Elegance and Enigma: The Quantum Interviews
By MAXIMILIAN SCHLOSSHAUER • Jan. 11, 2012 @ 15:41 GMT

To say that quantum theory is about describing how atoms behave would be like saying that all Hemingway ever did was show us how to write terse prose. Quantum theory, more than any other physical theory, seems to rub against what we have traditionally come to see as the mission of science: namely, to provide a tangible description of an objectively existing external reality. But rather than telling us what exists, quantum theory talks only about measurements and observation—and not even about what we will observe, but only about the probabilities of observing this or that result.

Many people, Einstein included, have felt that something must be missing from this picture—that a satisfactory, complete physical theory ought to be more than an instrument for computing probabilities of something so observer-focused as measurement outcomes. Much of the persistent and heated debate about the meaning of quantum theory has centered on this issue. Over the course of decades, people have responded to Einstein’s challenge in radically different ways. Personally, I’ve always found it intriguing how a theory can be so concisely formulated and inexhaustibly successful while fitting pretty much any worldview, from deep-seated realism to full-blown positivism. Perhaps this observation contains a lesson in itself.

Last year, I interviewed a bunch of physicists, philosophers, and mathematicians––many of whom are FQXi members––about the mysteries of quantum theory. I put the same set of questions to each of my interviewees, who are some of the most original thinkers working on quantum theory today. The answers, collected in my new book Elegance and Enigma: The Quantum Interviews, turned out to be marvels of bold thought and irresistible wit. They are deeply personal, providing rare glimpses into what motivates a group of scholars, all working off the same theory, to seek out drastically different approaches to the theory’s interpretation.

My first question asked how my interviewees became enamored with quantum theory. (Go to the end of this article to see the full list of questions.) It’s a question close to my heart, because I wouldn’t be a physicist today hadn’t it been for a chance encounter, in my last two years of high school, with Heisenberg’s and Schrödinger’s philosophical writings about quantum theory. Many of my interviewees told similar stories of decisive events: an eye-opening seminar they attended, or a book they had been given or picked up, or a radio broadcast they had heard (sometimes still as teenagers). Many had accepted, without giving it much thought, the standard presentation of quantum theory, only to be suddenly plunged into a sense of acute discomfort by something they happened to hear or read. They have never been the same since.

The first half of my interview questions focused on the core foundational problems of quantum theory. What is the best interpretation of the theory? How are we to understand the concept of measurement? What is the meaning of probabilities? Does quantum theory imply that nature is indeterministic? The second half of the questions looked at the bigger picture. What experiments may bring decisive progress to our understanding of quantum theory? What input may come from philosophy and from the search for a unified theory? How important are personal beliefs and values? What does the future hold?

The interview answers were a stark reminder of how little consensus has been reached in the century since quantum theory’s birth. They testified to a persistent disagreement about what the central problems are, how to address them, and about how much or little we ought to worry.

Take the infamous “measurement problem” as an example. It has its roots in an apparent clash between two ways in which measurement may appear in quantum theory. First, measurement is introduced axiomatically, as a primitive notion: quantum theory gives us a recipe for computing probabilities of measurement results, but without in turn reducing the act of measurement to an explicit account of the physical going-ons inside the measurement apparatus, like we would expect in classical physics. On the other hand, nothing prevents us from using the quantum formalism to describe these going-ons in the same way we describe the going-ons in any other physical system. But in such a description, the apparatus ends up in a strangely suspended state without any definitive measurement result.

So the measurement problem amounts to several different possible concerns. Should we regard the axiomatic notion of measurement as inadequate and instead seek a deeper explanation of the measurement process? Should we worry about the indefinite apparatus state? Is there an inconsistency between this state and how measurement-as-axiom operates?

The interviews not only showed that everybody has a different opinion on how to answer these questions and whether the measurement problem is, as I put it in my interview question, a “serious roadblock or dissolvable pseudo-issue.” They also showed that these opinions were strongly correlated with interpretive attitudes toward the quantum formalism as a whole. Those, such as Christopher Fuchs, a researcher at Perimeter Institute in Waterloo, Canada, and David Mermin, a professor emeritus of physics at Cornell University, who view quantum theory a man-made tool to help us structure and predict our experiences, tended to dismiss the measurement problem. Those, such as GianCarlo Ghirardi, a professor emeritus of physics at the University of Trieste, Italy, and Tim Maudlin, a philosopher at New York University, who believe a satisfactory physical theory ought to provide an observer-independent account of physical reality, were more likely to view the measurement problem as a real difficulty for quantum theory, calling for urgent remedy.

As far as interpretations of quantum theory are concerned, pretty much every possible interpretive flavor was represented among my interviewees. And some people were self-proclaimed agnostics. Lucien Hardy, a physicist at Perimeter Institute, was particularly blunt: “I do not believe any of the currently available interpretive programs.” And some interviewees didn’t think my question made sense to begin with. “The question is completely backward,” Fuchs retorted. “It acts as if there is this thing called quantum mechanics, displayed and available for everyone to see as they walk by it—kind of like a lump of something on a sidewalk. The job of interpretation is to find the right spray to cover up any offending smells.” Jeff Bub, a philosopher at the University of Maryland, College Park, had related concerns. “The program of interpreting quantum mechanics tends to treat the theory like a problem child in the family of theories and propose therapy,” he said. “The aim is to get quantum mechanics to conform to some ideal of classical comprehensibility. If this is what it means to ‘make the best sense of quantum mechanics,’ then I think the exercise is misguided.”

Over the past two decades or so, we have witnessed what has been called the “second quantum revolution.” One development is quantum information theory. It has given us a completely new view on quantum theory as a theory phrased in terms of the processing and communication of information in physical systems. Generations of physicists raised on Heisenberg’s uncertainty principle came away with the impression that quantum mechanics is about imposing all kinds of limits on what we can do in this world—like how we can’t simultaneously determine the position and momentum of a particle with full accuracy. Quantum information theory, if nothing else, has turned the tables by showing that in a world governed by quantum mechanics, we can do lots of things we can’t do in a classical world, like have completely secure communication or solve certain computational problems faster than any classical algorithm could ever do.

The question, of course, is whether quantum information theory has done anything to alleviate conceptual concerns about quantum theory. For Bub, “thinking about quantum mechanics from an information-theoretic standpoint has radically transformed the field of quantum foundations.” Those who see the task of physics as formulating theories that give an account of what exists tended to be more critical. “The notion that quantum information theory or quantum computational theory could contribute to the foundational questions has always puzzled me,” said Maudlin. “I have no concept of how one could turn the usual project on its head and derive or explain physics from information theory.” Whatever view one takes, for Tony Leggett, a Nobel Prize–winning physicist at the University of Illinois at Urbana–Champaign, quantum information theory is having a practical, political benefit: “It is now rather widely accepted that an active interest in the foundations of quantum mechanics does not disqualify one from being a ‘proper’ physicist.”

What might be next major development in the foundations of quantum mechanics? Some interviewees thought it will be the experimental demonstration that, as Leggett put it, “quantum mechanics is not the whole truth about the physical world”—in other words, that we will find a deeper, more general theory, with quantum mechanics simply reduced to an approximation. Daniel Greenberger, at City College of the CUNY, however, isn’t so sure of the prospects. “I think looking for the order in the universe is a noble enterprise, and I like to be part of it, but I am highly skeptical of the outcome,” he said. “Finding the ‘theory of everything’ is a pretty tall order for creatures who understand almost nothing.”

So, now that I have seen all the answers—all three hundred pages of them—what are my overall observations and conclusions about the state of quantum theory? Too many things to mention come to mind, and anyway I wouldn’t want to bias your own reading. But one observation has been robust and is worth mentioning. What the interview answers suggest is that what's happening today is not so much one interpretation fighting another, but rather a sharp contrast, in mindset and approach, between two camps, each encompassing a group of interpretations. The first camp wants to exorcise the observer from the theory and embed quantum theory into a realist interpretive framework with an explicit ontology (that is, with an explicit account of what *is*). The second camp looks at the quantum formalism as a tool for representing an observer’s knowledge, an attitude that in many cases goes along with a desire to understand why we have this formalism to begin with and what particular features of nature make it so successful.

I closed the interviews by asking my interviewees what single question about the foundations of quantum mechanics they would want to put to an omniscient being. But not everyone took the bait, and some gave the question a new spin. “There are no omniscient beings,” Fuchs said. “I believe this is one of the greatest lessons of quantum theory. For there to be an omniscient being, the world would have to be written from beginning to end like a completed book. But if there is no such thing as the universe in any completed and waiting-to-be-discovered sense, then there is no completed book to be read, no omniscient being.” Greenberger didn’t quite warm up to my question either. “Would you really want to live in a universe that was so simple that you could understand it, even if God himself tried to explain it to you?”

Caslav Brukner, a physicist at the University of Vienna, was even more curt. “Who cares about the foundations of quantum mechanics when offered an exclusive opportunity for posing a single question to an omniscient being?”

---

You can check out free samples of the book here, and order a copy here.

THE PARTICIPANTS

Guido Bacciagaluppi, Caslav Brukner, Jeffrey Bub, Arthur Fine, Christopher Fuchs, GianCarlo Ghirardi, Shelly Goldstein, Daniel Greenberger, Lucien Hardy, Anthony Leggett, Tim Maudlin, David Mermin, Lee Smolin, Antony Valentini, David Wallace, Anton Zeilinger, and Wojciech Zurek.

THE QUESTIONS

1. What first stimulated your interest in the foundations of quantum mechanics?

2. What are the most pressing problems in the foundations of quantum mechanics today?

3. What interpretive program can make the best sense of quantum mechanics, and why?

4. What are quantum states?

5. Does quantum mechanics imply irreducible randomness in nature?

6. Quantum probabilities: subjective or objective?

7. The quantum measurement problem: serious roadblock or dissolvable pseudo-issue?

8. What do the experimentally observed violations of Bell's inequalities tell us about nature?

9. What contributions to the foundations of quantum mechanics have, or will, come from quantum information theory? What notion of information could serve as a rigorous basis for progress in foundations?

10. How can the foundations of quantum mechanics benefit from approaches that reconstruct quantum mechanics from fundamental principles? Can reconstruction reduce the need for interpretation?

11. If you could choose one experiment, regardless of its current technical feasibility, to help answer a foundational question, which one would it be?

12. If you have a preferred interpretation of quantum mechanics, what would it take to make you switch sides?

13. How do personal beliefs and values influence one's choice of interpretation?

14. What is the role of philosophy in advancing our understanding of the foundations of quantum mechanics?

15. What new input and perspectives for the foundations of quantum mechanics may come from the interplay between quantum theory and gravity/relativity, and from the search for a unified theory?

16. Where would you put your money when it comes to predicting the next major development in the foundations of quantum mechanics?

17. What single question about the foundations of quantum mechanics would you put to an omniscient being?


Hanny's Voorwerp, and Other Year-End Goodies
By WILLIAM OREM • Dec. 30, 2011 @ 17:13 GMT



The new year is upon us: first step—according to the Gregorian calendar, anyway—of our next long loop around the sun. Time to look back over some of the most interesting Foundational stories from the previous trip.

Here, in no order, are five which caught my eye. Feel free to add your own.

Number one:

If the LHC has finally produced a Higgs boson, will a Higgs singlet be seen jumping backward in time?

“One of the major goals of the collider is to find the elusive Higgs boson: the particle that physicists invoke to explain why particles like protons, neutrons and electrons have mass. If the collider succeeds in producing the Higgs boson, some scientists predict that it will create a second particle, called the Higgs singlet, at the same time.

“According to Weiler and Ho's theory, these singlets should have the ability to jump into an extra, fifth dimension where they can move either forward or backward in time and reappear in the future or past.

"One of the attractive things about this approach to time travel is that it avoids all the big paradoxes," Weiler said. "Because time travel is limited to these special particles, it is not possible for a man to travel back in time and murder one of his parents before he himself is born, for example. However, if scientists could control the production of Higgs singlets, they might be able to send messages to the past or future."

Pet Peeve: Writing popular science, one becomes rapidly fatigued by seeing the grandfather paradox used to explain the logical problems with retrograde temporal motion. Not only is it a big yawn, there are several more interesting paradoxes out there. One, nicely touched on by FQXi member Brian Greene in his book “The Fabric of the Cosmos,” (recently televised, btw, and including interviews with Max) is a time-traveler’s evident ability to generate information ex nihilo.

In cartoonish form, imagine jumping forward in time, looking up a few articles in Physical Review that have your name in the byline, jumping back with copies of the articles, transcribing, and submitting them. Where did the information contained in the articles—information about how nature actually works—come from? No logical laws have been broken in this scenario: you wrote the articles; you read over your work; you just did these two things in reverse order. But, somehow, objective knowledge has been created here without any “access point.”

Given the importance of information itself to contemporary, physics-based ontologies, this kind of “information paradox” may be much more apposite than one more dead grandfather.

Two:

“’The key message of my paper is that dark matter may not exist and that phenomena attributed to dark matter may be explained by the gravitational polarization of the quantum vacuum,’ Hajdukovic told PhysOrg.com. ‘The future experiments and observations will reveal if my results are only (surprising) numerical coincidences or an embryo of a new scientific revolution.’

“Like his previous study about a cyclic universe successively dominated by matter and antimatter, Hajdukovic’s paper on a dark matter alternative is also an attempt to understand cosmological phenomena without assuming the existence of unknown forms of matter and energy, or of unknown mechanisms for inflation and matter-antimatter asymmetry. In the case of the fast rotational curves of galaxies, he explains that there are currently two schools of understanding the phenomenon.

“’The first school invokes the existence of dark matter, while the second school invokes modification of our law of gravity,’ he said. ‘I suggest a third way, without introducing dark matter and without modification of the law of gravity.’”



Readers of this blog know I am something of a dark matter skeptic myself—and a string skeptic, actually—though I wouldn’t go so far as to bet against either (skeptic, not denier). Still, we’ve talked before here about how dark matter is an absolutely ginormous revision to the very idea of what the cosmos is made of, based, largely, on what could equally well be a mistake over gravitation. “Dark” is certainly one way to explain rotational speeds, and gravitational lensing, but there are others—others that, as odd as they may be, don’t require us to assume that some 96% percent of the universe has never even been glimpsed.

That said, if there is any place on the web where “ginormous revisions” of our basic scientific views will find a rational audience, it’s FQXi.

Speaking of which . . .

“It is a concept that forms a cornerstone of our understanding of the universe and the concept of time – nothing can travel faster than the speed of light.

“But now it seems that researchers working in one of the world's largest physics laboratories, under a mountain in central Italy, have recorded particles travelling at a speed that is supposedly forbidden by Einstein's theory of special relativity.

“Scientists at the Gran Sasso facility will unveil evidence on Friday that raises the troubling possibility of a way to send information back in time, blurring the line between past and present and wreaking havoc with the fundamental principle of cause and effect.”

My money says ‘no’ on this one. The apparent violation—sixty billionths of a second—is still within the possibility of experimental error. Of course, if anything is going to be seen doing paradigm-splitting weirdness, it’s a neutrino. Could neutrinos briefly hop into another dimension, shortening their overall trip while not exceeding the local speed limit? Could they temporarily convert into tachyons . . . some kind of superluminal neutrino decay?

Four.

Are universal constants actually constant? And, are they universal?



“A cherished principle in science - the constancy of physics - may not be true, according to the research carried out at the University of New South Wales, Swinburne University of Technology and the University of Cambridge.

“The study found that one of the four fundamental forces, electromagnetism - measured by the so-called fine-structure constant and denoted by the symbol alpha - seems to vary across the Universe.”

One thing that is constant is the questioning of constancy, and it remains, for me at least, universally intriguing.

Number Five is not technically Foundational, but should be of interest to anyone invested in our ongoing quest to become a space-faring species—as well as the ambiguous feelings that come along with commercialization of that impulse:

“Spurred by a $30 million purse put up by Google, 29 teams have signed up for a competition to become the first private venture to land on the Moon. Most of them are unlikely to overcome the financial and technical challenges to meet the contest deadline of December 2015, but several teams think they have a good shot to win — and to take an early lead in a race to take commercial advantage of our celestial neighbor.”

On that note, just after the turn of last new year, NASA released some images that led to my favorite science news headline of the year: “Hubble Telescope Zeroes in on Green Blob in Space.” (Contenders include “Giant Space Blob Glows From Within” and the ever-popular “Is there a ring of debris around Uranus?”) The green blob story also gave rise to my currently favorite single line:

“The Hubble Space Telescope got its first peek at a mysterious giant green blob in outer space and found that it’s strangely alive.”

A living, galaxy-sized green blob named “Hanny’s Voorwerp”? The late Douglas Adams would have been proud.




Proving You Are Where You Say You Are: Position-Based Quantum Cryptography (Part II)
By GEORGE MUSSER • Dec. 20, 2011 @ 22:21 GMT

This is the second part of a post on garden hoses, quantum cryptography, and schemes to verify position. The first part, which details the first proposed scheme to verify your position--and exactly how you can cheat it with the help of two friends--is here.

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Having been cheated once (see Figures 1 and 2, in part I of this post), the two verifiers, to try to foil you, might ask you to perform a task that requires information from both of them (Figure 3). For instance, they each send you a number and demand that you calculate the sum, on the principle that you need to be where you say you are in order to receive the two numbers and add them up.

But this system, too, is fool-prone (Figure 4). Each of your comrades can intercept the signal, make a copy, and relay it to the other. That way, both can perform the calculation and send back the result just as if you were really there.

Because this fakery requires your comrades to copy the information, the verifiers figure they can ensnare you by exploiting ideas from quantum cryptography--namely, that a quantum state cannot be reliably copied (Figure 5). The verifier on the left sends you a photon in a certain quantum state and the one on the right sends you a single ordinary bit. To confirm your position, you need to return the photon to the left if the bit is 0 and send it on the right if the bit is 1. Unable to make a copy of the photon, your comrades are at a loss for how to cover for you.

But being good quantum physicists, not to mention good friends, they soon realize a trick. In advance, they prepare entangled pairs of particles (Figure 6 and Figure 7). These pairs can transmit the quantum state – and therefore, in effect, the photon itself -- between them using the process known as quantum teleportation. This allows them to set up a game of photon hot potato. The friend on the left side of your purported position intercepts the photon and teleports it to your friend on the right. If the bit is 1, the friend keeps it and sends it to the right verifier; if it is 0, the friend on the right teleports the photon back to the left.

Last year Buhrman and his colleagues proved that in general, no quantum cryptographic scheme is guaranteed to expose a location cheater (see arXiv:1009.2490v4) and recently they laid out a general recipe for evasion. That’s where the hoses come in (see arXiv:1109.2563v2). Teleporting a particle is like pouring water in a hose--it comes out the other side. Each of your friends connect the hoses in a certain way so that the photon flows back and forth and ends up on the side where it’s supposed to be. Buhrman demonstrated this on stage with hoses he’d bought at a garden supply store. We were all grateful he brought a towel.

The whole thing sounded very reminiscent of the holographic principle. (For more on the holographic principle, see this Scientific American article, “The Illusion of Gravity.) One of the deepest principles in quantum gravity, it holds that the amount of information within a region of space does not scale with the volume but with the area. It is always possible to simulate the goings-on within a region of space perfectly by manipulating its boundary. You really can read a book by its cover.

That is precisely what is happening in position evasion. Your two comrades are the boundary in one dimension, and they can connive to pretend that you’re in between them when you’re not. “By being on the surface, you can simulate any behavior on the inside,” Buhrman told me. So there is a very general reason why cheaters can always succeed. And this is in a system with no gravity.

It is mildly disturbing that it is always possible to fake a position. The verifiers do have the last laugh, though. They can place such exorbitant demands on your friendship, requiring them to share an impractically large number of entangled pairs of particles and execute elaborate schemes for tossing the photon back and forth between them, that even your BFF would probably sell you out.

I am grateful to Theiss Research, under whose auspices I received an FQXi mini-grant to help pay my way in Singapore.

Diagrams adapted from slides by Harry Buhrman.


Proving You Are Where You Say You Are: Position-Based Quantum Cryptography (Part I)
By GEORGE MUSSER • Dec. 20, 2011 @ 22:08 GMT

When a speaker brings a tangle of garden hoses, a bottle of water, and a towel to the podium, you know it’s going to be a fun talk. Computer scientist Harry Buhrman of the Centrum Wiksunde & Informatica in Amsterdam recently visited Singapore to help celebrate the fourth anniversary of the Centre for Quantum Technologies. He and his props gave quantum cryptography a whole new dimension--literally. Instead of encrypting a message or authenticating someone’s identity, Buhrman posed a deceptively simple question: Can someone be sure you are where you say you are?

Figure 1

Figure 2

Position verification isn’t just of interest to spouses who wonder about all those late nights at the office. It would tighten up secure communications channels and make it possible to send Mission Impossible-style messages that could be read only if someone visited a certain location (like geocaching puzzles). For fundamental physicists, the procedure raises an important question about the operational meaning of space. If you cannot confirm, even in principle, whether something is at a given location, does the concept of location have any objective meaning? In fact, the scenario Buhrman laid out bore a spooky resemblance to the holographic principle in quantum gravity.

Imagine trying to confirm your position in one dimension, along a straight line (Figure 1). A verifier can use the procedure of Einstein synchronization: send you a signal, to which you reply, and measure the round-trip time in order to gauge the distance between the two of you. It is the maximum distance; you might be closer, given the delays that could arise in the process. To narrow down your location, a second verifier on the opposite side of you also sends a signal that you must reply to. This technique goes by the name of distance bounding.

It’s easy to game the system, however (Figure 2). Two comrades intercept the signals, wait a short while (to simulate the travel time to your purported position), and reply on your behalf. Now you don’t need to be where you claim to be. With your friends covering for you, you can safely leave the office and have your affair.

(Images courtesy of Harry Buhrman.)

More nefarious scheming in the second part of this post, where things get quantum...


Greetings from Goa
By BRENDAN FOSTER • Dec. 17, 2011 @ 15:04 GMT

Greetings from Goa, India, on the shores of the Arabian Sea. I am here at the International Conference on Gravitation and Cosmology, hosted by the Indian Association for General Relativity and Gravitation and the International Centre for Theoretical Sciences, part of theTata Institute of Fundamental Research. Goa has changed a lot since the first ICGC meeting here in 1987, but the beaches and hospitality still make for a most pleasant spot to talk about physics.

In the next week or two, I hope to post more about the events and talks here at the conference. I am here in Goa representing FQXi, because we hope to expand our connections to the research community in India. A quick look at our Member list shows that we have just two members currently based in India, despite the many researchers here studying foundational questions.

My task here is to find out how FQXi can best help the Indian research community. One of our strengths as a funding agency is the flexibility of our funding -- we can support projects around the world, and we can support projects of many forms and function. Case in point, we were able to help with the conference by supporting the attendance of members of the Indian press. The worldwide public has a growing interest in science, and the attendance of the press at meetings like this help spread the word about good science.

Stay tuned for more.

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