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New theory unites Einstein’s gravity with quantum mechanics

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New theory unites Einstein’s gravity with quantum mechanics


A radical theory that consistently unifies gravity and quantum mechanics while preserving Einstein’s classical concept of spacetime is announced today in two papers published simultaneously by UCL (University College London) physicists.

Modern physics is founded upon two pillars: quantum theory on the one hand, which governs the smallest particles in the universe, and Einstein’s theory of general relativity on the other, which explains gravity through the bending of spacetime. But these two theories are in contradiction with each other and a reconciliation has remained elusive for over a century.

The prevailing assumptionhas been that Einstein’s theory of gravity must be modified, or “quantised,” in order to fit within quantum theory. This is the approach of two leading candidates for a quantum theory of gravity, string theory and loop quantum gravity.

But a new theory, developed by Professor Jonathan Oppenheim (UCL Physics & Astronomy) and laid out in a new paper in Physical Review X (PRX), challenges that consensus and takes an alternative approach by suggesting that spacetime may be classical — that is, not governed by quantum theory at all.

Instead of modifying spacetime, the theory — dubbed a “postquantum theory of classical gravity” — modifies quantum theory and predicts an intrinsic breakdown in predictability that is mediated by spacetime itself. This results in random and violent fluctuations in spacetime that are larger than envisaged under quantum theory, rendering the apparent weight of objects unpredictable if measured precisely enough.

A second paper, published simultaneously in Nature Communications and led by Professor Oppenheim’s former PhD students,looks atsome of the consequences of the theory, and proposes an experiment to test it: to measure a mass very precisely to see if its weight appears to fluctuate over time.

For example, the International Bureau of Weights and Measures in France routinely weigh a 1kg mass which used to be the 1kg standard. If the fluctuations in measurements of this 1kg mass are smaller than required for mathematical consistency, the theory can be ruled out.

The outcome of the experiment, or other evidence emerging which would confirm the quantum vs classical nature of spacetime, is the subject of a 5000:1 odds bet between Professor Oppenheim and Professor Carlo Rovelli and Dr Geoff Penington — leading proponents of quantum loop gravity and string theory respectively.

For the past five years, the UCL research group has been stress-testing the theory, and exploring its consequences.

Professor Oppenheim said: “Quantum theory and Einstein’s theory of general relativity are mathematically incompatible with each other, so it’s important to understand how this contradiction is resolved. Should spacetime be quantised, or should we modify quantum theory, or is it something else entirely? Now that we have a consistent fundamental theory in which spacetime does not get quantised, it’s anybody’s guess.”

Co-author Zach Weller-Davies, who as a PhD student at UCL helped develop the experimental proposal and made key contributions to the theory itself, said: “This discovery challenges our understanding of the fundamental nature of gravity but also offers avenues to probe its potential quantum nature.

“We have shown that if spacetime doesn’t have a quantum nature, then there must be random fluctuations in the curvature of spacetime which have a particular signature that can be verified experimentally.

“In both quantum gravity and classical gravity, spacetime must be undergoing violent and random fluctuations all around us, but on a scale which we haven’t yet been able to detect. But if spacetime is classical, the fluctuations have to be larger than a certain scale, and this scale can be determined by another experiment where we test how long we can put a heavy atom in superposition* of being in two different locations.”

Co-authors Dr Carlo Sparaciari and Dr Barbara Šoda, whose analytical and numerical calculations helped guide the project, expressed hope that these experiments could determine whether the pursuit of a quantum theory of gravity is the right approach.

Dr Šoda (formerly UCL Physics & Astronomy, now at the Perimeter Institute of Theoretical Physics, Canada) said: “Because gravity is made manifest through the bending of space and time, we can think of the question in terms of whether the rate at which time flows has a quantum nature, or classical nature.

“And testing this is almost as simple as testing whether the weight of a mass is constant, or appears to fluctuate in a particular way.”

Dr Sparaciari (UCL Physics & Astronomy) said: “While the experimental concept is simple, the weighing of the object needs to be carried out with extreme precision.

“But what I find exciting is that starting from very general assumptions, we can prove a clear relationship between two measurable quantities — the scale of the spacetime fluctuations, and how long objects like atoms or apples can be put in quantum superposition of two different locations. We can then determine these two quantities experimentally.”

Weller-Davies added: “A delicate interplay must exist if quantum particles such as atoms are able to bend classical spacetime. There must be a fundamental trade-off between the wave nature of atoms, and how large the random fluctuations in spacetime need to be.”

The proposal to test whether spacetime is classical by looking for random fluctuations in mass is complementary to another experimental proposal which aims to verify the quantum nature of spacetime by looking for something called “gravitationally mediated entanglement.”

Professor Sougato Bose (UCL Physics & Astronomy), who was not involved with the announcement today, but was among those to first propose the entanglement experiment, said: “Experiments to test the nature of spacetime will take a large-scale effort, but they’re of huge importance from the perspective of understanding the fundamental laws of nature. I believe these experiments are within reach — these things are difficult to predict, but perhaps we’ll know the answer within the next 20 years.”

The postquantum theory has implications beyond gravity. The infamous and problematic “measurement postulate” of quantum theory is not needed, since quantum superpositions necessarily localise through their interaction with classical spacetime.

The theory was motivated by Professor Oppenheim’s attempt to resolve the black hole information problem. According to standard quantum theory, an object going into a black hole should be radiated back out in some way as information cannot be destroyed, but this violates general relativity, which says you can never know about objects that cross the black hole’s event horizon. The new theory allows for information to be destroyed, due to a fundamental breakdown in predictability.

* Background information

Quantum mechanics background: All the matter in the universe obeys the laws of quantum theory, but we only really observe quantum behaviour at the scale of atoms and molecules. Quantum theory tells us that particles obey Heisenberg’s uncertainty principle, and we can never know their position or velocity at the same time. In fact, they don’t even have a definite position or velocity until we measure them. Particles like electrons can behave more like waves and act almost as if they can be in many places at once (more precisely, physicists describe particles as being in a “superposition” of different locations).

Quantum theory governs everything from semiconductors which are ubiquitous in computer chips, to lasers, to superconductivity to radioactive decay. In contrast, we say that a system behaves classically if it has definite underlying properties. A cat appears to behave classically — it is either dead or alive, not both, nor in a superposition of being dead and alive. Why do cats behave classically, and small particles quantumly? We don’t know, but the postquantum theory doesn’t require the measurement postulate, because the classicality of spacetime infects quantum systems and causes them to localise.

Gravity background: Newton’s theory of gravity, gave way to Einstein’s theory of general relativity (GR), which holds that gravity is not a force in the usual sense. Instead, heavy objects such as the sun, bend the fabric of spacetime in such a way that causes the earth to revolve around it. Spacetime is just a mathematical object consisting of the three dimensions of space, and time considered as a fourth dimension. General relativity predicted the formation of black holes and the big bang. It holds that time flows at different rates at different points in space, and the GPS in your smartphone needs to account for this in order to properly determine your location.

Historical context: The framework presented by Oppenheim in PRX, and in a companion paper with Sparaciari, Šoda and Weller-Davies, derives the most general consistent form of dynamics in which a quantum system interacts with a classical system. It then applies this framework to the case of general relativity coupled to quantum fields theory. It builds on earlier work and a community of physicists. An experiment to test the quantum nature of gravity via gravitationally mediated entanglement was proposed by Bose et. al. and by C. Marletto and V. Vadral. Two examples of consistent classical-quantum dynamics were discovered in the 90’s by Ph. Blanchard and A. Jadzyk, and by Lajos Diosi, and again by David Poulin around 2017. From a different perspective, in 2014 a model of Newtonian gravity coupled to quantum systems via a “measurement-and-feedback” approach, was presented by Diosi and Antoinne Tilloy in 2016, and by D Kafri, J. Taylor, and G. Milburn, in 2014. The idea that gravity might be somehow related to the collapse of the wavefunction, dates back to F. Karolyhazy (1966), L. Diosi (1987) and R. Penrose (1996). That classical-quantum couplings might explain localistation of the wavefunction has been suggested by others including M. Hall and M. Reginatto, Diosi and Tilloy, and David Poulin. The idea that spacetime might be classical dates back to I. Sato (1950), and C. Moller (1962), but no consistent theory was found until now.



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Bird flu: Diverse range of vaccines platforms ‘crucial’ for enhancing human pandemic preparedness

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Bird flu: Diverse range of vaccines platforms ‘crucial’ for enhancing human pandemic preparedness


Vaccination remains the most effective strategy for avian influenza prevention and control in humans, despite varying vaccine efficacy across strains.

That’s according to the authors of a new review which delves into existing research into bird flu vaccines for humans.

Published in the peer-reviewed journal Human Vaccines & Immunotherapeutics, the results of the paper are particularly timely following news last week (Wednesday 22nd May) that the bird flu strain H5N1 had once again, for a second time, jumped from cattle in America to a human — prompting fears of subsequent human-to-human infection, with possible critical consequences.

Instances of the avian influenza were first recognized in US cattle in March. Since then, this strain has mainly spread from cow-to-cow and scientists have discovered very high levels of virus in raw milk (pasteurized milk is safe, having shown viral RNA but not infectious virus). To-date two people, however, are known to have contracted the bird flu virus. Both patients — US farmers — only reported eye symptoms and with treatment they made a full recovery.

Following tests on the first human instance, it was seen that the strain had mutated to be better adapted to mammalian cells, but as long as that human didn’t pass it onto another person it likely stopped the spread at that point. With the second case, the CDC has released a statement to say it has been monitoring influenza surveillance systems intently, especially in impacted states. “There has been no sign of unusual influenza activity in people, including in syndromic surveillance,” they report.

The concern now, though, is that if H5N1 continues to be given the environment in which to mutate (such as in close quarter cattle farms) — and this continues long enough — it has the potential to find a combination that will easily spread to humans.

The results of this new research, carried out by a team at the University of Georgia, USA, suggests vaccines still remain our “primary defense” against potential spread of avian influenzas such as the H5N1 and others assessed.

“The H5N1, H7N9, and H9N2 subtypes of avian influenza virus pose a dual threat, not only causing significant economic losses to the global poultry industry but also presenting a pressing public health concern due to documented spillover events and human cases,” explains lead author Flavio Cargnin Faccin, who alongside his mentor Dr. Daniel Perez of the University of Georgia, USA, analyzed the current landscape of research into human vaccines for these bird flus.

“This deep delve into the landscape of avian influenza vaccines for humans shows vaccination remains the primary defense against the spread of these viruses.”

The team examined studies of vaccines tested in mice, ferrets, non-human primates, and clinical trials of bird flu vaccines in humans, and assessed both established platforms and promising new directions.

The review carried out suggests inactivated vaccines are a safe and affordable option that primarily activate humoral immunity — the part of our immune system that produces antibodies.

Live attenuated influenza vaccines (LAIVs) are known to induce a wider immune response than inactivated vaccines, activating not only antibody production but also mucosal and cellular defenses. In this review, the authors suggest this broader response may offer greater protection, though, the authors suggest further research is needed to fully understand and harness its potential benefits for both human and agricultural applications.

The review also examined alternatives, such as virus-like particle (VLP) vaccines and messenger RNA (mRNA) vaccines, that have emerged more recently. Although VLP vaccines for bird flu have limited clinical trial data in humans, results from studies in mice and ferrets showed promise, the authors found. mRNA vaccines against H5N1 and H7N9 bird flu subtypes also generated a rapid and strong immune response in mice and ferrets, and, while data in humans is scarce, results from a phase 1 study of an H7N9 mRNA vaccine in healthy humans were “encouraging.”

Overall, the team suggests “exploring and employing a diverse range of vaccine platforms,” will be “crucial for enhancing pandemic preparedness and mitigating the threat of avian influenza viruses.”



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When should you neuter or spay your dog?

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When should you neuter or spay your dog?


Researchers at the University of California, Davis, have updated their guidelines on when to neuter 40 popular dog varieties by breed and sex. Their recent paper in Frontiers in Veterinary Science adds five breeds to a line of research that began in 2013 with a study that suggested that early neutering of golden retrievers puts them at increased risk of joint diseases and certain cancers.

That initial study set off a flurry of debate about the best age to neuter other popular breeds. Professors Lynette and Benjamin Hart of the School of Veterinary Medicine, the study’s lead authors, set out to add more breed studies by examining more than a decade of data from thousands of dogs treated at the UC Davis veterinary hospital. Their goal was to provide owners with more information to make the best decision for their animals.

They specifically looked at the correlation between neutering or spaying a dog before 1 year of age and a dog’s risk of developing certain cancers. These include cancers of the lymph nodes, bones, blood vessels or mast cell tumors for some breeds; and joint disorders such as hip or elbow dysplasia, or cranial cruciate ligament tears. Joint disorders and cancers are of particular interest because neutering removes male and female sex hormones that play key roles in important body processes such as closure of bone growth plates.

For the most recent study, they focused on German short/wirehaired pointer, mastiff, Newfoundland, Rhodesian ridgeback and Siberian husky. Data was collected from the UC Davis veterinary hospital’s records that included more than 200 cases for each of these five breeds weighing more than 20 kg (or 44 pounds), spanning January 2000 through December 2020.

The Harts said their updated guidelines emphasize the importance of personalized decisions regarding the neutering of dogs, considering the dog’s breed, sex and context. A table representing guidelines reflecting the research findings for all 40 breeds that have been studied, including the five new breeds, can be found here.

Health risks different among breeds

“It’s always complicated to consider an alternate paradigm,” said Professor Lynette Hart. “This is a shift from a long-standing model of early spay/neuter practices in the U.S. and much of Europe to neuter by 6 months of age, but important to consider as we see the connections between gonadal hormone withdrawal from early spay/neuter and potential health concerns.”

The study found major differences among these breeds for developing joint disorders and cancers when neutered early. Male and female pointer breeds had elevated joint disorders and increased cancers; male mastiff breeds had increased cranial cruciate ligament tears and lymphoma; female Newfoundland breeds had heightened risks for joint disorders; female Ridgeback breeds had heightened risks for mast cell tumors with very early neutering; and Siberian huskies showed no significant effects on joint disorders or cancers.

“We’re invested in making contributions to people’s relationship with their animals,” said Benjamin Hart, distinguished professor emeritus. “This guidance provides information and options for veterinarians to give pet owners, who should have the final decision-making role for the health and well-being of their animal.”

Their combined research studies will soon be available with others in the open access journal, Frontiers of Veterinary Science, as a free e-book, Effective Options Regarding Spay or Neuter of Dogs.

Other researchers on this UC Davis study include: Abigail Thigpen, Maya Lee, Miya Babchuk, Jenna Lee, Megan Ho, Sara Clarkson and Juliann Chou with the School of Veterinary Medicine; and Neil Willits with the Department of Statistics.

The research received a small amount of funding from the Center for Companion Animal Health, but was primarily conducted by the above authors as volunteers.



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Why do Dyeing poison frogs tap dance?

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Why do Dyeing poison frogs tap dance?


The toe tapping behavior of various amphibians has long attracted attention from researchers and pet owners. Despite being widely documented, the underlying functional role is poorly understood. In a new paper, researchers demonstrate that Dyeing poison frogs modulate their taps based on specific stimuli.

Dyeing poison frogs, Dendrobates tinctorius, have been shown to tap their posterior toes in response to a range of prey sizes, from small fruit flies to large crickets. In the present study, the researchers hypothesized that if the tapping has a role in feeding, the frogs would adjust their behavior in response to different environmental cues.

To test their hypothesis, the researchers recorded the frogs under varying conditions. “I used the slow-motion camera on my iPhone to take minute-long videos of the frogs tapping. Afterwards, I went back to each video and counted the number of taps on each foot and how long they were visible since they were often hidden behind a leaf or the frog itself. I used those two numbers to get a “taps per minute” on each foot and added them up,” said Thomas Parrish, a former undergraduate student in the Fischer lab (GNDP), and the first author on the paper.

The researchers first tested whether the frogs tapped their toes more when they were feeding. To do so, the researchers fed the terrariums with half a teaspoon of fruit flies and recorded their hunting.

“We already knew the answer to this, but it was great to see that the tapping increased in the presence of the prey,” said Eva Fischer, an assistant professor of integrative biology. “We wanted to ask ‘Why?’ and we wondered whether it had a function in prey capture or it was just a excitatory response like how dogs wag their tails because they are excited.”

The researchers then used different surfaces to see whether the tapping behavior changed when the frogs could see the prey but not feed on it. They placed the fruit flies in small, clear Petri dishes in the frogs’ home and measured the rate of toe tapping. They found that the frogs had an average of 50 taps/minute when they couldn’t access the flies compared to 166 taps/minute when they fed on free-moving flies.

“The idea was that if they’re excited, we might see something different based on whether they can catch the flies,” Fisher said. “These results suggested that since they kept trying to eat in both cases, the tapping was not just out of excitement.”

The researchers wondered, then, whether the toe taps were a form of vibrational signaling where the frogs used it as a way to startle or distract the prey before they fed. They used four different surfaces to test this question: soil, leaf surfaces, gel, and glass.

“Soil and leaves are natural substances, but soil is not very responsive while leaves are. On the other hand, gels are responsive and glass is not, but they are both unnatural surfaces to frogs,” Fischer said.

They found that while the tap rate differed depending on the surface, with leaves being the highest at 255 taps/minute and glass the lowest at 64 taps/minute, there was no difference in the total number of feeding attempts or success.

“Although we saw that the frogs ate in every context, it was exciting to see that they changed their behavior based on what they’re standing on,” Fischer said. “We were surprised, however, that we didn’t see a difference in how successful they were at eating. It’s possible that the experiment is like sending them to a buffet instead of what happens in the forest where the tapping may help in stirring the prey.”

The researchers are now hoping to understand what other stimuli might trigger this behavior. “Although we’ve conclusively shown that it is important in feeding, it could also be important in other contexts. For example, we have seen that the frogs tap more when there are other frogs nearby, so there may be a social aspect to it,” Fischer said.

They are also interested in studying the underlying biomechanical aspects of the muscles. “It would be cool to look at the anatomy and see how the muscles work,” Fischer said. “Ultimately, we could ask whether all frogs can tap their toes if they have the right muscles or whether there’s something special about the anatomy of poison frogs.”



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