gravity

Next-generation black hole imaging may help us understand gravity better

Right now, we probably don’t have the ability to detect these small changes in phenomena. However, that may change, as a next-generation version of the Event Horizon Telescope is being considered, along with a space-based telescope that would operate on similar principles. So the team (four researchers based in Shanghai and CERN) decided to repeat an analysis they did shortly before the Event Horizon Telescope went operational, and consider whether the next-gen hardware might be able to pick up features of the environment around the black hole that might discriminate among different theorized versions of gravity.

Theorists have been busy, and there are a lot of potential replacements for general relativity out there. So, rather than working their way through the list, they used a model of gravity (the parametric Konoplya–Rezzolla–Zhidenko metric) that isn’t specific to any given hypothesis. Instead, it allows some of its parameters to be changed, thus allowing the team to vary the behavior of gravity within some limits. To get a sense of the sort of differences that might be present, the researchers swapped two different parameters between zero and one, giving them four different options. Those results were compared to the Kerr metric, which is the standard general relativity version of the event horizon.

Small but clear differences

Using those five versions of gravity, they model the three-dimensional environment near the event horizon using hydrodynamic simulations, including infalling matter, the magnetic fields it produces, and the jets of matter that those magnetic fields power.

The results resemble the sorts of images that the Event Horizon Telescope produced. These include a bright ring with substantial asymmetry, where one side is significantly brighter due to the rotation of the black hole. And, while the differences are subtle between all the variations of gravity, they’re there. One extreme version produced the smallest but brightest ring; another had a reduced contrast between the bright and dim side of the ring. There were also differences between the width of the jets produced in these models.

Delve into the physics of the Hula-Hoop

applied mathematics, gravity, hula hoop, mathematics, Physics, robotics, Science, Uncategorized / Kelly Newman / January 3, 2025

High-speed video of experiments on a robotic hula hooper, whose hourglass form holds the hoop up and in place.

Some version of the Hula-Hoop has been around for millennia, but the popular plastic version was introduced by Wham-O in the 1950s and quickly became a fad. Now, researchers have taken a closer look at the underlying physics of the toy, revealing that certain body types are better at keeping the spinning hoops elevated than others, according to a new paper published in the Proceedings of the National Academy of Sciences.

“We were surprised that an activity as popular, fun, and healthy as hula hooping wasn’t understood even at a basic physics level,” said co-author Leif Ristroph of New York University. “As we made progress on the research, we realized that the math and physics involved are very subtle, and the knowledge gained could be useful in inspiring engineering innovations, harvesting energy from vibrations, and improving in robotic positioners and movers used in industrial processing and manufacturing.”

Ristroph’s lab frequently addresses these kinds of colorful real-world puzzles. For instance, in 2018, Ristroph and colleagues fine-tuned the recipe for the perfect bubble based on experiments with soapy thin films. In 2021, the Ristroph lab looked into the formation processes underlying so-called “stone forests” common in certain regions of China and Madagascar.

In 2021, his lab built a working Tesla valve, in accordance with the inventor’s design, and measured the flow of water through the valve in both directions at various pressures. They found the water flowed about two times slower in the nonpreferred direction. In 2022, Ristroph studied the surpassingly complex aerodynamics of what makes a good paper airplane—specifically, what is needed for smooth gliding.

Girl twirling a Hula hoop, 1958 — Girl twirling a Hula-Hoop in 1958 Credit: George Garrigues/CC BY-SA 3.0

And last year, Ristroph’s lab cracked the conundrum of physicist Richard Feynman’s “reverse sprinkler” problem, concluding that the reverse sprinkler rotates a good 50 times slower than a regular sprinkler but operates along similar mechanisms. The secret is hidden inside the sprinkler, where there are jets that make it act like an inside-out rocket. The internal jets don’t collide head-on; rather, as water flows around the bends in the sprinkler arms, it is slung outward by centrifugal force, leading to asymmetric flow.

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