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Category: physics

Gravity follows Newton and Einstein’s rules, even at cosmic scales

Gravity, as most people understand it, is the familiar force that pulls a falling apple toward Earth. But for astronomers and theoretical physicists, it is also a vexing invisible architect that guides the shape and evolution of the largest cosmic structures across the universe.

For decades, puzzling observations of unusually fast-moving galaxies have forced cosmologists like the University of Pennsylvania’s Patricio A. Gallardo to revisit the fundamentals of physics, exploring, for example, whether the laws of gravity as described by Isaac Newton and Albert Einstein truly apply everywhere.

“Astrophysics has been plagued by a massive discrepancy in the cosmic ledger,” says Gallardo. “When we look at how stars orbit within galaxies or how galaxies move within galaxy clusters, some appear to be traveling way too fast for the amount of visible matter they contain.”

Scientists Make Breakthrough on 40-Year-Old 2D Physics Puzzle

Why do patterns emerge as surfaces grow, whether in crystals, flames, or living systems? Physicists have long turned to the Kardar–Parisi–Zhang (KPZ) equation, proposed in 1986, as a unifying description of these processes. This theory captures how randomness and nonlinear effects shape growth across vastly different systems, from spreading bacterial colonies to data-driven algorithms.

Now, researchers at the University of Würzburg have taken a major step toward confirming just how universal this idea really is. After earlier success in one dimension, they have demonstrated for the first time that KPZ behavior also governs growth in two-dimensional systems, a milestone that had remained experimentally out of reach.

Sean Carroll — Why Fine-tuning Seems Designed

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If all is random and our universe is the only universe, the chance existence of human awareness would seem incredible. Because the laws of physics would have to be so carefully calibrated to enable stars and planets to form and life to emerge, it would seem to require some kind of design. But there are other explanations.

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Sean Carroll is Homewood Professor of Natural Philosophy at Johns Hopkins University and fractal faculty at the Santa Fe Institute. His research focuses on fundamental physics and cosmology.

Droplet impacts reveal surprising physics in shear-thickening fluids

From ketchup to quicksand, non-Newtonian fluids have long fascinated and puzzled scientists. Unlike ordinary fluids, their flow properties change depending on how much force is applied, but the precise mechanics governing this behavior remain poorly understood—particularly under rapid deformation. Now, a team led by Xiang Cheng at the University of Minnesota has used droplet impacts to probe these dynamics in new detail, uncovering behaviors which have eluded physicists so far. Their findings appear in Physical Review Letters.

While ordinary Newtonian fluids maintain a constant viscosity regardless of the forces acting on them, non-Newtonian fluids behave very differently: with viscosities that can increase or decrease in response to stress. One classic example is a “shear-thickening” fluid, which can be made simply by mixing cornstarch into water. At high enough concentrations, these suspensions can jam almost completely solid under sudden impacts, even allowing a person to run across them without sinking.

In their study, Cheng’s team prepared cornstarch-water suspensions ranging from 30% to 43% cornstarch by volume, spanning regimes from mild to dramatic shear thickening. They then dropped millimeter-scale droplets of the fluids onto a metal plate at high speeds, producing particularly extreme shear thickening.

Physicists just witnessed pinpricks of darkness moving faster than the speed of light ‪—‬ without breaking the laws of relativity

For the first time, researchers measured singularities in combined light and sound waves moving faster than the speed of light. The findings have implications in fluid dynamics, optics and many other fields.

From ship wakes to soft tissues: Exploring fluid and solid surface-wave physics

A new study by scientists in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) shows that when a pressure disturbance moves across an ultrasoft elastic material, such as a gel or a biological tissue, it generates a V-shaped wake that’s strikingly similar to the waves that travel behind a boat.

Published in Physical Review Letters, the study offers a unified perspective, combining experiments and theory, on surface motion that spans fluids, solids, and the soft materials that lie between. It opens the door to new approaches to imaging and understanding the behavior of both natural and engineered soft materials.

The research was led by L. Mahadevan, the Lola England de Valpine Professor of Applied Mathematics, Organismic and Evolutionary Biology, and Physics, in SEAS and FAS, and includes first author and former postdoctoral researcher Aditi Chakrabarti; postdoctoral researcher Divya Jaganathan, and SEAS research associate Robert Haussman.

Rapid method uncovers hidden structures in materials—including elusive quasicrystals

An international team of scientists, including researchers from Loughborough University, has developed a method to dramatically speed up the discovery and design of advanced materials. The study, published in Physical Review Letters, shows how the new approach can map complex phase diagrams in as little as a day—rather than weeks or months—and pinpoint where important structures, including crystals and quasicrystals, are likely to form.

The method will enable scientists to “scout ahead” and identify where promising structures are likely to form and the conditions needed to create them, rather than using a trial-and-error approach. It could help accelerate the development of advanced materials and technologies that harness the unique properties of quasicrystal structures.

“Our approach is a day’s work for an expert—it’s much faster,” said Professor Andrew Archer, an expert in applied mathematics and theoretical physics at Loughborough University and one of the paper’s authors.

How young galaxies grew magnetic fields faster than expected

How fast can a galaxy build ordered magnetic fields spanning thousands of light-years? Existing theories say several billion years, but observations of galaxies in our universe imply shorter timescales. In a study published in the Physical Review Letters and highlighted in the Physics magazine, scientists propose an explanation that resolves this contradiction. They say that the collapse of plasma clouds during the formation of galaxies could significantly accelerate the growth of these magnetic fields.

Almost all visible matter in our universe is in the form of plasma, which can be stirred by forces related to gravity, temperature gradients and rotation. If these lead to turbulent flow, the dynamo theory predicts that the existing magnetic fields in the plasma are amplified. The dynamo theory is our primary framework for understanding the origin of cosmic magnetic fields.

“However, dynamo theory has its limitations,” says Pallavi, an assistant professor at the International Centre for Theoretical Sciences (ICTS) and an author of the study. “In particular, it struggles to explain observations of young galaxies with robust magnetic fields across thousands of light-years.”

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