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Iron Man talks quantum

Editor’s note: The following piece contains spoilers for Marvel’s Avengers: Infinity War (2018) and Avengers: Endgame (2019).

For those unfamiliar with the final two Avengers films, here’s a very quick, spoiler-heavy, synopsis. In Infinity War, the Avengers are trying to stop the villain Thanos from getting the Infinity Stones – powerful objects that each control an aspect of existence. Unfortunately, things don’t go well and when Thanos brings the Stones together using the Infinity Gauntlet, he wipes out half of the population of the universe with a snap of his fingers. In Endgame, the surviving heroes try to take the Stones off Thanos to perform a reverse snap, but he has already destroyed them. Years pass, and the remaining Avengers try to adapt, until Ant-Man (Scott Lang) reappears from the Quantum Realm where he was trapped and offers a potential method to travel back in time and collect the Infinity Stones from different points in history. And that’s where this mind- and time-bending letter fits in.

 

The following content is a transcription of a letter penned by an individual claiming to be Anthony Edward “Tony” Stark. The letter is dated April 1st, 2023.

I am Iron Man

My name is Anthony Edward Stark. My friends call me Tony. Everyone else calls me Iron Man.

I’m writing this because I’m at a bit of a crossroads. We’ve just completed a most revolutionary experiment – a test jump through time. And it worked. Now it’s time to take the leap back and begin fixing things. But why risk losing myself in a parallel timeline and forever separating myself from my wife and daughter? Because despite having everything I’ve ever wanted, and the two people I never knew I needed, the world is not right. It is not whole.

Five years ago, a half-god, half-raisin named Thanos snapped his fingers, adorned with six fundamentally powerful Infinity Stones. Instantly, half of all life in the universe disappeared. Friends, family and countless strangers were taken — simply erased from existence. We, the Avengers, with all our superpowers and supersuits, weren’t immune to it either. We lost…too much. We had failed. Even when we tried to get the Stones back to perform a reverse “snap”, we found Thanos had already destroyed them. Any hope we had was gone.

It had been five years since the Snap and we’d all tried to move on when, out of the blue, someone who we’d assumed had been “snapped” reappeared – the annoying and stubbornly naïve ex-con and friend to insects, Scott Lang, aka Ant-Man. Using Pym particles he can shrink down to quantum scales, and it turns out he had actually been trapped in the Quantum Realm since the original Snap and only just emerged. But for him, those five years were only five hours. Turns out time doesn’t play by the same rules in the Quantum Realm.

So, now, our ragtag group are going to use this revelation to take an unprecedented leap through time to locate (okay, steal) the six Infinity Stones, which, when united, will give us a chance to make everything right again.

The question is: how will we do it? How will we travel in time? Pure. Quantum. Magic.

You can’t go backwards in time… (but maybe you can go forward to the past)

To anyone with half a Stark brain, it should be obvious that time can only move in one direction: forward. There’s the thermodynamic arrow of time after all.

Time can only move forward because it is the dimension along which we keep track of change. The clock keeps ticking even for a pendulum, which, by almost every definition, seems to go back in time after each swing. However – and the irony does not escape me – we use pendulums to keep track of the passage of time moving forward. Even so, the pendulum has no trouble transitioning to what looks like its thermodynamic past without dragging the rest of the universe along with it. Time, after all, is relative – local to each observer.

Basically, maybe we don’t have to reverse time for everyone and everything in order to access the past. Maybe some of us can detach ourselves from this macro reality and enter a simulated one that looks close enough to some point in our thermodynamic past.

What I mean by simulating the past is a bespoke, fault-tolerant evolution of the quantum state of our time-travelling hero and their local surroundings. I know, it sounds awesome. And slightly dangerous. But, at least, all the paradoxes involving old us being offed vanish into thin air. You are always moving forward in time, even if you are not following time’s arrow like everyone else. How profound is that? You would be the tiniest rebel in recorded history.

So, forget about closed time-like curves and traversable wormholes running on exotic matter. If you want to go to the past, you just have to use good old Schrödinger to time-evolve your quantum state back in time. As your friendly neighbourhood quantum physicist keeps telling you, time goes both ways in the Quantum Realm. There, there is no past and no future.

If only… At best, you may be able to fool the universe for a few milliseconds, before the thermodynamic arrow of time takes over and you lose control of your quantum system. It doesn’t even matter that you are quantum-scale thanks to Pym particles. The second law of thermodynamics, which states that you can only transition to a state with higher entropy and is responsible for the aforementioned arrow of time, only gets more powerful as you shrink. In fact, down there, there is an infinite number of second laws of quantum thermodynamics. And they all point to the same conclusion – we are all going to die. Including the universe.

To be more precise, left to its own devices, a quantum system can only move closer to a version of itself that is completely useless. Quantum thermodynamicists call such states “passive”. You may know them as thermal states, Gibbs states, equilibrium states, whatever states. They all have one thing in common – they are extremely boring. In particular, they are a mixture of perfectly good quantum states, so that when you put them together in a special way, they become collectively useless. How is that possible, you may ask? How is it that even a few particles feel the gravitational pull of the arrow of time? Isn’t thermodynamics a theory about the collective behaviour of a large number of particles? These are interesting questions. The answer is so profound, of such fundamental importance, that even I found it surprising – for a second. Also, the answer should help us reverse the Snap.

So much time, so little to do

The frame rate of human perception is about 150 Hz (150 frames per second). This means that each frame we register represents the end result of an evolution involving, roughly, one trillion quadrillion quadrillion quantum events. Relative to the frame rate of the universe at the Planck scale, we are VERY slow. Don’t take it personally. Even an ultrashort pulse of light, the fastest thing in the universe, looks like a platypus out on a walk at 10 trillion frames per second. And that’s still 30 orders of magnitude slower than the true frame rate of the universe.

Most laws of physics probably only emerge at frame rates many orders of magnitude above the quantum limit at the Planck scale. It doesn’t matter how small you are. The problem with reversing time is the sheer magnitude of time you never get to experience. And what you can’t experience, you can’t control.

With such a large number of random events taking place within every femtosecond of existence, something amazing happens. The maths behind the infamous law of large numbers kicks in and real order can emerge out of pure chaos.

Emergence.

The answer to one of the most important and least understood question in all of science: who is to blame for the tyranny of truth? If all things are possible, how come the universe is so predictable, so boring? Where is my unicorn, damn it!

The world at Infinity

Each Infinity Stone unlocks a gate to a more fundamental aspect of physical reality. The Tesseract, aka the Space Stone, allows one to teleport, by inverting so-called bulk tensors responsible for bridging our world to an underlying space known as the Quantum Code. Physical reality is built on top of an underlying network of entanglement superhighways. That network, and the nodes in it, form the Quantum Code. To move from node to node down there, you teleport. How? Using quantum entanglement, of course. In other words, the illusion of physical movement up here is nothing but a reflection of your information teleporting from one quantum node to the next one down there.

It gets weirder. The Quantum Code is also an illusion, a ghost world. In particular, it is a hologram — a hologram of a world with no gravity. The world at Infinity.

Gravity

Did you know that gravity can exist even in a universe without any matter? As Albert Einstein pointed out over a century ago, gravity is a direct consequence of the curvature of space–time. Curvature measures how units of space and time change as they partially interconvert into each other while you move through space–time, even if you are standing perfectly still. Feels kind of like the pull of gravity, right? You don’t need to do anything to fall to the ground.

As time folds into space, the same second law of thermodynamics responsible for the one-way flow of time begins to infect dimensions of space. Because of the curvature of space–time, units of distance in space, say metres, turn into a mix of time and space — like, 10% second and 90% metre. How is that related to gravity, you ask? If each dimension of space is bi-directional (you can move forward and backwards at will), why can’t you just take off like Captain Marvel and fly out into the void? Here is why: the dimension of space extending from the centre of gravity of the Earth to you is now 10% one-way, because it is now partially composed of time. Sure, with sustained effort you can escape Earth’s gravity, but that’s because that radial dimension of space is only 10% time. Things change drastically if you find yourself near a black hole.

Near a black hole’s horizon, if you are brave enough to cross to the other side, you are no longer travelling along space towards the singularity at the centre – you are travelling along time. The dimension of space connecting you to the singularity while you were safely far away from the black hole’s event horizon, is now completely gone, turned into time. Hence the one-way trip to your doom at the centre of the black hole. You want to escape a black hole? You’ve got to beat the second law. And in order to beat anything, you first need to understand it.

The Time Stone

Whereas the Space Stone allows its wielder to unlock the power of teleportation native to the Quantum Code, the Time Stone (housed within the Eye of Agamotto) is like a construction crew that has the ability to repave the landscape of the Quantum Code with new entanglement superhighways. Destroy some here, create some there, and suddenly what used to be far is now near. When you drop a glass and it breaks, the chemical bonds that used to hold the molecules of the glass together break. Bonds are relationships between electrons at the boundary of each molecule. And every relationship in the microscopic world is a manifestation of entanglement. Rearrange how each node in the Quantum Code is entangled with every other node and you rearrange matter up here – you put the glass back together. Like running the clock backwards.

If the Time Stone allows its wielder to travel back in time, all I needed to do was reverse-engineer it. Unfortunately, for that I would need the Time Stone, and if we had that, none of this would have happened. Thankfully, the protectors and users of this stone – the Masters of the Mystic Arts – keep notes.

What jumped out at me right away was that the Time Stone could only make local temporal changes. I don’t mean local in time. I mean local in space. That may not seem like much, but it was a huge clue as to its inner workings. I already knew that the stone had to mess around with the topography of the Quantum Code, but I didn’t know how it did it. Well, apparently, it could only make local changes in the Quantum Code’s geometry at any given instance. Like a quantum computer that can only run a few quantum gates at a time. In other words, the Time Stone had the ability to replace a local patch of the Hamiltonian driving the dynamics of a region in space–time with a different Hamiltonian – one, which, when turned on could generate changes in the local state of the universe corresponding to different points in time. Of course, to do that, it needed to temporarily rewire the connections between that local patch of space–time and its surroundings. That’s the easy part.

Here comes the hard part: how do you tell the universe, which has plans of its own for how things should be glued together, that you have something else in mind? How can you beat an infinite number of second laws, which are each a consequence of emergent interactions between a quantum system and its vast environment? If you can turn off those interactions for long enough, you have yourself an isolated, coherent quantum system that is ripe for bespoke Hamiltonian dynamics of your choice. But how?!? The answer lies, of course, in the eigenvalue of an inverted Möbius strip.

Just kidding. But not really.

Stone and crystal

You see, the Time Stone is a topological time crystal. It can generate its own quantum dynamics at frame rates beyond our comprehension, despite being driven by the environment at whatever frequency, like the rest of us. More importantly, the stone has the incredible ability to create a temporary version of the world at Infinity (the world with no gravity) at the boundary of the local patch of space–time (and, hence, the underlying Quantum Code) it controls. It is effectively performing topological quantum computation on that local patch, which is impervious to constant poking by the environment. And what do you need in order to create a non-trivial topological phase of matter like the Time Stone? A Hamiltonian with twisted boundary conditions – a version of a quantum Hall system. And guess what has precisely such a twisted boundary… It rhymes with öbius.

It didn’t take long for me and Bruce Banner (my science buddy aka the Hulk) to recreate this effect. The key was having access to the Quantum Code, you know, the Quantum Realm. By creating a device that effectively replaced the boundary of a local patch of the Quantum Code with an environment of my choice, I could generate a new state in that local patch by simulating the time-inverted dynamics of a Hamiltonian whose low-energy states were topologically ordered. The state of the local patch I needed would be labelled by its eigenvalue in the Hamiltonian. And the local patch would include an Avenger shrunk using Pym particles.

But, and this is the last but, how should I choose the final target of the quantum simulation in order to effectively travel back, say, to the 80s? The answer is so simple it hurts — choose a state of the Quantum Code that looks like the 80s. Not the real 1980s. Just my version of it with enough detail to make it seem real to whoever is there. The second laws should kick in at that point and fill in the rest of the details – plus, the arrow of time will do the heavy lifting of simulating the future of the past. The idea is to create a tiny version of our macro world so I can control its initial state. This tiny world will look just like ours from the perspective of its inhabitants, but everything will run much faster there relative to our frame rate. Also, it will run backwards in time for a quick sec, before I let the arrow of time kick in again.

I plan to send Banner to go after the Time Stone, because he is the second smartest Avenger and I hate feeling small. Plus, he is a Hulk of a man. Here is how it’s going play out. He jumps into the Quantum Realm, I turn on my “world at Infinity” modulator until I observe the distinct quantum signature of a topological time crystal in the Quantum Code, I wait for a few minutes before reversing the process (hopefully he has the stone and is back in the Quantum Realm by then), and finally he emerges triumphantly in this timeline with the Time Stone. Easy peasy. We obviously then use the Time Stone to rewind time to near where we last saw the other Infinity Stones, we bring them back, reunite them, reverse Thanos’ snap, return them to their points in history and we are home free.

Simple. What can possibly go wrong? I hope the other Avengers don’t feel too left out. It would be such a pain if everyone decided they wanted to time travel too and go after all the stones at once.

Utrecht University Medical Center showcases flagship AI infrastructure

© AuntMinnieEurope.com
© AuntMinnieEurope.com

An artificial intelligence (AI) infrastructure developed at the Utrecht University Medical Center (UMC) has enabled easier deployment of AI algorithms and enhanced workflow in daily routine practice, according to a presentation given at the European Society of Medical Imaging Informatics (EuSoMII) annual meeting in Valenca, Spain.

Instead of using one-off AI solutions for different systems and modalities, the Utrecht team built a vendor-neutral infrastructure that supports the entire imaging workflow chain from patient selection, image planning, acquisition, reconstruction, analysis, reporting and prognosis, according to Tim Leiner, professor of radiology and chair of cardiovascular imaging at the department of radiology and nuclear medicine at Utrecht UMC. Their pioneering AI system can “listen in” to hospital information streams and then automatically select an appropriate algorithm to be used.

The IMAGR AI infrastructure took three years to build, finally going operational earlier this year, with the first application in January, and routine use since August. With an initial 150,000 euros in funding from Utrecht UMC, the project relied on many volunteer hours. Recently, however, it has received a grant of 620,000 euros from the university.

IMAGR AI system
A neural network on Utrecht UMC’s IMAGR AI system reveals white-matter hyperintensity for dementia questions from brain MRI. (Courtesy: Olivier Middendorp – NRC)

Vendor neutrality

“Companies are trying to step into this field by building cloud solutions, but they aren’t vendor neutral. As far as we know, Utrecht’s IMAGR infrastructure is the first of its kind as an onsite solution for either commercial or in-house algorithms that are integrated with PACS, the RIS and the hospital information system (HIS),” Leiner told AuntMinnieEurope.com ahead of EuSoMII 2019, where he discussed Utrecht’s experience in his keynote lecture, How to Bring AI to the Clinic.

Radiology departments should carefully think about how they deploy AI, he noted.

“Nobody wants to be locked into the solution of a single vendor. The full potential of AI will only be realized if algorithms are deployed in a vendor-neutral AI infrastructure.”

Vendor neutral AI
In his presentation, Tim Leiner detailed the advantages of vendor neutral archives and infrastructures for AI. (Courtesy: Tim Leiner)

Improved workflow

IMAGR listens in to information passing through the HIS and RIS and picks out what it needs to know, Leiner explained. For example, when it recognizes a brain MRI, it will select appropriate algorithms that can aid diagnosis and quantification. The infrastructure then retrieves the images for that specific patient from the PACS, including any priors, if necessary, and goes to work. Each so-called pipeline can opt to send resulting images back to the PACS, or present them in a dedicated AI viewer on the PACS station itself – or both, he noted.

Crucially, IMAGR makes AI fit into and improve workflow. For example, to check for dementia on the brain MRI, the infrastructure automatically starts a pipeline that applies a segmentation algorithm, before applying a white matter hyperintensity detection algorithm. When the radiologist selects the patient’s MRI in the PACS, the IMAGR infrastructure automatically follows and shows the corresponding AI results.

White-matter algorithm
Here, the IMAGR interface (left monitor) shows the results of the white-matter hyperintensity detection and quantification algorithm. The middle monitor shows the PACS interface. The right monitor shows the electronic health record. (Courtesy: Olivier Middendorp – NRC)

The department is continuing to feed algorithms into the system, according to Wouter Veldhuis, associate professor of radiology at Utrecht UMC and initiator of IMAGR, together with Edwin Bennink and Christian Mol.

Also speaking to AuntMinnieEurope.com before the EuSoMII meeting, Veldhuis pointed to the many algorithms that can run in parallel, such as a lung cancer screening AI program on chest CTs.

Furthermore, in this patient population, sarcopenia assessment – a labour-intensive and time-consuming task if performed by individuals – takes just seconds via another neural network on IMAGR that automatically performs a whole-body segmentation to determine fat and muscle content.

“By the time the patient is off the scanner, the radiologist can open the case and read it together with the AI results, because IMAGR has already automatically done the work,” Veldhuis said. “Our target is to have ready-to-read reports within 24 hours, which is perfect for outpatients.”

The AI in the system can also automatically select spine MRI and convert them into CT images, which brings the advantages of CT for orthopaedic pre-surgery planning, without having to irradiate patients, he noted, adding this was an example of a commercial solution (MRIGuidance) running within the infrastructure. Furthermore, the AI-created CT can then generate and display attenuation correction maps.

Quality and ethics

“With IMAGR up and running we can now deploy any AI algorithm directly into the clinical workflow at Utrecht UMC,” Leiner said. “However, we are continuously assessing which ones we want, and which ones we don’t, and how deep the level of clinical integration can be.”

Radiologists needed to learn how to deal with different types of AI errors, and know how to present data gleaned from AI to colleagues in order to make it most useful and actionable, Leiner stated.

It is also important to determine what approach to take for deciding when algorithms were good enough and how to measure quality, as well as thoroughly discussing the ethical implications of the results algorithms generate.

Whether imaging departments wait for US Food and Drug Administration (FDA) approval and/or CE certification for algorithms before implementing them remains a moot point. One view is that it is indeed possible, but with careful monitoring, according to Leiner.

Part of the solution lies in understanding an algorithm’s generalizability, he noted. For example, if an AI program predicts a 30% risk of a myocardial infarct due to a coronary calcium score, there needs to be clarity on how reliable this prediction is, potential variation and whether this statistic factors into exceptional cases. He pointed to the need for thorough algorithm testing on large patient databases before deployment.

Action call

“I would like to make this presentation a call to action on quality standards to ensure that our neural networks demonstrate reproducibility, applicability and generalizability in other patient groups,” Leiner stated.

This means defining quality standards as an imaging community before large-scale rollout. However, different algorithms need different standards, so this process will require a case-by-case approach, he noted, in tandem with regulatory requirements coming into play.

It is also vital that radiologists who are so far resistant to AI understand its potential power, Leiner continued. “Those that use AI will outperform those that don’t,” he said.

  • This article was originally published on AuntMinnieEurope.com ©2019 by AuntMinnieEurope.com. Any copying, republication or redistribution of AuntMinnieEurope.com content is expressly prohibited without the prior written consent of AuntMinnieEurope.com.

4D electron microscopy images charge density at sub-angstrom scales

Scanning transmission electron microscopy (STEM) can be used to detect electric charge density as well as image atoms. This new advance comes from researchers at the University of California, Irvine, who used the technique to show how interfacing a ferroelectric oxide with an insulating oxide can produce an electron-rich region in between the two materials. The work is an important step towards understanding and engineering material structures with strongly-correlated electrons.

Nearly all the physical properties of materials are determined by how electron charge is rearranged between nuclei when atoms aggregate together. Being able to directly visualize how electrons are distributed is therefore important. Compared to other diffraction methods, aberration-corrected STEM (AC-STEM) allows for atomic-scale imaging of a sample using an electron beam, or probe, focused to about half an angstrom in size. When electrons pass through the sample, they interact with the internal electric field in their path through the Lorentz force. This changes the beam’s momentum, which can then be measured by diffraction.

A team led by Xiaoqing Pan has now used a state-of-the-art AC-STEM and a high-speed pixelated electron camera to measure this change and delineate the electric field in a local region of the sample and so derive the electric charge density in this region. The researchers did their experiments on a composite material made from the ferroelectric oxide bismuth ferrite (BiFeO3) and the insulating oxide strontium titanite (SrTiO3).

“We can raster scan a 2D area of the sample using our electron camera and acquire a diffraction pattern at each point on the sample,” explains Pan. “The nice thing is that our detector can acquire 4D STEM images with 512 x 512 pixels at a speed of greater than 300 frames per second.”

Visualizing charge transfer

The researchers say they used their technique to visualize the mechanism of charge transfer between the BiFeO3 and  SrTiO3. “We did this by simply comparing the images we obtained from the two materials,” says Pan. “With the high resolution to determine the local charge distribution, we can see how the positive ionic cores and electrons are separated in BiFeO3.

“By then imaging the interface between the BiFeO3 and SrTiO3, we observe that an electric field from the former can leak through the interface towards the first few layers in the latter, causing the charge in SrTiO3 to accumulate at the interface between the two oxides.”

Although it is still early days for the technique and quantification is difficult, the researchers are confident that it could become a routine method in the future. “Many of our colleagues are developing 4D STEM tools like ours for a variety of purposes and these will eventually allow us to depict microstructures in detail down to the charge level,” Pan tells Physics World.

The team, reporting its work in Nature, says that it will now be looking to investigate the electronic structures in materials that are more inhomogeneous, containing boundaries and defects, for example, because these play an important role in many technologically important materials today – such as semiconductors and catalysts.

“As in any emerging techniques, however, there remain limitations on sample sizes and spatial resolutions in our current set up,” says Pan. “Better understanding the results from any 4D STEM imaging experiment will also require extensive electron scattering simulations and associated theory development.”

Kites that generate electricity, physics of spattered blood, origin of heavy elements

It this latest episode of the Physics World Weekly podcast we meet the physicist Rolf Luchsinger, who wants to use hi-tech kites to generate electricity. He is the co-founder and chief executive of TwingTec and he talks to Margaret Harris about the company’s vision for airborne wind energy.

If television police procedurals are anything to go by, forensic investigators can extract lots of useful information from blood spatter patterns. Sarah Tesh discusses “The physics of blood spatter”, which appears in the October issue Physics World and points out some of the challenges of analysing blood-spatter evidence.

Finally, we marvel at the fact that a recent study of light from the merger of two neutron stars is the first spectroscopic evidence that neutron stars are made mostly of neutrons. That same light also confirms that such mergers are the origins of some of the heaviest elements in the universe.

Student mentoring needs to be better recognized and rewarded, says report

Effective mentoring of graduate and undergraduate students in scientific fields play a key role in their professional success but more needs to be done to make sure it is better recognized and rewarded within US higher education. That is according to a report by National Academies of Sciences, Engineering and Medicine, which set out a series of actions that colleges and universities can take to strengthen their mentoring systems in science, technology, engineering, mathematics and medicine (STEMM) subjects.

The report – The Science of Effective Mentorship in STEMM – points out that well-designed mentoring has a positive influence on academic achievement and career success and satisfaction. However, the report states, most academic institutions have left the process to happen organically or on an ad hoc basis. Furthermore, studies indicate that STEMM students from underrepresented minorities typically receive less mentorship than other students.

There is an opportunity to provide mentors with training in good mentoring practices and that provide mentees with access to a network of mentors

Keivan Stassun

The report emphasizes that while every mentorship is unique, there are certain core behaviours by mentors and their subjects that are “more likely to yield effective mentoring relationships”. These include aligning expectations, building rapport and maintaining open communication. “Mentorship is a learned skill,” the report states, with programmes to foster mentorship skills having been shown “to help mentors and mentees advance their skills in multiple areas”.

Angela Byars-Winston from the University of Madison, Wisconsin, who chaired the 12-strong committee that wrote the report, says there is a gap between what we know about effective mentoring and how it is practiced in the US’s colleges and universities. “A growing body of evidence exists about how to sustain successful, inclusive mentoring relationships,” she says. “Our goal in this work was to bring together the multiple disciplinary ways that mentorship has been studied and investigated across many forms and across many disciplines – and to provide guidance for effective mentoring tools and resources on how to make relationships better.”

Regular assessments

The report provides nine recommendations that institutions and individual scientists should use to “build a culture of inclusive, effective mentorship”. The process starts with the adoption of a definition of STEMM mentorship as “a professional working alliance in which individuals work together over time to support the personal and professional growth, development, and success of the relational partners through the provision of career and psychosocial support”.

The report also recommends that institutions and departments adopt “evidence-based mentoring approaches” and establish structured feedback systems to improve the process at all levels. According to astrophysicist Keivan Stassun from Vanderbilt University, who sat on the committee, that recommendation has a particular resonance for physics. “Physics and astronomy have a strong tradition in the master/apprentice mode,” he says. “But this model traditionally involves a more one-sided approach than what we envision in the report.”

The report states that institutions should create systems that allow young scientists to receive support from more than one mentor, both inside and outside their department. Stassun notes the importance of this for “big science” projects in physics and astronomy. “Within large groups, it is reasonable to expect that it is easier for students to fall through the cracks, but there is an opportunity to provide mentors with training in good mentoring practices and that provide mentees with access to a network of mentors,” he says. “Mentees get more of their mentoring needs met, and each mentor can be clear about what types of mentoring they are, and are not, willing to provide.”

The report also asserts that individual mentors should recognize cultural diversity in their students. Specifically, it states, “they should learn about and make use of inclusive approaches…listening actively, working toward cultural responsiveness, and moving beyond ‘colour-blindness’”. Additionally, the report calls on programme leaders and department chairs to regularly assess the results of mentorship to identify and mitigate “negative experiences”. It concludes that institutions “should reward and visibly recognize mentors for documented, effective, and inclusive mentorship in the same manner as effective teaching is recognized, including through annual rewards”.

Surprisingly stable plutonium compound could affect nuclear waste storage

By accident, scientists in Germany, Russia and Sweden have discovered a compound containing a chemically-stable form of plutonium that has never been observed before. In a series of attempts to synthesize plutonium-containing nanoparticles, the team led by Kristina Kvashnina at the Helmholtz Centre Dresden-Rossendorf found that a compound containing the element in its fifth oxidation state can exist for long periods of time. Their discovery could provide new and crucial information that could help scientists predict the changing properties of harmful nuclear waste over millions of years.

Even when the plutonium waste produced by nuclear power plants is securely stored deep underground, it can eventually leak out into the surrounding environment. This happens when waste interacts with materials in the ground including clay and mineral ores to form suspended liquid mixtures. However, researchers studying the effect now understand that its damage is lessened by reactions that form stable plutonium compounds.

Kvashnina’s team synthesized nanoparticles of plutonium oxide (PuO2) using plutonium atoms in different oxidation states. Then they studied the nanoparticle’s structures using a combination of theory and X-ray observations at the European Synchrotron Radiation Facility in France. As expected, they found that plutonium in its third, fourth and fifth oxidation states formed nanoparticles very quickly. But when in the sixth oxidation state, the reaction seemed to take place over a more unusual, two-step process.

Significant change in our understanding

The researchers deduced that the intermediate step could involve a compound containing a stable form the fifth state. Such a form had never been observed experimentally, but could, in theory, remain solid and stable over periods of several months. If correct, this would be a significant change in our understanding of the chemical properties of nuclear waste, leading Kvashnina and colleagues to be sceptical of their result at first.

To confirm their prediction, the team used the spectroscopic technique of high-energy resolution fluorescence detection at two specific absorption edges of the intermediate compound. These edges are sharp distortions in compounds’ absorption spectra at wavelengths where photon absorption energies correspond to electron transitions — and are the only way to reveal the specific oxidation states of the elements they contain. When combined with theoretical calculations, Kvashnina’s team confirmed for the first time that a stable form of the fifth oxidation state had indeed been observed, within the compound NH4PuO2CO3.

The physicists then confirmed this compound’s stability by drying it out of liquid suspension and measuring the shape of its absorption spectrum over time. Even after three months, the shape of the line remained the same. The team’s result could offer significant new insights into the activity of nuclear waste over million-year periods. By accounting for stable solid phases containing the fifth oxidation state, theoretical predictions of these behaviours could become more accurate; potentially allowing researchers to update their ideas about how nuclear waste should be stored.

The study is described in Angewandte Chemie.

ProKnow DS: Distribute tasks, centralize knowledge, and harness the power of big data

Water splitting on haematite photoanodes changes with light intensity

A new study of water oxidation on the surface of iron oxide (Fe2O3) could lead to the development of more efficient water splitting devices. The work, which is a combined experimental and theory study, reveals that water splitting on Fe2O3 changes with light intensity and that the rate of water oxidation increases with the third power of the surface hole density.

An enzyme called photosystem II (PSII) in plants oxidizes water in the presence of sunlight to split water. This natural catalytic process extracts electrons and protons from water molecules and produces molecular oxygen – a process that has allowed for life to flourish on Earth. The electrons are immediately used to make a chemical fuel called adenosine triphosphate (ATP) that the plant stores and utilizes when needed.

Researchers are trying to mimic these natural photosynthetic mechanisms to harness sunlight and split water into molecular oxygen and hydrogen, or to reduce carbon dioxide to carbon-based molecular fuels. α-Fe2O3 (or haematite) is a promising material for making photoelectrodes for this application, but its catalytic functionality is little understood at the molecular level.

Measuring the kinetics of water oxidation

A team led by James Durrant at Imperial College London, Victor Batista at Yale University, Michael Grätzel of the Swiss Federal Institute of Technology Lausanne and Erwin Reisner at the University of Cambridge has now used optical spectroscopy to measure how the kinetics of water oxidation changes as more holes accumulate on the haematite surface when it exposed to sunlight. The researchers had already employed this technique in previous work to determine rate laws for water and methanol oxidation and proton reduction on various photoelectrocatalytic surfaces.

The technique allowed them to determine rate laws and rate constants for the water oxidation reaction and determine, for example, how many holes have to aggregate to access the rate-limiting step of the reaction and the activation energy for this reaction.

The new experimental studies, undertaken by Camilo Mesa and Laia Francàs at Imperial, show that water oxidation on iron oxide changes with light intensity and that the rate of oxidation increases with the third power of the density of holes on the surface of Fe2O3. “This implicates the equilibria of three holes with the water oxidation catalytic centre, reducing the activation energy for the water oxidation and appears to be a key factor in other studied metal oxides,” explains Mesa.

Thanks to accompanying theoretical work by Yales Ke Yang, the team identified the catalytic sites under low light intensity of an oxidizing Fe(OH)-O-Fe(OH) catalytic core that can extract electrons from water by accumulating up to three oxidizing equivalents (missing electrons or holes) under solar irradiation of 1 sun. This mechanism is similar to the proposed activation catalytic site in PSII, they say – even though haematite and PSII are radically different in composition and structure.

Full details of the research are reported in Nature Chemistry.

Researchers classify city hinterlands

The rural areas, or hinterlands, surrounding cities generally provide a mix of ecosystem services to the urban inhabitants. And that mix typically conforms to one of several archetypes. That’s the conclusion of scientists in Canada, who believe the archetypes reflect the relative prosperity of cities, as well as cities’ ability to exploit one another’s ecosystem services through globalization.

“The ecosystems surrounding wealthy cities tend to produce services like air quality, carbon storage, and water quality,” says Daniel Haberman of McGill University, Canada. “Conversely, ecosystems surrounding poorer cities tend to mainly produce services like crops and livestock, which are usually less resilient ecosystems.”

The ecosystem services provided for urban inhabitants by their city’s hinterlands range from basic supplies such as food and water to less obvious necessities such as climate regulation, air purification and recreation. These ecosystem services vary from place to place, partly due to the properties of the areas themselves. Deserts, for example, are always going to be relatively poor at food production.

Ecosystem services also vary due to socio-economic factors. For instance, it’s clear that as cities become wealthier, their wealth derives less from directly exploiting nearby ecosystems and more on manufacturing and intellectual services, even if the cities still require raw materials from elsewhere. But such links between socio-economics and ecosystem services are poorly understood.

To investigate, Haberman and McGill colleague Elena Bennett gathered dozens of different datasets, including those on climate, crop production and parkland. They combined them with models to create global maps of eight different ecosystem services: crops, water, livestock, carbon sequestration, carbon storage, air quality, water quality and recreation. Then they analysed the hinterlands of the world’s biggest cities to compare how they are exploited to produce the different services.

The archetypal ecosystem-service mixes the researchers found were: desert cities such as Las Vegas, which excel at water quality to the detriment of all other services; South Asian cities such as Shanghai and Mumbai, which are good at producing crops and livestock to the detriment of water quality; tropical cities such as Singapore, as well as others such as Berlin, which specialize in regulating services like carbon sequestration and storage but are poor at producing crops and other basic provisions; and cities all over the world such as Paris and Toronto, which produce average or below-average amounts of all services.

Not all hinterlands conformed exactly to the archetypes, however. “Trade-offs that we typically expect to see – such as high food production with low water quality – are not the case for all cities,” says Haberman. “There are a group of cities that actually manage to use their hinterlands to produce large amounts of food without sacrificing water quality. By looking at cities and their ecosystems around the world, we can spot groups of cities that are creating more ecosystem service synergies and minimising trade-offs, and study those areas more deeply to learn from them.”

The research is described in Environmental Research Letters.

Blood–brain barrier best breached by small molecules

Focused ultrasound (FUS) can be used to help drugs pass from the bloodstream into the brain, but the technique’s effectiveness depends on the ultrasound pressure and the size of the drug molecules. Michael Valdez and colleagues at the University of Arizona measured how thoroughly differently sized molecules diffused into mouse brains under a range of ultrasound intensities, and found that the largest molecules could not be delivered under any safe FUS regime. The results set a limit on the types of drugs that might one day be used to treat neurological conditions like Alzheimer’s and Parkinson’s disease (Ultrasound Med. Biol. 10.1016/j.ultrasmedbio.2019.08.024).

Usually, the brain is isolated from substances circulating in the bloodstream by the blood–brain barrier (BBB), a semipermeable layer of cells that permits only certain molecules to pass. This restricts the range of drugs that can be used in the brain to small, hydrophobic molecules (such as alcohol and caffeine), other small drugs like psychotropics and some antibiotics. Extending that range would open the door to new therapeutic possibilities, says Theodore Trouard, who led the team. “The ability to temporarily and safely open the BBB to allow drugs into the brain would help address a number of neurological diseases for which there is currently no effective treatment.”

Previous research has shown that such opening can be achieved by focusing an ultrasound beam in the brain while gas microbubbles circulate in the blood. The microbubbles – perfluorocarbon-filled lipid shells about 1 µm across – are inert while they move around the body, but rapidly expand and contract in the local pressure fluctuations caused by the ultrasound field. Mechanical forces exerted by this phenomenon create temporary gaps in the layer of cells that make up the BBB, giving larger molecules a chance to breach the brain’s defences.

More intense ultrasound fields produce a greater effect than weaker fields, and smaller molecules are more likely to diffuse into the brain than larger molecules. To quantify the relationship, Valdez and colleagues injected mice with dextran solutions containing molecules of three different weights: 3, 70 and 500 kilodaltons. They then administered a solution of microbubbles and subjected each mouse to FUS at one of three intensity levels.

When the procedure was complete, the researchers injected a fixing agent to preserve the distribution of the dextran molecules, then removed and sliced the mice’s brains for study. The method echoes past investigations conducted along similar lines, but in the earlier studies, each animal was given just one size of molecule, meaning inherent physiological differences between individuals made the results less certain. This time, Valdez and colleagues labelled each size of dextran molecule with a specific fluorescent marker, so that the distribution of all three could be measured simultaneously in each individual mouse.

Examining the brain slices using fluorescence microscopy, the researchers found that higher ultrasound pressures allowed the dextran to perfuse larger volumes of brain tissue. They noted, however, that the largest (500 kDa) molecules failed to penetrate the brain whatever the ultrasound intensity, and even the intermediate-weight (70 kDa) molecules only spread over small volumes.

Localized “hotspots” on the fluorescence images suggested that, though these dextran molecules had managed to breach the cells of the BBB, their size prevented them from diffusing further into the brain parenchyma. If large molecules like antibodies and other therapeutic proteins are restricted to the parts of the brain immediately next to where they exit the blood vessel, clinicians will need to get around the BBB at multiple sites simultaneously in order to access sufficient volumes of brain tissue.

Along with the three varieties of dextran, Valdez and colleagues also injected each mouse with a gadolinium-based MRI contrast agent. In some trials of the microbubble–FUS technique conducted until now, researchers have used this contrast as an easily detectable proxy for the therapeutic compound, confirming with MRI the volume perfused by the drug. Trouard’s team, however, showed that the small molecular weight of the contrast agent means that it diffuses into the brain much more readily than larger drug molecules, making it only a surrogate indicator of a procedure’s success.

Next, says Trouard, the researchers will investigate whether the BBB can be breached by a specific antibody that targets a protein associated with Parkinson’s disease, and whether it has any effect on the condition’s symptoms in mice.

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