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Philosophy Of Physics (@PhilosophyOfPhy) on X

The continuity equation was not the work of a single physicist. Its development grew from early hydraulic studies and the work of Daniel and Johann Bernoulli. In the eighteenth century, Jean le Rond d’Alembert produced the first partial-differential expression of mass conservation in fluid motion, and Leonhard Euler soon placed it in the general mathematical framework that became the foundation of modern fluid mechanics. It should therefore not be attributed solely to Giovanni Battista Venturi, whose later work concerned flow through constricted tubes. Its general form is ∂ρ/∂t + ∇·(ρv) = 0 where ρ is fluid density and v is the velocity field. The equation says that mass cannot simply appear or disappear: any change in the amount of fluid inside a region must be explained by fluid entering or leaving it. For steady flow through a pipe, this becomes ρ₁A₁v₁ = ρ₂A₂v₂ If the fluid is effectively incompressible, its density remains constant, giving the familiar form: A₁v₁ = A₂v₂ The meaning is simple. The same volume of fluid must pass through every section of the pipe each second. When the pipe becomes narrower, the fluid must move faster; when it becomes wider, the fluid slows down. This equation is fundamental to the study of pipes, nozzles, rivers, aircraft flow, circulation systems and computational fluid dynamics. More broadly, continuity equations appear throughout physics wherever something locally conserved, such as mass or electric charge, moves through space. The equation is not merely about fluids; it is the mathematical language of the principle that what flows into a region must either flow out or remain inside.

These ancient quasars shouldn’t exist so soon after the Big Bang

Scientists have found the oldest quasars ever seen, revealing giant black holes blazing across the universe when it was only 670 million years old. Astronomers have uncovered 31 of the oldest known quasars, including the two earliest ever detected, shining from a time when the universe was only about 670 million years old. Powered by supermassive black holes billions of times the Sun’s mass, these incredibly bright objects challenge scientists’ understanding of how such enormous black holes formed so quickly after the Big Bang.

Quasars rank among the brightest and most powerful objects in the universe. They are fueled by supermassive black holes that consume surrounding material at the centers of galaxies, producing so much energy that they can be seen across billions of light years.

Now, an international team of researchers has identified 31 of the oldest quasars ever discovered, including the two earliest known examples. These extraordinary objects were already shining with the light of roughly a trillion suns when the universe was only about 670 million years old. The discovery, published in Astronomy & Astrophysics, offers an unprecedented glimpse into one of the earliest chapters of cosmic history.

New radio-burst method helps locate universe’s missing ordinary matter

Stars and galaxies make up much of the universe’s ordinary, observable matter. But for decades, scientists have wrestled with a cosmic conflict: There should be much more.

Physicists have good estimates of how much matter was present in the early universe. Shortly after the Big Bang, roughly 83% of all matter in the universe was composed of invisible dark matter, with ordinary matter making up the rest. And yet, these estimates exceed the amount of ordinary matter seen in stars and galaxies today. Where, then, did all the missing ordinary matter go?

Now MIT scientists, as part of the CHIME/FRB Collaboration, are using far-off radio signals to reveal missing matter in the vast space between galaxies. The team has developed a new method to search out missing matter by combining locations of galaxies with detections of fast radio bursts.

Two-color lasers aim electron currents through semiconductor with no electric field

Researchers at the University of Michigan have created a device that enables them to control the flow of electrons through a semiconductor using only laser light—no electrical power source required. The device was built to explore fundamental physics and realize a previously unobserved behavior, but it could also open doors for new applications in areas that bridge optics and electronics, including sensing, imaging and telecommunications.

The paper is published in the journal Physical Review Letters.

The phenomenon could help improve how signals are sent through and between devices, as well as create new opportunities to store more information in those signals.

Scientists Broke a 160-Year-Old Law of Physics to Create Programmable Heat

There are certain laws of physics that heat must follow.

Take Kirchhoff’s law of thermal radiation, for example, which applies the idea of reciprocity to heat, and dictates that a surface’s ability to absorb heat at a specific angle and wavelength must also match its ability to emit heat at the same angle and wavelength.

It’s a rule that makes thermal energy difficult to control in ways we might like to, and although workarounds have been found before, they’re inefficient and volatile.

Programmable metasurface turns keyboard commands into dynamic holograms in milliseconds

Metasurfaces are ultrathin optical components engineered with arrays of nanoscale structures that can control light in ways that are difficult for conventional optics. Unlike traditional optical components, which typically rely on their shape and thickness, metasurfaces manipulate light using carefully designed nanostructures patterned on a flat surface.

“Active metasurfaces are extending the capabilities of flat optics by enabling optical functions to be dynamically reconfigured,” says Professor Laura Na Liu, director of the 2nd Physics Institute at the University of Stuttgart. “To tap into this potential, we need to learn how to address individual pixels within a two-dimensional metasurface at visible wavelengths.”

Researchers at the University of Stuttgart have developed an interactively addressable organic metadevice that uses electrically switchable organic materials to dynamically control light. The new platform enables every metasurface pixel to be electronically controlled independently, allowing user commands to be translated directly into dynamic holographic images.

How physics and mathematical modeling help us make better clothes

A new paper in the journal Nature Physics offers insights into the physics of liquid droplets—and while many people may not appreciate the mathematical accomplishment, they will benefit from the athletic wear and raincoats it makes possible. The recent article, “Tricky Tension,” explores the intersection of physics and textiles and how wetting is influenced by the structure of tiny individual liquid droplets.

In physics, the cohesive force between two phases is called surface tension. This allows small insects to walk on water.

In a three-phase system—where gas, liquid and solid objects all interact—there is a less-understood phenomenon called the line tension of a liquid droplet. This refers to the force acting at the boundary where the liquid droplet, the air and the solid surface on which the droplet sits all meet. Learning more about the mechanics of droplets on solid surfaces, known as sessile droplets, is important for understanding the wetting and drying of textiles, especially for very small droplets.

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