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The podcast features attendees at the New Directions in the Foundations of Physics meeting, held annually in Washington, DC. This meeting is one of the only recurring meetings that brings together physicists and philosophers in the same room to discuss the state of the art in their fields.
Following Sabine, Matt, and Dag, the group at large turned to Star Trek, tiny books, physics slogans, and more. On the recording, you’ll hear from Michael Fisher, Alexei Grinbaum, Jos Uffink, Alex Wilce, David Wallace, Melissa Jacquert, and Mile Gu.
Visit the podcast page to listen and find links to much more, including Sabine and Matt’s blogs.
Just to let you know that after a couple of special podcast editions from the quantum foundations meeting in Erice, Italy, Brendan and I are back with the regular podcast.
In this month's podcast, we're celebrating poetry about physics and maths. In April, I visited Penn State University, where I met with Emily Grosholz, a poet and philosopher of math and science, who works at the Institute for Gravitation and the Cosmos, which is headed up by FQXi’s Abhay Ashtekar. Grosholz shares three of her poems with us on the podcast: In Praise of Fractals, The Dissolution of the Rainbow, and Among Cosmologists. (Evelyn Lamb, who blogs over at Scientific American, has a nice review up of one of Groshloz’s anthologies here.)
At the COST quantum foundations meeting in Erice, Italy, that I attended back in March, I met with FQXi’s Sorin Paraoanu. You’ll know about his new research building an artificial space-time from superconducting quantum interference devices (SQUIDs), from Nicola Jones’ recent article for us. In an extended interview on the site, Paraonu talks a bit more about the progress of the experiment, as well as his recent publication in the New Journal of Physics, about testing the Landau-Zener formula for the transition probabilities between states of a qubit.
And finally, reporter Sophie Hebden talks to FQXi’s Jen Eisert about his quest to find undecidable problems in quantum mechanics. Again, you’ll be familiar with some this if you read Sophie’s article, “Searching for the Impossible” — but listen to he podcast piece for more details and to learn about Eisert’s unusual skill for dating architecture.
A couple of months ago we spoke with quantum physicists Martin Ringbauer and Alessandro Fedrizzi of the University of Queensland, in Australia, on the podcast, about their experiment looking into the nature of the wavefunction. Their results lend support (though not quite definitively) to “Psi-ontic models” that say that, if there’s an objective reality, then the wavefunction is real. Psi-ontic models include Many Worlds, Bohmian mechanics and collapse models. That’s as opposed to “Psi-epistemic models” that say that the wavefunction just reflects our ignorance about the state of reality. I recently wrote an article for Nature that rounds up various experiments—including that Australian one—that are trying to uncover quantum reality. The full article, “What is Really Real?” on Nature’s site, so please take a look. (The image is taken from Nature's twitter feed: @NatureNews.)
When I attended a quantum foundations meeting in Erice, Italy, sponsored by COST, last March, I was surprised to find that the participants were mainly split between Bohmians and Collapse Model fans. Anecdotally, I’d say that Many Worlders dominate FQXi meetings. I’ve already highlighted collapse models on the blog and podcast, but I wanted to take this chance to flag up a podcast special recorded in Erice with Shelly Goldstein of Rutgers University and Jean Bricmont of the Catholic University of Louvain, who explain what Bohmian mechanics is and present their case for why they believe that it makes the most sense — and talk about why they feel it hasn’t had a fair hearing over the years.
Bohmian Rhapsody: Physicists Shelly Goldstein and Jean Bricmont discuss features of the deterministic alternative to standard quantum mechanics proposed by David Bohm. From the Quantum Foundations meeting in Erice, Italy, supported by COST.
In the Nature article, I mention the oil droplet experiments that have been getting a lot of attention over the past couple of years because they appear to show droplets “walking” along an oil surface, guided by their own ripples, analogous to the predictions made by Bohmian mechanics. (See the “Pilot Wave Hydrodynamics" forum thread, suggested by John Merryman.)
But there were a couple of other tests to note, which didn’t make the final cut. Bricmont pointed me to the first, which was published in Science. Bohmian mechanics, unlike standard quantum theory, says that particles have definite locations, even before they are observed, and it makes predictions for the paths taken by these particles as they move through, for instance, the double slit experiment. In 2007, Howard Wiseman, a quantum physicist at Griffith University, came up with a way to sneakily track the paths taken by photons in a double-slit experiment using “weak measurements” that allow physicists to quickly peek at an experiment while it is in progress. This disturbs the photon’s location slightly, so no single measurement can indicate where the photon would have been, had it not been observed. But by repeating the test many times, it is possible to build up a statistical picture of the “classical trajectories” taken by photons through the apparatus, which is just what a team led by Aephraim Steinberg at the University of Toronto, Ontario, did, in 2011.
The paths they found tended to correspond with those calculated using the Bohm model, a similarity that Steinberg told me he found “thought-provoking”. The experiment, he said, “underscores the elegance of the Bohm model,” though he added that it does not serve as proof of the interpretation. That’s because the trajectories are not seen directly while the experiment is in progress, but inferred after the experiment has taken place and all the results are in. A Many-World’s fan could thus argue that the trajectories only represent where the particles would have been *if* they took classical trajectories, rather than showing the definite trajectories they actually took. Bricmont agrees that experiment doesn’t prove Bohmian mechanics, but feels that the similarities are striking enough that the results should inspire more physicists to take another look at Bohmian theory.
Owen Maroney, a physicist at Oxford University, also highlighted a paper by Samuel Colin and Antony Valentini at Clemson University in South Carolina, who last year analysed a modified version of the model and calculated that it would have led to a subtly different pattern of quantum fluctuations in the early universe than is predicted by conventional quantum theory and the Many-World’s interpretation (arXiv:1407.8262v1). These signatures could show up in measurements of the cosmic microwave background currently being made by the Planck satellite.
As I mention in the Nature story, there are hopes that it could be just a matter of months before an experiment is carried out that completely rules out Psi-epistemic models and favours Psi-ontic models. But even if that is successful, there is another quantum interpretation that I didn’t cover in the article that would not be affected one way or the other by such tests, even though it is not a Psi-ontic model: quantum Bayesianism (or QBism). This is a relatively recent model based on classical Bayesian probability theory that rejects the notion that the results of quantum experiments can directly access an external objective reality that is independent of the agent making the measurements. According to Q-Bists, the results of quantum measurements are intimately tied to the presence of an agent and serve to change the agent’s degree of belief about what their personal future experience will be.
I asked Ruediger Schack, a QBist at Royal Holloway University in London, about why QBism escapes. He explained that is because the definitions of Psi-epistemic and Psi-ontic — and the wavefunction reality tests — are valid within a specific framework, called an ontological model (by Nicholas Harrigan and FQXi’s Robert Spekkens). “In this framework, outcome probabilities of measurements are determined by some real property lambda. Such models used to be called hidden variable models,” said Schack. QBists, however, do not subscribe to this framework. The Copenhagen interpretation also lies outside this framework.
Maybe it’s time to do a survey. What’s your favourite quantum interpretation? And you think there will ever be a test that could help people choose between various interpretations? Could there be a test that would make you change your mind?
Yesterday afternoon at the quantum foundations meeting in Erice (supported by COST) we celebrated the 80th birthday (somewhat in advance) of GianCarlo Ghirardi who famously worked on collapse models, in an attempt to deal with the quantum measurement problem. He’s the “G” in GRW collapse theory. (Ghirardi is pictured here — a bit fuzzily —being presented with a gift by Catalina Oana Curceanu and Detleft Duerr.)
Physicists Angelo Bassi and GianCarlo Ghirardi discuss collapse models. From the COST quantum foundations meeting in Erice, Italy.
I’ve just posted a special podcast with interviews with physicist and meeting organiser Angelo Bassi and Ghirardi himself. Bassi talks a bit about the motivation behind collapse models and what they are, but they basically try to help explain why the classical world we see around us involves people and things in definite places, while one small scales, particles exist in a fuzzy uncertain realm.
The idea is that the wavefunction of particles can undergo spontaneous collapse, but in the case of individual particles, the odds of this happening are slim, so on the microscopic level you should see the same sort of things that standard quantum mechanics predicts. But when you bring lots of particles together in a macroscopic object, the probability of collapse shoots up — and hence they behave classically.
But how do you test this idea? There’s nothing in principle in standard quantum mechanics that prevents ever larger particles (even cats) being in quantum superposition, if you can prepare your experiment carefully enough (which is tough to do). By contrast, GRW predicts that above some certain mass limit, collapse is inevitable, no matter how pristine your experiment. So that gives you something different to search for.
In a previous post, I mentioned matter-wave interferometry experiments. Yesterday, FQXi’s Hendrik Ulbricht, of the University of Southampton, talked about efforts to see quantum effects in ever larger objects — cold atoms, molecules, metal clusters or nano particles, and even cantilevers — but at the moment they are not well-developed enough to be able to test collapse models. Another problem is that if you carry out such a test and you do see a loss of quantum effects, it might have been caused by problems with the experiment, and decoherence due to interactions with the environment, rather than revealing something fundamental.
The blurry image shows a dog who apparently loves physics — he gatecrashed the meeting for two days running (in search of Schrodinger’s cat?). The first time, he ran onto the stage with the lecturer, who was speaking about quantum biology. The second time, he stopped in front of the stage and barked loudly at the speaker, who was talking about string theory. Make of that what you will!
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