Spin(10) and Spin(18) chiral gauge theories) can be defined non-perturbatively via a 3+1D local lattice model of bosons or qubits, while the standard models from the SU(5) grand unification can be realized by a 3+1D local lattice model of fermions. This represents a unification of Matters and Forces by Quantum Information.

Weatherall, James Owen (2018) Theoretical Equivalence in Physics. [Preprint]

Daily briefing: Why the many-worlds interpretation of quantum mechanics is seductive, but wrong

Daily briefing: Why the many-worlds interpretation of quantum mechanics is seductive, but wrong, Published online: 19 October 2018; doi:10.1038/d41586-018-07130-5

At least in this timeline. Plus: we hear why a PhD isn’t for everyone and tempt you with the best science books of the season.

Quantum tunneling of a black hole into a white hole provides a model for the full life cycle of a black hole. The white hole acts as a long-lived remnant, providing a possible resolution to the information paradox. The remnant solution of the paradox has long been viewed with suspicion, mostly because remnants seemed to be such exotic objects. We point out that (i) established physics includes objects with precisely the required properties for remnants: white holes with small masses but large finite interiors; (ii) non-perturbative quantum gravity indicates that a black hole tunnels precisely into such a white hole, at the end of its evaporation. We address the objections to the existence of white-hole remnants, discuss their stability, and show how the notions of entropy relevant in this context allow them to evade several no-go arguments. A black hole’s formation, evaporation, tunneling to a white hole, and final slow decay, form a unitary process that does not violate any kno…

It is the most extraordinary, alluring and thought-provoking of all the ways in which quantum mechanics has been interpreted. In its most familiar guise, the many-worlds interpretation (MWI) suggests that we live in a near-infinity of universes, all superimposed in the same physical space but mutually isolated and evolving independently. In many of these universes there exist replicas of you and me, all but indistinguishable yet leading other lives.

The MWI illustrates just how peculiarly quantum theory forces us to think. It is an intensely controversial view. Arguments about the interpretation of quantum mechanics are noted for their passion, as disagreements that can’t be settled by objective evidence are wont to be. But when the MWI is in the picture, those passions can become so extreme that we must suspect a great deal more invested in the matter than simply the resolution of a scientific puzzle.

The MWI is qualitatively different from the other interpretations of quantum mechanics, although that’s rarely recognized or admitted. For the interpretation speaks not just to quantum mechanics itself but to what we consider knowledge and understanding to mean in science. It asks us what sort of theory, in the end, we will demand or accept as a claim to know the world.

After the Danish physicist Niels Bohr articulated and refined what became known as the Copenhagen interpretation — widely regarded as the orthodox view of quantum mechanics — in the 1930s and ’40s, it seemed that the central problem for quantum mechanics was the mysterious rupture created by observation or measurement, which was packaged up into the rubric of “collapse of the wave function.”

The wave function is a mathematical expression that defines all possible observable states of a quantum system, such as the various possible locations of a particle. Up until a measurement is made and the wave function collapses (whatever that means), there is no reason to attribute any greater a degree of reality to any of the possible states than to any other. It’s not that the quantum system is actually in one or other of these states but we don’t know which; we can confidently say that it is not in any one of these states, but is properly described by the wave function itself, which in some sense “permits” them all as observational outcomes. Where, then, do they all go, bar one, when the wave function collapses?

At first glance, the many-worlds interpretation looks like a delightfully simple answer to that mysterious vanishing act. It says that none of the states vanishes at all, except to our perception. It says, in essence, let’s just do away with wave function collapse altogether.

This solution was proposed by the young physicist Hugh Everett III in his 1957 doctoral thesis at Princeton, where he was supervised by John Wheeler. It purported to solve the “measurement problem” using only what we know already: that quantum mechanics works.

But Bohr and colleagues didn’t bring wave function collapse into the picture just to make things difficult. They did it because that’s what seems to happen. When we make a measurement, we really do get just one result out of the many that quantum mechanics offers. Wave function collapse seemed to be demanded in order to connect quantum theory to reality.

So what Everett was saying was that it’s our concept of reality that’s at fault. We only think that there’s a single outcome of a measurement. But in fact all of them occur. We only see one of those realities, but the others have a separate physical existence too.

In effect, this implies that the entire universe is described by a gigantic wave function that contains within it all possible realities. This “universal wave function,” as Everett called it in his thesis, begins as a combination, or superposition, of all possible states of its constituent particles. As it evolves, some of these superpositions break down, making certain realities distinct and isolated from one another. In this sense, worlds are not exactly “created” by measurements; they are just separated. This is why we shouldn’t, strictly speaking, talk of the “splitting” of worlds (even though Everett did), as though two have been produced from one. Rather, we should speak of the unraveling of two realities that were previously just possible futures of a single reality.

(The many-worlds interpretation is distinct from the multiverse hypothesis, which envisions other universes, born in separate Big Bangs, that have always been physically disconnected from our own.)

When Everett presented his thesis, and at the same time published the idea in a respected physics journal, it was largely ignored. It wasn’t until 1970 that people began to take notice, after an exposition on the idea was presented in the widely read magazine Physics Today by the American physicist Bryce DeWitt.

This scrutiny forced the question that Everett’s thesis had somewhat skated over. If all the possible outcomes of a quantum measurement have a real existence, where are they, and why do we see (or think we see) only one? This is where the many worlds come in. DeWitt argued that the alternative outcomes of the measurement must exist in a parallel reality: another world. You measure the path of an electron, and in this world it seems to go this way, but in another world it went that way.

That requires a parallel, identical apparatus for the electron to traverse. More, it requires a parallel you to observe it — for only through the act of measurement does the superposition of states seem to “collapse.” Once begun, this process of duplication seems to have no end: you have to erect an entire parallel universe around that one electron, identical in all respects except where the electron went. You avoid the complication of wave function collapse, but at the expense of making another universe. The theory doesn’t exactly predict the other universe in the way that scientific theories usually make predictions. It’s just a deduction from the hypothesis that the other electron path is real too.

This picture gets really extravagant when you appreciate what a measurement is. In one view, any interaction between one quantum entity and another — a photon of light bouncing off an atom — can produce alternative outcomes, and so demands parallel universes. As DeWitt put it, “Every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies.” In this “multiverse,” says the physicist and many-worlds proponent Max Tegmark, “all possible states exist at every instant” — meaning, at least in the popular view, that everything that is physically possible is (or will be) realized in one of the parallel universes.

In particular, after a measurement takes place, there are two (or more) versions of the observer where before there was one. “The act of making a decision,” says Tegmark — a decision here counting as a measurement, generating a particular outcome from the various possibilities — “causes a person to split into multiple copies.” Both copies are in some sense versions of the initial observer, and both of them experience a unique, smoothly changing reality that they are convinced is the “real world.” At first these observers are identical in all respects except that one observed this path of the electron (or whatever is being measured) and the other that path. But after that, who can say? Their universes go their separate ways, launched on a trajectory of continual unraveling.

You can probably see why the MWI is the interpretation of quantum mechanics that wins all the glamour and publicity. It tells us that we have multiple selves, living other lives in other universes, quite possibly doing all the things that we dream of but will never achieve (or never dare to attempt). There is no path not taken. For every tragedy, like Gwyneth Paltrow’s character being hit by a van in the many-worlds-inspired 1998 movie Sliding Doors, there is salvation and triumph.

Who could resist that idea?

There are, of course, some questions to be asked.

For starters, about this business of bifurcating worlds. How does a split actually happen?

That is now seen to hinge on the issue of how a microscopic quantum event gives rise to macroscopic, classical behavior through a process called “decoherence,” in which the wavelike states of a quantum system become uncoordinated and scrambled by their interactions with their environment. Parallel quantum worlds have split once they have decohered, for by definition decohered wave functions can have no direct, causal influence on one another. For this reason, the theory of decoherence developed in the 1970s and ’80s helped to revitalize the MWI by supplying a clear rationale for what previously seemed a rather vague contingency.

In this view, splitting is not an abrupt event. It evolves through decoherence and is only complete when decoherence has removed all possibility of interference between universes. While it’s popular to regard the appearance of distinct worlds as akin to the bifurcation of futures in Jorge Luis Borges’ story “The Garden of Forking Paths,” a better analogy might therefore be something like the gradual separation of shaken salad dressing into layers of oil and vinegar. It’s then meaningless to ask how many worlds there are — as the philosopher of physics David Wallace aptly puts it, the question is rather like asking, “How many experiences did you have yesterday?” You can identify some of them, but you can’t enumerate them.

What we can say a little more precisely is what kind of phenomenon causes splitting. In short, it must happen with dizzying profusion. Just within our own bodies, there must be at least as many splitting events affecting each of us every second as there are encounters between our molecules in the same space of time. Those numbers are astronomical.

The main scientific attraction of the MWI is that it requires no changes or additions to the standard mathematical representation of quantum mechanics. There is no mysterious, ad hoc and abrupt collapse of the wave function. And virtually by definition it predicts experimental outcomes that are fully consistent with what we observe.

But if we take what it says seriously, it soon becomes clear that the conceptual and metaphysical problems with quantum mechanics aren’t banished by virtue of this apparent parsimony of assumptions and consistency of predictions. Far from it.

The MWI is surely the most polarizing of interpretations. Some physicists consider it almost self-evidently absurd; “Everettians,” meanwhile, are often unshakable in their conviction that this is the most logical, consistent way to think about quantum mechanics. Some of them insist that it is the only plausible interpretation — for the arch-Everettian David Deutsch, it is not in fact an “interpretation” of quantum theory at all, any more than dinosaurs are an “interpretation” of the fossil record. It is simply what quantum mechanics is. “The only astonishing thing is that that’s still controversial,” Deutsch says.

My own view is that the problems with the MWI are overwhelming — not because they show it must be wrong, but because they render it incoherent. It simply cannot be articulated meaningfully.

I’ll attempt to summarize the problems, but first, let’s dispense with a wrong objection. Some criticize the MWI on aesthetic grounds: People object to all those countless other universes, multiplying by the trillion every nanosecond, because it just doesn’t seem proper. Other copies of me? Other world histories? Worlds where I never existed? Honestly, whatever next! This objection is rightly dismissed by saying that an affront to one’s sense of propriety is no grounds for rejecting a theory. Who are we to say how the world should behave?

A stronger objection to the proliferation of worlds is not so much all this extra stuff you’re making, but the insouciance with which it is made. Roland Omnès says the idea that every little quantum “measurement” spawns a world “gives an undue importance to the little differences generated by quantum events, as if each of them were vital to the universe.” This, he says, is contrary to what we generally learn from physics: that most of the fine details make no difference at all to what happens at larger scales.

But one of the most serious difficulties with the MWI is what it does to the notion of self. What can it mean to say that splittings generate copies of me? In what sense are those other copies “me?”

Brian Greene, a well-known physics popularizer with Everettian inclinations, insists simply that “each copy is you.” You just need to broaden your mind beyond your parochial idea of what “you” means. Each of these individuals has its own consciousness, and so each believes he or she is “you” — but the real “you” is their sum total.

There’s an enticing frisson to this idea. But in fact the very familiarity of the centuries-old doppelgänger trope prepares us to accept it rather casually, and as a result the level of the discourse about our alleged replica selves is often shockingly shallow — as if all we need contemplate is something like teleportation gone awry in an episode of “Star Trek.” We are not being astonished but, rather, flattered by these images. They sound transgressively exciting while being easily recognizable as plotlines from novels and movies.

Tegmark waxes lyrical about his copies: “I feel a strong kinship with parallel Maxes, even though I never get to meet them. They share my values, my feelings, my memories — they’re closer to me than brothers.” But this romantic picture has, in truth, rather little to do with the realities of the MWI. The “quantum brothers” are an infinitesimally small sample cherry-picked for congruence with our popular fantasies. What about all those “copies” differing in details graduating from the trivial to the utterly transformative?

The physicist Lev Vaidman has thought rather carefully about this matter of quantum youness. “At the present moment there are many different ‘Levs’ in different worlds,” he says, “but it is meaningless to say that now there is another ‘I.’ There are, in other words, beings identical to me (at the time of splitting) in these other worlds, and all of us came from the same source — which is ‘me’ right now.”

The “I” at each moment of time, he says, is defined by a complete classical description of the state of his body and brain. But such an “I” could never be conscious of its existence.

Consciousness relies on experience, and experience is not an instantaneous property: It takes time, not least because the brain’s neurons themselves take a few milliseconds to fire. You can’t “locate” consciousness in a universe that is frantically splitting countless times every nanosecond, any more than you can fit a summer into a day.

One might reply that this doesn’t matter, so long as there’s a perception of continuity threading through all those splittings. But in what can that perception reside, if not in a conscious entity?

And if consciousness — or mind, call it what you will — were somehow able to snake along just one path in the quantum multiverse, then we’d have to regard it as some nonphysical entity immune to the laws of (quantum) physics. For how can it do that when nothing else does?

David Wallace, one of the most ingenious Everettians, has argued that purely in linguistic terms the notion of “I” can make sense only if identity/consciousness/mind is confined to a single branch of the quantum multiverse. Since it is not clear how that can possibly happen, Wallace might then have inadvertently demonstrated that the MWI is not after all proposing a conceit of “multiple selves.” On the contrary, it is dismantling the whole notion of selfhood. It is denying any real meaning of “you.”

I shouldn’t wish anyone to think that I feel affronted by this. But if the MWI sacrifices the possibility of thinking meaningfully about selfhood, we should at least acknowledge that this is so, and not paper over it with images of “quantum brothers and sisters.”

The science-fiction vision of a “duplicated quantum self” has nevertheless delivered some fanciful, and undeniably entertaining, images. If splitting can be guaranteed by any experiment in which the outcome of a quantum process is measured, then one can imagine making a “quantum splitter”: a handheld device that measures, say, an electron’s intrinsic quantum angular momentum, or spin, which can be thought of as having two states, either pointing up or down; it then converts the result to a macroscopic arrow pointing on a dial to “Up” or “Down.” This conversion ensures that the initial superposition of spin states is fully decohered into a classical outcome. You can make these measurements as often as you like just by pushing the button on the device. Each time you do (so the story goes), two distinct “yous” come into being.

What can you do with this power to generate worlds and selves? You could become a billionaire by playing quantum Russian roulette. Your quantum splitter is activated while you sleep, and if the dial says Up then you’re given a billion dollars when you wake. If it shows Down then you are put to death painlessly in your sleep. Few people, I think, would accept these odds on a coin toss. But a committed Everettian should have no hesitation about doing so using the quantum splitter. For you can be certain, in this view, that you’ll wake up to be presented with the cash. Of course, only one of “you” wakes up at all; the others have been killed. But those other yous knew nothing of their demise. Sure, you might worry about the grief afflicted on family and friends in those other worlds. But that aside, the rational choice is to play the game. What could possibly go wrong?

You’re not going to play? OK, I see why. You’re worried about the fact that you’re going to die as a result, with absolute certainty. But look, you’re going to live and become rich with absolute certainty too.

Are you having trouble comprehending what that means? Of course you are. It has no meaning in any normal sense of the word. The claim is, in words aptly coined by the physicist Sean Carroll in another context (ironically, Carroll is one of the most vocal Everettians), “cognitively unstable.”

Some Everettians have tried to articulate a meaning nonetheless. They argue that, despite the certainty of all outcomes, it is rational for any observer to consider the subjective probability for a particular outcome to be proportional to the amplitude of that world’s wave function — or what Vaidman calls the “measure of existence” of that world.

It’s a misleading term, since there’s no sense in which any of the many worlds exists less. For the “self” that ends up in any given world, that’s all there is — for better or worse. Still, Vaidman insists that we ought rationally to “care” about a post-splitting world in proportion to this measure of existence. On this basis, he feels that playing quantum Russian roulette again and again (or even once, if there’s a very low measure of existence for the “good” outcome) should be seen as a bad idea, regardless of the morality, “because the measure of existence of worlds with Lev dead will be much larger than the measure of existence of the worlds with a rich and alive Lev.”

What this boils down to is the interpretation of probabilities in the MWI. If all outcomes occur with 100-percent probability, where does that leave the probabilistic character of quantum mechanics? And how can two (or for that matter, a thousand) mutually exclusive outcomes all have 100-percent probability?

There is a huge and unresolved literature on this question, and some researchers see it as the issue on which the idea stands or falls. But much of the discussion assumes, I think wrongly, that the matter is independent of questions about the notion of selfhood.

Attempts to explain the appearance of probability within the MWI come down to saying that quantum probabilities are just what quantum mechanics looks like when consciousness is restricted to only one world. As we saw, there is in fact no meaningful way to explain or justify such a restriction. But let’s accept for now — just to see where it leads — the popular view of the MWI that two copies of an observer emerge from the one who exists before a measurement, and that both copies experience themselves as unique.

Imagine that our observer, Alice, is playing a quantum version of a simple coin-toss gambling game — nothing as drastic or emotive as quantum Russian roulette — that hinges on measurement of the spin state of an atom prepared in a 50:50 superposition of up and down. If the measurement elicits up, she doubles her money. If it’s down, she loses it all.

If the MWI is correct, the game seems pointless — for Alice will, with certainty, both win and lose. And there’s no point her saying, “Yes, but which world will I end up in?” Both of the two Alices that exist once the measurement is made are in some sense present in the “her” before the toss.

But now let’s do the sleeping trick. Alice is put to sleep before the measurement is made, knowing she will be wheeled into one of two identical rooms depending on the outcome. Both rooms contain a chest. Inside one is twice her stake, while the other is empty. When she wakes, she has no way of telling, without opening the chest, whether it contains the winning money. But she can then meaningfully say that there is a 50-percent probability that it does. What’s more, she can say before the experiment that, when she awakes, these will be the odds deduced by her awakened self as she contemplates the still-closed chest. Isn’t that a meaningful concept of probability?

The notion here is that quantum events that occur for certain in the MWI can still elicit probabilistic beliefs in observers simply because of their ignorance of which branch they are on.

But it won’t work. Suppose Alice says, with scrupulous care, “The experience I will have is that I will wake up in a room containing a chest that has a 50-percent chance of being filled or empty.” The Everettian would say Alice’s statement is correct: It’s a rational belief.

But what if Alice were to say, “The experience I will have is that I will wake up in a room containing a chest that has a 100-percent chance of being empty”? The Everettian must accept this statement as a true and rational belief too, for the initial “I” here must apply to both Alices in the future.

In other words, Alice Before can’t use quantum mechanics to predict what will happen to her in a way that can be articulated — because there is no logical way to talk about “her” at any moment except the conscious present (which, in a frantically splitting universe, doesn’t exist). Because it is logically impossible to connect the perceptions of Alice Before to Alice After, “Alice” has disappeared. You can’t invoke an “observer” to make your argument when you have denied pronouns any continuity.

What the MWI really denies is the existence of facts at all. It replaces them with an experience of pseudo-facts (we think that this happened, even though that happened too). In so doing, it eliminates any coherent notion of what we can experience, or have experienced, or are experiencing right now. We might reasonably wonder if there is any value — any meaning — in what remains, and whether the sacrifice has been worth it.

Every scientific theory (at least, I cannot think of an exception) is a formulation for explaining why things in the world are the way we perceive them to be. This assumption that a theory must recover our perceived reality is generally so obvious that it is unspoken. The theories of evolution or plate tectonics don’t have to include some element that says “you are here, observing this stuff”; we can take that for granted.

But the MWI refuses to grant it. Sure, it claims to explain why it looks as though “you” are here observing that the electron spin is up, not down. But actually it is not returning us to this fundamental ground truth at all. Properly conceived, it is saying that there are neither facts nor a you who observes them.

It says that our unique experience as individuals is not simply a bit imperfect, a bit unreliable and fuzzy, but is a complete illusion. If we really pursue that idea, rather than pretending that it gives us quantum siblings, we find ourselves unable to say anything about anything that can be considered a meaningful truth. We are not just suspended in language; we have denied language any agency. The MWI — if taken seriously — is unthinkable.

Its implications undermine a scientific description of the world far more seriously than do those of any of its rivals. The MWI tells you not to trust empiricism at all: Rather than imposing the observer on the scene, it destroys any credible account of what an observer can possibly be. Some Everettians insist that this is not a problem and that you should not be troubled by it. Perhaps you are not, but I am.

Yet I have pushed hard against the MWI not so much to try to demolish it as to show how its flaws, once brought to light, are instructive. Like the Copenhagen interpretation (which also has profound problems), it should be valued for forcing us to confront some tough philosophical questions.

What quantum theory seems to insist is that at the fundamental level the world cannot supply clear “yes/no” empirical answers to all the questions that seem at face value as though they should have one. The calm acceptance of that fact by the Copenhagen interpretation seems to some, and with good reason, to be far too unsatisfactory and complacent. The MWI is an exuberant attempt to rescue the “yes/no” by admitting both of them at once. But in the end, if you say everything is true, you have said nothing.

We needn’t fear a scientific idea that changes our view of macroscopic reality. But an idea that, when we pursue it seriously, makes that view inchoate and unspeakable doesn’t fulfill the function of science. The value of the many worlds, then, is that they close off an easy way out. It was worth admitting them in order to discover that they are a dead end. But there is no point then sitting there insisting we have found the way out. We need to go back and keep searching.

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Explaining it requires neither supernatural intervention nor any new fundamental physics

— Read more on ScientificAmerican.com


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Author(s): Luca Mancino, Vasco Cavina, Antonella De Pasquale, Marco Sbroscia, Robert I. Booth, Emanuele Roccia, Ilaria Gianani, Vittorio Giovannetti, and Marco Barbieri

Theoretical bounds on irreversible entropy production in a thermalizing quantum system are supported by experiments simulating the thermalization of a qubit using a quantum photonic architecture.

[Phys. Rev. Lett. 121, 160602] Published Wed Oct 17, 2018

Azhar, Feraz and Kaiser, David I. (2018) Flows into inflation: An effective field theory approach. Physical Review D, 98 (6).

Exploring the Universe with matter waves

Exploring the Universe with matter waves, Published online: 17 October 2018; doi:10.1038/d41586-018-07009-5

An exotic ultracold gas known as a Bose–Einstein condensate has been produced and studied in space. Such gases could be used to build quantum sensors that probe the properties of the Universe with extreme precision.

Cabrera, Frank (2018) String Theory, Non-Empirical Theory Assessment, and the Context of Pursuit. Synthese. ISSN 1573-0964
Lewis, Peter J. (2018) Pragmatism and the content of quantum mechanics. In: UNSPECIFIED.

Authors: Nathaniel CraigIsabel Garcia Garcia

Stringent Swampland conjectures aimed at effective theories containing massive abelian vectors have recently been proposed (arXiv:1808.09966), with striking phenomenological implications. In this article, we show how effective theories that parametrically violate the proposed conjectures can be UV-completed into theories that satisfy them. The UV-completion is accessible through both the St\”uckelberg and Higgs mechanisms, with all dimensionless parameters taking $\mathcal{O}(1)$ values from the UV perspective. These constructions feature an IR limit containing a light vector that is parametrically separated from any other massive states, and from any cut-off scale mandated by quantum gravity consistency requirements. Moreover, the cut-off–to–vector–mass ratio remains parametrically large even in the decoupling limit in which all other massive states (including any scalar excitations) become arbitrarily heavy. We discuss how apparently strong constraints imposed by the proposed conjectures on phenomenologically interesting models, including specific production mechanisms of dark photon dark matter, are thereby circumvented.

Authors: Martin BojowaldSuddhasattwa BrahmaUmut BuyukcamJonathan GuglielmonMartijn van Kuppeveld

Weak magnetic monopoles with a continuum of charges less than the minimum implied by Dirac’s quantization condition may be possible in non-associative quantum mechanics. If a weakly magnetically charged proton in a hydrogen atom perturbs the standard energy spectrum only slightly, magnetic charges could have escaped detection. Testing this hypothesis requires entirely new methods to compute energy spectra in non-associative quantum mechanics. Such methods are presented here, and evaluated for upper bounds on the magnetic charge of elementary particles.

Authors: S. CarlipRicardo A. MosnaJ. P. M. Pitelli

Quantum fluctuations of the vacuum stress-energy tensor are highly non-Gaussian, and can have unexpectedly large effects on spacetime geometry. In this paper, we study a two-dimensional dilaton gravity model coupled to a conformal field theory, in which the distribution of vacuum fluctuations is well understood. By analyzing geodesic deviation, we show that a pencil of massive particles propagating on this fuzzy spacetime eventually converges and collapses. The collapse time depends on the velocity of the congruence of particles, but for ultra-relativistic particles the collapse probability as a function of time converges to an exponential distribution, consistent with our earlier analysis of null geodesics [Phys. Rev. Lett. 107, 021303 (2011)]. We thus find further evidence for the influence of vacuum fluctuations on the small scale causal structure of spacetime.

Authors: Christopher J. FewsterRainer Verch

The measurement process is considered for quantum field theory on curved spacetimes. Measurements are carried out on one QFT, the “system”, using another, the “probe” via a dynamical coupling of “system” and “probe” in a bounded spacetime region. The resulting “coupled theory” determines a scattering map on the uncoupled combination of the “system” and “probe” by reference to natural “in” and “out” spacetime regions. No specific interaction is assumed and all constructions are local and covariant.

Given any initial probe state in the “in” region, the scattering map determines a completely positive map from “probe” observables in the “out” region to “induced system observables”, thus providing a measurement scheme for the latter. It is shown that the induced system observables may be localized in the causal hull of the interaction coupling region and are typically less sharp than the probe observable, but more sharp than the actual measurement on the coupled theory. Post-selected states conditioned on measurement outcomes are obtained using Davies-Lewis instruments. Composite measurements involving causally ordered coupling regions are also considered. Provided that the scattering map obeys a causal factorization property, the causally ordered composition of the individual instruments coincides with the composite instrument; in particular, the instruments may be combined in either order if the coupling regions are causally disjoint. This is the central consistency property of the proposed framework.

The general concepts and results are illustrated by an example in which both “system” and “probe” are quantized linear scalar fields, coupled by a quadratic interaction term with compact spacetime support. System observables induced by simple probe observables are calculated exactly, for sufficiently weak coupling, and compared with first order perturbation theory.

Authors: Min ZhuangJiahao HuangYongguan KeChaohong Lee

Quantum adiabatic evolution, an important fundamental concept in physics, describes the dynamical evolution arbitrarily close to the instantaneous eigenstate of a slowly driven Hamiltonian. In most systems undergoing spontaneous symmetry-breaking transitions, their two lowest eigenstates change from non-degenerate to degenerate. Therefore, due to the corresponding energy gap vanishes, the conventional gap condition for quantum adiabatic evolution becomes invalid. Here we explore the existence of quantum adiabatic evolutions in spontaneous symmetry-breaking transitions and derive a symmetry-dependent adiabatic condition. Because the driven Hamiltonian conserves the symmetry in the whole process, the transition between different eigenstates with different symmetries is forbidden. Therefore, even if the gap vanishes, symmetry-protected quantum adiabatic evolution may appear when the driven system varies according to the symmetry-dependent adiabatic condition. This study not only advances our understandings of quantum adiabatic evolution and spontaneous symmetry-breaking transitions, but also provides extensive applications ranging from quantum state engineering, topological Thouless pumping to quantum computing.


In-principle restrictions on the amount of information that can be gathered about a system have been proposed as a foundational principle in several recent reconstructions of the formalism of quantum mechanics. However, it seems unclear precisely why one should be thus restricted. We investigate the notion of paradoxical self-reference as a possible origin of such epistemic horizons by means of a fixed-point theorem in Cartesian closed categories due to Lawvere that illuminates and unifies the different perspectives on self-reference.

We study an extension of spacetime across Schwarzschild’s central singularity and the behavior of the geodesics crossing it. Locality implies that this extension is independent from the future fate of black holes. We argue that this extension could be the ##IMG## [http://ej.iop.org/images/0264-9381/35/21/215010/cqgaae499ieqn001.gif] limit of the effective quantum geometry inside a black hole, and show that the central region contains causal diamonds with area satisfying Bousso’s bound for an entropy that can be as large as Hawking’s radiation entropy. This result sheds light on the possibility that Hawking radiation is purified by information crossing the internal singularity and supports the black hole to white hole transition scenario.

On Formalisms and Interpretations


on 2018-10-15 2:47pm GMT

Quantum 2, 99 (2018).

https://doi.org/10.22331/q-2018-10-15-99One of the reasons for the heated debates around the interpretations of quantum theory is a simple confusion between the notions of formalism $\textit{versus}$ interpretation. In this note, we make a clear distinction between them and show that there are actually two $\textit{inequivalent}$ quantum formalisms, namely the relative-state formalism and the standard formalism with the Born and measurement-update rules. We further propose a different probability rule for the relative-state formalism and discuss how Wigner’s-friend-type experiments could show the inequivalence with the standard formalism. The feasibility in principle of such experiments, however, remains an open question.

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