May 22, 2021 08:53 PM
(This post was last modified: May 22, 2021 08:54 PM by C C.)
‘Clyde’s Spot’ on Jupiter is starting to look pretty weird
https://gizmodo.com/clyde-s-spot-on-jupi...1846940380
EXCERPTS: Last May, a spot suddenly appeared in Jupiter’s southern hemisphere. But as new images from the Juno spacecraft show, the once circular feature has morphed into an enigmatic splotch. [...] Clyde’s Spot, as it’s informally known, is a convective outbreak—a plume of cloud that’s reaching out beyond the regular cloud tops—and is located to the southeast of Jupiter’s Great Red Spot. Such outbreaks are not uncommon within the gas giant’s South Temperate Belt.
[...] The updated view reminds us to not get attached to the beautiful shapes and colors that appear on Jupiter, as many of them are short lived. In the year since it was discovered, Clyde’s Spot has drifted away from the Great Red Spot and morphed into something that wouldn’t look out of place in a puddle on a gas station parking lot... (MORE - details)
Turn off the sound and read the captions if you don't understand Greek
https://www.youtube-nocookie.com/embed/QvIMFKMRD0I
How a weird theory of gravity could break cause-and-effect
https://www.livescience.com/modified-gra...ravel.html
EXCERPTS: Astronomers have known that galaxies across the universe are behaving badly. Some are spinning too fast, while others are just way too hot and still others glommed into super structures too quickly. But they don't know why. Perhaps some new hidden particle, like dark matter, could explain the weirdness. Or perhaps gravity is acting on these coalescing clusters of stars in a way scientists hadn't expected.
[...] cold dark matter (CDM), which is the name given to a hypothetical form of matter that is as yet unknown to physics. ... But the CDM hypothesis isn't perfect. ... What else could be going on?
An alternative to the whole CDM idea is a modified understanding of gravity. The simplest models fall under a class called MOND, for Modified Newtonian Dynamics. These models replace Newtonian physics (think Force = mass x acceleration) with other relationships that match the observed rotation rate of stars inside galaxies. While these models were popular when dark matter was first discovered in the 1970s and 1980s, they have failed to account for observations of galaxy clusters and the larger universe; as such, most scientists have all but rejected these models.
But the inadequacies of CDM to explain internal galactic dynamics provide an opening for MOND to survive. [...] "The only possibility to obtain something new [within the framework of relativity and quantum mechanics] is to add new degrees of freedom," Hertzberg told Live Science. In other words, in order to get MONDian theories to work with known physics, you have to add a whole bunch of funky stuff to theories. In examining that funky stuff, Hertzerg and collaborators found "some theoretical problems lurking in these attempts."
Local and causal
For instance, Hertzberg and his collaborators examined whether MONDian theories protect two principles: locality and causality. Locality is the concept that objects are directly influenced only by their surroundings — in order for one object to influence another, it must transmit that influence via something like a force that travels at a finite speed. Causality is the simple notion that all events have a cause.
If a theory violates locality and/or causality, it is unlikely to fit in with our theories of physics, which do protect both principles.
"If one gives up the principles of causality and locality, then it means we are essentially unable to explain the structure of the Standard Model of Particle Physics and General Relativity, as they are some of the central principles that go into constructing these theories in the first place," Hertzberg said. "In other words, if causality were badly broken in nature, we likely would have seen it already in various corrections to particle physics in the lab or tests of gravity in space."
In other words, we should've noticed by now. Since all available evidence indicates that locality and causality are preserved (at least at macroscopic scales), then they should be obeyed by any new theory of physics. The team of physicists put MONDian theories to the test and found that they contain features that allow for non-locality and acausality. In other words, if MONDian theories are correct, then it's possible for events to happen without a cause and for effects to travel instantaneously, which violates the speed-of-light limit in the universe.
[...] It might indeed be possible for locality and causality to be violated on galactic scales, but this would be extremely difficult to reconcile with everything else we know about physics.
As to the future of MONDian theories, Hertzberg speculated, "it motivates attempts to try to construct some classes of similar models that somehow maintain causality, but this looks difficult to achieve. In our paper, we show that a generalized form of these models fails the above tests for consistency."
Still, the "cold dark matter" paradigm has difficulty explaining the details of galactic physics. But there could be far more mundane reasons for this rather than upending all known physics. Modeling how galaxies form and evolve, even just accounting for all the messy processes where normal matter plays a role, is very difficult... (MORE - missing details)
https://gizmodo.com/clyde-s-spot-on-jupi...1846940380
EXCERPTS: Last May, a spot suddenly appeared in Jupiter’s southern hemisphere. But as new images from the Juno spacecraft show, the once circular feature has morphed into an enigmatic splotch. [...] Clyde’s Spot, as it’s informally known, is a convective outbreak—a plume of cloud that’s reaching out beyond the regular cloud tops—and is located to the southeast of Jupiter’s Great Red Spot. Such outbreaks are not uncommon within the gas giant’s South Temperate Belt.
[...] The updated view reminds us to not get attached to the beautiful shapes and colors that appear on Jupiter, as many of them are short lived. In the year since it was discovered, Clyde’s Spot has drifted away from the Great Red Spot and morphed into something that wouldn’t look out of place in a puddle on a gas station parking lot... (MORE - details)
Turn off the sound and read the captions if you don't understand Greek
How a weird theory of gravity could break cause-and-effect
https://www.livescience.com/modified-gra...ravel.html
EXCERPTS: Astronomers have known that galaxies across the universe are behaving badly. Some are spinning too fast, while others are just way too hot and still others glommed into super structures too quickly. But they don't know why. Perhaps some new hidden particle, like dark matter, could explain the weirdness. Or perhaps gravity is acting on these coalescing clusters of stars in a way scientists hadn't expected.
[...] cold dark matter (CDM), which is the name given to a hypothetical form of matter that is as yet unknown to physics. ... But the CDM hypothesis isn't perfect. ... What else could be going on?
An alternative to the whole CDM idea is a modified understanding of gravity. The simplest models fall under a class called MOND, for Modified Newtonian Dynamics. These models replace Newtonian physics (think Force = mass x acceleration) with other relationships that match the observed rotation rate of stars inside galaxies. While these models were popular when dark matter was first discovered in the 1970s and 1980s, they have failed to account for observations of galaxy clusters and the larger universe; as such, most scientists have all but rejected these models.
But the inadequacies of CDM to explain internal galactic dynamics provide an opening for MOND to survive. [...] "The only possibility to obtain something new [within the framework of relativity and quantum mechanics] is to add new degrees of freedom," Hertzberg told Live Science. In other words, in order to get MONDian theories to work with known physics, you have to add a whole bunch of funky stuff to theories. In examining that funky stuff, Hertzerg and collaborators found "some theoretical problems lurking in these attempts."
Local and causal
For instance, Hertzberg and his collaborators examined whether MONDian theories protect two principles: locality and causality. Locality is the concept that objects are directly influenced only by their surroundings — in order for one object to influence another, it must transmit that influence via something like a force that travels at a finite speed. Causality is the simple notion that all events have a cause.
If a theory violates locality and/or causality, it is unlikely to fit in with our theories of physics, which do protect both principles.
"If one gives up the principles of causality and locality, then it means we are essentially unable to explain the structure of the Standard Model of Particle Physics and General Relativity, as they are some of the central principles that go into constructing these theories in the first place," Hertzberg said. "In other words, if causality were badly broken in nature, we likely would have seen it already in various corrections to particle physics in the lab or tests of gravity in space."
In other words, we should've noticed by now. Since all available evidence indicates that locality and causality are preserved (at least at macroscopic scales), then they should be obeyed by any new theory of physics. The team of physicists put MONDian theories to the test and found that they contain features that allow for non-locality and acausality. In other words, if MONDian theories are correct, then it's possible for events to happen without a cause and for effects to travel instantaneously, which violates the speed-of-light limit in the universe.
[...] It might indeed be possible for locality and causality to be violated on galactic scales, but this would be extremely difficult to reconcile with everything else we know about physics.
As to the future of MONDian theories, Hertzberg speculated, "it motivates attempts to try to construct some classes of similar models that somehow maintain causality, but this looks difficult to achieve. In our paper, we show that a generalized form of these models fails the above tests for consistency."
Still, the "cold dark matter" paradigm has difficulty explaining the details of galactic physics. But there could be far more mundane reasons for this rather than upending all known physics. Modeling how galaxies form and evolve, even just accounting for all the messy processes where normal matter plays a role, is very difficult... (MORE - missing details)