Monthly Archives: September 2022

Quantum theory is perhaps the most successful scientific idea ever. So far, it has never been proved wrong. It is stupendously predictive, it has clarified the structure of the periodic table, the functioning of the sun, the colour of the sky, the nature of chemical bonds, the formation of galaxies and much more. The technologies we have been able to build as a result range from computers to lasers to medical instruments.

Yet, a century after its birth, something remains deeply puzzling about quantum theory. Unlike its illustrious predecessor, Newtons classical mechanics, it does not tell us how physical systems behave. Instead, it confines itself to predicting the probability that a physical system will affect us in one way or another. When an electron is fired from one side of a wall with two holes, for instance, quantum theory tells us where it will end up on the other side, stubbornly saying nothing plausible about which hole it has gone through. It treats any physical system as a black box: if you do this to it now, it will react like that later. What happens in between? The theory simply doesnt tell us.

Many scientists are content with this, but others are puzzled. Among the latter, some make hypotheses: they propose complicated stories about parts of nature that are hidden from us for ever, or multiple universes that underpin the part of reality we do see. Others resign themselves to the notion that science is not about what things really are: it is only about what we are able to directly observe.

Another idea has recently begun to catch on. Perhaps there is no need to make anything up about what lies behind quantum theory. Perhaps it really does reveal to us the deep structure of reality, where a property is no more than something that affects something else. Perhaps this is precisely what properties are: the effects of interactions. A good scientific theory, then, should not be about how things are, or what they do: it should be about how they affect one another.

The idea seems radical. It pushes us to rethink reality in terms of relations instead of objects, entities or substances. The possibility that this could be what quantum physics is telling us about nature was first suggested a quarter of a century ago. For a while it remained largely unnoticed, then several major philosophers picked it up and began to discuss it. Nowadays interest in the idea, called the Relational Interpretation of Quantum Mechanics, is steadily growing. It is a possible solution to the puzzle of quantum theory: what quantum phenomena are is evidence that all properties are relational.

There is a strikingly similar definition of existence at the root of the western philosophical tradition. Platos The Sophist contains the following phrase: Anything which possesses any sort of power to affect another, or to be affected by another, if only for a single moment, however trifling the cause and however slight the effect, has real existence; and I hold that the definition of being is simply action. [] And in the eastern tradition, the Buddhist philosopher Ngrjunas central notion of emptiness (nyat) tells us that nothing has independent existence: anything that exists, exists thanks to, as a function of, or according to the perspective of, something else.

So maybe this is not such a radical idea after all. We all know that a chemical substance is defined by how it reacts, a biological species is defined according to the niche it occupies in the biosphere, and what defines us as human beings is our relationships. Think of a simple object such as a blue teacup. Its being blue is not a property of the cup alone: colours happen in our brain as a result of the structure of the receptors in the retina of our eyes and as a consequence of the interactions between daylight and the cups surface. Its being a teacup refers to its potential function as a drinking vessel: for an alien who doesnt know about drinking tea, the very notion of a teacup is meaningless. What is more, its stability as an object depends on the timescale in which we consider it: take a longer view and it is just a fleeting aggregation of atoms. And are these atoms themselves independent elements of reality? No they are not, as quantum theory shows: they are defined by their physical interactions with the rest of the world.

So quantum physics may just be the realisation that this ubiquitous relational structure of reality continues all the way down to the elementary physical level. Reality is not a collection of things, its a network of processes.

If this is correct, I think it comes with a lesson. We understand reality better if we think of it in terms of interactions, not individuals. We, as individuals, exist thanks to the interactions we are involved in. This is why, in classic game theory, the winners in the long run are those who collaborate. Too often we foolishly measure success in terms of a single actors fortunes. This is both short-sighted and irrational. It misunderstands the true nature of reality, and is ultimately self-defeating. I believe, for example, that we make this mistake all the time in international politics. Prioritising individual countries, or groups of countries, over the common good, is a catastrophic error. It leads to the devastation of war and prevents us from addressing the true challenges that all of humankind a node in natures network faces as a whole.

Carlo Rovelli is a professor of physics. To support the Guardian and the Observer buy a copy at guardianbookshop.com. Delivery charges may apply.

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Helgoland: Making Sense of the Quantum Revolution by Carlo Rovelli (Allen Lane, 9.34)

The World According to Physics by Jim Al-Khalili (Princeton, 12.99)

Theaetetus & Sophist by Plato (Cambridge, 17.99)

Meeting the Universe Halfway: Quantum Physics and the Entanglement of Matter and Meaning by Karen Barad (Duke, 23.99)

More:

The big idea: why relationships are the key to existence - The Guardian

Mayer Hall recognized as the birthplace of density functional theory

(L-R): Dean Boggs, Professor Schuller, Professor Emeritus Sham and Executive Vice Chancellor Simmons hold the plaque commemorating Mayer Hall as a historic landmark. Photos by: Daniel Orren / UC San Diego Health.

Is there magic in the walls of Mayer Hall? This is the question Oleg Shpyrko, chair of the Department of Physics at the University of California San Diego, asked the audience gathered in the auditorium for a daylong series of events to celebrate the buildings designation as a historical site by the American Physical Society (APS).

Mayer Hall, after all, was named after famed theoretical physicist Maria Goeppert Mayerthe second woman ever to win the Nobel Prize in physics. It was also the birthplace of metamaterials which, among other things, have been used to create Harry Potter-like invisibility cloaks. In the labs of Mayer Hall, many novel high-temperature superconductors and quantum materials were developed. It was also in Mayer Hall where Walter Kohn and Lu Jeu Sham created the Kohn-Sham equation as part of their work in establishing density functional theory, or DFT.

Shpyrko concluded that, no, there wasnt magic inside the walls of Mayer Hall, but there was magic in the people who worked there.

And there was magic in the pivotal Kohn-Sham equation. Its subsequent impact on everything from new materials design to drug discovery led APS to designate Mayer Hall a historical site, stating that DFT is the most used technique for calculating the properties of nuclei, molecules, polymers, macromolecules, surfaces and bulk materials in the chemical, biological and physical sciences.

In the early part of the 20th century, the development of quantum mechanics allowed physicists to learn about the properties and behavior of atoms. Traditionally, the Schrdinger equation was used to determine the probabilistic location and behavior of a particle, including the complexity associated with quantum superposition, which is the basis of the famous Schrdingers cat paradox.

As a result, this equation requires a significant amount of computational effort for each individual electron as well as interactions with every other electron and nuclei. Even a single water molecule contains 10 electrons. Thus, determining the electron behavior of larger molecules quickly becomes prohibitive, akin to controlling the behavior of hundreds of quantum-mechanical Schrdingers kittens who are actively interacting with each other while occupying many locations at once.

From 1964-1966, Kohn and Sham laid the foundation of a computation method based on a single-particle approach, which became known as the Kohn-Sham equation and formed the basis of density functional theory.

DFT simplified the previous process by using the density of all the electrons in the system to determine electron behavior. Researchers no longer needed to focus on each individual electron, but used their collective density as the single variable to solve for, transforming the way quantum mechanics research was performed.

DFT is known as an ab initio, or first principle method, because it can predict material properties for unknown systems without any experimental input. So while it does not precisely solve the Schrdinger equation, it does offer a close approximation at a fraction of the computational effort.

Understanding the electronic properties of complex systems is essential to the design and engineering of new materials and drugs. DFT has been used to study and develop the properties of important materials such as novel semiconductors, new catalysts, neuromorphic materials and complex molecules.

For instance, drug discovery uses DFT as a fast and efficient method to limit the number of drugs that must be experimentally tested for their efficacy in the treatment of many diseases. Thanks to DFT, the time and cost of drug development have been considerably reduced.

The UC San Diego School of Physical Sciences and the physics department worked together to create an engaging, informative day of events to celebrate Mayer Halls designation. Although APS officially named Mayer Hall a historic site in 2021, the celebration was postponed until now due to the pandemic.

Distinguished Professor of Physics Ivan Schuller and Shpyrko welcomed attendees before opening the day with a series of lectures on the impacts of DFT. Researchers and experts from around the world provided insight into the ways DFT continues to shape science, engineering and medicine. The talks touched on everything from materials physics and molecular dynamics to drug discovery and supercomputing.

Dean Boggs spoke about the spirit of discovery that exists in the School of Physical Sciences.

We were thrilled to welcome everyone in-person for this event, stated Dean of the School of Physical Sciences Steven E. Boggs. More than just background on DFT itself, these talks highlighted the spirit of discovery that is still present on our campus. The School of Physical Sciences has lived at the heart of that spirit since the universitys founding.

After the lectures and a panel discussion, the university held a dedication ceremony and plaque unveiling. From APS, President Jon Bagger and former President Jim Gates commented on how meaningful the designation was and the continuing importance of DFT.

UC San Diegos Executive Vice Chancellor Elizabeth H. Simmons noted that the groundbreaking work of Kohn, Sham and colleague Pierre Hohenberg was only one example of the extraordinary talent found in the School of Physical Sciences.

The efforts of faculty like Kohn, Sham, Mayer, Roger Tsien, Sally Ride, Harold Urey and others are testament to our universitys remarkable history as a community of visionaries who push boundaries and break barriers to change the world, she said. Their transformative impacts across academic disciplines and in the lives of student and faculty colleagues will continue to reverberate into the future.

Excerpt from:

Honoring a UC San Diego Landmark and Its Lasting Impact on Physics - University of California San Diego

Sabine Hossenfelder is a theoretical physicist and creator of the popular YouTube series Science Without the Gobbledygook. In her new book Existential Physics, she argues that some of her colleagues may have gotten a little too excited about wild ideas like multiverse theory or the simulation hypothesis.

If you want to discuss them on the level of philosophy, or maybe over a glass of wine with dinner because its fun to talk about, thats all fine with me, Hossenfelder says in Episode 525 of the Geeks Guide to the Galaxy podcast. I have a problem if they argue that its based on a scientific argument, which is not the case.

Multiverse theory states that an infinite number of alternate universes are constantly branching off from our own. Hossenfelder says its possible to create mathematical models that are consistent with multiverse theory, but that doesnt necessarily tell you anything about reality. I know quite a lot of cosmologists and astrophysicists who actually believe that other universes are real, and I think its a misunderstanding of how much mathematics can actually do for us, she says. There are certainly some people who have been pushing this line a little bit too farprobably deliberately, because it sellsbut I think for most of them theyre genuinely confused.

Hossenfelder is also skeptical of the simulation hypothesis, the idea that were living in a computer simulation. Its an idea thats been taken increasingly seriously by scientists and philosophers, but Hossenfelder says it really amounts to nothing more than a sort of techno-religion. If people go and spit out numbers like, I think theres a 50 percent chance were living in a simulation, Im not having it, she says. As a physicist who has to think about how you actually simulate the reality that we observe on a computer, Im telling you its not easy, and its not a problem that you can just sweep under the rug.

While theres currently no scientific evidence for multiverse theory or the simulation hypothesis, Hossenfelder says there are still plenty of cool ideas, including weather control, faster-than-light communication, and creating new universes, that dont contradict known science. This is exactly what I was hoping to achieve with the book, she says. I was trying to say, Physics isnt just something that tells you stuff that you cant do. It sometimes opens your mind to new things that we might possibly one day be able to do.'

Listen to the complete interview with Sabine Hossenfelder in Episode 525 of Geeks Guide to the Galaxy (above). And check out some highlights from the discussion below.

Sabine Hossenfelder on entropy:

Entropy is a very anthropomorphic quantity. The way its typically phrased is that entropy tells you something about the decrease of order or the increase of disorder, but this is really from our perspectivewhat we think is disorderly. I think that if you were not to use this human-centric notion of order and disorder, you would get a completely different notion of entropy, which brings up the question, Why is any one of them more tenable than any other? Theres just too much that we dont really understand about space and timeand entropy in particular, gravity, and so onto definitely make the statement. I dont think the second law of thermodynamics is as fundamental as a lot of physicists think it is.

Sabine Hossenfelder on creating a universe:

There is nothing in principle that would prevent us from creating a universe. When I talked about this the first time, people thought I was kidding, because Im kind of known to always say, No, this is bullshit. You cant do it. But in this case, its actually correct. I think the reason people get confused about it is, naively, it seems you would need a huge amount of mass or energy to create a universe, because where does all the stuff come from? And this just isnt necessary in Einsteins theory of general relativity. The reason is that if you have an expanding spacetime, it basically creates its own energy. How much mass youd need to create a new universe turns out to be something like 10 kilograms. So thats not all that much, except that you have to bring those 10 kilograms into a state that is very similar to the conditions in the early universe, which means you have to heat it up to dramatically high temperatures, which we just currently cant do.

Sabine Hossenfelder on faster-than-light communication:

I think that physicists are a little bit too fast to throw out faster-than-light communication, because theres a lot that we dont understand about locality. Im not a big fan of big wormholes, where you can go in one end and come out on the other end, but if spacetime has some kind of quantum structureand pretty much all physicists I know believe that it doesits quite conceivable that it would not respect the notion of locality that we enjoy in the macroscopic world. So on this microscopic quantum level, when youre taking into account the quantum properties of space and time, distance may just completely lose meaning. I find it quite conceivably possible that this will allow us to send information faster than light.

Sabine Hossenfelder on community:

When I was at the Perimeter Institute in Canada, they had a weekly public lecture. It was on the weekendso a time when people could actually come, not during work hoursand afterward there was a brunch that everyone would have together, and I know that the people who would attend those lectures would go there regularly, and they would appreciate the opportunity to just sit together and talk with other people who were interested in the same things. This is something that I think scientists take for granted. We have all our friends and colleagues that we talk to about the stuff that were interested in, but its not the case for everybody else. Some people are interested in, I dont know, quantum mechanics, and maybe they dont know anyone else whos interested in quantum mechanics. To some extent there are online communities that fulfill this task now, but of course its still better to actually meet with people in person.

Continued here:

Have Some Scientists Gotten Too Excited About the Multiverse? - WIRED

By Dalian Institute of Chemical Physics, Chinese Academy SciencesSeptember 9, 2022

Lattice distortion in lead halide perovskite quantum dots leads to a fine structure gap and coherent exciton quantum beating. Credit: DICP

Scientists just reported the utilization of lattice distortion in lead halide perovskite quantum dots to control their exciton fine structure. The researchers were led by Prof. Kaifeng Wu from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Dr. Peter C. Sercel from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE). CHOISE is an Energy Frontier Research Center (EFRC) of the U.S. Department of Energys Office of Science.

The study was published in Nature Materials on September 8, 2022.

It is well established that shape or crystal anisotropy in quantum dots, which are tiny semiconductor nanoparticles, results in energy splitting of their optically bright excitons (bound electron-hole pairs). This is known as fine structure splitting (FSS). These excitons form an important playground for quantum information science researchers. For instance, the excitons FSS can be exploited for coherent control of quantum states for quantum computing, or for polarization-entangled photon pairs in quantum optics, although for the latter it is important to suppress the magnitude of splitting.

Traditionally, studying FSS usually requires a single or just a few quantum dots at liquid-helium temperature, because of its sensitivity to quantum dot size and shape. Measuring FSS at an ensemble level, let alone controlling it, appears impossible unless all the dots are made to be nearly identical.

In this study, by using ensemble-level femtosecond polarized transient absorption, the scientists observed clear bright-exciton FSS in solution-processed CsPbI3 perovskite quantum dots, which is manifested as exciton quantum beats (periodic oscillations of kinetic traces).

Even more amazingly, the beat frequency, as determined by the FSS energy, of a given sample can be continuously controlled by changing the temperature. This is an unprecedented result, meaning that now scientists can facilely control FSS through temperature, said Prof. Wu.

The researchers also discovered that the temperature-dependent FSS was related to the interesting, highly-dynamic lattice of lead halide perovskites. Lowering the temperature led to a more distorted lead-iodide octahedral framework.

Calculations suggested that, because these orthorhombic-phase quantum dots were actually still bounded by the pseudocubic family of crystal planes, the lattice distortion results in an avoided crossing fine-structure gap between bright exciton. This gap was responsible for the observed FSS, and it could be detected in spite of quantum dot size and shape heterogeneity across an ensemble sample.

Lattice distortion in CsPbI3 perovskites is well known in the photovoltaic community, as it is connected to the issue of phase stability of perovskite solar cells, but nobody has previously connected it experimentally to the exciton fine structure, said Prof. Wu. Our study demonstrates that this material property can actually be harnessed to control the bright-exciton splitting in quantum dots for quantum information technologies.

Reference: Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI3 perovskite quantum dots by Yaoyao Han, Wenfei Liang, Xuyang Lin, Yulu Li, Fengke Sun, Fan Zhang, Peter C. Sercel and Kaifeng Wu, 8 September 2022, Nature Materials.DOI: 10.1038/s41563-022-01349-4

Link:

Coherent Quantum Beating Induced by Lattice Distortion of Perovskite Quantum Dots - SciTechDaily